Multiwavelength constraints on the unidentified Galactic TeV sources HESS J1427-608, HESS J1458-608, and new VHE γ-ray source candidates
Justine Devin, Matthieu Renaud, Marianne Lemoine-Goumard, Georges Vasileiadis
AAstronomy & Astrophysics manuscript no. TeVsources © ESO 2021January 20, 2021
Multiwavelength constraints on the unidentified GalacticTeV sources HESS J1427 − − γ -ray source candidates J. Devin , M. Renaud , M. Lemoine-Goumard , and G. Vasileiadis Univ. Bordeaux, CNRS, CENBG, UMR 5797, F-33170 Gradignan, Francee-mail: [email protected], [email protected] Laboratoire Univers et Particules de Montpellier, CNRS / IN2P3, Université de Montpellier, F-34095 Montpellier, FranceReceived 30 September 2020, Accepted 6 January 2021
ABSTRACT
Aims.
Among the γ -ray sources discovered at high and very-high energies, a large fraction still lack a clear identification. In particular,the H.E.S.S. Galactic Plane Survey (HGPS) revealed 78 TeV sources among which 47 are not clearly associated with a known object.Multiwavelength data can help identify the origin of the very-high energy γ -ray emission, although some bright TeV sources have beendetected without clear counterparts. We present a multiwavelength approach to constrain the origin of the emission from unidentifiedHGPS sources. Methods.
We present a generic pipeline that explores a large database of multiwavelength archival data toward any region in theGalactic plane. Along with a visual inspection of the retrieved multiwavelength observations to search for faint and uncatalogedcounterparts, we derive a radio spectral index that helps disentangle thermal from nonthermal emission and a mean magnetic fieldthrough X-ray and TeV data in case of a leptonic scenario. We also search for a spectral connection between the GeV and the TeVregimes with the
Fermi -LAT cataloged sources that may be associated with the unidentified HGPS source. We complete the associationprocedure with catalogs of known objects (supernova remnants, pulsar wind nebulae, H ii regions, etc.) and with the source catalogsfrom instruments whose data are retrieved. Results.
The method is applied on two unidentified sources, namely HESS J1427 −
608 and HESS J1458 − B (cid:46) µ G, which is consistent with that obtained from ancient PWNe. We place both sourceswithin the context of the TeV PWN population to estimate the spin-down power and the characteristic age of the putative pulsar. Wealso shed light on two possibly significant γ -ray excesses in the HGPS: the first is located in the south of the unidentified sourceHESS J1632 −
478 and the second is spatially coincident with the synchrotron-emitting supernova remnant G28.6 − γ -ray excesses make these promising candidates for being new very-high energy γ -raysources. Key words. gamma rays: ISM – X-rays: ISM – Radio continuum: ISM – ISM: supernova remnants – ISM: individual objects(HESS J1427 − −
1. Introduction
Supernova remnants (SNRs) and pulsar wind nebulae (PWNe)are considered the best candidates to accelerate the bulk ofGalactic cosmic rays (CRs) at least up to the knee of the CRspectrum ( ∼ × eV). Although no evidence for particleacceleration up to PeV energies has been found so far, e ffi cientparticle acceleration in SNRs and PWNe has been observed andcan be explained by the widely adopted di ff usive shock acceler-ation mechanism (DSA; Bell 1978). Accelerated electrons andpositrons radiate synchrotron emission and also γ rays throughBremsstrahlung and inverse Compton (IC) scattering on photonfields while accelerated protons and heavier nuclei colliding withambient matter emit γ rays through neutral pion decay. Thus, γ -ray studies probe the population of both electrons and protonsand allow us to disentangle di ff erent origins of the emission.However, γ -ray instruments usually have lower angular reso-lution than most radio and X-ray instruments, and their obser-vations often su ff er from source confusion. Thus, probing theaccelerated leptons through the expected synchrotron emission mainly in the radio and X-ray bands helps constrain the natureof γ -ray sources.The identification of γ -ray emitting sources in the high-energy (HE; 0.1 < E <
100 GeV) and very-high energy (VHE;0.1 TeV < E <
100 TeV) domains often relies on spatial correla-tion with identified objects at other wavelengths. While catalogsof known γ -ray emitters in the Galaxy such as SNRs, PWNe,and pulsars can provide hints of the origin of the emission,multiwavelength data exploration is often necessary to pinpointthe nature of the HE / VHE sources (Aliu et al. 2014; MAGICCollaboration et al. 2014, 2020). For instance, the TeV shellHESS J1534 −
571 was found to be spatially coincident with theSNR candidate G323.7 − − − γ -ray energy-dependent mor-phology shrinking to the energetic pulsar PSR J1826 − Article number, page 1 of 19 a r X i v : . [ a s t r o - ph . H E ] J a n & A proofs: manuscript no. TeVsources
HESS J1356 −
645 was classified as an evolved PWN as a resultof the exploitation of multiwavelength data that revealed a non-thermal, center-filled radio and X-ray emission surrounding theenergetic pulsar PSR J1357 − Centred’Analyse des Données Étendues and the Space Science DataCenter provide interactive databases, such as the Skyview tool,which enables the production of images at multiple wavelengthsfor any position in the sky. The interactive tool gamma sky per-mits us to superimpose catalogs such as those of the Fermi -LATand known SNRs.However, using multiwavelength data to identify γ -raysources is not always straightforward. The best example is thenearby and evolved PWN Vela X, which exhibits a changingmorphology at di ff erent wavelengths. The brightest part of theVHE emission is spatially coincident with the X-ray PWN, butits largest extent is consistent with that of the surrounding ex-tended radio nebula of 2 ◦ × ◦ (Abramowski et al. 2012). Emis-sion in the north of the pulsar is also seen with the Fermi -LAT(Grondin et al. 2013), illustrating how the interpretation of mul-tiwavelength data can be di ffi cult and puzzling. Moreover, mul-tiwavelength associations relying on spatial correlations is chal-lenging if the γ -ray emission is produced by an ancient PWNor a molecular cloud (MC) illuminated by CRs that have es-caped from a nearby SNR. In the former case, the pulsar canbe largely o ff set from the TeV emission produced by old elec-trons (de Jager et al. 2009) and, in the latter case, the MC canalso be distanced from the SNR from which CRs have escaped(Gabici et al. 2009). Thus, there may be no spatial correlationbetween the driven-phenomenon of the emission (here pulsar orSNR) and the TeV emission itself, making the association withmultiwavelength data di ffi cult. Finally, nonthermal radio and X-ray emission from ancient PWNe and MCs illuminated by CRsis expected to be faint, challenging the identification process.With 2700 hours of observations, the H.E.S.S. experiment,through a Galactic Plane Survey (HGPS; H. E. S. S. Collabora-tion et al. 2018b) covering the inner part of the Galaxy ( l = ◦ to 65 ◦ and | b | < . ◦ ), has reported 78 TeV sources, among which12 PWNe, 16 SNRs (or composite SNRs), 3 binaries, and 47sources that are not firmly identified. With a point spread func-tion (PSF) of ∼ ◦ (68% containment radius) and a point-source sensitivity of (cid:46) γ -ray mor-phology. The association procedure made use of the 3FGL and2FHL Fermi -LAT catalogs (Acero et al. 2015; Ackermann et al.2016), the “SNRcat” (Ferrand & Safi-Harb 2012), the ATNF pul-sar catalog (Manchester et al. 2005, version 1.54), and 20 exter-nal analyses involving data at other wavelengths. When a firmidentification was not possible, the H.E.S.S. sources were asso-ciated with all objects whose cataloged position is at an angu-lar distance smaller than the H.E.S.S. source spectral extractionradius (noted R spec ). For a total of 47 unidentified sources, the http://cade.irap.omp.eu/dokuwiki/doku.php?id=start https://skyview.gsfc.nasa.gov/current/cgi/titlepage.pl http://gamma-sky.net/map HGPS association procedure reported 11 sources that are not as-sociated with any cataloged sources and 36 sources for whichthe origin of the emission is unclear, mainly owing to severalpossible scenarios and source confusion.Given the large number of TeV sources without firm identi-fication, we developed a generic pipeline that retrieves archivalmultiwavelength data toward any region of the Galactic plane toconstrain the origin of the γ -ray emission. Although extragalac-tic sources can be located near the Galactic plane (as exemplifiedby the blazar HESS J1943 + −
63 (e.g., Chernyakova & Malyshev 2020). Wefirst describe our multiwavelength approach in Section 2. In Sec-tion 3, we apply this method on two unidentified sources, namelyHESS J1427 −
608 and HESS J1458 − −
478 and toward the SNR G28.6 − γ -ray analyses to confirm these as newVHE sources.
2. Multiwavelength approach
We present in this section a generic pipeline that retrievesarchival radio, X-ray, infrared, and GeV data to search for un-cataloged, presumably faint counterparts of the TeV unidentifiedsources. The association procedure is made with the instrumentsource catalogs whose data are retrieved and with catalogs ofknown objects that are more numerous than those used for theHGPS association procedure. After automatically downloadingall the archival data and performing the association procedure,we took prior attention to the visual inspection of all the archivalmultiwavelength images to search for faint excesses within theTeV source extent and to define the regions of interest for flux ex-traction. This is the only step that needs manual intervention. Af-ter this, the pipeline automatically derives a radio spectral indexthat allows us to disentangle thermal from nonthermal emissionand a mean magnetic field using X-ray and TeV data, assum-ing that the γ -ray emission is produced by IC scattering o ff thecosmic microwave background (CMB) photon field. Finally, thepipeline automatically plots the spectra of the Fermi -LAT cata-loged sources found within the TeV source extent together withthat of the H.E.S.S. source to search for a spectral connectionbetween the HE and VHE regimes.
Given a position in the sky and a search radius, we developed acode that automatically extracts radio continuum data from 13single-dish telescopes or interferometers that survey the north-ern and the southern sky. This includes the second epoch of theMolongo Galactic Plane Survey (MGPS-2, 843 MHz; Murphyet al. 2007), the VLA Galactic Plane Survey (VGPS, 1.4 GHz;Stil et al. 2006), the Southern Galactic Plane Survey (SGPS, 1.4GHz, a combination of the Australia Telescope Compact Arrayand the Parkes Radio Telescope; McClure-Gri ffi ths et al. 2005),the TIFR GMRT Sky Survey (TGSS, 150 MHz; Intema et al.2017), the NRAO-VLA Sky Survey (NVSS, 1.4 GHz; Condon Article number, page 2 of 19. Devin et al.: Multiwavelength constraints on unidentified TeV sources et al. 1998), the Canadian Galactic Plane Survey (CGPS, 1.4GHz; Taylor et al. 2003), the Green Bank Telescope (8.35 at14.35 GHz and the 87GB survey at 4.85 GHz; Langston et al.2000; Gregory & Condon 1991), and the HI / OH / Recombinationline survey of the inner Milky Way (THOR, 1, 1.3, 1.4, 1.6,and 1.8 GHz; Beuther et al. 2016). We also used data from theParkes 64 m radio telescope (2.4 GHz; Duncan et al. 1995) andthose from the Parkes-MIT-NRAO survey (PMN, 4.85 GHz;Gri ffi th & Wright 1993). Archival data from the HI Parkes All-Sky Survey (HIPASS) and the HI Zone of Avoidance (HIZOA)survey were reprocessed into a new continuum map that we re-trieved (CHIPASS, 1.4 GHz; Calabretta et al. 2014). We alsoretrieved data from the Multi-Array Galactic Plane Imaging Sur-vey (MAGPIS, 0.325, 1.5 and 5 GHz; Helfand et al. 2006). Thetechnical details of the 13 radio instruments and data retrievalare given in Appendix A (and references therein). In addition,data from the Planck satellite are also used (30, 44, 70, 100, and143 GHz, Planck Collaboration et al. 2016).In the X-ray range, we retrieved data from Chandra / ACIS(0.5–7 keV),
XMM-Newton (0.2–12 keV), ASCA (0.4 − −
10 keV),
Integral / IBIS-ISGRI (17 −
60 keV),
Swift / XRT(0.2–10 keV),
Swift / BAT (15–150 keV), NuSTAR (5–80 keV),
Suzaku (0.2–12 keV), and ROSAT / PSPC (0.1–2.4, 0.5–2.0, 0.1–0.4, 0.4–0.9 and 0.9–2.4 keV). We automatically extracted allthe observations whose center is comprised within an angulardistance from the source of interest. Except for large field-of-view instruments (such as ROSAT / PSPC,
Integral / IBIS-ISGRIand
Swift / BAT), we built a mosaic of these images in case ofmultiple observations. When possible, we created background-subtracted and exposure-corrected images. Appendix A gives thetechnical description of the X-ray instruments and data retrieval.We completed our data set with infrared data fromSpitzer / GLIMPSE (3.6, 4.5, 5.8, 8, 21, 24, 870, and 1100 µ m)and Fermi -LAT data (see Appendix A for more details).For the association procedure, we used catalogs of objectsreporting pulsars, PWNe, SNRs, H ii regions, etc. This includesthe ATNF pulsar catalog (Manchester et al. 2005, version 1.58),the “SNRcat” (Ferrand & Safi-Harb 2012), the Galactic SNRcatalog (Green 2017), and the catalog of 76 Galactic SNR can-didates revealed with THOR data (Anderson et al. 2017). Weused Fermi -LAT pulsar, point-source, and extended-source cata-logs (2PC, 3FGL, 2FHL, 3FHL, FGES, and 4FGL catalogs ) aswell as the HAWC catalog (2HWC; Abeysekara et al. 2017). Wenote that energetic pulsars are considered indirect associationssince we do not expect them to produce TeV emission but ratherto generate a PWN that could be seen at TeV energies. We alsoused the catalogs of H ii regions obtained with WISE data (An-derson et al. 2014), of MCs in the Milky Way (Rice et al. 2016),of Galactic O stars (Maíz Apellániz et al. 2013), and informationreported in the TeV source catalog . The details of the catalogsare given in Appendix A. To reduce potential missing associa-tions, we automatically requested the Simbad database, whichprovides information on the listed astronomical objects. This survey was made using the Parkes 64 m radio telescope with theNRAO multibeam receiver at a frequency of 4.85 GHz. References: Abdo et al. (2013); Acero et al. (2015); Ackermann et al.(2016); Ajello et al. (2017); Ackermann et al. (2017); Abdollahi et al.(2020) “TeVcat”: http://tevcat2.uchicago.edu http://simbad.u-strasbg.fr/simbad/ When extended radio emission is found toward the unidentifiedTeV source, we calculated a radio spectral index α (defined as S ν ∝ ν α , where S ν and ν are the flux density and frequency,respectively) that helps identify thermal from nonthermal (i.e.,synchrotron) emission. While α (cid:38) ii regions, its average value amounts to ∼ − / − ∼ − / ii regions can exhibita similar spectral index, they di ff er in shape with a shell-likeand a Gaussian-like morphology for H ii regions and PWNe, re-spectively. In each archival image, we masked the cataloged ra-dio sources to estimate the noise and we calculated the flux ina given region of interest (called R ON ) by summing up the fluxin the pixels and correcting from the instrument beam. The fluxextraction region was chosen after a thorough visual inspectionof the data. The background-corrected fluxes, calculated at dif-ferent frequencies, were fit with a power law to derive the ra-dio spectral index. Upper limits on flux were derived at the 3 σ confidence level. We followed the same method as for the PWNHESS J1356 −
645 (H.E.S.S. Collaboration et al. 2011), which isnow implemented in a generic way using all the retrieved radioobservations. Detailed explanations on the radio spectral indexderivation, with an illustration on the PWN HESS J1356 − Nonthermal X-ray emission probes the presence of the highest-energy electrons that also radiate TeV γ rays through IC scatter-ing on photon fields. Although ROSAT / PSPC detects soft X-rayphotons (up to 2.4 keV), which can be heavily absorbed by theinterstellar gas, we took advantage of its large field of view ( ∼ ◦ × ◦ ) to derive constraints on the X-ray flux within any spectralextraction region defined in the HGPS. We masked the catalogedsources (reported in the 1RXS and 2RXS catalogs; Voges et al.1999; Boller et al. 2016) to estimate the background with the ring and reflected background methods (Berge et al. 2007). Thesignificance is calculated following Li & Ma (1983), taking acorrelation radius equal to the spectral extraction radius ( R spec )used in the HGPS. For a given X-ray spectral index Γ X and acolumn density N H , the source count rate (or its 5 σ upper limit )in the 0.9 − . Assuming aone-zone model and that the TeV emission is produced by ICscattering on the CMB in the Thomson regime ( Γ X = Γ TeV = Γ ),the X-ray and TeV flux measurements ( F sync , F IC ) can be usedto constrain the mean magnetic field, which is expressed as B µ G ∝ (cid:18) F sync F IC × E − Γ , IC , TeV − E − Γ , IC , TeV E − Γ , sync , keV − E − Γ , sync , keV (cid:19) / Γ , (1)where Γ = ( p + / p is theparticle spectral index. More details on the mean magnetic field In case there is no significant ( > σ ) detection, we simulate ten X-ray sources at a given count rate and with the same morphology as thatfound in the HGPS. The significance of each of these simulated sources,once added to the ROSAT image, is calculated by applying the samemethods as described in the text. After repeating this procedure for sev-eral source count rates, the 5 σ upper limit is derived when it leads to amean significance of 5. https://heasarc.gsfc.nasa.gov/Tools/w3pimms_help.html Article number, page 3 of 19 & A proofs: manuscript no. TeVsources estimate are given in Appendix C, with an illustration on theSNR RX J1713.7 − µ G (Parizot et al. 2006), the expected meanmagnetic field value in case of linear DSA is roughly B ∼ − µ G for SNRs (Renaud 2009) and B ∼ − µ G for evolved PWNe(Torres et al. 2014).
The GeV spectra show di ff erent features depending on the originof the emission. γ -ray emission from pulsars is usually describedby a power law with an exponential cuto ff occurring below tensof GeV or by a logarithmic parabola. The second Fermi -LATpulsar catalog (2PC; Abdo et al. 2013) contains 117 pulsars for2796 radio-detected pulsars. This di ff erence can be explained bythe pulsar spin-down power ˙ E , which needs to be high enoughto provide e ffi cient particle acceleration up to γ -ray emitting en-ergies. The emission from PWNe is usually characterized by apower law with a hard spectral shape ( Γ <
2, where Γ is thephoton spectral index) originating from IC scattering on pho-ton fields. This hard spectral shape limits the detection of PWNewith the Fermi -LAT and makes PWNe the most numerous VHE-emitting objects in our Galaxy. While a hard spectral shape isindicative of a leptonic origin of the emission, a roll-o ff in thephoton spectrum below 100 −
200 MeV is the signature of accel-erated protons colliding with ambient matter. In this case, thephoton spectrum at (cid:38)
GeV energies is usually found to be soft( Γ (cid:38)
2) in SNRs interacting with MCs such as IC 443, W44,or W51C (Jogler & Funk 2016, and references therein). Giventhe crucial information brought by GeV spectra, we retrievedthe
Fermi -LAT spectrum of all cataloged sources located withinthe R spec of the HGPS unidentified source to search for a spectralconnection between GeV and TeV energies.
3. Application on HESS J1427 −
608 andHESS J1458 − − HESS J1427 −
608 is not spatially resolved in the HGPS and istherefore defined as a point source with a significance of √ TS ∼ . . Using a two-dimensional symmetric Gaussian, Aharonian et al. (2008) de-rived a size of σ = ◦ ± ◦ while the HGPS reported σ = ◦ ± ◦ , a value just below the significant extensionthreshold. The upper limit at the 95% confidence level on theGaussian extent is σ = ◦ . Taking a spectral extraction ra-dius R spec = ◦ , the spectrum is represented by a power lawwith a spectral index Γ TeV = . ± .
22 and an integrated en-ergy flux F −
10 TeV = (1 . ± . × − erg cm − s − . Thesource is associated with an extended nonthermal X-ray emis-sion (Suzaku J1427 − σ = . (cid:48) ± . (cid:48) ) with an energyflux F X = . + . − . × − erg cm − s − and a spectral index Γ X = . + . − . (Fujinaga et al. 2013). Such a soft spectrum indi-cates that the photon cuto ff energy is likely below the Suzaku en-ergy band. The derived column density N H = . + . − . × cm − points toward a possible large distance that could explain why it The test statistic (TS) is the ratio of the logarithm of two likelihoodsobtained with and without the source model. The TS follows a χ dis-tribution with n additional degrees of freedom. appears as point-like with H.E.S.S. No associated X-ray pointsource with similar column density was reported using XMM-Newton data (Fujinaga et al. 2013). Above 3 GeV, a
Fermi -LATpoint source was detected at the center of HESS J1427 −
608 witha hard spectral index Γ GeV = . ± .
17 and an integrated fluxof Φ GeV = (3 . ± . × − photon cm − s − (Guo et al.2017). Figure 1 (left) shows the HGPS significance map withthe associated X-ray and GeV sources overlaid. The extendedX-ray emission and the hard spectral shape of the Fermi -LATsource, both spatially coincident with the TeV emission, makethe leptonic scenario plausible as the origin of the TeV emission.
We extracted archival radio continuum data toward the regionof HESS J1427 − Suzaku overlaid. Two MGPS-2 sources (MGPS J142755 − − R spec ofHESS J1427 −
608 and a H ii region (reported in the PMN cat-alog) lies in the northwest (outside) of the H.E.S.S. source ex-tent. The compact source MGPS J142755 − F = . ± . B = µ G, the predicted radio flux was significantly larger than thatof MGPS J142755 − B = µ G, but the modelfails to account for the flux measured with
Suzaku . We de-fined the ON region such as it encompasses the brightest partof the emission (located between the two MGPS-2 sources)and MGPS J142755 − Suzaku source), excluding the compact source MGPS J142714 − − − . (cid:48) , compared to 45 (cid:48)(cid:48) with the MGPS-2). The best-fit powerlaw gives a spectral index of α = − . ± .
36, consistent withnonthermal emission. We also derived the spectral index in theregion encompassing only the brightest part of the emission (lo-cated between the two MGPS-2 sources) and we found a softerspectrum α = − . ± .
34 that may be due to contamina-tion from the compact source MGPS J142714 − ff erentinjection and radiative loss timescales. Freshly injected particlesproduce X-ray emission while the oldest particles have alreadycooled and di ff used away from the pulsar, producing a relic ra-dio PWN. We therefore naturally expect the radio extent of anevolved PWN to be larger than the X-ray extent, which is some- Article number, page 4 of 19. Devin et al.: Multiwavelength constraints on unidentified TeV sources
Fig. 1. (Left) HGPS significance map (obtained with a correlation radius of 0.1 ◦ ) with the Fermi -LAT cataloged sources (within 95% uncer-tainties) overlaid. The blue circle corresponds to the position uncertainty (1 σ ) of the Fermi -LAT source detected by Guo et al. (2017) above 3GeV. The H.E.S.S. upper limit on the TeV source extent and the spectral extraction radius R spec are represented by the dashed and solid yellowcircles, respectively. The green contours represent the emission seen by Suzaku between 2 keV and 8 keV (Suzaku J1427 − − − R spec , while the cyan and magenta ellipsesrepresent the two MGPS-2 sources inside R spec . The FWHM of the MGPS-2 is given at the top left of the image. The radio flux extraction regionis represented by a white circle (called R ON ). The yellow circles and green contours are the same as those in the left panel. what at odds with that seen in HESS J1427 − − − ff ected by this instrumental limitation. The displacement ofthe centroid of the radio emission from that of the X-ray emis-sion is found in other evolved PWNe such as Vela X and can beexplained by di ff erent particle populations. This could explainwhy the brightest part of the radio emission is displaced from theSuzaku J1427 − − / PSPC significance map shows no emission to-ward HESS J1427 − F X < . × − erg cm − s − ). This is con-sistent with the high column density value derived in Fujinagaet al. (2013), which leads to a large absorption of soft X-rayphotons along the line of sight. Thus, we derived a loose con-straint on the mean magnetic field, that is, B < . µ G atthe 5 σ confidence level with N H < . × cm − (Fujinagaet al. 2013) and Γ TeV < .
7. Assuming that Suzaku J1427 − − Γ X ≈ Γ TeV ,with Γ X = . + . − . and Γ TeV = . ± .
22) and B amountsto 10 . + . − . µ G (from Equation 1), which is compatible withthat obtained for PWNe and SNRs. The latest
XMM-Newton source catalog contains 4XMM J142756.7 − −
608 (black cross in Figure 1, left),which was not detected in the study of Fujinaga et al. (2013).A dedicated X-ray analysis of this region would allow us to de-termine if 4XMM J142756.7 − − Fermi -LAT sources are located inside the R spec ofHESS J1427 −
608 (3FHL J1427.9 − − Fermi -LAT cat-alogs. The spectrum of the point source detected above 3 GeV(Guo et al. 2017) smoothly connects to the H.E.S.S. measure-ments while the 4FGL catalog reports a source with a pulsar-likespectrum represented by a logarithmic parabola. If the dominantemission above 3 GeV originates from a PWN powered by a pu-tative pulsar whose emission is reported in the 4FGL catalog, wenote that the PWN spectrum is very soft in the TeV range and hasa relatively low energy cuto ff (in the ∼
500 GeV range), whichalso points toward an evolved PWN scenario.
Since HESS J1427 −
608 is unresolved at TeV energies, a blazaror binary origin could have been considered. However, giventhe existence of an extended X-ray emission spatially coinci-dent with the H.E.S.S. source, these two scenarios are clearlydisfavored. With an extended and center-filled nonthermal ra-dio emission detected within HESS J1427 −
608 and a 4FGL
Fermi -LAT cataloged source exhibiting a spectrum reminiscentof a pulsar, our multiwavelength data exploitation strengthensthe scenario of an evolved PWN.To constrain the physical parameters in more detail, we mod-eled the broadband nonthermal spectrum of HESS J1427 − Naima package (Zabalza 2015) assuming a one-zoneleptonic model. The electron spectrum is in the form d N e / d E e ∝ E − pe exp( − E e / E cut ) for E e ∈ [ E min , E max ], where p is the par-ticle spectral index. The minimum and maximum energy of theparticles are set to E min = E max = d is unknown, we arbitrarily set d = Article number, page 5 of 19 & A proofs: manuscript no. TeVsourcesHESS J1427–608 HESS J1458–608MGPS-2 SGPS PMN MGPS-2 Parkes PMNFrequency (GHz) 0.843 1.4 4.85 0.843 2.4 4.85rms (mJy / beam) 2.2 8.0 21.5 1.6, 1.6 75.0, 219.5 11.7, 17.4Flux density (mJy) 197.1 ± ± < . ± ± < . < . ± ± Table 1.
Estimated radio fluxes toward HESS J1427 −
608 and HESS J1458 −
608 with the associated statistical errors. The flux upper limits aregiven at the 3 σ confidence level. For HESS J1458 − R ON1 and R ON2 shown in Figure3 (right). -2 -13 -12 -11 -10 -2 Energy (GeV)10 -13 -12 -11 -10 E d N / d E ( e r g c m - s - ) HESS J1427-6084FGL J1427.8-60513FHL J1427.9-6054Guo et al. 17 E (MeV)10 E d N / d E ( e r g c m s ) HESS J1427 608
Suzaku IC CMB ( E min = 1 GeV)Sync + IC tot ( E min = 1 GeV)Sync + IC tot ( E min = 25 GeV) radioFermi-LATH.E.S.S. Fig. 2. (Left) Spectral energy distribution of HESS J1427 −
608 with those of the associated
Fermi -LAT sources. The
Fermi -LAT upper limits arecalculated with a confidence level of 95%. (Right) Broadband nonthermal spectrum of HESS J1427 −
608 in a leptonic scenario. The solid blackline corresponds to the model with E min = E cut = p = B = µ G, and W e = . × × (d / erg. The model with E min =
25 GeV, E cut = p = B = µ G, and W e = . × × (d / erg is represented by the dashed black line. (2008) included within the GALPROP project. We obtained T IR =
25 K, U IR = − , T opt = U opt = − . We used the radio fluxes reported in Table 1 (exclud-ing the upper limit derived with PMN data, see Section 3.1.2),the X-ray measurements from Suzaku (Fujinaga et al. 2013), theGeV data points from Guo et al. (2017), and the TeV SED re-ported in the HGPS. To simultaneously fit the radio and X-raydata, a hard particle spectral index such as p = . E cut = . + . − . TeV and B = . + . − . µ G,which is consistent with the magnetic field value inferred in Sec-tion 3.1.2. As shown in Figure 2 (right), the low-energy
Fermi -LAT data points cannot be reproduced with such a hard particlespectral index. Using p = . E min =
25 GeV, the broadbandnonthermal emission can be explained with B = . + . − . µ G and E cut = . + . − . TeV (Figure 2, right). Although the low-energycuto ff in the electron spectrum is questionable, a value of E min =
13 GeV was also required to simultaneously explain the ra-dio and X-ray measurements of the TeV PWN HESS J1356 − E min are alsoin the range of those considered in Kennel & Coroniti (1984)and Ackermann et al. (2011). We note however that significantradio emission could have been suppressed during the data re-duction. Finally, it is worth noting that if HESS J1427 −
608 isan ancient PWN, a more detailed model is likely needed to ex-plain its broadband nonthermal emission, as this is the case forthe Vela X PWN whose multiwavelength modeling requires twoelectron populations (Hinton et al. 2011).Deeper radio observations and dedicated TeV analyseswould be helpful to obtain better insight into the source mor-phology. Pulsation searches on 4FGL J1427.8 − Fermi -LAT source https://galprop.stanford.edu as a pulsar and dedicated X-ray observations and analyses arerequired to search for this putative compact source. If a pul-sar is detected with a spin-down luminosity high enough topower a detectable nebula despite its likely large distance,HESS J1427 −
608 could thus be firmly identified as a TeV PWN. − In the HGPS, HESS J1458 −
608 is described by a two-dimensional symmetric Gaussian with σ = . ◦ ± ◦ andhas a significance of √ TS = Γ TeV = . ± .
14 and F −
10 TeV = (5 . ± . × − erg cm − s − obtained with R spec = ◦ . At the center of HESS J1458 −
608 lies a
Fermi -LAT γ -ray pulsar PSR J1459 − E = . × erg s − (Ray et al. 2011) and a characteristic ageof τ c =
64 kyr (Marelli et al. 2011). Data taken with
Swift / XRT(6 ks) revealed a point source, o ff set from the γ -ray pulsar by ∼ − XMM-Newton observationsof 50 ks confirmed the nonthermal X-ray emission from the pul-sar with a derived column density of N H = . + . − . × cm − (Pancrazi et al. 2012). Since the pulsar is not detected in the ra-dio band, the distance of the system is unknown. No associatedX-ray and γ -ray emissions originating from a PWN have beendetected so far. Figure 3 (left) shows the H.E.S.S. significancemap, indicating that the TeV morphology may be more complexthan a symmetric Gaussian and that two Fermi -LAT catalogedsources lie in the western part of HESS J1458 − Article number, page 6 of 19. Devin et al.: Multiwavelength constraints on unidentified TeV sources
Fig. 3. (Left) HGPS significance map (obtained with a correlation radius of 0.1 ◦ ) centered on HESS J1458 − R spec of HESS J1458 −
608 are represented by the dashed and solid yellow circles, respectively. The blue and white circles correspond tothe 95% position uncertainties of the
Fermi -LAT cataloged sources, while the green cross represents the position of PSR J1459 − − R ON1 and R ON2 ) and the FWHM of the PMN is represented at the top left of the image. The yellow circles arethe same as those in the left panel.
We extracted archival radio continuum data that show emis-sion in the vicinity of the pulsar and in the westernpart of HESS J1458 − Fermi -LAT sources 3FGL J1456.7 − − ff use emissions within the H.E.S.S. source:one in the vicinity of the pulsar (called R ON1 ) and one toward thewestern part of the H.E.S.S. source (called R ON2 ). The estimatedradio fluxes using MGPS-2, Parkes, and PMN are given in Ta-ble 1 whose best-fit power law gives α = − . ± .
18 (for R ON1 ) and α = − . ± .
20 (for R ON2 ). Both spectral indexesindicate nonthermal radio emission and it is not clear whetherthese two regions originate from the same system. The spectralindexes of R ON1 and R ON2 are compatible with those expectedfrom both PWNe and shell-type SNRs. The spectral index of R ON1 is even closer to that of a SNR than a PWN, but the ab-sence of a shell-like morphology and the location of the radioemission close to the energetic PSR J1459 − Chandra , Swift / XRT,
XMM-Newton ,and
Suzaku data but no extended emission appears in these im-ages. The ROSAT / PSPC data show no significant emission andgive a tight constraint on the X-ray flux F X < . × − ergcm − s − (with N H < . × cm − , Pancrazi et al. 2012, and Γ TeV < . B < . µ G at the 5 σ confidence level.The pulsar PSR J1459 − Fermi -LAT catalogs also con-tain two sources located in the western part of HESS J1458 − − − − Γ GeV = . ± .
28) and connectedto the spectrum of HESS J1458 − − γ -ray pulsar, as might be the case for the SED of3FGL J1456.7 − − HESS J1458 −
608 is a puzzling source, with a complex and elon-gated TeV morphology, and shelters the energetic X-ray and γ -ray pulsar PSR J1459 − − −
608 could originate from a PWN. We also reportednonthermal radio emission in the vicinity of PSR J1459 − − − −
608 assuming a one-zone lep-tonic scenario and a distance of d = T IR =
25 K, U IR = .
88 eV cm − , T opt = U opt = .
94 eV cm − (estimated using GALPROP code). We used the radio fluxes(from both regions) listed in Table 1, the upper limit on the X-rayflux derived in Section 3.2.2 and the TeV SED from the HGPS.Owing to the lack of X-ray data points and the absence of a VHEcuto ff , B and E cut cannot be accurately derived. As shown in Fig-ure 4 (right), the data can be reproduced with E min = E cut =
60 TeV, B = µ G, p = W e = × × ( d / erg.For illustration, the best-fit spectrum of 4FGL J1456.7 − Article number, page 7 of 19 & A proofs: manuscript no. TeVsources -2 -13 -12 -11 -10 -2 Energy (GeV)10 -13 -12 -11 -10 E d N / d E ( e r g c m - s - ) HESS J1458-6084FGL J1459.5-60534FGL J1456.7-6050c3FGL J1456.7-6046 E (MeV)10 E d N / d E ( e r g c m s ) ROSAT
HESS J1458 608 IC CMB
Sync + IC tot radioH.E.S.S.
Fig. 4. (Left) Spectral energy distribution of HESS J1458 −
608 with those of the associated
Fermi -LAT sources. The
Fermi -LAT upper limits arecalculated with a confidence level of 95%. Since the γ -ray pulsar PSR J1459 − Fermi -LAT catalogs, only its best-fitspectrum and SED from the 4FGL catalog (4FGL J1459.5 − −
608 in a leptonic scenario with E min = E cut =
60 TeV, B = µ G, p =
2, and W e = × × ( d / erg. spectrum connected to that of HESS J1458 − −
608 and HESS J1458 −
608 within thecontext of the population of TeV PWNe
We found extended nonthermal radio emission towardHESS J1427 −
608 and HESS J1458 − −
608 and HESS J1458 − −
608 could be a composition ofmultiple sources, since the hard spectrum seen by the
Fermi -LAT (although this detection needs to be confirmed) originatesfrom the western edge of the TeV source with a nonthermalradio counterpart. Therefore, it is not clear whether this part isconnected to the main emission region of HESS J1458 − − −
608 and HESS J1458 − S and the pulsar spin-down power ˙ E as follows: S = (30 . ± . × ˙ E . ± . , (2)where S and ˙ E are in units of erg pc − s − and 10 erg s − ,respectively. The surface brightness can be written as S = L −
10 TeV π R ≈ F −
10 TeV σ , (3)where L −
10 TeV , F −
10 TeV , R PWN and σ are, respectively, the lu-minosity (erg s − ) and the integral energy flux (erg pc − s − ) be-tween 1 and 10 TeV, the radius of the PWN (pc) and the Gaussian σ extent (radians). Since no pulsar was detected in the vicin-ity of HESS J1427 −
608 and the distance of PSR J1459 − −
608 and HESS J1458 −
608 werenot considered in the H.E.S.S. PWN population study. ForHESS J1427 − F −
10 TeV = (1 . ± . × − erg cm − s − and σ < ◦ , which gives ˙ E > × ergs − . Assuming that Suzaku J1427 − − σ X = ff used farther away from the pulsar than thoseemitting X-ray synchrotron. With σ > E < × erg s − . For HESS J1458 − F −
10 TeV = (5 . ± . × − erg cm − s − and σ = . ◦ ± ◦ , weobtained ˙ E = . + . − . × erg s − , which is only 2 σ away fromthe measurement of ˙ E = × erg s − for PSR J1459 − − −
608 areassociated, this di ff erence could indicate that either the integralenergy flux of the TeV source was underestimated or its Gaus-sian σ extent was overestimated. Given the complex TeV mor-phology of HESS J1458 −
608 (Figure 3, left), possible sourceconfusion could lead to a σ extent larger than that associatedwith the PWN produced by PSR J1459 − τ c is expressed as τ c = ( n − × ( t age + τ ) yr , (4)where n is the braking index, t age the age of the system, and τ theinitial characteristic age of the pulsar (Gaensler & Slane 2006).Assuming n = τ c ≈ t age for evolved systems ( t age >> τ ). Since these relicPWNe no longer inject particles into the system, the maximumenergy reached by particles is limited by radiative losses. Thebreak energy E b (above which electrons significantly su ff er fromsynchrotron losses) is found equating t age = τ sync , where τ sync isthe synchrotron loss time τ sync = (1 . × ) × E − B − µ G yr , (5)where E TeV and B µ G are the particle energy and mean mag-netic field in units of TeV and 100 µ G, respectively (Parizotet al. 2006). In evolved systems limited by radiative losses, wecan therefore assume E b = E cut . Our estimate on τ c relies onthe assumption that HESS J1427 −
608 and HESS J1458 − τ c = t age . ForHESS J1427 − B = . + . − . µ G and E cut = . + . − . Article number, page 8 of 19. Devin et al.: Multiwavelength constraints on unidentified TeV sources Characteristic age c (kyr) 10 Sp i n - d o w n p o w e r E ( e r g s ) HESS J1825-137MSH15-52Kes75 N157BCrab G0.9+0.13C58 CTA1 HESS J1458-608 (measured)HESS J1427-608 (estimated)HESS J1458-608 (estimated)Identified PWNeCandidate PWNe (HGPS)Unidentified sources
Fig. 5.
Spin-down power ˙ E and characteristic age τ c of pulsars witheither a firmly identified TeV PWN (blue diamonds) or a PWN candi-date (orange circles). Assuming a PWN scenario, the values obtainedfor the unidentified sources HESS J1427 −
608 and HESS J1458 − E and τ c for the pulsarPSR J1459 − − −
608 and HESS J1458 − TeV (combining the constraints obtained with E min = τ c = t age = . . , .
6] kyr (Equation 5). For HESS J1458 − E cut >
30 TeV since no break or cuto ff is visi-ble in Figure 4 (right). In order to bound the cuto ff energy andthe mean magnetic field, we assumed E cut <
100 TeV and B > µ G, which are appropriate for evolved PWNe. With B < µ G (derived in Section 3.2.2), we found 1 . < τ c = t age < . −
608 andHESS J1458 − − −
608 are contained within those of thesample of identified PWNe and PWN candidates. However,HESS J1427 −
608 and HESS J1458 −
608 appear to be similarto a relatively young PWN (with a high ˙ E ) and an evolvedPWN (with an ˙ E estimate lying among the lowest values of theTeV PWN population), respectively, whereas their γ -ray spec-trum (shown in Figure 2 and 4) may suggest the opposite. It isnevertheless possible that the extent of HESS J1458 −
608 wasoverestimated as a result of source confusion and that the hardTeV spectral shape originates from multiple source contribution.Owing to the inherent uncertainties in such approach, we can-not draw precise conclusions except that HESS J1427 −
608 andHESS J1458 −
608 seem to broadly follow the general trend ofthe population of TeV PWNe.Finally, we note that the characteristic age ofHESS J1427 −
608 is comparable to that of the evolved PWNHESS J1356 −
645 ( τ c = −
608 is similar to that of HESS J1356 − R ≈ d . pc, where d . is the distance in units of 2.4kpc), the distance to HESS J1427 −
608 would be d ≈
10 kpcusing an angular TeV size of σ = ◦ (or d > σ < ◦ ). HESS J1458 −
608 would be more comparableto the PWN HESS J1825 −
137 ( τ c =
21 kyr, R ≈ d pc).Assuming that their intrinsic sizes are similar, that would placeHESS J1458 −
608 at a distance d ≈ σ = ◦ . The distances to HESS J1427 − −
608 could therefore be compatible with therelatively large column densities derived for their associatedX-ray sources (Suzaku J1427 − − ff usion or advection, as discussed forHESS J1825 −
137 in H. E. S. S. Collaboration et al. 2019),HESS J1427 −
608 and HESS J1458 −
608 could share similarcharacteristics with the well-known and bright TeV PWNeHESS J1356 −
645 and HESS J1825 −
4. New VHE γ -ray source candidates Although very robust, the HGPS detection pipeline, relying onan iterative template fitting in which the γ -ray sources are treatedas two-dimensional symmetric Gaussians, faced the di ffi cultyof revealing multiple components in regions with large sourceconfusion. The visual inspection of the HGPS images led usto focus on two possible γ -ray excesses lying in such complexregions, which eluded detection by the algorithm. The first islocated south of the unidentified source HESS J1632 −
478 andthe other lies at the position of the synchrotron-emitting SNRG28.6 − − γ -raysources without strong prior morphological assumptions: struc-tural information is extracted using an edge detection operatorand the detected objects are found as local maxima after ap-plying the Hough circle transform. This algorithm, applied onthe HGPS maps, has provided a list of objects that warrant fur-ther investigation. Each of the two above-mentioned excesses,which we independently identified after examining the HGPSmaps, has a counterpart in the catalog of Remy et al. (2020) asa detected object uncataloged in the HGPS (labeled HC_147 and
HC_382 , respectively). Although this strengthens the interest ofthese two excesses, only a dedicated H.E.S.S. data analysis couldconfirm these as proper VHE sources. In the following, we con-sider them as such and investigate their origin through the exist-ing multiwavelength data. − The unidentified source HESS J1632 −
478 is described in theHGPS by a Gaussian component with σ = . ◦ ± ◦ . TheHGPS significance map is shown in Figure 6 (left) and shows a γ -ray excess with a peak significance at about 8 σ in the southof HESS J1632 −
478 (outside of the HGPS R spec ). The pulsarPSR J1633 − E = × erg s − and d = . − γ -ray excess.We explored archival radio continuum data toward the southof HESS J1632 − ii regions reported in the WISE cata-log (Anderson et al. 2014) and one MC (Rice et al. 2016) arespatially coincident with the γ -ray excess. Using data from theMGPS-2, SGPS and Parkes, we derived a radio spectral index of α = . ± .
04 in the ON region encompassing the radio emis-sion (white circle in Figure 6, right). The spectral index indicates
Article number, page 9 of 19 & A proofs: manuscript no. TeVsources G a l a c t i c L a t i t u d e RONMGPS-2 J y / b e a m Fig. 6. (Left) HGPS significance map (with a correlation radius of 0.1 ◦ ) toward the region of HESS J1632 − σ extent and the R spec of HESS J1632 − − R spec . The H.E.S.S. contours from 3 σ to 11 σ appear in cyan. The γ -ray excess studied in this work is located within thewhite box (with a peak significance of ∼ − − R ON . (Right) MGPS-2 map at 843 MHz of the region within the white box seen in the left panel. The instrument beamis given on the top left corner. The dashed orange and red circles correspond to the cataloged MC (Rice et al. 2016) and H ii regions (Andersonet al. 2014), respectively. The pink cross represents a source reported in the Galactic O star catalog (Maíz Apellániz et al. 2013), while the greencircles indicate the σ extent (containing the PSF) of the XMM-Newton extended sources (Webb et al. 2020). The contours and solid circles are thesame as in the left panel. that the radio emission is thermal in nature and could originatefrom the H ii regions. Since these H ii regions do not constitutethe entire radio emission, it is possible that there is a subdomi-nant nonthermal emission.We also found a X-ray point-source detected byROSAT / PSPC (2RXS J163352.2 − ASCA / GIS although not cataloged. Four
XMM-Newton extended sources are also reported in this region (see Fig-ure 6, right): 4XMM J163353.3 − σ = . (cid:48)(cid:48) ,containing the PSF of ∼ (cid:48)(cid:48) ), 4XMM J163350.8 − σ = . (cid:48)(cid:48) ), 4XMM J163354.9 − σ = . (cid:48)(cid:48) ), and4XMM J163350.4 − σ = . (cid:48)(cid:48) ). These X-ray sourceshave unknown origin and the Simbad database does not reportany known object at these positions. A source, reported inthe Galactic O star catalog (Maíz Apellániz et al. 2013), islying at the center of the four extended XMM-Newton sources,which is spatially coincident with a
Swift / XRT cataloged source(2SXPS J163352.3 − Fermi -LAT extended source FGES J1633.0 − r = ◦ ; Ackermann et al. 2017)encompasses the TeV γ -ray excess but its large extent points to-ward possible source confusion. The Fermi -LAT residual countmap above 10 GeV shows emission that is spatially coincidentwith the TeV γ -ray excess, but a dedicated analysis would berequired to potentially reveal an extended component.To conclude, we found radio emission that is spatiallycoincident with the significant γ -ray excess in the south ofHESS J1632 − ii regions located within the γ -ray excess indicate thatthe TeV emission could arise from a star-forming region, whichcan accelerate particles up to γ -ray emitting energies (with col-liding stellar winds). Such regions were recently proposed assources of Galactic CRs (Aharonian et al. 2019). Extended X-ray sources are found in the western part of this region with un- known origin. Deeper X-ray observations would help understandthe nonthermal processes occurring in this region. Detailed HEand VHE γ -ray data analyses would be of great interest to in-vestigate the morphology and spectrum of this γ -ray excess andpotentially reveal a new VHE source. − − The unidentified source HESS J1843 −
033 is defined as a largeextended Gaussian component ( σ = . ◦ ± ◦ ) and resultsfrom the merging of two Gaussian components, which werepreviously detected by the HGPS pipeline, as illustrated in theHGPS significance map (Figure 7, left). Several SNR candi-dates revealed in the THOR radio survey (Anderson et al. 2017)overlap with HESS J1843 −
033 (Figure 7, right). The radio SNRG28.6 − − E = × erg s − and τ c = σ at the positionof the synchrotron-emitting SNR G28.6 − − ≈ ASCA source spatially coincidentwith G28.6 − Γ X = . + . − . ,a column density of N H = . + . − . × cm − and a distanceestimate between 6 kpc and 8 kpc. Chandra observations ofG28.6 − − − N H = + − × cm − , similar to that obtained for theSNR (Zyuzin et al. 2018). The transverse velocity of the pulsarcan be written as v t = . µ D km s − , where µ is the proper ve-locity of the pulsar (mas / yr) and D the distance (kpc) (Lyne & Article number, page 10 of 19. Devin et al.: Multiwavelength constraints on unidentified TeV sources
Fig. 7. (Left) HGPS significance map of HESS J1843 −
033 (obtained with a correlation radius of 0.1 ◦ ). The dashed and solid yellow circles indicatethe Gaussian σ extent and the R spec . The dashed black circles represent the two Gaussian components previously detected by the HGPS pipeline,which were finally merged into HESS J1843 − σ to 9 σ appear in cyan. (Right) Same as in the left panel; theSNR candidates found in the THOR radio survey (Anderson et al. 2017) are shown as green circles and the synchrotron-emitting SNR G28.6 − − Chandra map of G28.6 − Lorimer 1995). Since µ has not been measured, we can approxi-mate µ ≈ θτ c , where θ is the angular distance between the currentposition of the pulsar and its place of birth and τ c its charac-teristic age. Assuming a distance of 4.3 kpc (estimated from theempirical relation obtained for γ -ray pulsars; Saz Parkinson et al.2010) and 6 kpc (the closest estimate for G28.6 − v t ≈ − and ≈ − (Zyuzin et al. 2018), while typical pulsar velocities range be-tween 400 −
500 km s − (Lyne & Lorimer 1995). The associationbetween PSR J1844 − − − α = − . ± .
01, fully consistent with thatusually measured for shell-type SNRs. The brightest parts ofthe shell are reported as sources in the TGSS catalog (at 150MHz). By summing up their respective fluxes we obtained ∼ ± Chandra image, which shows afaint shell-like morphology, previously revealed by Ueno et al.(2003). No
Fermi -LAT cataloged sources are reported close tothe SNR. We also explored the archival multiwavelength data to-ward the pulsar PSR J1844 − Spitzer show a brightextended emission southeast of PSR J1844 − ii regions, whose distance is estimated to be 5.1 kpcor 9.7 kpc; the former is the most likely distance (see Anderson& Bania 2009; Dirienzo et al. 2012). With an angular distancebetween N49 and PSR J1844 − θ = µ = . t − . mas yr − , assuming t ≈ τ c = d = . v t ∼ d . t − . km s − , makingthe association of PSR J1844 − − − α = − . ± .
01. Multiwavelength data do not show any emission from a potential PWN originating from thenearby and energetic pulsar PSR J1844 − − − − γ -ray emitting energies in a leptonic scenario. Moregenerally, the unidentified source HESS J1843 −
033 is an inter-esting source to investigate at VHE, given the numerous overlap-ping SNR candidates. Among these, we note that there is anotherexcess (at ∼ − σ in the HGPS maps) lying on the eastern partof the shell of G28.78 − γ -ray emission from G28.6 − −
5. Conclusions
Given the large number of HGPS sources that are not firmlyidentified, we developed a generic code aiming to constrain theorigin of their TeV emission through the exploitation of mul-tiwavelength data. The algorithm is based on an automaticallyretrieval of archival data from radio continuum, X-ray, infrared,and GeV instruments toward any region of the sky to search forfaint counterparts through a careful visual inspection in everyavailable image. The pipeline was completed by an associationprocedure using the catalogs of known objects (SNRs, PWNe,H ii regions, etc.) and those related to the instruments whosedata are exploited. The main constraints on the origin of the TeVemission are obtained through the derivation of a radio spectralindex that helps us disentangle thermal from nonthermal emis-sion and through the estimate of the mean magnetic field underthe assumption of a leptonic scenario. Finally, the Fermi -LATcataloged source spectra are used to search for a smooth con-nection between GeV and TeV energies. This algorithm is wellsuited for isolated sources but faces some limitations for large
Article number, page 11 of 19 & A proofs: manuscript no. TeVsources and complex regions, although it gives an overall insight into thepossible multiple components as the origin of the TeV emission.We applied this pipeline on two unidentified sources reportedin the HGPS: HESS J1427 −
608 and HESS J1458 − Fermi -LAT data revealed a pulsar-like spectrumand a PWN-like spectrum in the vicinity of HESS J1427 − − B (cid:46) µ G, which is consistent with that obtained from ancientPWNe. We estimated the spin-down power and the characteristicage of the putative pulsars and we found that these are broadly inline with those of the population of known TeV PWNe. DeeperX-ray observations are necessary to potentially reveal a compactsource in the vicinity of HESS J1427 − − − γ -ray data anal-yses are required to reveal in details the morphology of thisunidentified VHE source.We also shed light on a possibly significant, yet uncata-loged, γ -ray excess in the HGPS data, located in the south of theunidentified source HESS J1632 − γ -ray emitting energies. We also highlightedanother uncataloged γ -ray excess, that is spatially coincidentwith the synchrotron-emitting shell-type SNR G28.6 − − γ -raysource detection based on image processing and pattern recog-nition techniques and provided a list of promising HGPS TeVsource candidates. Future work will include the application ofthis pipeline on these source candidates. If interesting multi-wavelength counterparts are to be found, detailed VHE data re-analyses would then be warranted to firmly confirm these candi-dates as new VHE sources. Our next improvements of the codewill include the use of H i and CO data to assess the environmentof the sources and the analysis of X-ray observations to derivebetter constraints on the synchrotron spectrum. The convolutionof the radio and X-ray maps with the H.E.S.S. PSF would bealso needed to quantify the spatial correlations at di ff erent wave-lengths.The next generation of Cherenkov telescopes CTA(Cherenkov Telescope Array Consortium et al. 2017) will cer-tainly detect a significant number of new sources, leading to ahigher degree of source confusion and hence, a larger numberof unidentified sources than what is currently observed. Dubuset al. (2013) estimated that 20 −
70 shell-type SNRs and 300 − γ -ray sky and the synergy with multiwavelength observations will be necessary to constrain theorigin of the observed TeV emission. The large field-of-view X-ray instrument eROSITA , recently launched, will soon give anunprecedented high-energy map up to 10 keV (Merloni et al.2012), allowing us to search for faint X-ray counterparts alongthe whole Galactic plane. Data from the Square Kilometre Ar-ray (Weltman et al. 2018) are also very promising to help iden-tify γ -ray sources. Thus, future VHE observations and multi-wavelength data exploitation supported by new generation in-struments will probably reveal the nature of a significant numberof unidentified TeV sources in order to assess their importancewithin the issue of the origin of the Galactic CRs. Acknowledgements.
We thank the anonymous referee for his / her comments thatimproved the manuscript. JD and MLG acknowledge support from Agence Na-tionale de la Recherche (grant ANR-17-CE31-0014). GMRT is run by the Na-tional Centre for Radio Astrophysics of the Tata Institute of Fundamental Re-search. This research has made use of data from the Karl G. Jansky Very LargeArray and from the MOST, which is operated by The University of Sydney withsupport from the Australian Research Council and the Science Foundation forPhysics within The University of Sydney. The National Radio Astronomy Ob-servatory is a facility of the National Science Foundation operated under coop-erative agreement by Associated Universities, Inc. The research presented in thispaper has used data from the Canadian Galactic Plane Survey, a Canadian projectwith international partners, supported by the Natural Sciences and EngineeringResearch Council. The Australia Telescope Compact Array and the Parkes radiotelescope are part of the Australia Telescope National Facility which is fundedby the Australian Government for operation as a National Facility managed byCSIRO. This research has made use of data from the Green Bank telescope, thePlanck satellite, CHIPASS and the THOR survey. This research has made use ofdata obtained from the Chandra
Source Catalog, provided by the Chandra X-rayCenter (CXC) as part of the Chandra Data Archive, and from the Suzaku satel-lite, a collaborative mission between the space agencies of Japan (JAXA) andthe USA (NASA). This research has made use of data obtained from the 4XMMXMM-Newton serendipitous source catalogue compiled by the 10 institutes ofthe XMM-Newton Survey Science Centre selected by ESA. This work has madeuse of data supplied by the UK Swift Science Data Centre at the University ofLeicester, and is based on observations with INTEGRAL, an ESA project withinstruments and science data centre funded by ESA member states (especiallythe PI countries: Denmark, France, Germany, Italy, Switzerland, Spain), and withthe participation of Russia and the USA. We have also used data from the NASAsatellite NuSTAR and from ASCA, a collaborative mission between ISAS andNASA. This work has made use of the ROSAT Data Archive of the Max-Planck-Institut für extraterrestrische Physik (MPE) at Garching, Germany. This work isbased in part on observations made with the Spitzer Space Telescope, which isoperated by the Jet Propulsion Laboratory, California Institute of Technology un-der a contract with NASA. This research has made use of the SIMBAD database,operated at CDS, Strasbourg, France, and of data and / or software provided bythe High Energy Astrophysics Science Archive Research Center (HEASARC),which is a service of the Astrophysics Science Division at NASA / GSFC.
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Appendix A: Instrument characteristics and catalogs
Table A.1 gives the description of the radio instruments whose archival data are retrieved through the pipeline.
Instruments Type Coverage Frequency (GHz) Sensitivity (mJy) PSF (FWHM) Data links ReferencesTGSS I δ > − ◦ ∼ . (cid:48) * [1]* Intema et al. (2017)MGPS-2 I 245 ◦ < l < ◦ b < ◦ THOR I 14 . ◦ ≤ l ≤ . ◦ / / ∼ | b | < . ◦ / ∼ δ < ◦ ◦ < l < ◦ | b | < . − . ◦ NVSS I δ ≥ − ◦ . ◦ < l < . ◦ ∼ − . ◦ < b < . ◦ SGPS I 253 ◦ ≤ l ≤ ◦ ffi ths et al. (2005) | b | < . ◦ Parkes S 238 ◦ ≤ l ≤ ◦ | b | ≤ ◦ PMN S − . ◦ < δ < ◦ ffi th & Wright (1993)GBT / GPA S − ◦ < l < ◦ | b | < ◦ ◦ < δ < ◦ ∼ . ◦ < l < . ◦ | b | < ◦ I 5 ◦ < l < . ◦ ∼ . (cid:48) | b | < . ◦ I 350 ◦ < l < ◦ | b | < . ◦ Table A.1.
Properties of the radio surveys. The type "S" and "I" denotes a "single-dish telescope" or "interferometers", respectively. The point-source sensitivity is given, as well as the FWHM. If the FWHM is asymmetric, the major axis is given. For the TGSS and the CGPS, the FWHMdepends on the declination (indicated by the asterisk). The links to retrieve these data are labeled with numbers and given below. Numbers followedby an asterisk indicate that a source catalog, associated with the instrument, is available and used.Archival data are retrieved at the following links: – TGSS: [1] http://vo.astron.nl/tgssadr/q_fits/imgs/form – MGPS-2: [2] – THOR: [3] – CHIPASS: [4] – VGPS: [5] – NVSS: [6] – CGPS: [7] – SGPS: [8] – Parkes: [9] – PMN: [10] ftp://ftp.atnf.csiro.au/pub/data/pmn/maps/PMN/ – GBT / GPA: [11] – ftp://ftp.atnf.csiro.au/pub/data/pmn/maps/87GB/ – MAGPIS: [13] https://third.ucllnl.org/gps/index.html
Article number, page 14 of 19. Devin et al.: Multiwavelength constraints on unidentified TeV sourcesTable A.2 summarizes the description of the X-ray instruments whose archival data are retrieved through the pipeline. We automatically extractall the observations whose center is comprised within an angular distance from the source of interest. For ROSAT / PSPC and
XMM-Newton , wecreate background-subtracted and exposure-corrected maps, while the count maps of
Swift / XRT, ASCA and
Suzaku are only corrected fromthe exposure because the corresponding background maps are not available. Except for large field-of-view instruments (such as ROSAT / PSPC,
Integral / IBIS-ISGRI, and
Swift / BAT), we create a mosaic of these images in case of multiple observations. For
Chandra data, we directly retrievestacked and subtracted-background images (combining multiple observations), which are also corrected from the exposure and the vignetting. For
Integral / IBIS-ISGRI and
Swift / BAT, we extract a subregion of the flux, error flux, and significance maps.
Satellite Detector Field of view Energy (keV) PSF (FWHM) Data link Catalog names
Chandra
ACIS 17 (cid:48) × (cid:48) (cid:48)(cid:48) [1]* CSC2 a Suzaku
XIS 19 (cid:48) × (cid:48) < . (cid:48) [2] – XMM-Newton
EPIC (MOS1-2) 30 (cid:48) × (cid:48) (cid:48)(cid:48) [3]* 4XMM-DR9 b EPIC (PN) 30 (cid:48) × (cid:48) (cid:48)(cid:48) ASCA
SIS 22 (cid:48) × (cid:48) (cid:48) [4]* GIS / SIS / Galactic Plane Survey c GIS 20 (cid:48) × (cid:48) (cid:48) Swift
XRT 23 . (cid:48) × . (cid:48) (cid:48)(cid:48) * [5]* 1SWXRT / d BAT 5400 (cid:48) × (cid:48)
14 – 195 17 (cid:48) [6]* BS105 months e Integral IBIS 1140 (cid:48) × (cid:48)
17 – 60 12 (cid:48) [7]* 8-1 yr All sky and 9-14 Galactic sky f NuSTAR – 13 (cid:48) × (cid:48) . (cid:48)(cid:48) [8]* Survey source catalog (Fornasini et al. 2017)ROSAT PSPC 114 (cid:48) × (cid:48) (cid:48)(cid:48) [9]* 1RXS (Voges et al. 1999)2RXS (Boller et al. 2016) a http://cxc.cfa.harvard.edu/csc2/ b http://xmmssc.irap.omp.eu/Catalogue/4XMM-DR9/4XMM_DR9.html , Webb et al. (2020) c https://heasarc.gsfc.nasa.gov/W3Browse/all/ascasis.html (same for ascagis.html and ascagps.html) d , , D’Elia et al. (2013), Evans et al. (2020) e https://swift.gsfc.nasa.gov/results/bs105mon/ f https://heasarc.gsfc.nasa.gov/W3Browse/integral/ibiscat.html , https://heasarc.gsfc.nasa.gov/w3browse/all/intibisvhd.html , https://heasarc.gsfc.nasa.gov/w3browse/all/intibisgal.html , http://hea.iki.rssi.ru/integral/fourteen-years-galactic-survey/ Table A.2.
Properties of the X-ray instruments. The
Swift / XRT PSF (noted with an asterisk) corresponds to the half-power diameter. The links toretrieve these data are labeled with numbers and given below. Numbers followed by an asterisk indicate that a source catalog, associated with theinstrument, is available and used.Archival X-ray data are retrieved through the following links: – Chandra : [1] http://cxc.cfa.harvard.edu/csc2/data_products/ – Suzaku : [2] – XMM-Newton : [3] http://nxsa.esac.esa.int/nxsa-sl/ – ASCA : [4] https://darts.isas.jaxa.jp/pub/asca/data/ – Swift / XRT: [5] ftp://legacy.gsfc.nasa.gov/swift/data/ – Swift / BAT: [6] https://skyview.gsfc.nasa.gov/ – Integral: [7] http://hea.iki.rssi.ru/integral/fourteen-years-galactic-survey/ – NuSTAR: [8] https://heasarc.gsfc.nasa.gov/FTP/nustar/data/ – ROSAT: [9] ftp://ftp.mpe.mpg.de/rosat/archive/
To complete our data set, we extract infrared data from Spitzer / GLIMPSE at 3.6, 4.5, 5.8, 8, 21, 24, 870, and 1100 µ m (available on theMAGPIS website https://third.ucllnl.org/gps/ ). Using ten years of Fermi -LAT data, we also created a binned count map with energiesbetween 10 and 500 GeV, setting a pixel size of 0.05 ◦ and 10 energy bins (version 1.0.10 of the Fermitools ). The map encompasses the HGPSwith latitude | b | ≤ ◦ . The SOURCE event class was selected, which is a compromise between background rejection and statistics and we imposeda maximum zenith angle on the photon arrival direction of 90 ◦ to reduce the contamination of the Earth limb. We excluded time intervals duringwhich the satellite passed through the South Atlantic Anomaly and when the rocking angle was more than 52 ◦ . We modeled the contribution ofthe Galactic and isotropic di ff use emissions using the files gll_iem_v07.fits and iso_P8R3_SOURCE_V2.txt (with the normalizations fixedto 1 and the spectral index to 0) using the instrument response functions P8R3_SOURCE_V2 , and we created a residual count map between 10 GeVand 500 GeV. Available at https://fermi.gsfc.nasa.gov/ssc/data/access/lat/BackgroundModels.html
Article number, page 15 of 19 & A proofs: manuscript no. TeVsourcesTable A.3 reports the catalogs of known objects used for the association procedure.
Objects Name and details ReferencesPulsars ATNF (version 1.58) Manchester et al. (2005) a Galactic SNRs Green catalog Green (2017)SNRs / PWNe SNR cat Ferrand & Safi-Harb (2012) b Galactic SNR candidates found with THOR data Anderson et al. (2017)
Fermi -LAT sources 4FGL (50 MeV − −
300 GeV) Acero et al. (2015)2FHL (50 GeV − − Fermi -LAT extended sources FGES ( >
10 GeV) Ackermann et al. (2017)
Fermi -LAT pulsars 2PC Abdo et al. (2013)HAWC sources 2HWC Abeysekara et al. (2017)H ii regions obtained with WISE data Anderson et al. (2014) c Molecular clouds Galactic MCs Rice et al. (2016)Galactic O-Stars GOSC Maíz Apellániz et al. (2013) a b c http://astro.phys.wvu.edu/wise/ Table A.3.
Catalogs used for the association procedure.
Appendix B: Radio spectral index derivation
To estimate the radio spectral index, we calculate the flux at di ff erent frequencies with the available archival continuum data. We mask all thecataloged radio sources in the images that are located outside the flux extraction region (called R ON ), which is defined after a visual inspection ofthe images. The maps are in Jy / beam (hereafter Φ ) and the associated beam is the full width at half maximum (FWHM) for which the e ff ectivearea is given by B area = π ab (2 √ deg , (B.1)where a and b are the half major and minor axes of the instrument PSF. The number of pixels per beam is thus written as N pix , psf = B area c c , (B.2)where c and c are the pixel size in degrees in both directions. The mean background B and the standard deviation (rms) are obtained by aGaussian fit on the flux distribution of unmasked pixels outside the ON region. The total flux in the ON region and its associated error are givenby F tot = (cid:80) N pix , on i Φ i N pix , psf Jy , (B.3) σ tot = rms × (cid:115) N pix , on N pix , psf Jy , (B.4)where N pix , on is the total number of pixels inside the ON region and Φ i the flux in units of mJy / beam per pixel. The total background in the ONregion is written as B tot = B × N pix , on N pix , psf Jy . (B.5)The source flux in the ON region is thus F ON = F tot − B tot . If the significance is below 3 σ , we derive an upper limit at the 3 σ confidence level asUL = × σ tot . We then fit the calculated fluxes at di ff erent frequencies with a power law. We only consider the data points (not the upper limits)for the fit, since in some cases the source masking in radio maps with moderate PSF can lead to an underestimation of upper limits.The radio flux is expressed as S ν ∝ ν α , where ν and α are the frequency and spectral index, respectively. The spectral index α allows us todisentangle thermal from nonthermal emission for which we have α (cid:38) α (cid:46) , respectively. Radio spectral index of SNRs has a mean valueof α ∼ − . / − . α ∼ − . / −
645 in H.E.S.S. Collaboration et al. (2011), for which thederived spectral index was α = − . ± .
07. Below we illustrate the method, which is now implemented in a generic way involving more radioinstruments, on the PWN HESS J1356 − . (cid:48) and 10 . (cid:48) , respectively) prevents us from deriving a meaningfulArticle number, page 16 of 19. Devin et al.: Multiwavelength constraints on unidentified TeV sourcesflux due to a too small number of remaining unmasked pixels. The source masking is then the main limitation of this method . Figure B.3 depictsthe distribution of the unmasked pixels in the OFF region (used for the background estimation) of the MGPS-2 data (left), and the SED with thebest-fit power law on the calculated fluxes. We found a radio spectral index of α = − . ± .
06 that is compatible with the value obtained inH.E.S.S. Collaboration et al. (2011). The di ff erence can be explained by a slightly di ff erent background estimation method and a small di ff erencein the size and position adopted for the ON region. Galactic longitude (°) G a l a c t i c l a t i t u d e ( ° ) - . - . - . ParkesPSR J1357-6429HESS J1356-645 - . - . - . PMNPSR J1357-6429HESS J1356-645 - - . - . - . MGPS-2PSR J1357-6429HESS J1356-645
Fig. B.1.
MGPS-2, Parkes, and PMN maps of HESS J1356 − / black square. The PSF (FWHM) of each instrument is illustrated at the top right of the images. The cataloged radio sources arerepresented by red circles. The dashed and solid yellow circles indicate the Gaussian σ extent and the flux extraction radius of HESS J1356 − σ , 5 σ , and 7 σ in the HGPS significance map. MGPS-2 CHIPASS Parkes PMNFrequency (GHz) 0.843 1.4 2.4 4.85rms (mJy / beam) 1.76 637.2 235.1 10.57Flux density (mJy) 538.6 ± < . < . ± Fig. B.2.
Estimated fluxes with the associated statistical errors. The upper limits are given at the 3 σ confidence level. rms(fit) = 1.76 mJy/beam -5 0 5Flux (mJy/beam)1101001000 N u m be r o f p i x e l s F = 538.61 +/- 42.34 mJy F l u x ( < R , m Jy ) HESSJ1356-645: α = -0.03 +/- 0.06 ν (GHz)10100100010000 F ν ( m Jy ) Fig. B.3. (Left) Distribution of the flux in the MGPS-2 unmasked pixels located outside of the ON region and used for the background estimation.The best-fit Gaussian is represented in red. (Right) Calculated fluxes (black points) with the best-fit power law overlaid in red. The upper limits(black arrows) are given at the 3 σ confidence level and are not considered in the fit. The source masking sometimes prevented us from using data (for the flux calculation) from relatively low angular resolution instruments suchas CHIPASS and Parkes toward HESS J1427 − − − & A proofs: manuscript no. TeVsources
Appendix C: Mean magnetic field estimation
We use ROSAT / PSPC data in the 0.9–2.4 keV band to estimate the flux (or the flux upper limit) and to constrain the value of the mean magneticfield, given the measured flux at TeV energies. We take advantage of this large field-of-view all-sky instrument, which covers the entire HGPSregion. We use the 1RXS catalog reporting the brightest sources detected with ROSAT / PSPC (Voges et al. 1999) and the 2RXS catalog (Bolleret al. 2016) to mask the sources located outside the ON region. We use the ring background and the reflected background methods, and we calculatethe significance following the prescription in Li & Ma (1983), using a correlation radius equal to the flux extraction radius defined in the HGPS( R spec ). For the ring background method, the OFF region is defined between R in = R spec + R out = (cid:113) (10 R + R ) ◦ so that the OFF toON area ratio amounts to ∼
10 for large sources. For the reflected background method, we use four circular regions (with a radius R spec ) locatedat an angular distance of 3 × R spec from the source center. If the X-ray emission is not significant, an upper limit on the flux is calculated at a 5 σ confidence level. The X-ray flux is simulated using the HEASARC tools WebPimms and defined as F obs = F × e − σ ( E ) N H with F = N (cid:18) EE (cid:19) − Γ , (C.1)where F is the non-absorbed flux, σ ( E ) the photoelectric absorption cross section (in units of cm ) and N H the column density (in units of cm − ).For each simulated flux, a combination of ( Γ , N H ) corresponds to a count rate between 0.9 and 2.4 keV that we simulate in the image with Monte-Carlo sample. The simulation stops when the significance seen in the ROSAT / PSPC data is reached (or when the 5 σ -threshold is reached in caseof a non-detection) and gives the X-ray flux (or upper limit on flux) in the parameter space ( Γ , N H ).Then, we estimate the mean magnetic field assuming a one-zone model and that electrons radiating synchrotron and IC emission (scatteringon the cosmic microwave background) are in the Thompson regime ( Γ sync = Γ IC = Γ , where Γ is the photon spectral index). If we describe theelectron spectrum with a power law ( K γ − p , γ and p being the Lorentz factor and the electron spectral index), the ratio of the synchrotron to ICfluxes is given by F sync F IC = U B U CMB × (cid:18) γ − p , sync − γ − p , sync γ − p , IC − γ − p , IC (cid:19) (C.2)where U B ∼ . B − eV cm − and U CMB = − are the magnetic field and cosmic microwave background energy densities respectively(with B − = B µ G ). The synchrotron and IC radiations occur at the characteristic energies of γ sync (cid:39) . × E / , keV B − / − and γ IC (cid:39) . × E / , TeV (C.3)Using the above equations, the mean magnetic field can be expressed as B − (cid:119) G( Γ ) × (cid:18) F sync F IC × E − Γ , IC , TeV − E − Γ , IC , TeV E − Γ , sync , keV − E − Γ , sync , keV (cid:19) / Γ (C.4)where F is the flux between E and E , G( Γ ) (cid:39) (0 . × Γ − ) / Γ and Γ = ( p + /
2. If
Γ =
2, the mean magnetic field is simply expressed as F sync F IC (cid:39) B − (C.5)We can thus estimate the mean magnetic field using the X-ray flux derived with ROSAT / PSPC data and the TeV flux and spectral indexreported in the HGPS. Below the method is applied on the SNR RX J1713.7 − − R spec = ◦ and the spectral parameters reported in the HGPS are F = (2 . ± . × − cm − s − TeV − and Γ TeV = . ± .
02. We masked in the ROSAT / PSPC images all the 1RXS and 2RXS cataloged sourceswhose centroid are located outside R spec . Figure C.1 (left) shows the significance map using the reflected background method, which is similar tothat obtained using the ring background method. As expected, the significance is well above the 5 σ threshold. The mean value of the magnetic fieldis given in Figure C.1 (right) with respect to the TeV photon spectral index and the column density. Following Sano et al. (2015), who estimatedthe variation of the column density over the entire SNR, we took N H = (0 . − . × cm − . Within these TeV spectral index and column densityranges, we determined a mean magnetic field of B = [10 . − . µ G, which is consistent with B = . ± . µ G, obtained when modelingthe broadband nonthermal emission of the SNR with a leptonic model (H. E. S. S. Collaboration et al. 2018b). https://heasarc.gsfc.nasa.gov/Tools/w3pimms_help.html Article number, page 18 of 19. Devin et al.: Multiwavelength constraints on unidentified TeV sources × cm -2 )1.61.82.02.22.42.62.83.0 Γ B - f i e l d ( µ G ) Fig. C.1. (Left) ROSAT / PSPC significance map using the reflected background method and a correlation radius of R spec = ◦ (represented by ayellow circle). The dashed green circles are the 1RXS and 2RXS sources that have been masked in the ROSAT / PSPC count map (0.9 −−