Identification of HESS J1303-631 as a Pulsar Wind Nebula through gamma-ray, X-ray and radio observations
H.E.S.S. Collaboration, A. Abramowski, F. Acero, F. Aharonian, A.G. Akhperjanian, G. Anton, S. Balenderan, A. Balzer, A. Barnacka, Y. Becherini, J. Becker, K. Bernlöhr, E. Birsin, J. Biteau, A. Bochow, C. Boisson, J. Bolmont, P. Bordas, J. Brucker, F. Brun, P. Brun, T. Bulik, I. Büsching, S. Carrigan, S. Casanova, M. Cerruti, P.M. Chadwick, A. Charbonnier, R.C.G. Chaves, A. Cheesebrough, G. Cologna, J. Conrad, C. Couturier, M. Dalton, M.K. Daniel, I.D. Davids, B. Degrange, C. Deil, H.J. Dickinson, A. Djannati-Ataï, W. Domainko, L.O'C. Drury, G. Dubus, K. Dutson, J. Dyks, M. Dyrda, K. Egberts, P. Eger, P. Espigat, L. Fallon, C. Farnier, S. Fegan, F. Feinstein, M.V. Fernandes, A. Fiasson, G. Fontaine, A. Förster, M. Füßling, M. Gajdus, Y.A. Gallant, T. Garrigoux, H. Gast, L. Gérard, B. Giebels, J.F. Glicenstein, B. Glück, D. Göring, M.-H. Grondin, S. Häffner, J.D. Hague, J. Hahn, D. Hampf, J. Harris, M. Hauser, S. Heinz, G. Heinzelmann, G. Henri, G. Hermann, A. Hillert, J.A. Hinton, W. Hofmann, P. Hofverberg, M. Holler, D. Horns, A. Jacholkowska, C. Jahn, M. Jamrozy, I. Jung, M.A. Kastendieck, K. Katarzyński, U. Katz, S. Kaufmann, B. Khélifi, D. Klochkov, W. Kluźniak, T. Kneiske, Nu. Komin, K. Kosack, R. Kossakowski, F. Krayzel, et al. (98 additional authors not shown)
AAstronomy & Astrophysics manuscript no. HESSJ1303-631 c (cid:13)
ESO 2012October 26, 2012
Identification of HESS J1303 −
631 as a Pulsar Wind Nebula through γ -ray, X-ray and radio observations H.E.S.S. Collaboration, A. Abramowski , F. Acero , F. Aharonian , , , A.G. Akhperjanian , , G. Anton ,S. Balenderan , A. Balzer , A. Barnacka , , Y. Becherini , , J. Becker , K. Bernl¨ohr , , E. Birsin , J. Biteau ,A. Bochow , C. Boisson , J. Bolmont , P. Bordas , J. Brucker , F. Brun , P. Brun , T. Bulik , I. B¨usching , ,S. Carrigan , S. Casanova , , M. Cerruti , P.M. Chadwick , A. Charbonnier , R.C.G. Chaves , , A. Cheesebrough ,G. Cologna , J. Conrad , C. Couturier , M. Dalton , , M.K. Daniel , I.D. Davids , B. Degrange , C. Deil ,H.J. Dickinson , A. Djannati-Ata¨ı , W. Domainko , L.O’C. Drury , G. Dubus , K. Dutson , J. Dyks , M. Dyrda ,K. Egberts , P. Eger , P. Espigat , L. Fallon , C. Farnier , S. Fegan , F. Feinstein , M.V. Fernandes , A. Fiasson ,G. Fontaine , A. F¨orster , M. F¨ußling , M. Gajdus , Y.A. Gallant , T. Garrigoux , H. Gast , L. G´erard ,B. Giebels , J.F. Glicenstein , B. Gl¨uck , D. G¨oring , M.-H. Grondin , , S. H¨a ff ner , J.D. Hague , J. Hahn ,D. Hampf , J. Harris , M. Hauser , S. Heinz , G. Heinzelmann , G. Henri , G. Hermann , A. Hillert , J.A. Hinton ,W. Hofmann , P. Hofverberg , M. Holler , D. Horns , A. Jacholkowska , C. Jahn , M. Jamrozy , I. Jung ,M.A. Kastendieck , K. Katarzy´nski , U. Katz , S. Kaufmann , B. Kh´elifi , D. Klochkov , W. Klu´zniak ,T. Kneiske , Nu. Komin , K. Kosack , R. Kossakowski , F. Krayzel , H. La ff on , G. Lamanna , J.-P. Lenain ,D. Lennarz , T. Lohse , A. Lopatin , C.-C. Lu , V. Marandon , A. Marcowith , J. Masbou , G. Maurin ,N. Maxted , M. Mayer , T.J.L. McComb , M.C. Medina , J. M´ehault , U. Menzler , R. Moderski , M. Mohamed ,E. Moulin , C.L. Naumann , M. Naumann-Godo , M. de Naurois , D. Nedbal , D. Nekrassov , N. Nguyen ,B. Nicholas , J. Niemiec , S.J. Nolan , S. Ohm , , , E. de O˜na Wilhelmi , B. Opitz , M. Ostrowski , I. Oya ,M. Panter , M. Paz Arribas , N.W. Pekeur , G. Pelletier , J. Perez , P.-O. Petrucci , B. Peyaud , S. Pita ,G. P¨uhlhofer , M. Punch , A. Quirrenbach , M. Raue , A. Reimer , O. Reimer , M. Renaud , R. de los Reyes ,F. Rieger , , J. Ripken , L. Rob , S. Rosier-Lees , G. Rowell , B. Rudak , C.B. Rulten , V. Sahakian , ,D.A. Sanchez , A. Santangelo , R. Schlickeiser , A. Schulz , U. Schwanke , S. Schwarzburg , S. Schwemmer ,F. Sheidaei , , J.L. Skilton , H. Sol , G. Spengler , Ł. Stawarz , R. Steenkamp , C. Stegmann , F. Stinzing ,K. Stycz , I. Sushch , A. Szostek , J.-P. Tavernet , R. Terrier , M. Tluczykont , K. Valerius , C. van Eldik , ,G. Vasileiadis , C. Venter , A. Viana , P. Vincent , H.J. V¨olk , F. Volpe , S. Vorobiov , M. Vorster , S.J. Wagner ,M. Ward , R. White , A. Wierzcholska , M. Zacharias , A. Zajczyk , , A.A. Zdziarski , A. Zech , andH.-S. Zechlin (A ffi liations can be found after the references) Received June 14, 2012; accepted October 12, 2012
ABSTRACT
Aims.
The previously unidentified very high-energy (VHE; E >
100 GeV) γ -ray source HESS J1303 − XMM-Newton
X-raysatellite and from the PMN radio survey are also examined.
Methods.
Detailed morphological and spectral studies of VHE γ -ray emission as well as of the XMM-Newton
X-ray data are performed. Radiodata from the PMN survey are used as well to construct a leptonic model of the source. The γ -ray and X-ray spectra and radio upper limit are usedto construct a one zone leptonic model of the spectral energy distribution (SED). Results.
Significant energy-dependent morphology of the γ -ray source is detected with high-energy emission ( E >
10 TeV) positionally coincidentwith the pulsar PSR J1301 − E < ∼ . ◦ to the South-East of the pulsar. The spectrum ofthe VHE source can be described with a power-law with an exponential cut-o ff N = (5 . ± . × − TeV − cm − s − , Γ = . ± .
2) and E cut = (7 . ± .
2) TeV. The PWN is also detected in X-rays, extending ∼ − (cid:48) from the pulsar position towards the center of the γ -ray emissionregion. A potential radio counterpart from the PMN survey is also discussed, showing a hint for a counterpart at the edge of the X-ray PWN trailand is taken as an upper limit in the SED. The extended X-ray PWN has an unabsorbed flux of F − ∼ . + . − . × − erg cm − s − and isdetected at a significance of 6 . σ . The SED is well described by a one zone leptonic scenario which, with its associated caveats, predicts a verylow average magnetic field for this source. Conclusions.
Significant energy-dependent morphology of this source, as well as the identification of an associated X-ray PWN from
XMM-Newton observations enable identification of the VHE source as an evolved PWN associated to the pulsar PSR J1301 − γ -ray and X-ray radiation are such that they may have asimilar origin in the pulsar nebula. However, the large discrepancy in emission region sizes and the low level of synchrotron radiation suggest amulti-population leptonic nature. The low implied magnetic field suggests that the PWN has undergone significant expansion. This would explainthe low level of synchrotron radiation and the di ffi culty in detecting counterparts at lower energies, the reason this source was originally classifiedas a “dark” VHE γ -ray source. Key words.
Gamma-rays: observations – Pulsars: individual: PSR J1301 − −
631 1 a r X i v : . [ a s t r o - ph . H E ] O c t . Introduction In recent years, nearly a hundred VHE γ -ray sourceshave been discovered by various experiments, includingmany di ff erent types of sources. Generally, sources fromthese di ff erent classes also exhibit radio and X-ray radi-ation, however, the discovery of TeV J2032 + γ -raysources without obvious counterparts at other wavelengths.HESS J1303 − −
63 (Aharonian et al.2005a), in observations taken between January and Juneof 2004 (Aharonian et al. 2005b). HESS J1303 −
631 wasthe first so-called “dark source” discovered by H.E.S.S.More of these sources were discovered by the H.E.S.S. col-laboration in the following years (Aharonian et al. 2008;Tibolla et al. 2009). Identifying and understanding this newclass of sources has become an important task for modern γ -ray astronomy.A growing number of extended VHE γ -ray sources,without (or with significantly fainter or less extended)X-ray / radio counterparts, appear to be associated withenergetic pulsars in the Galactic Plane. Some re-cent examples of this class of objects include HESSJ1825 −
137 (Aharonian et al. 2006a) and HESS J1356 − O ff set PWNe wherethe pulsar is located at or near the edge of the γ -ray andX-ray emission regions. These configurations may form intwo ways. First, a high spatial velocity pulsar, possibly su-personic (in which case a bow shock nebula may form),leaves behind a “trail” of high-energy electrons in the am-bient medium. Alternatively, an o ff set PWN may form ifits expansion is blocked on one side by the reverse shockof the supernova remnant (SNR) in which the pulsar wasborn. Due to inhomogeneous densities in the ISM, the ex-pansion of the supernova remnant may proceed asymmet-rically, or the motion of the pulsar may place it near theedge of the SNR and the expanding PWN may then be dis-rupted asymmetrically by the reverse shock of the SNR, ascenario known as a Crushed PWN (Blondin et al. 2001).At the time of discovery, HESS J1303 −
631 was foundto have a large intrinsic Gaussian extent of ∼ . ◦ , as-suming a 2-dimensional symmetric Gaussian distribution,and a flux of ∼
17% of the Crab flux above 380 GeV.Originally, the source had no known extended counter-parts at other wavelengths and was, therefore, classified
Send o ff print requests to : Matthew L. Dalton, e-mail: [email protected] as a dark source. As is the case with many such darksources, HESS J1303 −
631 is found to have a pulsar lyingnear the edge of the emission region with a high enoughspin-down luminosity to account for the γ -ray emission.PSR J1301 − E = . × erg s − , is the most pow-erful pulsar within 6 ◦ of the H.E.S.S. source (Manchesteret al. (2005), see Table 1 for a list of known pulsars within0 . ◦ of HESS J1303 − τ c =
11 kyr, and a rotation period of184 ms.Originally, the distance to PSR J1301 − γ -ray conver-sion e ffi ciency of 37% in the 0.3 to 10 TeV range. Using anewer model of the Galactic electron distribution, NE2001(Cordes & Lazio 2002), however, yields a much closerdistance of 6 . γ -ray spectrum at the time of discov-ery yields an integrated flux in the 1 to 30 TeV band of Φ = . × − erg cm − s − or 3.7% of the current spin-down luminosity of this pulsar, ( F . = ˙ E / π (6 . = . × − erg cm − s − ), a γ -ray conversion e ffi ciencywhich is comparable to other VHE PWNe (typically 0-7%,see e.g. Mattana et al. (2009)).A 5 ksec Chandra
X-ray observation, partially cover-ing the VHE peak emission region (Mukherjee & Halpern2005), revealed several point sources within the field ofview, but no extended emission corresponding to the γ -rayemission region was found, and none of the radio pulsars inthe field of view of the Chandra observation were detected.The possibility of an annihilating clump of dark matter asthe origin of the γ -ray signal was explored by Ripken et al.(2008). Such a model could explain the lack of detection oflower energy counterparts. However, it was found that thespectrum obtained for this source would require an unrea-sonably high mass for the candidate dark matter particles( ∼
40 TeV). Also, as mentioned in that study, the inferredlateral density distribution does not support a dark matterscenario. Ripken et al. (2008), therefore, concluded this tobe an unlikely candidate for the explanation of the VHEsource.To build a complete picture of the γ -ray emission pro-cess in this source, data from recent re-observations ofHESS J1303 −
631 with the H.E.S.S. telescope array wereanalysed, enabling studies of energy-dependent morphol-ogy. Also, follow-up observations by the
XMM-Newton
X-ray satellite, performed in 2005, showing a detection of acompact source slightly o ff set from the pulsar position anda significantly extended PWN, are presented.In Section 2, the H.E.S.S. instrument, data and anal-ysis methods are discussed as well as the light curve.Section 3 describes the studies of energy-dependent mor- . Abramowski et al. (H.E.S.S. Collaboration): Identification of HESS J1303 −
631 as a pulsar wind nebula phology of the γ -ray source, followed by a discussion ofthe spectrum of the source in Section 4. Section 5 presentsthe results of the XMM-Newton
X-ray follow-up obser-vations, showing an X-ray PWN associated with the pul-sar PSR J1301 − −
631 with the pulsarPSR J1301 −
2. H.E.S.S. observations and analysis
H.E.S.S. is an array of four imaging atmosphericCherenkov telescopes located in the Khomas Highlandof Namibia (23 ◦ (cid:48) (cid:48)(cid:48) S, 16 ◦ (cid:48) (cid:48)(cid:48) E) at an altitudeof 1800 m above sea-level. The telescopes image theCherenkov light emitted by charged particles in the exten-sive air shower created when a γ -ray is absorbed in the at-mosphere. They are optimized for detection of VHE γ -rayinitiated showers in the energy range of hundreds of GeVto tens of TeV by Each telescope has a 107 m tessellatedmirror surface and is equipped with a 960 photomultipliertube camera with a field of view (FoV) diameter of ∼ ◦ (Bernl¨ohr et al. 2003; Cornils et al. 2003). The telescopesare triggered in coincidence mode (Funk et al. 2004) assur-ing that an event is always recorded by at least two of thefour telescopes allowing stereoscopic reconstruction of theshowers. More information about H.E.S.S. can be found inHinton (2004). HESS J1303 −
631 was originally discovered during an ob-servation campaign for PSR B1259 −
63. Follow-up obser-vations of the two sources between 2004 and 2008 led to atotal dataset of 108.3 hours of live time, using only obser-vations which passed standard H.E.S.S. data quality selec-tion which rejects observations taken during periods of badweather or with instrumental irregularities. The data weretaken in wobble mode at an average zenith angle of 43 . ◦ ,with an average o ff set of 0 . ◦ from the position reported inthe discovery paper (Aharonian et al. 2005b).The data were analyzed using H.E.S.S. standard Hillasreconstruction (Aharonian et al. 2006b). Cuts were ap-plied to the shower image parameters to minimize back-ground, primarily due to cosmic-ray protons. For spectrumextraction, standard cuts (also defined in Aharonian et al.(2006b)), were used together with the Reflected-RegionBackground method (Berge et al. 2007) to subtract resid-ual cosmic ray background, which resulted in an averageenergy threshold of ∼
720 GeV. The resulting excess forthis analysis was found to be 12085 photons for a detec-tion significance of 33 σ . Some of these early observationswere made with telescope pointings coincident with the Right Ascension D ec li n a t i on (cid:176) -64 30’ (cid:176) -63 00’ (cid:176) -63 050100150200250300350400 m h m h
13 PSR J1301-6305PSR B1259-63Galactic Plane
Fig. 1.
The HESS J1303 −
631 VHE γ -ray excess map, producedusing hard cuts and the ring background method, was smoothedwith a Gaussian kernel with σ = . ◦ . Coordinates are J2000.0.The high spin-down power pulsar, PSR J1301 − − −
63, is seenin the bottom of the FoV. The size of the H.E.S.S. PSF, alsosmoothed with a Gaussian kernel with σ = . ◦ , is shown inthe white box to the lower left. The blue / red transition occurs ata detection significance of ∼ σ . HESS J1303 −
631 emission region, rendering them unsuit-able for spectral analysis since placement of reflected re-gions for background estimation is not possible. For themorphology studies, hard cuts were applied to further re-duce background contamination and improve image recon-struction, and hence the point spread function (PSF) of theinstrument, at the expense of a higher energy threshold,together with the Ring Background method, resulting inan average energy threshold of ∼
840 GeV. Cross-checkswere performed using a multi-variate analysis (Ohm et al.2009), with background suppression based on boosted-decision trees, leading to compatible results. γ -ray map and light curve The VHE γ -ray excess map (Fig. 1) of theHESS J1303 −
631 FoV shows extended emission tothe South-East of PSR J1301 − α = ± stat , δ = − ◦ (cid:48) (cid:48)(cid:48) ± (cid:48)(cid:48) stat (J2000.0), with major / minoraxis Gaussian widths of σ x = . ◦ ± . ◦ and σ y = . ◦ ± . ◦ , with a position angle (counterclockwise from north) of φ = ◦ ± ◦ . The χ / NDF of the fit was 390 /
3. Abramowski et al. (H.E.S.S. Collaboration): Identification of HESS J1303 −
631 as a pulsar wind nebula the source extension was found to be small and have anegligible a ff ect on the resulting source position. The fittedposition is consistent with the one quoted in the originaldiscovery paper (Aharonian et al. 2005b), but slightlyshifted towards the pulsar position due to the (comparedto the discovery paper) higher energy threshold of thehard cuts used and the presence of energy-dependentmorphology (see Section 3).The nightly flux was determined using a flux extrac-tion region of radius 0 . ◦ to ensure full enclosure of thesource, around the best fit position given above assum-ing a power-law spectrum with an index of 1.5. Studieswere performed to account for influences from the nearbyVHE source PSR B1259 −
63. As expected for an extendedsource, with an estimated diameter of 40 pc at a distance of6.6 kpc, the nightly flux is consistent with constant emis-sion, with χ / NDF = /
69, verifying the stability of theH.E.S.S. instrument over the period of data taking.
3. Energy-dependent morphology
To test for the presence of energy-dependent morphologyin the VHE source, excess images were generated in thefollowing energy bands: E = (0.84 - 2) TeV, E = (2 -10) TeV and E >
10 TeV (Fig. 2, left, top to bottom).The radial acceptance of the FoV was determined from thedata, thus naturally accounting for the energy dependence.Slices were made on the uncorrelated excess images hav-ing dimensions of 1 . ◦ × . ◦ and centered at the best fit po-sition of the VHE excess. The orientation is chosen alongthe fitted position angle (see Sec. 2.2). A Gaussian func-tion was then fit to each slice as shown in Fig. 2 (right).The intrinsic source width was obtained by fitting the con-volution of a Gaussian with the energy dependent H.E.S.S.PSF.The resulting parameters of the PSF convolvedGaussian fits, mean c and intrinsic Gaussian width w int , foreach energy band (Table 2) were then plotted as a functionof energy (Fig. 3). A fit of a constant to these parametersyielded very bad quality fits, which establishes the exis-tence of strong energy-dependent morphology. This mor-phology implies a spectral steepening in γ -rays away fromthe pulsar, a physical property predicted to be present inevolved PWNe. Fitting a linear function yielded much bet-ter quality fits (Tab. 3) and a model of the morphology pa-rameterized by a projected center of emission, c ( E ), cal-culated with respect to the pulsar position, and an intrin-sic source Gaussian width, w int ( E ), which is calculated bytaking into account the (energy-dependent) finite angularresolution of the instrument: c = (0 . ± . ◦ − (0 . ± . ◦ × E TeV w int = (0 . ± . ◦ − (0 . ± . ◦ × E TeV
Table 1.
All known pulsars within 0 . ◦ of HESS J1303 − δ
10 TeV is the distance from the givenpulsar to the E >
10 TeV peak position.
Pulsar ˙ E / erg / s δ [arc min]PSR J1301 − − − − − Right Ascension J2000.0 D ec li n a t i on J2000 . (cid:176) -63 00’ (cid:176) -63 h m h m h m h ] (cid:176) Distance from pulsar [ -0.5 0 E xcess Right Ascension J2000.0 D ec li n a t i on J2000 . (cid:176) -63 00’ (cid:176) -63 h m h m h m h ] (cid:176) Distance from pulsar [ -0.5 0 E xcess Right Ascension J2000.0 D ec li n a t i on J2000 . (cid:176) -63 00’ (cid:176) -63 -202468101214m h m h m h m h ] (cid:176) Distance from pulsar [ -0.5 0 E xcess -202468101214 Fig. 2.
Left: uncorrolated excess images of the HESS J1303 − = (0.84 - 2) TeV, E = (2 - 10) TeVand E >
10 TeV (from top to bottom). Coordinates are J2000.0.All images were smoothed with a Gaussian kernel of width0 . ◦ . Slices are indicated by the rectangles, taken in the direc-tion of the semi-major axis of the fitted asymmetric Gaussianfunction. Right: the slices on the uncorrelated excess imagesare then fitted with a Gaussian function. The pulsar position ismarked by a green star in the sky maps and a dashed line in theprofiles. The dashed curves show the energy-dependent PSF ofthe H.E.S.S. instrument. Table 2.
Results of the Gaussian fit to the slices on the excessimages in the energy bands E = (0.84 - 2) TeV, E = (2 - 10) TeVand E >
10 TeV. c is the mean of the Gaussian, w img is theGaussian width and w int is the intrinsic Gaussian width of thesource after correcting for the PSF, P is the p-value of the χ fit. Band c w img w int P E − . ◦ ± . ◦ . ◦ ± . ◦ . ◦ ± . ◦ E − . ◦ ± . ◦ . ◦ ± . ◦ . ◦ ± . ◦ E − . ◦ ± . ◦ . ◦ ± . ◦ . ◦ ± . ◦ −
631 as a pulsar wind nebula
Energy [TeV]0 2 4 6 8 10 12 14 16 ] (cid:176) G a u ss i a n M ea n D i s t a n ce F r o m PS R [ ] (cid:176) G a u ss i a n W i d t h [ Image WidthPSF Corrected Width
Fig. 3.
Left: HESS J1303 −
631 fitted Gaussian mean, c ( E ), mea-sured from the pulsar position, as a function of energy. Right:the PSF corrected intrinsic Gaussian extension ( w int ( E ), blue) isoverlaid with the fitted uncorrected excess Gaussian extension( w img ( E ), black dashed) as a function of energy. The points areplaced at the average energy of the photons falling in the corre-sponding energy bin (indicated by the horizontal error bars, notused in fit). Fig. 4.
HESS J1303 −
631 spectrum derived using an integrationregion of radius 0 . ◦ . The spectrum is well fit with a power-lawfunction with spectral index Γ = . ± . ff energy of E cut = (7 . ± .
2) TeV. The fit resulted in a χ / NDF of (20 /
8) corresponding to 1% p-value. A fit of a power-lawspectrum with no cuto ff , shown by the dashed line, resulted in ap-value of 7 × − . The last spectrum point deviates ∼ σ fromthe fitted curve. It was removed from the residuals plot for bettervisibility.
4. Energy spectrum
The spectrum was derived using the Reflected-Regionbackground method with an integration region of radius0 . ◦ , roughly three times the intrinsic Gaussian extent atlow energies to avoid e ff ects of energy-dependent mor-phology, centered at the fitted source position. The de-rived spectrum for the entire dataset, excluding observa-tions where the o ff set of the pointing position to the center of the source is less than 0 . ◦ (reducing the total live time to70.3 hours), is shown in Fig. 4. The spectrum was fit with apower-law function, dN / dE = N ( E / − Γ , with a re-sulting photon index of Γ = . ± . stat and a normaliza-tion constant N = (5 . ± . stat ) × − TeV − cm − s − .This normalization is larger than that found in the orig-inal discovery paper due to a larger integration region.However, with the inclusion of the additional data takensince the source discovery, the p-value of a chi-squaredminimization is rather poor (7 × − ). A chi-square fit toa power-law spectrum with a cut-o ff at the energy E cut , dNdE = N E − Γ e − E / E cut , yielded a better p-value of 1%, with fitted parameters N = (5 . ± . stat ) × − TeV − cm − s − , Γ = . ± . stat and E cut = (7 . ± . stat ) TeV. This spectrum yields anintegrated flux in the 1 −
30 TeV band of (2 . ± . × − erg cm − s − or 7.7% of F . . Monte-Carlo studieswere preformed to test for a possible contribution from thesource PSR B1259 −
63 (spill over events), due to the po-sition and size of the integration region and the exclusionregion for PSR B1259 −
63. E ff ects from this source are es-timated to be about 2% on the integrated flux, smaller thanstatistical and systematic errors. XMM-Newton
X-ray observations
In a search for counterparts of the VHE γ -ray sourcein the keV energy band, two XMM-Newton observa-tions, each about 30 ksec, were carried out on July12 th and 14 th , 2005, in satellite revolution number 1024(ObsID 0303440101, “Observation 1”) and revolution1025 (ObsID 0302340101, “Observation 2”) respectively.All three X-ray imaging CCD cameras (EPIC MOS1,MOS2, and pn) were operated in full-frame mode, witha medium filter to screen out optical and UV light, withthe exception of the pn camera during the first observation,where the Large Window mode with the Thin1 filter wasused. For the data analysis of these observations, the
XMM-Newton
Science Analysis Software (SAS), version 9.0, wasused (http: // xmm.esac.esa.int / sas / ). Cleaning the data andremoving periods of high background due to soft protonflares resulted in a combined data set of about 52 ksec ex-posure. For this analysis, the energy band 2 - 8 keV wasused to optimize the signal-to-noise ratio, since few eventsare expected at lower energies due to high absorption. TheSAS task emosaicproc was used to combine the observa-tions and perform source detection, resulting in the de-tection of 73 point sources within the combined field ofview above the maximum likelihood threshold of 10. The
5. Abramowski et al. (H.E.S.S. Collaboration): Identification of HESS J1303 −
631 as a pulsar wind nebula
05 04 13:03 02-63:00051015
PSR
IRAS 13010-6254 ] (cid:176) Position Angle [ C oun t s Fig. 5.
Top: The 2 - 8 keV
XMM-Newton
X-ray count map inthe region of the pulsar smoothed by a Gaussian kernel of width σ = (cid:48)(cid:48) . The horizontal axis is Right Ascension and verticalaxis is Declination in J2000.0 coordinates. The count map is notexposure corrected, thus the apparent enhanced emission nearthe center is only an artifact. Overlaid is the projection annu-lus, shown in cyan, used to determine the direction of the X-rayextension, with inner radius of 48 (cid:48)(cid:48) and outer radius of 120 (cid:48)(cid:48) ,centered on the pulsar position. The 8, 14 and 20 σ TeV signifi-cance contours are shown in white.The direction of the extension is found to be within 1 σ from thedirection of the star formation region, IRAS 13010 − . ◦ , indicated by a magenta linein the sky map and a dashed line in the projection. The cyanlines in the sky map show the 1 σ errors in the fitted directionof the extension. Bottom: The X-ray azimuthal projection in po-sition angle from the pulsar location. The projected on-countswere fitted with the sum of a Gaussian and a flat backgroundgiving a position angle of 101 . ◦ ± . ◦ and a Gaussian width of30 ◦ ± ◦ . The point source located at ∼ ◦ is an unidentifiedX-ray source. X-ray PWN associated to PSR J1301 − − F −
12 keV = (7 . ± . × − erg cm − s − but peaked 15 (cid:48)(cid:48) ± . (cid:48)(cid:48) to the East of the pulsar with an ex-tension of 6 (cid:48)(cid:48) at a maximum likelihood of 7.7 (sources withlikelihood < − (cid:48) from the pulsar position towards thecenter of the VHE γ -ray emission region (Fig. 5). A de-tailed analysis of this feature is presented in the followingsection.While Observation 1 has the pulsar position closer toon-axis than Observation 2, it is unfortunately not suitedfor studying the extended X-ray source since the extendedregion found in Observation 2 lies directly on / betweenthe edges of the CCD chips in all three detectors inObservation 1, thereby obscuring the view of this feature.Therefore, only Observation 2 was used for further analy-sis. To determine the direction of the X-ray feature, possiblyassociated to PSR J1301 − (cid:48)(cid:48) and an outer radius of 120 (cid:48)(cid:48) (Fig. 5 top). The projectedcounts were fitted with the sum of a Gaussian and a flatbackground giving a position angle of 101 . ◦ ± . ◦ and aGaussian width of 30 ◦ ± ◦ (Fig. 5 bottom). The statisticswere too low to warrant individual examination of the threecameras. The direction of the extension as determined herewas used for the orientation of the slice on the count map,as presented below.The direction of the X-ray extension is consistent towithin 1 σ with the direction of the star formation regionIRAS 13010 − σ significance contour of the VHE source, as indicated inFig. 5 top. This potential birthplace for the pulsar is con-sidered in more detail in Sec. 7.In order to determine the extension of the extended X-ray feature, a slice on the count map containing the pulsarwas taken (Fig. 6, top) in the direction determined by theazimuthal projection, with a slice width of 88 (cid:48)(cid:48) and a lengthof 238 (cid:48)(cid:48) (on slice). A background slice of the same size andorientation was chosen in a source free region at roughlyequal o ff set to the center of the FOV as the on slice to en-sure equal exposure. The slices are completely containedwithin single chips in the MOS1 and MOS2 cameras andextend ∼ (cid:48)(cid:48) over the edges of neighboring chips in the pncamera. Profiles of the on slice and background slice areshown in Fig. 6 (middle and bottom).A point source located just West of the pulsar,2XMM J130141.3 − − . − . ± . . ± .
12 for the source associated to thepulsar).The slice (Fig. 6, middle) does not exhibit enoughstatistics to precisely determine the morphology of the X-
6. Abramowski et al. (H.E.S.S. Collaboration): Identification of HESS J1303 −
631 as a pulsar wind nebula
Distance from Pulsar [arc sec]-180 -160 -140 -120 -100 -80 -60 -40 -20 0 20 40 C oun t s B G C oun t s Fig. 6.
Smoothed, exposure corrected XMM-Newton X-ray fluxmap (cm − s − ) in the 2 − − σ γ -ray significance). Chipedges are shown in black for all three detectors and XMM cata-log sources are shown in magenta. The green box shows the sliceused to create the profile (middle) and the red box shows the sliceused for background estimation (bottom). Both slices were takenfrom the un-exposure corrected count map. A presumably unre-lated soft point source, 2XMM J130141.3 − ff use region” left of the pulsar) a King profile forthe unrelated soft source to the right of the pulsar position and aconstant background from the bottom slice. The red dotted binsin the projections lie directly on a chip edge in the pn camerain both slices and are excluded from the analysis. The dashedhorizontal line indicates the fitted background level. ray extension, but the extension appears to consist of amore compact region near the pulsar position, referred to inthis section as the “compact region”, and extending ∼ (cid:48)(cid:48) to the left, corresponding to the 2XMMi catalog source2XMM J130145.7 − ∼ − (cid:48)(cid:48) to ∼ − (cid:48)(cid:48) , referred to here as the “di ff use”emission region.A simultaneous fit of the slices was performed, con-sisting of a fit to the unrelated soft point source to theWest of the pulsar, a Gaussian to the “compact” regionnear the pulsar position, a larger Gaussian to the “di ff use”region extending to the East and a constant to the countsin the background slice. The fit resulted in a di ff use emis-sion centered at − (cid:48)(cid:48) ± (cid:48)(cid:48) with a Gaussian width of σ = (cid:48)(cid:48) ± (cid:48)(cid:48) while the compact region was found tobe centered at − (cid:48)(cid:48) ± (cid:48)(cid:48) with a width of 16 (cid:48)(cid:48) ± (cid:48)(cid:48) . Theunrelated point source was fitted with a King profile f ( x ) = C (cid:16) + ( x − x R ) (cid:17) α , (1)with R = . (cid:48)(cid:48) and α = .
5, corresponding to the PSF ofthe
XMM-Newton pn camera at 1.5 keV and at ∼ (cid:48) o ff setfrom the center of the field of view. For the other cameras,the PSF is slightly narrower than this.The total X-ray extension is found to extend roughly170 (cid:48)(cid:48) (di ff use center + σ width) beyond the pulsar posi-tion, however, the tail of the extension may be cut shortby the edge of the pn chip. However, taking an integra-tion region from the edge of the pn chip, to avoid e ff ectsof changing sensitivity across chips, to the pulsar position(avoiding the soft point source to the west), for a total inte-gration length of 145 (cid:48)(cid:48) gives total on-counts of 950, and to-tal background counts 689 with the on / o ff area ratio α = σ . For the spectral extraction, placement of a ring for back-ground determination was not possible due to multiplenearby sources. A small elliptical region covering the ex-tension region was taken and an identical ellipse was usedfor background extraction (Fig. 7). The extraction regionsare well contained within single chips for the MOS and pncameras. The spectra were obtained for the three camerasindepedently and then fit simultaneously.The obtained spectrum was fit using the spectral fit-ting software XSPEC with an absorbed power-law modelwhich yielded a column density N H = . + . − . × cm − ,a photon index Γ = . + . − . , and a flux normalization at1 keV of 6 . + − . × − keV − cm − s − . The integrated un-absorbed flux in the 2 −
10 keV energy band was found tobe F −
10 keV = . + . − . × − erg cm − s − .
7. Abramowski et al. (H.E.S.S. Collaboration): Identification of HESS J1303 −
631 as a pulsar wind nebula
Fig. 7.
Zoom of Fig. 6 to the region around PSR J1301 − σ significancecontours are shown in white.
6. Radio observations
The region of HESS J1303 −
631 was covered by a sur-vey of the southern sky by the Parkes, MIT and NRAO(PMN) radio telescopes at 4.85 GHz (Condon et al. 1993).Calibrated maps were obtained from the NASA SkyViewonline tool, shown in Fig. 8. There is a radio feature justEast of the X-ray nebula and near the peak of the VHEsource, the apparent position of which may be shiftedslightly to the North-East due to a strong gradient in theFOV from the strong unidentified radio sources to theNorth-East. The feature is found to have a peak flux of0 .
03 Jy / beam. The flux resolution (rms) of the PMN sur-vey is 0.01 Jy / beam so that the significance of this feature isonly 3 σ and is at the detection limit of the survey (and thusnot reported in the catalog). Therefore, the flux is taken asan upper limit. The feature is consistent with the size of thePSF of the survey (7 (cid:48) FWHM) in the North-East to South-West direction, but may be slightly elongated in the North-West to South-East direction, roughly parallel to the X-rayextension. Since the feature is not significant, no definitiveconclusions about its morphology can be made.Although it is unclear whether this radio feature doesindeed represent a counterpart of the γ -ray and X-raysources, since this lies in a rather complicated region ofthe radio sky, the location is promising due to its similari-ties with other known PWNe having a radio peak just be-yond the X-ray nebula, as in, for example, PSR B1929 + Fig. 8. (cid:48) ) in the HESS J1303 −
631 region. The horizontal axisis Right Ascension and the vertical axis is Declination in J2000.0coordinates and scale is Jy / beam. H.E.S.S. contours are shown ingreen, XMM-Newton
X-ray contours are shown in black and theradio contours are shown in white. A radio feature peaks about3’ East of the pulsar position, just beyond the extended
XMM-Newton
X-ray source and near the center of the H.E.S.S. γ -raysource at a peak value of 0.03 Jy / beam. The apparent position ofthe radio feature may be slightly shifted to the North East due toa strong gradient in background from neighbouring sources.
7. Discussion
Having analysed the morphology and spectra in VHE γ -rays, X-rays and radio data available for the region, it isnow possible to consider HESS J1303 −
631 in a full multi-wavelength scenario. First, an energy mosaic of the VHEemission was created using the three smoothed excess im-ages from Fig. 2. These images were overlaid, as shown inFig. 9, along with the contours of the extended X-ray PWN.This energy mosaic is rather reminiscent of the known o ff -set PWN association HESS J1825 −
137 (Aharonian et al.2006a) where the low-energy VHE γ -ray emission regionis quite extended with the pulsar laying towards the edge ofemission and with the higher energy emission more com-pact and found centered closer to the pulsar.Taking the spectra and fluxes obtained in previous sec-tions, it is now possible to consider the SED of the sourcein a PWN scenario. Although a time-dependent model, in-cluding the evolution of the lepton populations over time,would be required to accurately describe the emission ob-served in the various wave bands, for simplicity, and dueto the limited number of multi-wavelength data available,a simple stationary “one zone” leptonic model (Aharonian& Atoyan 1999) was used to fit the VHE γ -ray and X-ray spectra as well as the single PMN upper limit in radio(Fig. 10). The leptonic model assumes that the same elec-tron population, with an energy distribution in the form ofa single power-law with an exponential cut-o ff , creates ra-
8. Abramowski et al. (H.E.S.S. Collaboration): Identification of HESS J1303 −
631 as a pulsar wind nebula dio and X-ray emission via synchrotron emission as wellas VHE γ -rays via inverse Compton (IC) scattering onCosmic Microwave Background photons. Inclusion of ICscattering on infrared and optical target photons (as ob-tained from GALPROP (Moskalenko et al. 2002), assum-ing a pulsar distance of 6.6 kpc) had a negligible e ff ect onthe model parameters.The fit of the radio upper limit, and the X-ray and γ -rayfluxes with this model yielded an electron spectral index of α = . + . − . , a cut-o ff energy of E cut = + − TeV, a normal-ization of K e = . + . − . × cm − and an average mag-netic field of 1 . + . − . µ G, which is similar to the inferredmean line-of-sight magnetic field strength of ∼ µ G, asdetermined from the pulsar’s rotation measure (Crawford& Ti ff any 2007), but larger than the magnetic field of ∼ . µ G predicted by the γ -ray to X-ray luminosity scal-ing law given in Aharonian & Atoyan (1999). The p-valueof the fit was 0.02 and the model predicts a total energyin electrons of ∼ . × erg. It is worth noting that theresulting model spectrum in the radio band is steeper thantypically observed in PWNe since the X-ray spectral indexis not constrained and the fit of the single electron popu-lation is dominated by the narrow peak in TeV energies.Since the fluxes at the various energies described by thismodel are extracted from regions of di ff ering size, the fit-ted magnetic field represents only an average and shouldbe interpreted with caution.The di ff ering sizes of the γ -ray and X-ray emission re-gions imply the existence of di ff ering electron populationsso that the entire PWN cannot be accurately modeled bya single population. The simple approach presented here,therefore, su ff ers from the caveat that a model with twoelectron populations could reproduce the observed spectrawith a significantly di ff erent magnetic field than obtainedwith a one zone model. Incorporating a strong cuto ff in anolder electron population at high energies would suppressthe X-ray synchrotron emission, even in the face of a muchhigher magnetic field, and still reproduce the VHE peak.Indeed, the higher energy synchrotron emitting electronsmay have been e ff ectively extinguished precisely becauseof the high magnetic field. The morphology in VHE γ -raysshows no evidence of a distinct break in the populations ofelectrons caused by passage of an SNR shock, but ratherappears to show a more continuous transition from lowerto higher energies in VHE γ -rays and on up to the high-est energy synchrotron X-ray emitting electrons closer tothe pulsar. This would imply a continuous transition fromolder to younger electrons which may require not a twozone electron model, but a continuously changing popu-lation making modeling quite di ffi cult. Due to scant spec-tra available at lower energies, the precise details of theelectron populations cannot be distinguished, and this firstorder approximation model serves as a starting point forfuture studies and searches. The VHE γ -ray morphology presented here favorsthe association of HESS J1303 −
631 with the high spin-down power pulsar PSR J1301 − As stated before, the distance of 6.6 kpc toPSR J1301 − N H / DM =
85 whichis much higher than the values seen for all other X-raydetected pulsars, for which typically we observe N H / DM ≈ −
10 (Gaensler et al. 2004). For PSR J1301 − N H / DM ∼
23, one of the highest known N H / DM ratios amongPWNe, which could imply that the distance obtained fromDM is an underestimate for this source as well.On the other hand, the star formation regionIRAS 13010 − σ from the direction of the star formation re-gion, IRAS 13010 − . ◦ , the only other identified object within the VHEemission region besides lower energy pulsars and stars.IRAS 13010 − γ -rays in Fermi observations by Wu et al. (2011)which could indicate the presence of evolved massive stars,which are the progenitors to pulsars, with colliding winds,or the presence of an SNR within the star forming region,or potentially a counterpart to a part of the VHE sourcegiven the bulge in the VHE significance contours at theposition of the star forming region (see Fig. 5, top). As theonly star formation region within more than a degree of thepulsar, and in the absence of another plausible SNR associ-ation, this provides a plausible candidate for the birthplaceof the pulsar. This could be similar to the case of the X-rayfeature G359.95 − − V SLR = (33 . ± .
2) km / s (Bronfman et al. 1996), which corre-sponds to a distance of ∼ . − .
9. Abramowski et al. (H.E.S.S. Collaboration): Identification of HESS J1303 −
631 as a pulsar wind nebula with the Galactic structure parameters of Reid et al. (2009).This is nearly double the distance of 6.6 kpc based on DM,placing the source close to the edge of the Galaxy. Thiskinematic distance is corroborated by the measure of thecolumn density from X-rays, which is larger than the totalintegrated Galactic HI column density in that direction of1 . × cm − (Dickey & Lockman 1990).If the pulsar was born in IRAS 13010 − . ◦ or ∼
62 pc, implying a veryhigh transverse velocity of ∼ ,
000 km / s if the charac-teristic age of 11 kyr is taken as the true age. This age es-timate is, however, often considered to be unreliable andthe true ages may di ff er by a factor of 2-3. The true ageof the pulsar, assuming constant braking index, is given as(Manchester & Taylor 1977) τ = P ( n −
1) ˙ P (cid:34) − (cid:18) P P (cid:19) n − (cid:35) (2)The characteristic age τ c is calculated assuming a brakingindex of n = P is much less than the current period, P .The braking index has only been reliably measured for ahandful of young pulsars (Alpar & Baykal 2006) and wasfound to be less than 3 in every case, with the extreme caseof with n = . ± . τ c if the assumption of P << P still holds, or PSR J1734 − n = . ± . ff ects of quantum vacuum frictionon the spindown of pulsars and found a braking index de-creasing as 1 − (1 − n ) e − At with A = ¨ P / ˙ P + ˙ P / P for pe-riod P and predict the braking index of the Crab pulsarat birth of just below 3 and that it will fall to ∼ +
32 is likely ∼
40% youngerthan its characteristic age, implying a non-negligible birthperiod.The very large “darkness ratio” of γ -ray to X-ray lumi-nosity (in the 1 −
30 TeV and 2 −
10 keV bands respectively)for this source of 156, makes this the darkest identifiedPWN to date (the darker HESS J1702-420, darkness ratio1,500, is now believed to be an SNR, Giacani et al. (2011)).This could imply a relatively old age for PSR J1301 − − ∼ , ,
000 km / s. This high velocity is not unreasonablegiven the two component pulsar velocity model byArzoumanian et al. (2002) which predicts ∼
15% ofall pulsars to have a space velocity greater than 1 , / s, but would place PSR J1301 − + +
50, PSR B1800-21,PSR B1757-24 and PSR B1610-50 all believed to have avelocity of (cid:38) / s (see Cordes & Cherno ff (1998);Caraveo (1993)). In the case that the pulsar is much olderthan 11 kyr, IC cooling may play an important role forthe oldest electrons, i.e. those created nearest the place ofbirth, leading to strong energy-dependent morphology asobserved here.Adopting the kinematic distance of IRAS 13010 − γ -ray luminosity would represent about28% of the current spin-down luminosity of the pulsar( ˙ E . = ˙ E / π (12 . = . × − erg cm − s − ).This γ -ray conversion e ffi ciency is higher than for typicalPWNe ( (cid:46)
10% for PWNe with known γ -ray and X-rayluminosities, Mattana et al. (2009)). However, high γ -rayconversion e ffi ciency may not be unreasonable consider-ing the very high “darkness ratio” of for this source of156, which implies low synchrotron losses. Thus, an as-sociation of PSR J1301 − − N H / DM, the large absolute value of N H con-sistent with the entire integrated galactic column density inthat direction, the possible bump seen in the TeV emissionat the location of IRAS 13010 − − γ -ray to X-ray luminosity. This larger distance may alsohelp explain the di ffi culty of detecting counterparts at otherwavelengths.
8. Conclusions
PWNe now appear to constitute the largest class ofGalactic VHE γ -ray emitters. The first dark source, andconsidered “prototypical” dark source, TeV J2032 + γ -rays (Abdo et al. 2009) and thenin radio (Camilo et al. 2009). The work presented herehas successfully identified energy-dependent morphologyin VHE γ -rays as well as an X-ray PWN counterpart ofHESS J1303 −
10. Abramowski et al. (H.E.S.S. Collaboration): Identification of HESS J1303 −
631 as a pulsar wind nebula
Table 3.
Quality of fit of a constant vs. a line to the source in-trinsic Gaussian extension and mean, measured from the pulsarposition, as a function of energy. The much improved p-valuesof the linear fits as compared to the constant fits indicate thepresence of significant energy-dependent morphology.
Constant Fit χ / NDF p-value w int / . × − c / . × − Linear fit w int / . c / . IRAS 13010-6254
E > 10 TeVE 2-10 TeVE < 2 TeV
PSR J1301-6305
Fig. 9.
Energy mosaic of HESS J1303-631. The horizontal axisis Right Ascension and the vertical axis is Declination in J2000.0coordinates. Red: E = (0.84 - 2) TeV, Green: E = (2 - 10) TeVand Blue: E >
10 TeV. The highest energy photons origi-nate nearest the pulsar, PSR J1301 − σ signifi-cance contour of the entire source. XMM-Newton
X-ray con-tours are shown in black. A potential birthplace for the pul-sar, IRAS 13010 − E [eV] -7 -5 -3 -1
10 10 ] - s - [ e V c m d E d N E -4 -3 -2 -1 Parkes XMM H.E.S.S.
Synchrotron radiationIC scattering on CMB photonsSum
Fig. 10.
Spectral Energy Distribution of HESS J1303 −
631 fittedwith a simple stationary leptonic model. The required magneticfield is ∼ . µ G. the “not-so-dark”, or “synchrotron under-luminous” classof VHE γ -ray sources having peak synchrotron energyfluxes that are much lower than the peak fluxes in the VHEregime. The observations presented here support the inter-pretation of this source as a large cloud of electrons, ac-celerated by the pulsar, which emit γ -ray radiation throughthe IC mechanism. These electrons can have an IC emis-sion lifetime of the order of the pulsar age, and can, there-fore, reflect the total energy output of the pulsar since birth,while the X-ray part of the PWN, generated by higher en-ergy synchrotron emitting electrons with a much shorter in-teraction time, decreases rapidly in time and reflects onlythe more recent spin-down power of the pulsar (de Jageret al. 2009). While an association of the pulsar with thestar formation region IRAS 13010 − − ff orts to extend the radioand X-ray measurements of this source will be crucial fora deeper understanding of the processes at play.Many other extended Galactic γ -ray sources whichwere previously unidentified are also finding associationswith pulsars and PWNe as this class of sources continues toexpand. The results obtained here also support the hypoth-esis that this “not-so-dark” source may be understood inthe context of very low magnetic field, possibly in combi-nation with a large distance to the source, causing relativeextinction of the X-ray counterpart. Acknowledgements.
The support of the Namibian authorities and of theUniversity of Namibia in facilitating the construction and operation of H.E.S.S.is gratefully acknowledged, as is the support by the German Ministryfor Education and Research (BMBF), the Max Planck Society, the FrenchMinistry for Research, the CNRS-IN2P3 and the Astroparticle InterdisciplinaryProgramme of the CNRS, the U.K. Particle Physics and Astronomy ResearchCouncil (PPARC), the IPNP of the Charles University, the South AfricanDepartment of Science and Technology and National Research Foundation, andby the University of Namibia. We appreciate the excellent work of the technicalsupport sta ff in Berlin, Durham, Hamburg, Heidelberg, Palaiseau, Paris, Saclay,and in Namibia in the construction and operation of the equipment. M. Dalton ac-knowledges the support of the European Research Council (ERC-StG-259391). References
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