On the origin of two unidentified radio/X-ray sources discovered with XMM-Newton
Federico García, Jorge A. Combi, María C. Medina, Gustavo E. Romero
aa r X i v : . [ a s t r o - ph . H E ] O c t Astronomy & Astrophysicsmanuscript no. 2014GCMR_AA_25355_am c (cid:13)
ESO 2018January 8, 2018
On the origin of two unidentified radio/X-ray sources discoveredwith XMM-Newton
Federico García , ,⋆ , Jorge A. Combi , , María C. Medina , and Gustavo E. Romero , Instituto Argentino de Radioastronomía (CCT La Plata, CONICET), C.C.5, (1894) Villa Elisa, Buenos Aires, Argentinae-mail: [fgarcia,jcombi,clementina,romero]@iar-conicet.gov.ar Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque, B1900FWA La Plata,ArgentinaReceived; accepted
ABSTRACT
Aims.
We aim at clarifying the nature of the emission of two spatially related unidentified X-ray sources detected with XMM − Newton telescope at intermediate-low Galactic latitude
Methods.
We use the imaging and spectral capabilities of XMM − Newton to study the X-ray properties of these two sources. Inaddition, we complement our study with radio data obtained at di ff erent frequencies to analyze a possible physical association betweenthe sources. Results.
Observations reveal a point-like source aligned with elongated di ff use emission. The X-ray spectra of these sources is best-fitted by an absorbed power law with photon index Γ ∼ . ∼ . .
200 pc and on the linear size of the system of . & erg s − and & × erg s − , for the point-like and extended sources, respectively. Second, an extra-Galacticnature is discussed, where the point-like source might be the core of a radio galaxy and the extended source its lobe. In this case,we compare derived fluxes, spectral indices, and spatial correlation with those typical from the radio galaxy population, showing thefeasibility of this alternative astrophysical scenario. Conclusions.
From the available observational evidence, we suggest that the most promising scenario to explain the nature of thesesources is a system consisting of a one-sided radio galaxy, where the point-like source is an active galactic nucleus and the extendedsource corresponds to the emission from its lobe. Other possibilities include a pulsar and its associated pulsar wind nebula, wherethe radio / X-ray emission originates from the synchrotron cooling of relativistic particles in the pulsar’s magnetic field or a casualalignment between two unrelated sources, such as an active galactic nucleus and a Galactic X-ray blob. Deeper dedicated observationsin both radio and X-ray bands are needed to fully understand the nature of these sources.
Key words.
X-rays: ISM – Radiation mechanisms: nonthermal – Radio continuum: general
1. Introduction
With the advent of the new generation of high-sensitivity X-ray observatories, such as XMM-Newton, which operates in the0.2–12 keV energy range, a large number of point-like and ex-tended unidentified X-ray sources were discovered at middle andhigh Galactic latitudes (Watson et al. 2009). An instrument withthis kind of capability o ff ers a good opportunity to detect and in-vestigate X-ray emission from distant and obscured high-energysources. These sources range from undetected pulsars, accret-ing black holes or supernova remnants (SNRs), to active galacticnuclei (AGNs). A system composed of a point-like source andextended X-ray emission with comet-like structure and nonther-mal radio / X-ray emission could have two possible origins, eitherGalactic or extra-Galactic.In our Galaxy, this kind of system can be the result of a pul-sar moving at supersonic velocity with respect to its surround-ings (Bucciantini & Bandiera 2001; van der Swaluw 2003). Inthis case, the radio / X-ray emission originates from synchrotron
Send o ff print requests to : F. García ⋆ Fellow of CONICET, Argentina. radiation of the high-energy particles injected by a spinningneutron star through its own magnetic field, namely, a pulsarwind nebula (PWN). The ∼
50 X-ray PWNe detected so far(Kargaltsev & Pavlov 2008) show several di ff erent observationalproperties: X-ray spectral indices in the 1–2 range, X-ray lumi-nosities ranging from 10 to 10 erg s − , and radio / X-ray mor-phologies going from regular, clearly toroidal or bow-shock-likeshapes in young SNRs to irregular shapes in older systems (for adetailed review, see Gaensler & Slane 2006). In some cases, be-cause of a density gradient in the interstellar medium (ISM), thePWN can detach from the pulsar, resulting in a relic PWN (e.g.,van der Swaluw et al. 2004). Althought most of PWNe had beenassociated with pulsars and / or SNRs, some of them remain can-didates because of the lack of conclusive observational evidence(Matheson et al. 2013; Kargaltsev et al. 2013).In an extra-Galactic scenario, these systems can be associ-ated with AGNs. Possible candidates are FR I radio galaxies(Beckmann & Shrader 2012), where nonthermal radio and X-ray emission is produced by charged particles accelerated withinthe jet. The broadband spectra (from UV to X-rays) of theirjets can be fitted by a simple synchrotron model (e.g., 3C 66B, Article number, page 1 of 6 & Aproofs: manuscript no. 2014GCMR_AA_25355_am point-like sourceextended sourcebkg
Fig. 1.
XMM–
Newton
X-ray image of the emission in the 0.5–8.0 keV energy range of the extended source. Soft X-rays photons (0.5–2.0 keV)are shown in red, medium X-ray photons (2.0–3.5 keV) in green, and hard X-ray photons (3.5–8 keV) in blue.
Left panel: radio contours at 843MHz are overlaid in steps of 0.01, 0.03, 0.07, and 0.18 mJy beam − in magenta, and the nominal beam ellipse of 43 ×
57 arcsec are indicated inthe upper right corner in magenta.
Right panel:
X-ray spectra extracting regions are indicated with white and yellow ellipses. Background regionis indicated with a yellow dashed line.
Hardcastle et al. 2001) with a steep X-ray component. How-ever, similar X-ray morphology has also been found in sev-eral BL Lac objects; e.g., AP Lib (Kaufmann et al. 2013), S52007 +
777 (Sambruna et al. 2008), OJ 287 (Marscher & Jorstad2011), PKS 0521–365 (Birkinshaw et al. 2002), 3C371, andPKS 2201 +
044 (Sambruna et al. 2007). The spectra of the jetsin these objects are generally flat and they are considered to beinverse-Compton (IC) dominated (synchrotron self Compton orthe result of scattering of cosmic microwave background pho-tons, Harris & Krawczynski 2002). There is a correlation be-tween the jet and the core spectral indices, which means thatIC radiation should also dominate the core X-ray emission. Thecore of these BL Lac objects has been detected by Fermi, pre-senting softer spectra in gamma-rays (spectral index rangingfrom 2 to 2.4), opening the possibility that the IC componentcould reach TeV energies.In this paper, we report a radio / X-ray study of two possiblyassociated unidentified X-ray sources. One source is point-likeand the other has a di ff use and elongated morphology, detectedwith the XMM-Newton Observatory. We analyze the two possi-bilities on the origin of this emission (Galactic or extra-Galactic)mentioned above. The structure of our paper is as follows: inSect. 2 we describe the observations and the data reduction pro-cess. X-ray analysis of the sources is presented in Sect. 3. InSect. 4, we discuss the results found in the X-ray band togetherwith those coming from the radio analysis of pre-existent data,and we suggest some possible astrophysical explanations for thesources, either in the context of Galactic or an extra-Galactic ori-gin. Finally, in Sect. 5, we summarize our main conclusions.
2. X-ray observations and data reduction
The point-like and extended X-ray sources under study in thiswork were observed on August 2012 by the XMM-
Newton
X-ray observatory (ObsID. 0692830301). The observation waspointed toward the unidentified γ -ray source 2FGL J1536.4– 4949 ( α J2000 . = h m s . δ J2000 . = − ◦ ′ ′′ .
9) with a 68%error ellipse of 1 . × .
58 arcmin and an inclination of 90 ◦ ′ with respect to north. Data was acquired with the European Pho-ton Imaging Camera (EPIC) by means of MOS (Turner et al.2001) and PN (Strüder et al. 2001) detectors, operating in the0.2–15 keV energy range, and for a net exposure time of17.91 ks. The observation was performed using a medium fil-ter for MOS1 / .We obtained raw data of this observation from the XMM- Newton
Science Archive (XSA) , which we calibrated usingXMM- Newton
Science Analysis System (SAS) version 13.5.0and latest calibration files. We extracted light curves of photonsabove 10 keV for the entire field of view of the EPIC camerasand discarded high-background time intervals to produce a GoodTime Interval (GTI) file in order to exclude periods of high back-ground due to soft-proton flares, which could eventually a ff ectthe observations. We ended up with a total live time of 17.44 ks,17.45 ks, and 17.82 ks for the MOS-1, MOS-2, and PN cameras,respectively. To create spectra and light curves images, we se-lected events with flag
0, and patterns ≤
12 and 4 for MOS andPN cameras, respectively.After a quick inspection of the datasets we found twounidentified, spatially associated X-ray sources in the northwestpart of the detector: a point-like source and an extended source.Both sources had been recently included in the 3XMM-DR5catalog (Rosen et al. 2015). According to this catalog, thepoint-like source 3XMM J153611.2–494500 is located at( α J2000 . = h m s . δ J2000 . = − ◦ ′ ′′ ) with a position er- http: // XMM-Newton.vilspa.esa.es / externalXMM-Newton_user_support / documentation / http: // XMM-Newton.vilspa.esa.es / xsa / http: // XMM-Newton.vilspa.esa.es / externalXMM-Newton_sw_cal / sas.shtmlArticle number, page 2 of 6. García et al.: On the origin of two unidentified radio / X-ray sources discovered with XMM-
Newton
Table 1.
Spectral parameters of the X-ray emission of point-likeSource 1 and extended Source 2.
Model & Regions Source 1 Source 2TBABS*POWERLAWN H [10 cm − ] 0.38 ± ± Γ ± ± χ ν / d.o.f. 1.02 /
145 0.97 / − ± ± Notes.
Error values are 1- σ confidence levels for every single param-eter. Fluxes are absorption corrected and they are given in units of10 − erg cm − s − . ror of 0 . ′′ and the extended source 3XMM J153559.6-494306is centered at ( α J2000 . = h m s . δ J2000 . = − ◦ ′ ′′ ) elon-gated ∼ ′ in the northwest-southeast direction with a positionerror of 1 . ′′ . The o ff -angle of both sources with respect to thepointing of the observation is about ∼ ′ and 8 ′ , respectively,which implies a reduction of the e ff ective area of about 9%and 11%, respectively. In which follows, we focus our workon the study of these two previously unindentified X-ray sources.
3. Results
We produced X-ray images for MOS and PN cameras that wecombined to increase the signal-to-noise ratio (S / N) by meansof the emosaic
SAS task. We prepared the corresponding set ofexposure maps for each camera to account for spatial quantume ffi ciency and mirror vignetting by running the SAS task eexmap .We performed exposure vignetting corrections dividing each im-age by its corresponding superimposed exposure map. Finally,we smoothed the images with a three-pixel Gaussian filter.In Fig. 1 we show the resulting narrowband X-ray im-ages of the region for soft (0.5–2.0 keV), medium (2.0–3.5keV), and hard (3.5–8 keV) energy bands in red, green, andblue, respectively. Above 8.0 keV, no significant X-ray emis-sion was detected. In the left panel, radio contours at 843 MHz(Whiteoak & Green 1996) from the Molonglo Observatory Syn-thesis Telescope (MOST) were superimposed in magenta, and anominal beam size of 43 ×
57 arcsec is indicated in the upperright corner. In the right panel, we overlaid X-ray spectra ex-traction regions. We used solid lines to indicate source regionsand dashed line for the background. In the images, north is upand east is to the left. As it can be easily observed, a point-likesource (hereafter Source 1) seems to be connected with an elon-gated di ff use X-ray source (hereafter Source 2), which extendsalong the southeast / northwest axis. Both X-ray sources spatiallycorrelate very well with a continuum radio source with twopeaks: Source 1 with MGPS J153611–494456 and Source 2 withMGPS J153559–494301 (Murphy et al. 2007). The extended X-ray emission is elongated along a position angle of approx. -45 ◦ ,with an apparent angular size of roughly 1.5 × In order to analyze the physical conditions of the X-ray emis-sion of these sources, we first extracted spectra from a circular −4 −3 no r m a li z ed c oun t s s − k e V − Point−like source spectra χ Energy (keV)10 −3 no r m a li z ed c oun t s s − k e V − Extended source spectra χ Energy (keV)
Fig. 2.
XMM-
Newton
PN (black) MOS1 (red), and MOS2 (green) spec-tra of the point-like Source 1 (upper panel) and the extended Source 2(lower panel). Solid lines indicate the best-fit, absorbed power-lawmodel (see Table 1). Lower panels show χ fit residuals. region (PS) for Source 1 and an elliptical region (excluding PS)for Source 2. We indicated both regions in the right panel ofFig. 1 with white solid lines. Ancillary response files (ARFs)and redistribution matrix files (RMFs) were produced by meansof the arfgen and rmfgen SAS tasks, respectively. In the caseof PS, the point spread function (PSF) correction was applied.Background was subtracted using the dashed line region shownin the same figure. We performed the spectral analysis via theXSPEC package (Arnaud 1996) working in the 0.5–8.0 keV en-ergy range because no significant X-ray emission was detectedabove this limit.We fitted EPIC spectra with a single nonthermal model, de-scribed by a power law, a ff ected by an interstellar-absorptioncomponent ( tbabs ; Wilms et al. 2000). The goodness of themodel fit was derived according to the χ -test statistics. In Fig. 2we show the background subtracted X-ray spectra of Source 1and 2 in upper and lower, panels, respectively. In solid lineswe plot the best-fit model obtained by fitting simultaneouslyPN (black) and MOS1 / χ fit residuals. The parameters of the bestfit to the spectra of Source 1 and 2 are presented in Table 1,where errors are quoted at 1- σ confidence limits. We tried to fitthe spectra using a single thermal blackbody component, but wewere not able to obtain a good fit (reduced χ ν = . N H ∼ Article number, page 3 of 6 & Aproofs: manuscript no. 2014GCMR_AA_25355_am
Table 2.
Spectral parameters of the X-ray emission of the selected elliptical regions.
Model & Regions PS E E E E TBABS*POWERLAWN H [10 cm − ] 0.38 ± ± ± ± ± Γ ± ± ± ± ± χ ν / d.o.f. 1.02 /
145 0.69 /
71 1.07 /
281 0.99 /
325 0.98 / ± ± ± ± ± Notes.
Error values are 1- σ confidence levels for each parameter. Fluxes are absorptioncorrected and they are given in units of 10 − erg cm − s − . This did not improve the fit significantly, since the fits obtainedfor a single nonthermal component already yielded χ ≈ .
15% for the unab-sorbed thermal flux detected in the 0.5–8.0 keV energy range,using a black body with its temperature fixed at the best-fit value kT = .
14 keV.To better understand the physical structure of the di ff useemission, we also performed spatially resolved X-ray spec-troscopy. For this purpose, following Bocchino & Bykov (2001)and Gaensler et al. (2004), we split the elongated emission intoseveral increasing elliptical annuli, E i for i = ... E whose outer ellipse coincides with the white ellipse of the regionSource 2(shown in yellow in the right panel of Fig. 1). Spectrawas produced and fitted with the same approach and data wasagain best fitted with a single power-law component. In Table 2we show the best-fit parameters obtained in our analysis.According to the results presented in Tables 1 and 2, Source 1appears harder ( Γ ∼ .
7) than Source 2 ( Γ ∼ . ff ects caused by variationsfound in the absorption column for the best-fit parameters, wealso performed the same analysis but keeping N H fixed to thebest-fit value of Source 1. Following this approach, we foundconsistent results both in the spectral indices and total flux. Thespectrum of E region could possibly be contaminated by theresidual PSF of the point-like source, and while regions E to E show an increasing power-law index with the distance to thepoint-like source, all of the spectral indices are consistent withinthe error bars.To obtain a statistical assessment of possible X-ray pulsa-tions of Source 1, we used the 17.8 ks EPIC-PN observation tocompare the time arrival distribution of source photons. We usedthe PS extraction region centered in the point-like source, whichgave us a total of ∼ / s. We produced the FFT of the light curve via powspec with di ff erent approaches for both unbinned and binneddata and we found no significant peak in the resulting powerspectra. We also performed a pulsation search based on an H-test (de Jager et al. 1989) on the phase of unbinned events. Foran independent period search of 10 trials over the Nyquist limit,we found one peak with an H-probability . − , which is fullycompatible with a by-chance detection for the number of trialsused. Thus, we discarded pulsations with a period greater thantwice the read-out time of the EPIC-PN camera in the FF mode,which corresponds to ∼ χ -squaretest via lcstats and we found no significant variability. In ad- dition, we analyzed three observations available in the Swift archive from June and August 2009 and January 2010 of 1000,750 and 5200 s exposures, respectively. From data adquired inthe photon-counting mode, using xselect task, we obtained thecount rate of Source 1 in the 0.5–10 keV energy band, resultingin 0.011 ± ± ± − , re-spectively, which also discards variability at 1- σ confidence lev-els at these timescales. Finally, we compared Swift count rateswith XMM-
Newton spectral results (that we converted to ex-pected count rates via PIMMS), resulting consistent betweeneach other within the errors. Hence, no clear evidence of vari-ability was detected at any timescale. γ -ray data analysis We used data from the
Fermi Large Area Telescope (LAT) tosearch for putative γ -ray emission from these sources in a re-gion centered at ( α = . ◦ , δ = − . ◦ ) within a circle of 3 ◦ of radius of interest (ROI). The data analysis was performedusing the Science Tools package (v9r32p5) available from theFermi Science Support Center (FSSC). Data were obtained fromthe reprocessed Fermi Pass 7 database and the instrumental re-sponse function P7REP SOURCE V15 was used. We analyzedsix years of data, from August 2008 to August 2014 (2008-08-04T15:43:36 to 2014-08-28T08:34:21, UTC). To prevent con-tamination by the Earth’s albedo, the events with zenith anglegreater than 100 ◦ or rocking angle greater than 52 ◦ were filtered.We performed unbinned likelihood analysis via the gtlike task.To model background source contributions, we included all two-year Fermi
Gamma-ray LAT (2FGL) point sources (Nolan et al.2012) associated with the extended source templates within 3 ◦ from the ROI center, including the unidentified γ -ray source2FGL J1536.4–4949, possibly associated with the recently dis-covered millisecond pulsar MSP J1536–4948 (Ray et al. 2012),together with the Galactic di ff use background (gll_iem_05) andthe isotropic di ff use background (iso_source_v05). We mod-eled the di ff erential γ -ray flux expected from a point source lo-cated at the center of our ROI with a simple power law. Wedid not detect significant emission in the chosen energy bandat the location of the X-rays sources, so only an upper limit of3.53 × − ph cm − s − for the 3–30 GeV energy range couldbe imposed.
4. Discussion
The morphological and spectral information gathered from radioand X-ray frequencies about the compact point-like Source 1 andthe elongated extended Source 2 can be interpreted in at least http: // swift.gsfc.nasa.gov / archive / http: // fermi.gsfc.nasa.gov / Article number, page 4 of 6. García et al.: On the origin of two unidentified radio / X-ray sources discovered with XMM-
Newton three di ff erent astrophysical scenarios. Assuming a physical as-sociation between them, in a Galactic setting we could be ob-serving a system composed of a pulsar and its associated PWN,which is moving at supersonic velocity with respect to its sur-roundings. In an extra-Galactic context, the system could be aradio galaxy composed of an AGN and an extended one-sidedlobe. Finally, if the physical connection is not real, we might justbe observing a casual alignment between a background AGNand a Galactic X-ray blob.Concerning our X-ray spectral analysis, we found that boththe point-like and di ff use sources are well fitted by a singlepower law with photon indices Γ= ± Γ= ± N H = × cm − , McClure-Gri ffi ths et al. 2009;Kalberla et al. 2010). Moreover, no emission lines are presentin the spectra, which would be a clear indication of a thermalplasma origin. We also looked at data from 2MASS , but wedid not find infrared counterparts for any of the sources. Thus,the observational evidence points to a nonthermal origin for theX-ray emission of both sources.Since the beam-sizes of radio-continuum data available inthe literature are very di ff erent from each other, we can onlyroughly estimate a mean spectral index for the whole system(the point-like plus the extended source). From the available ra-dio fluxes of 1230 ±
40 mJy at 408 MHz (Large et al. 1981), witha nominal beam-width of 2 . × .
81 arcmin, 883 ± ×
57 arc-sec (including both sources), and 156 ±
12 mJy at 4850 MHz(Gregory et al. 1994), with a beam size of 4.8 arcmin, we derivea mean spectral index of α ∼ − .
78, confirming a nonthermalorigin of the radio sources.The high value of radio flux at 408 MHz and the spectral in-dex are typical of an AGN. If the mean spectral index comesfrom a combination of the emission of a pulsar / PWN system(roughly -1.8 and -0.3–0.0, respectively), a high fraction of theflux at 408 MHz has to be assigned to the pulsar. In this case, itwould be very hard to explain how it was not detected yet in anyParkes pulsar survey. Despite that in our X-ray analysis we couldnot find pulsations with high confidence, and the high Nyquistlimit of 0.4 s does not allow us to discard this possibility.As we mentioned above, in the extra-Galactic case if there isa physical association between both sources, the system mightbe a one-sided radio galaxy composed of a core (the point-likesource) and a jet (the extended source). Generally, the core isa compact component, unresolved when observed with low an-gular resolution ≥ http: // / / by the mean spectral index of S ∝ ν − . estimated at the ra-dio frequencies. An optical / IR counterpart was not detected; thismay be because of the limited resolution of the available dataand the low Galactic latitude where the source is located. De-pending on the jet and ambient medium parameters, most radiogalaxies display sizes below 100 kpc (Begelman et al. 1984). Ifthis is the case, using standard Friedmann-Lemaître-Robertson-Walker formulae, the ∼ ′ size of our extended source translatesinto a distance smaller than 80 Mpc, which yields radio lumi-nosities of 1 . × W Hz − sr − at 4590 MHz and an X-rayflux at 3 . × W Hz − sr − at 2 keV, in agreement with Worrall(2002).In the case of a Galactic origin, if both sources are phys-ically connected, it is possible to constrain their distance withH i data from the Parkes Galactic All-Sky Survey (GASS;McClure-Gri ffi ths et al. 2009; Kalberla et al. 2010) with a nom-inal angular resolution of ∼
16 arcmin to analyze the H i line in-tensity of the medium pointing to the direction of these sourcesand to search for possible absorption features caused by the ob-served radio-continuum emission. From the velocity profile ofthe H i line in the −
400 to +
400 km s − range, we found threeminima at velocities of − − , and − . − . Based onthe Galactic rotation curve obtained by Fich et al. (1989), thesevelocities correspond to heliocentric distances of ∼ ∼
11 kpc,for −
39 km s − and < ∼ &
13 kpc for the other cases. An eyeinspection of the H i datacube suggests that the minimum in theprofile at −
12 km s − can be an absorption feature, whereas theminima in the other cases is related to bright clouds in the east-ern side of the maps. Based on the flux and angular size of thedi ff use source, distances &
10 kpc can be safely discarded for aGalactic origin. Hence, we suggest that the radio / X-ray sourcesshould be at a distance < ∼ < ∼
200 pc and on its linearsize of & & erg s − and & × erg s − for the point-like Source 1 and extendedSource 2, respectively. These sources are compatible with typi-cal luminosities of pulsar / PWN associations (see lower panel ofFig. 5 from Kargaltsev & Pavlov 2008). Following the compar-ison with the pulsar / PWN population, the power-law indices Γ of both the putative pulsar and its PWN fall in the typical 1.0–2.0 range. However, the lack of pulsations together with the highradio flux make this scenario rather unlikely.The unidentified γ -ray source 2FGL J1536.4–4949 locatedat the center of the XMM- Newton observation field of view hasbeen detected by
Fermi -LAT with a significance of 40- σ and aflux of 1.2 × − ph cm − s − in the 1 and 100 GeV energy range.Sources 1 and 2 lie about 0.15 ◦ outside the 95% confidence limitof the γ -ray source, which makes a physical association unlikely.Moreover, in the position where the radio / X-ray sources lie, onlyan upper limit of 3.53 × − ph cm − s − for the 3–30 GeV en-ergy range could be derived.
5. Conclusions
We have conducted a morphological and spectral analysis of twospatially associated unidentified X-ray sources (a point-like anda di ff use-extended source) based on an XMM- Newton observa-tion and focused in the 0.5–8.0 keV energy range. From the in-formation gathered, the system can be interpreted in at least threedi ff erent ways. Assuming a physical association, if the system isGalactic, we might be detecting a pulsar / PWN pair. In an extra-
Article number, page 5 of 6 & Aproofs: manuscript no. 2014GCMR_AA_25355_am
Galactic scenario, the system may be a radio galaxy composedof an AGN and an extended one-sided jet. Otherwise, if thereis no physical connection, we might just be observing a casualalignment between a background AGN and an elongated Galac-tic X-ray blob. In this case, the nature of the extended sourcewould remain unsolved.In the case of an extra-Galactic origin, the most likely config-uration is that of a FR I radio galaxy with a single side outflow.The X-rays and radio data for the extended emission of FR Igalaxies is generally explained by synchrotron emission from asingle population of relativistic electrons gyrating in a magneticfield inside the jet. This is supported by the fact that the spectralindex of the X-rays is > ffi cult togo further into this interpretation. However taking into accountthe synchrotron model for the observed radiation, no detectionat high energies should be expected as a cuto ff must appear atharder X-rays.For a Galactic origin, based on the H i data available, wesuggest that the radio / X-ray system might be at a distance < ∼ .
200 pc and on its linear size in . & erg s − and & × erg s − for the point-like Source 1 and extendedSource 2, respectively, would also be compatible with typicalluminosities and luminosity ratios of pulsar / PWN associations.Complementary studies are needed to confirm or reject anyof the scenarios we explored here regarding the point-like andextended di ff use X-ray sources. High-resolution X-ray observa-tions from Chandra satellite and radio observations performedwith Parkes observatory are needed to compare X-ray spectraand morphology with those at the radio-continuum bands and totry to detect or put constraints on pulsed emission coming fromthe point-like source. Acknowledgements.
FG is a fellow of CONICET. JAC, MCM and GER areCONICET researchers. JAC was supported in di ff erent aspects of this work byConsejería de Economía, Innovación, Ciencia y Empleo of Junta de Andalucíaunder excellence grant FQM-1343 and research group FQM-322, as well asFEDER funds. GER is supported by grants from ANPCyT (PICT 2012-00878)and Spanish MICINN (AYA 2013-47447-C3-1-P). References
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