Multi-frequency study of the Large Magellanic Cloud Supernova Remnant J0529-6653 near Pulsar B0529-66
L. M. Bozzetto, M. D. Filipović, E. J. Crawford, F. Haberl, M. Sasaki, D. Urosević, W. Pietsch, J. L. Payne, A. Y. De Horta, M. Stupar, N. Tothill, J. Dickel, Y.-H. Chu, R. Gruendl
MMon. Not. R. Astron. Soc. , 1–8 (2002) Printed 9 October 2018 (MN LaTEX style file v2.2)
Multi-frequency study of the Large Magellanic CloudSupernova Remnant J0529–6653 near Pulsar B0529–66
L. M. Bozzetto, M. D. Filipovi´c, E. J. Crawford, F. Haberl, M. Sasaki, D. Uroˇsevi´c, , W. Pietsch, J. L. Payne, A. Y. De Horta, M. Stupar, , N. Tothill, J. Dickel, Y.-H. Chu, and R. Gruendl, School of Computing and Mathematics, University of Western Sydney Locked Bag 1797, Penrith South DC, NSW 1797, Australia Max-Planck-Institut f¨ur extraterrestrische Physik, Giessenbachstraße, D-85748 Garching, Germany Institut f¨ur Astronomie und Astrophysik T¨ubingen, Sand 1, D-72076 T¨ubingen, Germany Department of Astronomy, Faculty of Mathematics, University of Belgrade, Studentski trg 16, 11000 Belgrade, Serbia Isaac Newton Institute of Chile, Yugoslavia Branch Department of Physics, Macquarie University, Sydney, NSW 2109, Australia Australian Astronomical Observatory, P.O. Box 296, Epping, NSW 1710, Australia Physics and Astronomy Department, University of New Mexico, MSC 07-4220, Albuquerque, NM 87131, USA Department of Astronomy, University of Illinois, 1002 West Green Street, Urbana, IL 61801, USA
Released 2011 Xxxxx XX
ABSTRACT
We report the ATCA and
ROSAT detection of Supernova Remnant (SNR) J0529–6653in the Large Magellanic Cloud (LMC) which is positioned in the projected vicinityof the known radio pulsar PSR B0529–66. In the radio-continuum frequencies, thisLMC object follows a typical SNR structure of a shell morphology with brightenedregions in the south-west. It exhibits an almost circular shape of D=33 ×
31 pc (1 pcuncertainty in each direction) and radio spectral index of α =–0.68 ± ∼ ±
7% at 6 cm. An investigation of
ROSAT images produced from mergedPSPC data reveals the presence of extended X-ray emission coincident with the radioemission of the SNR. In X-rays, the brightest part is in the north-east. We discussvarious scenarios in regards to the SNR-PSR association with emphasis on the largeage difference, lack of a pulsar trail and no prominent point-like radio or X-ray source.
Key words: supernova remnants – pulsars – Large Magellanic Cloud –SNR J0529–6653.
The Large Magellanic Cloud (LMC) provides an excellentlaboratory to study supernova remnants (SNRs) at a knowndistance of 50 kpc (di Benedetto 2008). The line of sight tothe LMC lies well away from the Galactic plane, minimisingthe obscuration and confusion from the foreground gas, dustand stars.A distinguishing characteristic of SNRs in radio wave-lengths is their predominantly non-thermal continuum emis-sion. Generally, SNRs display a radio spectral index of α ∼ − . S ∝ ν α ), although α may vary signifi-cantly, as there exists a wide variety of types of SNRs in dif-ferent environments and stages of evolution (Filipovi´c et al.1998). For example, younger remnants can have a spectralindex of α ∼ − .
8, while older SNRs and Pulsar Wind Nebu-lae (PWN) tend to have flatter radio spectra with α ∼ − . ∼ ± M (cid:12) (Smartt 2009), and may leave behind compact central ob-jects, such as neutron stars that may be observable as pul-sars. However, not many SNRs from Type II supernovae areobserved to host pulsars. Among the 51 confirmed and 25possible candidate SNRs in the LMC (Klimek et al. 2010;Desai et al. 2010, Filipovi´c et al. in prep), only three (N 49,30 Dor B, B0540–693) have documented associations witha pulsar (Manchester et al. 2006). There are over 70 wellstudied SNRs in the Magellanic Clouds (MCs) as well as19 detected pulsars. Ridley & Lorimer (2010) modelled thepotentially observable radio pulsars in the MCs and pre-dicted some 1.79 × pulsars. The Milky Way (MW) has274 SNRs (Green 2009) and ∼ c (cid:13) a r X i v : . [ a s t r o - ph . H E ] N ov L. M. Bozzetto et al. observed SNR/PSR ratio in the MCs is much less than inthe MW ( ∼ ∼ (10/50) would result in the two fractions being similar.This rarity of pulsar-SNR connections may be attributedto the fact that many young neutron stars in SNRs exhibitdifferent properties from the traditional radio pulsars (Got-thelf & Vasisht 2000). Radio pulsars remain detectable wellafter their SNRs have merged into the ISM; therefore, mostpulsars are not in SNRs and their general properties may bedifferent from those young ones in SNRs.PSR B0529–66 in the LMC was the first extra-galacticpulsar discovered. It was found by McCulloch et al.(1983) using the 64-m radio telescope at the ParkesObservatory. The pulsar was studied by Costa et al.(1991) at a frequency of 600 MHz with the intent ofusing a polarimeter to collect integrated profiles, rota-tion and dispersion measures. Crawford et al. (2001) usedthe Parkes telescope at 20 cm and improved the radiopositional accuracy to RA (J2000)=05 h m s ( ± s )and DEC (J2000)= − ◦ (cid:48) (cid:48)(cid:48) ( ± (cid:48)(cid:48) ). Manchester et al.(2005) then included new and more detailed informationon the pulsar in a catalogue along with 1532 other pulsarsand in 2006 re-observed the pulsar at Parkes at a wave-length of 20 cm. Filipovi´c et al. (1998) detected the radiosource LMC B0530–6655 and suggested it to be an SNR can-didate based on its non-thermal spectral index. We notethat the SNR J0529–6653 (or LMC B0530–6655) lies just onthe northeast edge of the arc of H α nebulosity DEM L214(Davies et al. 1976). Finally, Haberl & Pietsch (1999,hereafter HP99) detected a nearby ROSAT
X-ray source([HP99] 440) at a position of RA (J2000)=05 h m s andDEC (J2000)= − ◦ (cid:48) (cid:48)(cid:48) .Here, we report on radio-continuum, X-ray and opticalobservations of the candidate LMC SNR J0529–6653 withits possible association with the LMC pulsar PSR B0529–66.Observations, data reduction and imaging techniques are de-scribed in Sect. 2. The astrophysical interpretation of themoderate-resolution total intensity and polarimetric imagesare discussed in Sect. 3. We used radio observations at four frequencies (Table 1) tostudy and measure flux densities of SNR J0529–6653. For the36 cm (Molonglo Synthesis Telescope – MOST) flux densitymeasurement given in Table 1 we used unpublished imagesas described by Mills et al. (1984) and for the 20 cm weused image from Hughes et al. (2007). Two Australia Tele-scope Compact Array (ATCA) projects (C634 and C797; at6/3cm) observations were combined with mosaic observa-tions from project C918 (Dickel et al. 2005). Data for projectC634 were taken by the ATCA on 1997 August 2, using thearray configuration EW375. Four days of observations weretaken from project C797: 1999 May 1–2 (array configura-tion: 1.5C), 1999 July 21 (array configuration 750D), and1999 July 31 (array configuration: 1.5D). For the final image(stokes parameter I ) we exclude baselines created with the6 th ATCA antenna, leaving the remaining five antennas to
Figure 1.
ATCA image of SNR J0529–6653 at 3 cm overlaid with6 cm contours. The contours are from 3 until 17 σ with spacings of1 σ (0.11 mJy). The black ellipse in the lower left corner representsthe synthesised beam width of 17.3 (cid:48)(cid:48) × (cid:48)(cid:48) . The circle in thecentre marks the midpoint of the SNR. The circle in the eastshows the catalogued X-ray location of [HP99] 440 and the circlein the north is the position of the pulsar PSR B0529–66. Table 1.
Integrated flux densities of SNR J0529–6653. The fluxdensity at λ =36 cm was estimated using images from Mills et al.(1984) and at λ =20 cm from Hughes et al. (2007). ν λ Beam Size R.M.S S
Total (MHz) (cm) ( (cid:48)(cid:48) ) (mJy) (mJy)843 36 43.0 × × × × be arranged in a compact configuration. C634 observationswere carried out in “snap-shot” mode, totalling ∼ miriad (Sault& Killeen 2006) and karma (Gooch 2006) software packageswere used for reduction and analysis. The 6 and 3 cm images(Fig. 1) were constructed using miriad multi-frequency syn-thesis (Sault & Wieringa 1994). Deconvolution was achievedwith the clean and restor tasks with primary beam cor-rection applied using the linmos task. Similar procedureswere used for the U and Q stokes parameters. We have examined images (Fig. 2) from the MagellanicClouds Emission Line Survey (MCELS) (Smith et al. 2006)and higher-resolution H α images (Fig. 3) obtained with theMOSAIC ii camera on the Blanco 4-m telescope at the CerroTololo Inter-American Observatory. The extended arc of theH ii region DEM L214 is readily seen but we do not find anyoptical nebulosity associated with the SNR candidate or thePSR B0529–66. Additional long-slit observations were per- c (cid:13) , 1–8 tudy of the LMC SNR J0529–6653 near PSR B0529–66 formed on 2010 September 20, using the 1.9-m telescope andCassegrain spectrograph at the South African Astronomi-cal Observatory (SAAO) in Sutherland. While the observingconditions were good, we did not detect any emission fromthe SNR candidate. ROSAT performed a raster of pointed observations withthe Position Sensitive Proportional Counter (PSPC) to mapthe soft X-ray emission from the hot gas in the LMC su-per giant shell-4 (Meaburn 1980; Bomans & Dennerl 1999,SGS LMC-4). A number of these observations with typicalexposures of 1000 s include SNR J0529–6653 (Figs. 3 and 4).The HP99 catalogue includes a very weak detection withinthe extent of the radio emission from SNR J0529–6653 whichdoes not allow to derive much about the X-ray proper-ties of the source (the position of [HP99] 440 is shown inFig. 1). The hardness ratios are undefined and thereforegive no information on the spectrum. The catalogue ofHP99 was derived from the individual PSPC observations,not utilising the combined exposure of overlapping fields.To investigate [HP99] 440 and its possible association withSNR J0529–6653 and PSR B0529–66 in more detail we se-lected 13 observations from the SGS LMC-4 raster whichcovered the SNR/PSR within 24 (cid:48) of the optical axis (to avoidthe degraded point spread function at larger off-axis angles).We produced images in different energy bands (broad: 0.1–2.4 keV, soft: 0.1–0.4 keV, hard1: 0.5–0.9 keV and hard2:0.9–2.0 keV) from the merged data. A colour image of thearea around the SNR with net exposure (vignetting cor-rected) of 11.0 ks is shown in Fig. 4 with red, green andblue representing the X-ray intensities in the soft, hard1and hard2 bands. The resolution of the
ROSAT
PSPC varieswith energy but the point spread function is always less than1 (cid:48) . The radio-continuum remnant SNR J0529–6653 exhibitsa ring-like morphology, indicative of a shell struc-ture, with brightened region along the southwest rim(Fig. 1). It is centred at RA (J2000)=05 h m s andDEC (J2000)=–66 ◦ (cid:48) (cid:48)(cid:48) . We estimate the spatial extentof SNR J0529–6653 (Fig. 5) at the 3 σ (Table 1; Col. 4)level (0.33 mJy) along the major (N-S) and minor (E-W)axes. Its size at 6 cm is 137 (cid:48)(cid:48) × (cid:48)(cid:48) ± (cid:48)(cid:48) (33 ×
31 pc with1 pc uncertainty in each direction). We also estimate theSNR J0529–6653 ring thickness to < (cid:48)(cid:48) (5 pc) at 6 cm,about 30% of the SNR’s radius.An extended X-ray source is clearly seen at the loca-tion of the SNR with a brighter spot right at the positionof [HP99] 440. Our images are based on the combination ofdata from 13 observations, which had too short exposuresto allow an astrometric alignment to a common referencesystem. Therefore, in order to verify the source extent wecompared the projected profiles of the extended source andother nearby sources, which appear point-like. For the pro-file, we projected the counts in the 0.1 − (cid:48)(cid:48) width and position angle of 330 ◦ (to investi-gate the profile of the bright spot). We measure a FWHM of Figure 2.
MCELS H α ( top ), [S ii ] ( middle ) and [O iii] ( bottom )images of the area around SNR J0529–6653 overlaid with 6 cm 3 σ (0.33 mJy) radio-continuum contour.c (cid:13) , 1–8 L. M. Bozzetto et al.
Figure 3.
The MCELS-2 H α image of the area SNR J0529–6653overlaid with the ROSAT
PSPC contours from the broad-band(0.1–2.4 keV) image (pixel size: 15 (cid:48)(cid:48) ). Contours are from 3 to7 cts/pixel with 0.5 cts/pixel spacings. (cid:48)(cid:48) ± (cid:48)(cid:48) for the extended source and 75 (cid:48)(cid:48) ± (cid:48)(cid:48) for the sourcevisible at the southern rim of the image in Fig. 4, which ex-hibits a similar peak intensity. The extended profile shows apeak on top of the broader intensity distribution confirmingthe brightest part of the X-ray emission in the north-east.The presence of extended X-ray emission is coincident withthe radio emission of the SNR (Fig. 6).The latter is also supported from the relation betweenX-ray luminosity and energy loss for rotation- poweredpulsars (L x =(10 − –10 − ) × ˙E; Becker & Truemper (1997)).With a value of ˙E=6.6 × erg s − (from online ATNF pul-sar catalogue ) for PSR B0529–66 we expect an X-ray lumi-nosity for the pulsar of well below 10 erg s − , several ordersof magnitude below the detection limit of ∼ erg s − fora 10 ks ROSAT
PSPC observation. Also, from the estimatedspin down age of ∼ yr (see also below) one would not ex-pect the existence of a PWN.The net counts of 337 ±
25 cts s − derived from the SNRin the 0.1 − × H cm − we can roughly estimate the absorption-corrected X-ray lu-minosity to 5.7 × erg s − .We estimate the radio spectral index α = − ± ∼ ◦ from 30 Dor) and there-fore, conceivable that it is expanding in a very low density Figure 4.
The
ROSAT
PSPC RGB colour image of the areaaround SNR J0529–6653. The energy bands are: red (0.1–0.4keV), green (0.5–0.9 keV) and blue (0.9–2.0 keV). The imagehas a pixel size of 15 (cid:48)(cid:48) and is smoothed with a σ of one pixel.A white ellipse is centred on the position of the SNR J0529–6653with extent of 137 (cid:48)(cid:48) × (cid:48)(cid:48) (33 ×
31 pc).
Figure 6.
ATCA image of SNR J0529–6653 at 6 cm overlaid withthe contours from
ROSAT
PSPC image (0.1–2.4 keV). Contoursare as in Fig. 3. The ellipse in the bottom left corner representthe beam of 17.3 (cid:48)(cid:48) × (cid:48)(cid:48) . environment, which causes a slightly steeper spectral indexfor its age (see Sect. 3; Para. 10).Fig. 7 shows a surface brightness–diameter (Σ − D )diagram at 1 GHz with theoretically-derived evolutionarytracks (Berezhko & V¨olk 2004) superposed. SNR J0529–6653lies at ( D, Σ) = (32 pc, 2 . × − W m − Hz − Sr − ) on c (cid:13)000
PSPC image (0.1–2.4 keV). Contoursare as in Fig. 3. The ellipse in the bottom left corner representthe beam of 17.3 (cid:48)(cid:48) × (cid:48)(cid:48) . environment, which causes a slightly steeper spectral indexfor its age (see Sect. 3; Para. 10).Fig. 7 shows a surface brightness–diameter (Σ − D )diagram at 1 GHz with theoretically-derived evolutionarytracks (Berezhko & V¨olk 2004) superposed. SNR J0529–6653lies at ( D, Σ) = (32 pc, 2 . × − W m − Hz − Sr − ) on c (cid:13)000 , 1–8 tudy of the LMC SNR J0529–6653 near PSR B0529–66 Figure 5.
The left image shows the 6 cm image overlaid with the major and minor axis labels. The centre and right images show theI-Profile of the major and minor axis respectively, with an overlaid black line at 3 σ . the diagram. Its position tentatively suggests that it is inthe early Sedov phase of evolution — expanding into a verylow density environment with the higher initial energy ofa supernova explosion (2 − × ergs) and the age of ∼
25 000 yr. We acknowledge that such scenario couldn’t ex-plain for the lack of SNR’s optical emission. Alternatively,this SNR could be bit older ( ∼
70 000 yr; the end of adiabaticphase) and expanding into the medium dense environmentwith the minimal start-up energy.A linear polarisation image was created for 6 cm wave-length using Q and U parameters (Fig. 8). The mean frac-tional polarisation was calculated using flux density and po-larisation: P= (cid:112) S Q + S U S I · S Q , S U and S I areintegrated intensities for the Q , U and I Stokes parameters.Our estimated peak value is 17% ±
7% (3 σ ) at 6 cm and noreliable detection at 3 cm. Along the south side of the SNRshell there is a pocket of uniform polarisation (Fig. 8), possi-bly indicating varied dynamics along the shell. The polarisa-tion appear to be uniformly distributed (tangential orienta-tion) coinciding with the SNR total intensity. As the wholeSNR is nearly round the only irregularity is the brightnessvariations. While the distance from the nearby H α filament(DEM L214) seems bit far away it is most likely that theSNR interacted with some less obvious local clouds so theradio-continuum shape has not yet been disturbed.Without reliable polarisation measurements at a sec-ond frequency (3 cm) we cannot determine the Faraday ro-tation and thus cannot deduce the magnetic field strength.However, by using the new equipartition formula for SNRs(Arbutina et al. 2011), we can estimate the magnetic fieldstrength for the SNR J0529–6653. The derivation of the newequipartition formula is based on the Bell (1978) diffuseshock acceleration (DSA) theory. This derivation is purelyanalytical, accommodated especially for the estimation ofmagnetic field strength in SNRs. Using this new formula,the calculated magnetic field strengths for SNRs are be-tween those calculated by using classical equipartition (Pa-cholczyk 1970) and revised equipartition (Beck & Krause2005). The average equipartition field over the whole shell ofSNR J0529–6653 is ∼ µ G (see Arbutina et al. (2011); and
Figure 7.
Surface brightness-to-diameter diagram from Berezhko& V¨olk (2004), with SNR J0529–6653 added. The evolutionarytracks are for ISM densities of N H = 3, 0.3 and 0.003 cm − andexplosion energies of E SN = 0.25, 1 and 3 × erg. corresponding ”calculator” ), corresponding those of youngto middle-aged SNRs where the interstellar magnetic field iscompressed and amplified by the strong shocks.The Parkes pulsar catalogue suggests that the pul-sar PSR B0529–66 is 9.97 ± . × yr old. SNRs nor- The calculator is available onhttp://poincare.matf.bg.ac.rs/˜arbo/eqp/c (cid:13) , 1–8
L. M. Bozzetto et al.
Figure 8.
SNR J0529–6653 at 6 cm overlaid with polarisation ( E ) vectors that peak at 17 ± (cid:48)(cid:48) × (cid:48)(cid:48) and the line under it representing a polarisation E vector of 100%. mally merge into the ISM and become unrecognisable af-ter ∼
150 000 yr. We point out that a measured radius of ∼
17 pc would imply a mean expansion speed of only about0.1 km s − for a million year age which is clearly unreason-able. Therefore, this factor of ∼ τ c = P/2 ˙P) could be uncertain when com-pared to its corresponding SNR age. Also for nearby Galacticisolated neutron stars large differences between the charac-teristic age and the dynamic age estimated from their propermotion and the likely birth place are found (a factor of ∼ ∼ ± ∼
14 km s − , an absolute minimum kick velocity. If weassume a canonical SNR age of 25 000–50 000 yr then thepulsar velocity in the sky plane would be in a range of 550to 275 km s − , in agreement with the typical pulsar kick ve-locities (Lyne & Lorimer 1994). The direction of the pulsar’stravel is not necessarily in the sky plane, so this is a lower limit. However, unless the direction of the pulsar motion issignificantly out of the sky plane, its real velocity will notbe much greater. Klinger et al. (2002) found a jet-like struc-ture and a point X-ray source within the LMC SNR N 206travels at a speed of 800 km s − and Owen et al. (2011)estimated the kick velocity of the pulsar candidate insideSMC PWN IKT 16 to be approximately 580 km s − . Thepossible pulsars have left prominent radio trails in N 206 andIKT 16, but no such trail can be seen in our radio images ofSNR J0529–6653. This makes a SNR-pulsar connection lesslikely. Nevertheless, if PSR B0529–66 is related to the SNR,it is likely to be travelling more slowly and thus any trailwould be less prominent. We acknowledge that even if thepulsar is not traveling, there may still be a prominent PWNin the X-rays, although it will not be trailed. Unfortunately,the lack of evidence of PWN in the ROSAT data is purelybecause of resolution, and therefore, cannot be taken as anevidence of lacking a PWN.The absence of detectable optical emission (Fig. 2) fromthis SNR is not unique as other well-studied SNRs, such asLMC SNR J0528–6714 (Crawford et al. 2010) or the Galac-tic Vela Jr. (Stupar et al. 2005), also do not exhibit opti-cal emission. The ISM in the interior of SGS LMC-4 musthave very low density and hence the lack of optical emission.We argue that, as with the majority of other SNRs in theMagellanic Clouds, this intriguing SNR is most likely in theadiabatic phase of its evolution (Payne et al. 2008) simplybecause of its modest size. c (cid:13) , 1–8 tudy of the LMC SNR J0529–6653 near PSR B0529–66 In the MCELS-2 H α image (Fig. 3) we clearly see thelarge extent of H ii region DEM L214 in the south, south-west and south-east. We also see smaller scale filamentarystructures in the north of the remnant which, together withDEM L214 and the H α emission around the cluster in theeast, seem to form a large scale shell. This and the distribu-tion of the massive stars (see next paragraph) indicate thatsome heating must have already taken place where the SNRis located. A shock expanding in a already hot thin mediumis not efficient and expands quickly. This may explain thefaint X-ray appearance of the remnant SNR J0529–6653.As massive stars rarely form in isolated environments,core-collapse supernovae are most likely superposed on astellar population rich in massive stars. We have usedthe Magellanic Cloud Photometric Survey (Zaritsky et al.2004, MCPS) data to construct colour-magnitude diagrams(CMDs) and identify blue stars more massive than ∼ M (cid:12) within the area of SNR J0529–6653. The CMD in Fig. 9(left) indicates that most of the blue massive stars are mainsequence B stars. Their spatial distribution marked in blueFig. 9 (right) shows higher concentrations toward a clusterto the east and the nebulosity to the southeast of the SNR,but not within the SNR and its immediate vicinity. The pro-genitor of SNR J0529–6653 could be a B star (core-collapsesupernova) or an accreting white dwarf in a binary system(Type Ia supernova). To distinguish between these two pos-sibilities, deep XMM-Newton
X-ray observations with suffi-cient counts for spectral analysis of plasma abundances areneeded.
We have carried out the first detailed multi-frequency studyon a recently-detected LMC SNR J0529–6653, which pre-viously had records for a pulsar at this position. We esti-mated a diameter of 137 (cid:48)(cid:48) × (cid:48)(cid:48) ± (cid:48)(cid:48) (33 ×
31 pc with 1 pcuncertainty in each direction), a spectral index ( α = − . ± ∼ ± ii ]/H α> ACKNOWLEDGEMENTS
We used the karma software package developed by theATNF. The ATCA is part of the Australia Telescope whichis funded by the Commonwealth of Australia for operationas a National Facility managed by CSIRO. We thank theMCELS team for access to the optical images. Travel to theSAAO was funded by Australian Government AINSTO AM-NRF grant number 10/11-O-06. This research is supportedby the Ministry of Education and Science of the Republic ofSerbia through project No. 176005. We thank the referee fortheir excellent comments that improved this manuscript.
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Figure 9.
Left panel shows a B − V vs. V colour-magnitude diagram from the MCPS (Zaritsky et al. 2004). Stellar evolutionary tracksfrom Lejeune & Schaerer (2001) are shown with solid red lines (5, 15, 25 and 60 M (cid:12) ) and dashed red lines (3, 9, 20 and 40 M (cid:12) ). Theselection criteria used for probable B-star candidates is shown as a heavy dashed blue lines. On the right is the MCELS-2 H α image ofthe area around SNR J0529–6653 overlaid with probable B-star candidates (blue crosses) from the MCPS. A 90 (cid:48)(cid:48) radius circle (red) iscentred on the position of the SNR. Migliazzo J. M., Gaensler B. M., Backer D. C., StappersB. W., van der Swaluw E., Strom R. G., 2002, ApJ, 567,L141Mills B. Y., Turtle A. J., Little A. G., Durdin J. M., 1984,Australian Journal of Physics, 37, 321Owen R. A., Filipovi´c M. D., Ballet J., Haberl F., CrawfordE. J., Payne J. L., Sturm R., Pietsch W., Mereghetti S.,Ehle M., Tiengo A., Coe M. J., Hatzidimitriou D., BuckleyD. A. H., 2011, A&A, 530, A132+Pacholczyk A. G., 1970, Radio astrophysics. Nonthermalprocesses in galactic and extragalactic sourcesPayne J. L., White G. L., Filipovi´c M. D., 2008, MNRAS,383, 1175Ridley J. P., Lorimer D. R., 2010, MNRAS, 406, L80Sault B., Killeen N., 2006, Miriad Users Guide. AustraliaTelescope National FacilitySault R. J., Wieringa M. H., 1994, A&AS, 108, 585Smartt S. J., 2009, ARA&A, 47, 63Smith C., Points S., Winkler P. F., 2006, NOAO Newslet-ter, 85, 6Stupar M., Filipovi´c M. D., Jones P. A., Parker Q. A., 2005,Advances in Space Research, 35, 1047Tetzlaff N., Eisenbeiss T., Neuhaeuser R., Hohle M. M.,2011, ArXiv e-printsZaritsky D., Harris J., Thompson I. B., Grebel E. K., 2004,AJ, 128, 1606 c (cid:13)000