We present a high resolution Chandra X-ray observation of PSR B0540-69, the
Crab-like 50 msec pulsar in the Large Magellanic Cloud. We use phase-resolved
imaging to decompose the extended X-ray emission, as expected of a synchrotron
nebula, from the point-like emission of the pulsar. The image of the pulsed
X-ray emission shows a well-defined point-spread function of the observation,
while the resolved nebula has a morphology and size remarkably similar to the
Crab nebula, including evidence for a jet-like feature from PSR B0540-69. The
patchy outer shell, which most likely represents the expanding blast-wave of
the supernova, is reminiscent of that seen in radio. Based on morphology, size,
and energetics there can be little doubt that PSR B0540-69 is an analogous
system to the Crab but located in our neighboring galaxy.Comment: 5 pages with 5 figures, LaTex, emulateapj.sty. Revision based on data
  corrected for instrumental artifacts; main result unchanged. This version to
  appear in the Astrophysical Journal Letter

Gotthelf, E. V.

Wang, Q. Daniel

English

arXiv

This is the pre-published version harvested from ArXiv. The published version is located at http://iopscience.iop.org/1538-4357/532/2/L117/We present a high-resolution Chandra X-ray observation of PSR B0540-69, the Crab-like 50 ms pulsar in the Large Magellanic Cloud. We use phase-resolved imaging to decompose the extended X-ray emission, as expected of a synchrotron nebula, from the pointlike emission of the pulsar. The image of the pulsed X-ray emission shows a well-defined point-spread function of the observation, while the resolved nebula has a morphology and size remarkably similar to the Crab nebula, including evidence for a jetlike feature from PSR B0540-69. The patchy outer shell, which most likely represents the expanding blast wave of the supernova, is reminiscent of that seen in radio. Based on morphology, size, and energetics, there can be little doubt that SNR B0540-69 is an analogous system to the Crab but located in our neighboring galaxy.L117

Gotthelf, EV

Wang, QD

ScholarWorks@UMass Amherst

A Spatially Resolved Plerionic X-ray Nebula Around Psr B0540–69

We present a high-resolution Chandra X-ray observation of PSR B0540-69, the Crab-like 50 ms pulsar in the Large Magellanic Cloud. We use phase-resolved imaging to decompose the extended X-ray emission, as expected of a synchrotron nebula, from the pointlike emission of the pulsar. The image of the pulsed X-ray emission shows a well-defined point-spread function of the observation, while the resolved nebula has a morphology and size remarkably similar to the Crab nebula, including evidence for a jetlike feature from PSR B0540-69. The patchy outer shell, which most likely represents the expanding blast wave of the supernova, is reminiscent of that seen in radio. Based on morphology, size, and energetics, there can be little doubt that SNR B0540-69 is an analogous system to the Crab but located in our neighboring galaxy

University of Massachusetts AmherstScholarWorks@UMass AmherstAstronomy Department Faculty Publication Series Astronomy2000A Spatially Resolved Plerionic X-Ray Nebulaaround PSR B0540–69EV GotthelfQD WangUniversity of Massachusetts - AmherstFollow this and additional works at: https://scholarworks.umass.edu/astro_faculty_pubsPart of the Astrophysics and Astronomy CommonsThis Article is brought to you for free and open access by the Astronomy at ScholarWorks@UMass Amherst. It has been accepted for inclusion inAstronomy Department Faculty Publication Series by an authorized administrator of ScholarWorks@UMass Amherst. For more information, pleasecontact scholarworks@library.umass.edu.Recommended CitationGotthelf, EV and Wang, QD, "A Spatially Resolved Plerionic X-Ray Nebula around PSR B0540–69" (2000). The Astrophysical JournalLetters. 1055.10.1086/312568arXiv:astro-ph/0002166v2  10 Mar 2000To appear in the Astrophysics Journal LettersPreprint typeset using LATEX style emulateapjA SPATIALLY RESOLVED PLERIONIC X-RAY NEBULA AROUND PSR B0540−69E. V. Gotthelf1 and Q. Daniel Wang2To appear in the Astrophysics Journal LettersABSTRACTWe present a high resolution Chandra X-ray observation of PSR B0540−69, the Crab-like 50 msecpulsar in the Large Magellanic Cloud. We use phase-resolved imaging to decompose the extended X-rayemission, as expected of a synchrotron nebula, from the point-like emission of the pulsar. The imageof the pulsed X-ray emission shows a well-defined point-spread function of the observation, while theresolved nebula has a morphology and size remarkably similar to the Crab nebula, including evidencefor a jet-like feature from PSR B0540−69. The patchy outer shell, which most likely represents theexpanding blast-wave of the supernova, is reminiscent of that seen in radio. Based on morphology, size,and energetics there can be little doubt that SNR B0540−69 is an analogous system to the Crab butlocated in our neighboring galaxy.Subject headings: pulsars: general — pulsars: individual (PSR B0540−69) — X-rays: general —supernova remnant — stars: neutron1. INTRODUCTIONThe X-ray-bright 50 ms pulsar PSR B0540−69 was dis-covered in the N158A nebula of the Large MagellanicCloud (LMC) and has long been compared to the Crabpulsar (Seward, Harnden, & Helfand 1984). Based on itstiming and spectral properties, the two rotation poweredpulsars are very similar with a spin period of 50 vs. 33 msand a spin-down rate of 4.8×10−13 vs. 4.0×10−13 s/s forPSR B0540−69 and the Crab, respectively. From thesequantities, assuming the standard magnetic dipole pulsarmodel, one can infer a characteristic age (1.7 vs. 1.2 kyr),spin-down energy (2.0× 1038 vs. 6.5× 1038 ergs s−1), andsurface magnetic field strength (5.0 × 1012 vs. 3.8 × 1012G) for PSR B0540−69 and the Crab, respectively. Thissimilarity suggests that PSR B0540−69 could be accom-panied by a “plerion”, a pulsar driven wind nebula (Weiler& Sramek 1988), reminiscent of that seen for the Crab.Indeed, there are several lines of evidence indicating thepresence of a plerion in the vicinity of PSR B0540−69.Chanan, Helfand, & Reynolds (1984) detected a polar-ized optical nebula of half-power diameter ∼ 4′′ aroundthe pulsar. This apparent synchrotron nebula was alsoresolved (though barely) in a radio image presented byManchester et al. (1993) and in a ROSAT high resolutionimager observation by Seward & Harnden (1994). Fur-thermore, the overall X-ray spectrum of PSR B0540−69and its remnant (SNR B0540−69) is well characterized bya power law, as expected if the emission is predominantlynon-thermal (Clark et al. 1982; Wang & Gotthelf 1998a).The ROSAT observation also revealed a faint X-rayemitting shell, ∼ 15 pc in size surrounding the pulsar.This shell contributes less than 20% to the total lumi-nosity of ∼ 1.0 × 1037 ergs s−1 in the ROSAT 0.1-2 keVband and is likely the SNR associated with the pulsar (Se-ward & Harden 1994). However, no evidence has yet beenfound for a similar X-ray-emitting shell or a shell-like SNRaround the Crab (e.g. see discussion in Jones et al. 1998).In this Letter, we report new results on SNR B0540−69based on a recent observation acquired with the Chan-dra High Resolution Camera. This observation enables usfor the first time to distinguish morphological details ofthe nebula around PSR B0540−69. We analyze phase de-pendent images and resolve the expected plerion-like neb-ula from the point-like pulsar emission. This allows usto identify features similar to those seen from the Crabnebula; we present morphological evidence for a torus ofX-ray emission, which most likely represents shocked pul-sar wind materials and a likely X-ray jet emanating fromthe pulsar. We discuss the implications of the results inthe context of the pulsar-nebula system. Throughout thepaper we adopt a distance of 51 kpc for the LMC.2. OBSERVATIONThe Chandra observatory (Weisskopf et al. 1996) ob-served SNR B0540−69 on Aug 31 1999 as part of the initialcalibration of the High Resolution Camera (HRC; Murrayet al. 1997). A total of 19.4 ksec of data were collectedduring a portion of the orbit which avoided regions of highbackground contamination such as from bright Earth andradiation belt passages. The remnant was centered on theon-axis position of the HRC where the point-spread func-tion (PSF) has a half power radius (the radius enclosing50% of total source counts) of ∼ 0.′′5. Time-tagged pho-tons were acquired with 15.6 µs precision, and the arrivaltimes were corrected to the solar system barycenter us-ing a beta version of AXBARY available from the ChandraX-ray Center FTP site (A. Rots 1999, private communi-cation). While the detector is sensitive to X-rays over a0.1 − 10.0 keV range, there is essentially no energy infor-mation available. We analyzed event data calibrated bythe initial processing and dated 1999 September 12, whichwas made available through the Chandra public archive.In addition to the standard processing, the event data wasfurther filtered to reduce the instrumental background andto remove “ghost image” artifacts using a beta version of1Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, New York, NY 10027, USA; evg@astro.columbia.edu2Astronomy Department, University of Massachusetts, B-524 LGRT, Amherst, MA 01003, USA; wqd@astro.umass.edu12 X-ray nebula around PSR B0540−69HRC SCREEN (S. Murray 2000, private communication).We extracted 1024 × 1024 pixel images centered on thepulsar rebinned by a factor of two from the native HRCpixel size of 0.′′13175 per side.3. RESULTSA global view of N158A and its environment as seen bythe Chandra HRC is shown in Figure 1 (contours) andFigure 2 (greyscale). The large-scale X-ray enhancementon scales up to ∼ 1′ is the previously resolved shell-likeemission (Seward & Harnden 1994). In fact, the X-ray andradio emission together outlines a nearly circular morphol-ogy around PSR B0540−69. Clearly, the shell representsthe blastwave of SNR B0540−69. The X-ray intensity dis-tribution within the remnant appears rather patchy. Whilethe southwest X-ray enhancement is a good tracer of theradio and optical emission peaks, there is no general cor-relation between fainter radio and X-ray features.The superb spatial resolution of the Chandra observa-tion further allows for a close-up of the immediate vicinityof PSR B0540−69 (Fig. 2b). The presence of a diffuseplerion-like nebula around the pulsar is apparent. To de-compose the nebula emission from the pulsar contribution,we conducted phase-resolved image analysis. This enablesus to estimate the local PSF based on the pulsed, point-like emission from the pulsar and to quantify the extended,unpulsed nebula radiation.First, we must determine the pulse period at the cur-rent epoch. We constructed a periodigram around a nar-row range of periods centered on the expected period ±0.1ms, sampled in increments of 0.05(P 2/T ), were T is theobservation duration, and P is the test period. For eachtrial period, we folded photons extracted from a 1.′′0 aper-ture centered on the bootstrapped pulsar position (see be-low) into 20 phase bins and computed the χ2 of the resul-tant profile. We find a highly significant signal (> 56σ)at P = 50.508132(6) ms at Epoch 51421.630741 MJD;the uncertainty is estimated according to the method ofLeahy (1987). We have assumed a period derivative ofP˙ = 4.789342 × 10−13 for the data epoch (Deeter et al.1999). In Figure 3 we display the resultant light-curvefolded at the peak period which, as expected, is roughlysinusoidal and modulated with a ∼ 40% pulse fraction (de-fined as the amplitude divided by the mean).Next, we defined two regions in the phase space, on-and off-pulse, by selecting eight adjacent phase bins corre-sponding to the peak and trough of the pulse profile. Theon-pulse image with the off-pulse image subtracted is pre-sented in Figure 4b. This image reproduced the expectedPSF with no evidence of asymmetric deviations, as mightbe caused by poor aspect reconstruction, like that typi-cally found for ROSAT images. Figure 5 presents averageradial intensity distributions around the centroid of thepoint-like source.By subtracting the normalized pulsar image (Fig. 4b),we are able to construct an image of the nebula emission(Fig. 4c) alone. The subtracted image is scaled to com-pensated for both the relative phase coverage and for a21% unpulsed emission contribution, estimated by min-imize a pointlike contribution at the pulsar position ofthe nebula image. As shown in Figure 4c, the extendedemission is distinctly different from the pointlike imageof the pulsed emission from the pulsar. The primarilyfeature is the NE-SW elongated feature, which is mor-phologically symmetric relative to the pulsar and extendsabout ∼ 2.′′5 on both sides of the pulsar. However, theobserved emission on the SW side appears twice as strongcompared to the NE side, with an average intensity of∼ 7.5 × 10−2 counts s−1 arcsec−2. Because the centralcore of the distribution is significantly brighter than theextended features and the subtraction of the pulsar con-tribution is somewhat arbitrary, the exact intensity distri-bution is uncertain.There is also marginal evidence for a jet-like feature em-anating from the pulsar. This emission, most apparent inthe NW and extending about 3′′, is nearly perpendicular tothe NE-SW elongated nebula and is slightly bent towardNorth. The integrated emission of the jet-like feature isroughly ∼ 3.1×10−2 counts s−1. The configuration of thejet feature relative to the nebula is remarkably similar tothat of the Crab nebula as seen by ROSAT and which isnow clearly resolved with Chandra (see Chandra publicityphoto).In short, the X-ray emission can be decomposed intothree major morphological components: a point-likesource, the surrounding nebula which shows evidence fora jet feature, and a patchy supernova remnant shell.4. DISCUSSIONSA comparison between the X-ray-emitting nebulaaround PSR B0540−69 and the Crab nebula (see Chan-dra publicity photo3) is very informative. The Crab neb-ula image shows a torus of X-ray-emitting loops, whichmost likely represents shocked pulsar wind materials con-sisting of magnetic waves and ultra-relativistic particles.Also clear are the two jets of X-ray-emitting material em-anating from the Crab pulsar, in the direction perpendic-ular to the major axis of the torus. We speculate that thenebula around PSR B0540−69 has a similar structure. Infact, at the spatial resolution of Chandra at the LMC, thesize, morphology, and surface intensity of the two nebulaeare all remarkably similar (Fig. 2b).Assuming a power law spectrum for the X-ray emis-sion from the SNR B0540−69 nebula of photon index 2.0and NH = 4 × 1021cm2 (Finley et al. 1993, Wang &Gotthelf 1998a), the conversion between the count rateand the energy flux in the standard 1.0-10 keV band is∼ 1 × 10−10 ergs s−1 cm−2/counts s−1. The correspond-ing total luminosity of the nebula is ∼ 2.7×1037 ergs s−1,which is 13% of the spin-down energy of PSR B0540−69.The fraction is again similar to that of the Crab.N157B (PSR J0537−6910) is the only other LMC SNRwith a detected pulsar (16 ms) and also shows both anextended (resolved by ROSAT HRI), non-thermal neb-ula and a partial X-ray-emitting shell (Wang & Gotthelf1998a; 1998b). The upcoming Chandra observations willmake a detailed comparison between these two youngCrab-like SNRs possible.We gratefully acknowledge the Chandra team for mak-ing available the public data used herein. In particularwe thank A. Rots and S. Murray for kindly making avail-3Available at http://xrtpub.harvard.edu/photo/0052/0052 hand.htmlGotthelf & Wang 3Table 1HRC Spatial Components of SNR B0540−69Componenta Count Rateb Size and ShapePulsarcpulsed 0.14 point-likeunpulsed 0.18 ”Nebula 0.8 5′′ × 3′′ NE-SWJet 0.03 3′′ long SE-NWSNR Shell 0.2 ∼ 1′ diameteraSee §3; bCount/s; cX-ray emission from a 2′′ ra-dius aperture around the pulsar.able their beta software. We thank U. Hwang for point-ing out an instrumental artifact (“ghost image”) in theoriginal HRC image of SNR B0540-69. We thank DickManchester for sending us the radio image and FernandoCamilo, David Helfand, and Jules Halpern for carefullyreading the manuscript. This work was funded in part byNASA LTSA grants NAG5-7935 (E.V.G.) and NAG5-6413(Q.D.W.). This is contribution #690 of the Columbia As-trophysics Laboratory.REFERENCESCaraveo, P., Mignani, R., & Bignami, G. B. 1998, in Memorie dellaSocieta Astronomia Italiana, Vol. 69, p.1061Chanan, G. A., Helfand, D. J., & Reynolds, S. P. 1984 ApJ, 287, L23Clark, D. H., Tuohy, I. R., Dopita, M. A., Mathewson, D. S., Long,K. S., Szymkowiak, A. E., Culhane, J. L. 1982, ApJ, 255, 440Deeter, J. E., Nagase, F., Boynton, P. E. 1999, ApJ, 512, 300Finley, J. P., Oegelman, H., Hasinger, G., Truemper, J. 1993 ApJ,410, 323Jones, T. W. et al. 1998, PASP, 110, 125Leahy, D. A., 1987, A&A, 180, 275Manchester, R. N., Staveley-Smith, & Kesteven, M. J. 1993, ApJ,411, 756Murray, S. S., et al., 1997, SPIE, 3114, 11Seward, F., D., Harnden, Jr., F. R., & Helfand, D. J. 1984 ApJ, 287,L19Seward, F. D., & Harnden, Jr., F. R. 1994, ApJ, 421, 581Wang, Q. D. & Gotthelf, E. V. 1998a, ApJ, 494, 623.Wang, Q. D. & Gotthelf, E. V. 1998b, ApJL, 509, 109Weisskopf, M. C. O’Dell, S. L., van Speybroeck, L. P. 1996, Proc.SPIE 2805, Multilayer and Gazing Incidence X-ray/EUV OpticsIII, 2.Weiler, K. W. & Sramek, R. A. 1988, ARAA, 25, 2954 X-ray nebula around PSR B0540−69Fig. 1.— The overall X-ray and radio view of the region of SNR 0540-69 containing the central 50 ms pulsarPSR B0540−69. The contours denote the Chandra HRC X-ray intensity distribution; the levels are 0.001, 0.005, 0.008,0.013, 0.03, 0.1, 0.3, 1., 3, 10, and 30 counts s−1 arcsec−2. The greyscale map shows the 6 cm radio emission which tracesthe approximately circular remnant. The radio map is from Manchester et al. (1993) and has a beam size with half powerwidth of ∼ 5”.Fig. 2.— X-ray intensity distribution around the region containing the 50 ms pulsar PSR B0540−69. Left Panel)the Chandra HRC broad energy-band image centered on the pulsar. The image has been adaptively smoothed using aminimum signal-to-noise of 6 ratio and the intensity scale chosen to highlight the diffuse SNR emission surrounding thebright pulsar nebula, which is fully saturated in this image. The central box delineates the region enlarged and displayedin the right panel. Right Panel) Close up image around the pulsar, displayed scaled by the square root of intensity. Insert)Crab nebula image, placed at the distance of the LMC (assuming a distance to the Crab of 2 kpc) and blurred to theHRC resolution. Notice the similar size, shape, and overall brightness morphology compared to PSR B0540−69.Gotthelf & Wang 5Fig. 3.— HRC light curve (0.4− 10 keV) for PSR B0540−69 folded at the ephemeris given in the text. Two periods areshown for clarity. The two sets of vertical bars denote the two phase regions (on/off) used to isolate the nebula from thepulsar emission. The light curve has been extracted from a 1′′ radius aperture centered on the pulsar.Fig. 4.— Phase-subtracted X-ray intensity maps of PSR B0540−69. Left Panel) Before phase-subtraction, image of thepulsar and nebula (all photons), same as the right panel of figure 2. Middle Panel) the pulsar image containing the pulsedphotons only. This is in excellent representation of the HRC point-spread function, consistent with the ground calibration.Right Panel) same region after subtracting the pulsar’s contribution to the total flux (see text for details) – this providesa good estimate of the nebula emission surrounding the pulsar. The cross marks the pulsar’s centroid. The three mapsare identically sized and linearly scaled in intensity.Fig. 5.— HRC radial intensity distributions around the point-like emission peak of PSR B0540−69. The pulsar radialprofile (solid line) is consistent with the HRC point-spread function. The nebula profile (dotted line) is significantlyextended, the enhancement between 0.1− 1.5′′ is due to the relatively bright SW nebula region.

A Spatially Resolved Plerionic X-Ray Nebula around PSR B0540–69

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A spatially resolved plerionic X-ray nebula around PSR B0540-69

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