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Diffusive synchrotron radiation from pulsar wind nebulae
Diffusive synchrotron radiation (DSR) is produced by charged particlesas they random walk in a stochastic magnetic field. The spectrum of theradiation produced by particles in such fields differs substantiallyfrom those of standard synchrotron emission because the correspondingparticle trajectories deviate significantly from gyration in a regularfield. The Larmor radius, therefore, is no longer a good measure of theparticle trajectory. In this paper, we analyse a special DSR regimewhich arises as highly relativistic electrons move through magneticfields which have only random structure on a wide range of spatialscales. Such stochastic fields arise in turbulent processes, and arelikely present in pulsar wind nebulae (PWNe). We show that DSR generatedby a single population of electrons can reproduce the observedbroad-band spectra of PWNe from the radio to the X-ray, in particularproducing relatively flat spectrum radio emission as is usually observedin PWNe. DSR can explain the existence of several break frequencies inthe broad-band emission spectrum without recourse to breaks in theenergy spectrum of the relativistic particles. The shape of theradiation spectrum depends on the spatial spectrum of the stochasticmagnetic field. The implications of the presented DSR regime for PWNphysics are discussed.
| Discovery of a Compact X-Ray Source in the LMC Supernova Remnant N23 with Chandra
An X-ray compact source was discovered with Chandra in a supernovaremnant (SNR) N23, located in the Large Magellanic Cloud (LMC). Thecompact source (CXOU J050552.3-680141) is seen in only the hard band(>2 keV) image of N23, while the soft-band image (< 2 keV) showsdiffuse emission of the SNR, with an extent of~60''×80''. The compact source is located atalmost the center of N23, and there is no identifiable object for thesource from previous observations at any other wavelength. The sourcespectrum is best explained by a power-law model with a photon index ofΓ=2.2+0.5-0.3 and an absorption-correctedluminosity of LX=1.0×1034 ergss-1 in the 0.5-10 keV band for a distance of 50 kpc. Neitherpulsation nor time variability of the source was detected with thisobservation with a time resolution of 3.2 s. These results correspondwith those of Hughes et al., who carried out analysis independentlyaround the same time as our work. Based on information from the best-fitpower-law model, we suggest that the source emission is most likely froma rotation-powered pulsar and/or a pulsar wind nebula. It is generallyinferred that the progenitor of N23 is a core-collapsed massive star.
| The Duck Redux: An Improved Proper-Motion Upper Limit for the Pulsar B1757-24 near the Supernova Remnant G5.4-1.2
``The Duck'' is a complicated nonthermal radio system, consisting of theenergetic radio pulsar B1757-24, its surrounding pulsar wind nebulaG5.27-0.90, and the adjacent supernova remnant (SNR) G5.4-1.2. PSRB1757-24 was originally claimed to be a young (~15,000 yr) andextreme-velocity (>~1500 km s-1) pulsar, which hadpenetrated and emerged from the shell of the associated SNR G5.4-1.2 butrecent upper limits on the pulsar's motion have raised seriousdifficulties with this interpretation. We here present 8.5 GHzinterferometric observations of the nebula G5.27-0.90 over a 12 yrbaseline, doubling the time span of previous measurements. These datacorrespondingly allow us to halve the previous upper limit on thenebula's westward motion to 14 mas yr-1 (5 σ), allowinga substantive reevaluation of this puzzling object. We rule out thepossibility that the pulsar and SNR were formed from a common supernovaexplosion ~15,000 yr ago, as implied by the pulsar's characteristic age,but conclude that an old (>~70,000 yr) pulsar/SNR association, or asituation in which the pulsar and SNR are physically unrelated, are bothstill viable explanations.
| XMM-Newton observations of SN 1987 A
Context: .We report on XMM-Newton observations of SN 1987 A in the LargeMagellanic Cloud. Aims: .The large collecting area telescopestogether with the European Photon Imaging Cameras (EPIC) provide X-rayspectra with unprecedented statistical quality and make it possible toinvestigate the spectral evolution during the brightening observed sincethe discovery in X-rays. High resolution spectra from the ReflectionGrating Spectrometers yield a complementary view and allow us to performmore detailed investigations of prominent emission lines. Methods:.The X-ray spectra were modeled with two-temperature emission componentsfrom a hot plasma in collisional ionization equilibrium and innon-equilibrium (NEI). Results: .We find a temperature for theequilibrium component of 0.24 ± 0.02 keV in January 2000 andApril 2001 which increased to 0.30 ± 0.02 keV in May 2003 andalso an indication for a temperature increase in the hot NEI componentfrom ~2 keV to ~3 keV. Emission line ratios inferred from the RGSspectra suggest temperatures as low as 100 eV and an increase in theionization state of oxygen and neon consistent with the observedtemperature increases. The fast readout of the EPIC-pn instrument yieldsX-ray fluxes free of CCD pile-up effects which we used to normalizepile-up corrections for the published Chandra fluxes. The correctedX-ray light curve of SN 1987 A in the 0.5-2.0 keV energy band is bestrepresented by a linear increase up to about day 4000 after theexplosion and an exponential rise afterwards until the last publishedChandra observation on day 6716. Modeling the light curve by emissionfrom the inner ring which is approximated by a circular torus a centraldensity nH}=1.15×10^4 cm^-3 is found. In this model the forwardshock has just passed the center of the torus. Conclusions: .SN1987 A continues to brighten exponentially in soft X-rays. The X-rayspectra can be represented by pure thermal emission without significantcontribution from a compact object yet.
| Massive young stellar objects in the Large Magellanic Cloud: water masers and ESO-VLT 3-4 μm spectroscopy
We investigate the conditions of star formation in the Large MagellanicCloud (LMC). We have conducted a survey for water maser emission arisingfrom massive young stellar objects in the 30 Doradus region (N157) andseveral other HII regions in the LMC (N105A, N113 and N160A). We haveidentified a new maser source in 30Dor at the systemic velocity of theLMC. We have obtained 3-4 μm spectra, with the European SouthernObservatory (ESO)-Very Large Telescope (VLT), of two candidate youngstellar objects. N105AIRS1 shows H recombination line emission, and itsSpectral Energy Distribution (SED) and mid-infrared colours areconsistent with a massive young star ionizing the molecular cloud.N157BIRS1 is identified as an embedded young object, based on its SEDand a tentative detection of water ice. The data on these four HIIregions are combined with mid-infrared archival images from the SpitzerSpace Telescope to study the location and nature of the embedded massiveyoung stellar objects and signatures of stellar feedback. Our analysisof 30Dor, N113 and N160A confirms the picture that the feedback from themassive O- and B-type stars, which creates the HII regions, alsotriggers further star formation on the interfaces of the ionized gas andthe surrounding molecular cloud. Although in the dense cloud N105A starformation seems to occur without evidence of massive star feedback, thegeneral conditions in the LMC seem favourable for sequential starformation as a result of feedback. In an Appendix, we present watermaser observations of the galactic red giants RDoradus and WHydrae.
| Chandra ACIS Spectroscopy of N157B: A Young Composite Supernova Remnant in a Superbubble
We present a Chandra ACIS observations of N157B, a young supernovaremnant (SNR) located in the 30 Doradus star formation region of theLarge Magellanic Cloud. This remnant contains the most energetic pulsarknown (PSR J053747.39-691020.2 E˙=4.8×1038 ergss-1), which is surrounded by a X-ray-bright nonthermal nebulathat likely represents a toroidal pulsar wind terminal shock observededge-on. Two of the eight pointlike X-ray sources detected in theobservation are shown to have near-IR and optical counterparts (within0.5" offsets), which are identified as massive stellar systems in theCloud. We confirm the nonthermal nature of the comet-shaped X-rayemission feature and show that the spectral steepening of this featureaway from the pulsar is quantitatively consistent with synchrotroncooling of shocked pulsar wind particles flowing downstream at a bulkvelocity close to the speed of light. Around the cometary nebula weunambiguously detect a spatially resolved thermal component, whichaccounts for about 1/3 of the total 0.5-10 keV flux from the remnant.This thermal component is distributed among various clumps ofmetal-enriched plasma embedded in the low surface brightnessX-ray-emitting diffuse gas. The relative metal enrichment patternsuggests that the mass of the supernova progenitor is >~20Msolar. A comparison of the X-ray data with Hubble SpaceTelescope optical images now suggests that the explosion site is closeto a dense cloud, against which a reflection shock is launched. Theinteraction between the reflected material and the nebula has likelyproduced both its cometary shape and the surrounding thermal emissionenhancement. SNR N157B is apparently expanding into the hot low-densityinterior of the surrounding superbubble formed by the young OBassociation LH 99, as revealed by Spitzer mid-infrared images. Thisscenario naturally explains the exceptionally large sizes of both thethermal and nonthermal components, as well as the lack of an outer shellof the SNR. However, the real situation in the region is likely to bemore complicated. We find that a partially round soft X-ray-emittingclump with distinct spectral properties may result from a separateoxygen-rich remnant. These results provide a rare glimpse into the SNRstructure and evolution in a region of recent star formation.
| Supernova Remnants in the Magellanic Clouds. VII. Infrared Emission from Supernova Remnants
We have used the instruments on the Spitzer Space Telescope to study theLarge Magellanic Cloud supernova remnants (SNRs) N11L, N44, N49, N206,N63A, and N157B. The two large SNRs N44 and N206 were not detected inany Spitzer IRAC or MIPS wave bands; the remainder were detected at oneor more wavelengths. In particular, the SNRs N49 and N63A each hadfeatures that were evident in all available IRAC and MIPS bands. Each ofthese two also displayed faint limb emission in the MIPS 24 μm bandonly. Spitzer IRS spectra obtained for the N49 SNR showed a number ofprominent lines, with little continuum contribution. We thereforesuggest that N49, and possibly N63A, are dominated by line emission,with thermal emission from hot dust being at most a secondary component.
| Discovery of 14 Radio Pulsars in a Survey of the Magellanic Clouds
A systematic survey of the Large and Small Magellanic Clouds for radiopulsars using the Parkes radio telescope and the 20 cm multibeamreceiver has resulted in the discovery of 14 pulsars and the redetectionof five of the eight previously known spin-powered pulsars believed tolie in the Magellanic Clouds. Of the 14 new discoveries, 12 are believedto lie within Clouds, three in the Small Cloud and nine in the LargeCloud, bringing the total number of known spin-powered pulsars in theClouds to 20. Averaged over all positions within the survey area, thesurvey had a limiting flux density of about 0.12 mJy. Observeddispersion measures suggest that the mean free electron density in theMagellanic Clouds is similar to that in the disk of our Galaxy. Theobserved radio luminosities have little or no dependence on pulsarperiod or characteristic age and the differential luminosity function isconsistent with a power-law slope of -1, as is observed for Galacticpulsars.
| Probing the Pulsar Wind Nebula of PSR B0355+54
We present XMM-Newton and Chandra X-ray observations of the middle-agedradio pulsar PSR B0355+54. Our X-ray observations reveal emission notonly from the pulsar itself, but also from a compact diffuse componentextending ~50" in the opposite direction to the pulsar's proper motion.There is also evidence for the presence of fainter diffuse emissionextending ~5' from the point source. The compact diffuse feature is wellfitted with a power law, the index of which is consistent with thevalues found for other pulsar wind nebulae. The morphology of thediffuse component is similar to the ram-pressure-confined pulsar windnebulae detected for other sources. The X-ray emission from the pulsaritself is well described by a thermal plus power-law fit, with thethermal emission most likely originating in a hot polar cap.
| The Spin Periods and Rotational Profiles of Neutron Stars at Birth
We present results from an extensive set of one- and two-dimensionalradiation-hydrodynamic simulations of the supernova core-collapse,bounce, and postbounce phases and focus on the proto-neutron star (PNS)spin periods and rotational profiles as a function of initial iron coreangular velocity, degree of differential rotation, and progenitor mass.For the models considered, we find a roughly linear mapping betweeninitial iron core rotation rate and PNS spin. The results indicate thatthe magnitude of the precollapse iron core angular velocities is thesingle most important factor in determining the PNS spin. Differences inprogenitor mass and degree of differential rotation lead only to smallvariations in the PNS rotational period and profile. Based on ourcalculated PNS spins at ~200-300 ms after bounce and assuming angularmomentum conservation, we estimate final neutron star rotation periods.We find periods of 1 ms and shorter for initial central iron coreperiods of <~10 s. This is appreciably shorter than what previousstudies have predicted and is in disagreement with current observationaldata from pulsar astronomy. After considering possible spin-downmechanisms that could lead to longer periods, we conclude that there isno mechanism that can robustly spin down a neutron star from ~1 msperiods to the ``injection'' periods of tens to hundreds of millisecondsobserved for young pulsars. Our results indicate that, given currentknowledge of the limitations of neutron star spin-down mechanisms,precollapse iron cores must rotate with periods of around 50-100 s toform neutron stars with periods generically near those inferred for theradio pulsar population.
| A detailed observation of a LMC supernova remnant DEM L241 with XMM-Newton
We report on an XMM-Newton observation of the supernova remnant (SNR)DEM L241 in the Large Magellanic Cloud. In the softband image, the emission shows an elongated structure, like a killifish,with a central compact source. The compact source is point-like, andnamed XMMU J053559.3-673509. The source spectrum is reproduced well by apower-law model with a photon index of Γ = 1.57 (1.51-1.62); andthe intrinsic luminosity is 2.2 ×1035~erg~s-1 in the 0.5-10.0 keV band, with anassumed distance of 50 kpc. The source has neither significant coherentpulsations in 2.0 × 10-3-8.0 Hz nor time variabilities.Its luminosity and spectrum suggest that the source might be a pulsarwind nebula (PWN) in DEM L241. The spectral feature classifies thissource as rather bright and hard PWN, which is similar to those in Kes75 and B0540-693. The elongated diffuse structure can be divided into a"Head" and "Tail", and both have soft and line-rich spectra. Theirspectra are reproduced well by a plane-parallel shock plasma (vpshock)model with a temperature of 0.3-0.4 keV, over-abundance in O and Ne, anda relative under-abundance in Fe. Such an abundance pattern and themorphology imply both that the emission is from the ejecta of the SNRand that the progenitor of DEM L241 is a very massive star, more than 20M_ȯ. This result is also supported by the existence of the centralpoint source and an OB star association, LH 88. The total thermal energyand plasma mass are ~4 × 1050 erg and ~200~M_ȯ,respectively.
| G359.95-0.04: an energetic pulsar candidate near Sgr A*
We report the discovery of a prominent non-thermal X-ray feature locatednear the Galactic centre that we identify as an energetic pulsar windnebula. This feature, G359.95-0.04, lies 1-lyr north of Sgr A* (inprojection), is comet like in shape, and has a power-law spectrum thatsteepens with increasing distance from the putative pulsar. The distinctspectral and spatial X-ray characteristics of the feature are similar tothose belonging to the rare class of ram-pressure confined pulsar windnebulae. The luminosity of the nebula at the distance of Sgr A*,consistent with the inferred X-ray absorptions, is Lx~ 1× 1034ergs-1 in the 2-10 keV energy band.The cometary tail extends back to a region centred at the massivestellar complex IRS 13 and surrounded by an enhanced diffuse X-rayemission, which may represent an associated supernova remnant.Furthermore, the inverse Compton scattering of the strong ambientradiation by the nebula consistently explains the observed TeV emissionfrom the Galactic centre. We also briefly discuss plausible connectionsof G359.95-0.04 to other high-energy sources in the region, such as theyoung stellar complexes IRS 13 and SNR Sgr A East.
| A Chandra ACIS Study of 30 Doradus. II. X-Ray Point Sources in the Massive Star Cluster R136 and Beyond
We have studied the X-ray point-source population of the 30 Doradus (30Dor) star-forming complex in the Large Magellanic Cloud using highspatial resolution X-ray images and spatially resolved spectra obtainedwith the Advanced CCD Imaging Spectrometer (ACIS) on board the ChandraX-Ray Observatory. Here we describe the X-ray sources in a17'×17' field centered on R136, the massivestar cluster at the center of the main 30 Dor nebula. We detect 20 ofthe 32 Wolf-Rayet stars in the ACIS field. The cluster R136 is resolvedat the subarcsecond level into almost 100 X-ray sources, including manytypical O3-O5 stars, as well as a few bright X-ray sources previouslyreported. Over 2 orders of magnitude of scatter in LX is seen among R136O stars, suggesting that X-ray emission in the most massive starsdepends critically on the details of wind properties and the binarity ofeach system, rather than reflecting the widely reported characteristicvalue LX/Lbol~=10-7. Such a canonicalratio may exist for single massive stars in R136, but our data are tooshallow to confirm this relationship. Through this and future X-raystudies of 30 Dor, the complete life cycle of a massive stellar clustercan be revealed.
| A Chandra ACIS Study of 30 Doradus. I. Superbubbles and Supernova Remnants
We present an X-ray tour of diffuse emission in the 30 Doradusstar-forming complex in the Large Magellanic Cloud using high spatialresolution X-ray images and spatially resolved spectra obtained with theAdvanced CCD Imaging Spectrometer on board the Chandra X-RayObservatory. The dominant X-ray feature of the 30 Doradus nebula is theintricate network of diffuse emission generated by interacting stellarwinds and supernovae working together to create vast superbubbles filledwith hot plasma. We construct maps of the region showing variations inplasma temperature (T=3-9 million degrees), absorption[NH=(1-6)×1021 cm-2], andabsorption-corrected X-ray surface brightness[SX=(3-126)×1031 ergs s-1pc-2]. Enhanced images reveal the pulsar wind nebula in thecomposite supernova remnant N157B, and the Chandra data show spectralevolution from nonthermal synchrotron emission in the N157B core to athermal plasma in its outer regions. In a companion paper we show thatR136, the central massive star cluster, is resolved at the arcsecondlevel into almost 100 X-ray sources. Through X-ray studies of 30 Doradusthe complete life cycle of such a massive stellar cluster can berevealed.
| An empirical calibration of sulphur abundance in ionised gaseous nebulae
We have derived an empirical calibration of the abundance of S/H as afunction of the S{23} parameter, defined using the bright sulphur linesof [SII] and [SIII]. Contrary to the case for the widely used O{23}parameter, the calibration remains single valued up to the abundancevalues observed in the disk HII regions. The calibration is based on alarge sample of nebulae for which direct determinations of electrontemperatures exist and the sulphur chemical abundances can be directlyderived. ICFs, as derived from the [SIV] 10.52 μ emission line (ISOobservations), are shown to be well reproduced by Barker's formula for avalue of α = 2.5. Only about 30% of the objects in the samplerequire ICFs larger than 1.2. The use of the proposed calibration opensthe possibility of performing abundance analysis with red to IRspectroscopic data using S/H as a metallicity tracer.
| Chandra Observation of the Magellanic Cloud Supernova Remnant 0454-67.2 in N9
A Chandra observation has defined the extent of the SNR 0454-67.2 in theLMC H II region N9. The remnant has dimension 2.3 arcmin×3.6 arcminand is elongated in the north-south direction. The brightest emissioncomes from a north-south central ridge that includes three brightpatches. There is good agreement between X-ray and [O III] and [S II]morphology. The remnant is old enough so that optical data give moreinformation about dynamics than do the X-ray data. The supernova (SN)energy release was >=5×1050 ergs, and the age is~3×104 yr. There are several unresolved sources nearby,but none are clearly associated with the remnant. The X-ray spectrum issoft and indicates enhanced Fe abundance in the central region,consistent with a Type Ia SN origin, but a Type II origin cannot beruled out.
| Searches for diffuse X-ray emission around millisecond pulsars: an X-ray nebula associated with PSR J2124-3358
We report on diffuse X-ray emission associated with the nearby solitarymillisecond pulsar PSR J2124-3358 detected with XMM-Newton and Chandra.The emission extends from the pulsar to the northwest by ~ 0.5 arcmin.The spectrum of the nebular emission can be modeled with a power-law ofphoton index 2.2±0.4, in line with the emission originating fromaccelerated particles in the post shock flow. For PSR J0437-4715, PSRJ0030+0451 and PSR J1024-0719, which all have spin parameters comparableto that of PSR J2124-3358, no diffuse emission is detected down to a3-σ limiting flux of ~ 4-7×10-15 ergs-1 cm-2.
| A photoionization-modelling study of 30 Doradus: the case for small-scale chemical inhomogeneity
Photoionization models of the giant HII region (GHIIR) 30 Doradus arebuilt and confronted to available ultraviolet (UV), optical, infrared(IR) [Infrared Space Observatory (ISO)] and radio spectra, underblackbody or CoStar spectral energy distributions for the primary sourceand various density distributions for the nebular gas. Chemicallyhomogeneous models show very small rms electron-temperature fluctuationsand fail to reproduce the heavy-element optical recombination line (ORL)spectrum of the nebula. Dual-abundance models incorporating small-scalechemical inhomogeneities in the form of hydrogen-deficient inclusionswhich are in pressure balance with the normal-composition ambient gas,provide a better fit to the observed heavy-element ORLs and othernebular lines, while most spectral features are satisfactorily accountedfor. The inclusions, whose mass is ~2per cent of the total gaseous mass,are 2-3 times cooler and denser than the ambient nebula. Their O/Habundance ratio is ~0.9dex larger than in the normal-composition gas andhave typical mass fractions of X= 0.687, Y= 0.273 and Z= 0.040. Heliumis found to be about as deficient as hydrogen in the inclusions, whileelements heavier than neon, such as sulphur and argon, are quitepossibly enhanced in proportions similar to oxygen, as indicated by themost satisfactory dual-abundance model obtained. This suggests that theposited hydrogen-deficient inclusions may have arisen from partialmixing of matter which was nucleosynthetically processed in a supernovaevent with gas of normal Large Magellanic Cloud composition. The averagegaseous abundances of the chemically inhomogeneous models are ~0.08dexhigher than those of the homogeneous models, yet they are lower by asimilar ~0.08dex than those derived from standard empirical methods(fully corrected for inaccuracies in ionization correction factors anddifferences in atomic data) which postulate temperature fluctuations ina chemically homogeneous medium. Attention is drawn to a bias in thedetermination of HII region (HIIR) helium abundances in the presence ofhydrogen-deficient inclusions. It is argued that these results provideevidence for incomplete small-scale mixing of the interstellar medium(ISM). The case for the existence of abundance inhomogeneities in HIIRsis examined in the light of current theoretical considerations regardingthe process of chemical homogenization in the ISM.
| An ATCA radio-continuum study of the Small Magellanic Cloud - III. Supernova remnants and their environments
A total of 717 sources from the Australia Telescope Compact Array (ATCA)catalogue of the Small Magellanic Cloud (SMC) have been classified inPaper II (of this series) by Payne et al. Here, we present a statisticalanalysis of all 16 confirmed supernova remnants (SNRs) and five newcandidate remnants. Included is a detailed discussion of the latter andthree other sources that have some SNR characteristics. We have alsofound a new microquasar candidate (ATCA J005523-721055) in addition toATCA J004718-723947 reported in Paper II. Source diameter comparisonssuggest that SNRs as a group are of similar size in radio, optical andX-ray with surface brightness values in the range of Galactic remnants.Remnant spectral indices, α[defined asSν~να, with Sν (fluxdensity) and ν (frequency)], have a mean of -0.63 [standard deviation(s.d.) = 0.43] and ROSAT X-ray hardness ratios confirm them to be softX-ray sources compared to background objects. We could not find anymeaningful correlation between SNR surface brightness and diameter; wealso discuss the number-diameter relation. A Venn diagram summarizesthat most SNRs emit radiation in all three of the radio, optical andX-ray domains. HII region diameter comparisons between radio and opticalsources show them to be a very diverse group that defies any simplerelationship, preventing any meaningful calculation of flux density orspectral index. To better understand environments containing SNRs, wehave scaled Hα images of four SMC regions and subtracted theirflux from the ATCA 2.37-GHz radio image. These 2.37 GHz-Hαsubtraction (or difference) images reveal some new sources withpredominantly non-thermal emission, exposing SNRs confused with HIIregions.
| Limits from the Hubble Space Telescope on a Point Source in SN 1987A
We observed supernova 1987A (SN 1987A) with the Space Telescope ImagingSpectrograph (STIS) on the Hubble Space Telescope (HST) in 1999September and again with the Advanced Camera for Surveys (ACS) on theHST in 2003 November. Our spectral observations cover ultraviolet (UV)and optical wavelengths from 1140 to 10266 Å, and our imagingobservations cover UV and optical wavelengths from 2900 to 9650 Å.No point source is observed in the remnant. We obtain a limiting flux ofFopt<=1.6×10-14 ergs s-1cm-2 in the wavelength range 2900-9650 Å for anycontinuum emitter at the center of the supernova remnant (SNR). Thiscorresponds to an intrinsic luminosity ofLopt<=5×1033 ergs s-1. It islikely that the SNR contains opaque dust that absorbs UV and opticalemission, resulting in an attenuation of ~35% due to dust absorption inthe SNR. Correcting for this level of dust absorption would increase ourupper limit on the luminosity of a continuum source by a factor of 1.54.Taking into account dust absorption in the remnant, we find a limit ofLopt<=8×1033 ergs s-1. Wecompare this upper bound with empirical evidence from point sources inother supernova remnants and with theoretical models for possiblecompact sources. We show that any survivor of a possible binary systemmust be no more luminous than an F6 main-sequence star. Bright youngpulsars such as Kes 75 or the Crab pulsar are excluded by optical andX-ray limits on SN 1987A. Other nonplerionic X-ray point sources haveluminosities similar to the limits on a point source in SN 1987A; RCW103 and Cas A are slightly brighter than the limits on SN 1987A, whilePup A is slightly fainter. Of the young pulsars known to be associatedwith SNRs, those with ages <=5000 yr are all too bright in X-rays tobe compatible with the limits on SN 1987A. Examining theoretical modelsfor accretion onto a compact object, we find that spherical accretiononto a neutron star is firmly ruled out and that spherical accretiononto a black hole is possible only if there is a larger amount of dustabsorption in the remnant than predicted. In the case of thin-diskaccretion, our flux limit requires a small disk, no larger than1010 cm, with an accretion rate no more than 0.3 times theEddington accretion rate. Possible ways to hide a surviving compactobject include the removal of all surrounding material at early times bya photon-driven wind, a small accretion disk, or very high levels ofdust absorption in the remnant. It will not be easy to improvesubstantially on our optical-UV limit for a point source in SN 1987A,although we can hope that a better understanding of the thermal infraredemission will provide a more complete picture of the possible energysources at the center of SN 1987A.
| X-Ray Spectroscopy of PSR B1951+32 and Its Pulsar Wind Nebula
We present spatially resolved X-ray spectroscopy of PSR B1951+32 and itspulsar wind nebula (PWN) in supernova remnant (SNR) CTB 80 from aChandra observation. The Chandra X-ray map clearly reveals variouscomponents of a ram-pressure-confined PWN embedded in the SNR ejecta: apoint source representing the pulsar, X-ray emission from the bow shock,a luminous X-ray tail, a 30" diameter plateau whose northwestern part isabsent, and the more diffuse outer X-ray emission. The plateau isclosely surrounded by the radio, [O III], [S II], and [N II] shells, andthe outer diffuse emission is mostly within the Hα shells. Whilethe spectra of all the features are well fitted with power-law models, apower-law plus blackbody model can fit the spectrum of the pulsarsignificantly better than using a power-law model alone. Generally, thespectra of these components obey the trend of steepening from the insideto the outside. However, the edge of the plateau probably has a harderspectrum than the central region of the plateau. The cause of theapparent hard spectrum of the plateau edge is unclear, but we speculatethat it might be due to a shock between the PWN and the SNR ejecta. Thepossible blackbody radiation component from the pulsar has a temperatureof 0.13+/-0.02 keV and an equivalent emitting radius of2.2+1.4-0.8 (d/2 kpc) km, and is thus probablyfrom the hot spots on the pulsar. We also show in this paper that theblackbody temperature of the entire surface of PSR B1951+32 is muchlower than those predicted by the standard neutron star cooling models.
| Supernova Remnants in the Magellanic Clouds. V. The Complex Interior Structure of the N206 Supernova Remnant
The N206 supernova remnant (SNR) in the Large Magellanic Cloud has longbeen considered a prototypical ``mixed morphology'' SNR. Recentobservations, however, have added a new twist to this familiar plot: anelongated, radially oriented radio feature seen in projection againstthe SNR face. Utilizing the high resolution and sensitivity availablewith the Hubble Space Telescope, Chandra, and XMM-Newton, we haveobtained optical emission line images and spatially resolved X-rayspectral maps for this intriguing SNR. Our findings present the SNRitself as a remnant in the middle to late stages of its evolution. X-rayemission associated with the radio linear feature strongly suggests itto be a pulsar wind nebula (PWN). A small X-ray knot is discovered atthe outer tip of this feature. The feature's elongated morphology andthe surrounding wedge-shaped X-ray enhancement strongly suggest a bowshock PWN structure.
| Presupernova Evolution of Differentially Rotating Massive Stars Including Magnetic Fields
As a massive star evolves through multiple stages of nuclear burning onits way to becoming a supernova, a complex, differentially rotatingstructure is set up. Angular momentum is transported by a variety ofclassic instabilities and also by magnetic torques from fields generatedby the differential rotation. We present the first stellar evolutioncalculations to follow the evolution of rotating massive starsincluding, at least approximately, all these effects, magnetic andnonmagnetic, from the zero-age main sequence until the onset ofiron-core collapse. The evolution and action of the magnetic fields isas described by Spruit in 2002, and a range of uncertain parameters isexplored. In general, we find that magnetic torques decrease the finalrotation rate of the collapsing iron core by about a factor of 30-50when compared with the nonmagnetic counterparts. Angular momentum inthat part of the presupernova star destined to become a neutron star isan increasing function of main-sequence mass. That is, pulsars derivedfrom more massive stars rotate faster and rotation plays a moreimportant role in the star's explosion. The final angular momentum ofthe core has been determined-to within a factor of 2-by the time thestar ignites carbon burning. For the lighter stars studied, around 15Msolar, we predict pulsar periods at birth near 15 ms, thougha factor of 2 range is easily tolerated by the uncertainties. Severalmechanisms for additional braking in a young neutron star, especially byfallback, are explored.
| An XMM-Newton survey of M 31
In an analysis of XMM-Newton archival observations of the bright LocalGroup spiral galaxy M 31 we study the population ofX-ray sources (X-ray binaries, supernova remnants) down to a 0.2-4.5 keVluminosity of 4.4~×~ 1034 . EPIC hardness ratios andoptical and radio information are used to distinguish between differentsource classes. The survey detects 856 sources in an area of 1.24 squaredegrees. We correlate our sources with earlier M 31X-ray catalogues and use information from optical, infra-red and radiowavelengths. As sources within M 31 we detect 21supernova remnants (SNR) and 23 SNR candidates, 18 supersoft source(SSS) candidates, 7 X-ray binaries (XRBs) and 9 XRB candidates, as wellas 27 globular cluster sources (GlC) and 10 GlC candidates, which mostlikely are low mass XRBs within the GlC. Comparison to earlier X-raysurveys reveal transients not detected with XMM-Newton, which add to thenumber of M 31 XRBs. There are 567 sources classifiedas hard, which may either be XRBs or Crab-like SNRs in M31 or background AGN. The number of 44 SNRs and candidatesmore than doubles the X-ray-detected SNRs. 22 sources are new SNRcandidates in M 31 based on X-ray selection criteria.Another SNR candidate may be the first plerion detected outside theGalaxy and the Magellanic Clouds. On the other hand, six sources areforeground stars and 90 foreground star candidates, one is a BL Lac-typeactive galactic nucleus (AGN) and 36 are AGN candidates, one sourcecoincides with the Local Group galaxy M 32, one with a background galaxycluster (GCl) and another is a GCl candidate, all sources not connectedto M 31.
| Young Core-Collapse Supernova Remnants and Their Supernovae
Massive star supernovae can be divided into four categories, dependingon the amount of mass loss from the progenitor star and the star'sradius: red supergiant stars with most of the H envelope intact (SNIIP), stars with some H but most lost (IIL and IIb), stars with all Hlost (Ib and Ic), and blue supergiant stars with a massive H envelope(SN 1987A-like). Various aspects of the immediate aftermath of thesupernova are expected to develop in different ways, depending on thesupernova category: mixing in the supernova, fallback on the centralcompact object, expansion of any pulsar wind nebula, interaction withcircumstellar matter, and photoionization by shock breakout radiation.The observed properties of young supernova remnants allow many of themto be placed in one of the supernova categories; all the categories arerepresented except for the SN 1987A-like type. Of the remnants withcentral pulsars, the pulsar properties do not appear to be related tothe supernova category. There is no evidence that the supernovacategories form a mass sequence, as would be expected in a single-starscenario for the evolution. Models for young pulsar wind nebulaeexpanding into supernova ejecta indicate initial pulsar periods of10-100 ms and approximate equipartition between particle and magneticenergies. Ages are obtained for pulsar nebulae, including an age of2400+/-500 yr for 3C 58, which is not consistent with an origin in SN1181. There is no evidence that mass fallback plays a role in neutronstar properties.
| Discovery of an OH (1720 MHz) Maser in the Large Magellanic Cloud
We report the first study of OH (1720 MHz) masers in the LMC in order toprobe regions where supernova remnants interact with adjacent molecularclouds. Using the Australia Telescope Compact Array, we observed foursources in the LMC and detected a single OH (1720 MHz) maser with a fluxdensity of 377 mJy toward 30 Dor. No main line emission at 1665 or 1667MHz was detected. The observed OH (1720 MHz) maser emission from 30 Dorshows characteristics similar to the well-known collisionally pumpedsupernova remnant masers found in the Galaxy, though 30 Dor is known asa star-forming region. It is possible that shocks driven by a nearbysupernova remnant or by strong stellar winds from young stars areresponsible for production of OH (1720 MHz) maser in 30 Dor. Futurestudies are required to distinguish between collisional and radiativepumping mechanisms for the 30 Dor OH (1720 MHz) maser.
| A 4.8 and 8.6 GHz Survey of the Large Magellanic Cloud. I. The Images
Detailed 4.8 and 8.6 GHz radio images of the entire Large MagellanicCloud with half-power beamwidths of 33" at 4.8 GHz and 20" at 8.6 GHzhave been obtained using the Australia Telescope Compact Array. A totalof 7085 mosaic positions were used to cover an area of 6° on a side.Full polarimetric observations were made. These images have sufficientspatial resolution (~8 and 5 pc, respectively) and sensitivity (3σ of 1 mJy beam-1) to identify most of the individualsupernova remnants and H II regions and also, in combination withavailable data from the Parkes 64 m telescope, the structure of thesmooth emission in that galaxy. In addition, limited data using thesixth antenna at 4.5-6 km baselines are available to distinguish brightpoint sources (<3" and 2", respectively) and to help estimate sizesof individual sources smaller than the resolution of the full survey.The resulting database will be valuable for statistical studies andcomparisons with X-ray, optical, and infrared surveys of the LMC withsimilar resolution.
| Search for the elusive optical counterpart of PSR J0537-6910 with the HST Advanced Camera for Surveys
We present the results of deep, high-resolution, multi-band opticalobservations of the field of the young (˜ 5000 yr) 16 ms X-raypulsar PSR J0537-6910 performed with the Advanced Camera for Surveys(ACS) aboard the Hubble Space Telescope (HST). Although a few newpotential counterparts have been detected within or close to the revisedChandra X-ray error circle (≃ 1 arcsec) of the pulsar, only two ofthem (with magnitudes m_814W ≈ 23.9 and m_814W ≈ 24.2) showindications of a peculiar spectrum which could be related to opticalemission from the pulsar. This might be true also for a third, fainter,candidate detected only in one filter (with magnitude m_814W ≈ 26.7).If either of the two brighter candidates is indeed the actualcounterpart, the optical output of PSR J0537-6910 would make it similarto young Crab-like pulsars. If not, it would mean that PSR J0537-6910 issignificantly underluminous with respect to all pulsars detected in theoptical.
| Candidates for pulsar/supernova remnant associations
We list a sample of 50 pairs of pulsar (PSR) and supernova remnant (SNR)associations which have been suggested from over 230 SNRs and 1300 PSRs.At least 20 pairs of them are real because the pulsar wind nebulaearound the PSRs have been detected. We analyze factors which may resultin the deficit of associations. Considering recent observationalprogress, we simply discuss reality of the 50 pairs catalogedassociations and the possibility to detect more PSR/SNR associations. Wegive a detailed discussion aimed at three pairs of likely PSR/SNRassociations PSR J1811-1925/SNR G11.2-0.3, PSR J1932+2020/SNR G55.0+0.3and PSR J1706/SNR G343.1-2.3 in our sample.
| XMM-Newton survey of the Local Group galaxy M 33
In an XMM-Newton raster observation of the bright Local Group spiralgalaxy M 33 we study the population of X-ray sources(X-ray binaries, supernova remnants) down to a 0.2-4.5 keV luminosity of1035 erg s-1 - more than a factor of 10 deeperthan earlier ROSAT observations. EPIC hardness ratios and optical andradio information are used to distinguish between different sourceclasses. The survey detects 408 sources in an area of 0.80 squaredegree. We correlate these newly detected sources with earlierM 33 X-ray catalogues and information from optical,infra-red and radio wavelengths. As M 33 sources wedetect 21 supernova remnants (SNR) and 23 SNR candidates, 5 super-softsources and 2 X-ray binaries (XRBs). There are 267 sources classified ashard, which may either be XRBs or Crab-like SNRs in M33 or background AGN. The 44 confirmed and candidate SNRs morethan double the number of X-ray detected SNRs in M33. 16 of these are proposed as SNR candidates from the X-raydata for the first time. On the other hand, there are several sourcesnot connected to M 33: five foreground stars, 30foreground star candidates, 12 active galactic nucleus candidates, onebackground galaxy and one background galaxy candidate. Extrapolatingfrom deep field observations we would expect 175 to 210 backgroundsources in this field. This indicates that about half of the sourcesdetected are within M 33.XMM-Newton is an ESA Science Mission with instruments and contributionsdirectly funded by ESA Member States and the USA (NASA).Full Table \ref{master} is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/426/11
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