Supernova Remnant Candidates Research Articles

Filamentary Shell Structures from the AAO/UKST Hα Survey

AbstractHere we present the first results of a search for new optical supernova remnant candidates and other filamentary objects on films produced by the Anglo-Australian Observatory/UK Schmidt Telescope Há Survey. Sixty-one fields, or 26% of the Galactic plane survey fields, have been visually examined. This has resulted in the detection of four newlarge diameter filamentary structures, and the discovery of extensive new optical emission in two previously known optical supernova remnant candidates.

G106.3+2.7: A Supernova Remnant in a Late Stage of Evolution

We report on observations of the candidate supernova remnant (SNR) G106.3+2.7 with the Dominion Radio Astrophysical Observatory's Synthesis Telescope in the continuum at both 408 and 1420 MHz and in the 21 cm line of neutral hydrogen. The general morphology of the object and its spectral index (α ≈ 0.57 ± 0.04, where Sν ∝ ν-α) confirm it as an SNR. The object consists of two distinct components, the head and the tail. The tail component is of lower surface brightness and has a marginally steeper spectral index than the head component. A deficiency of neutral hydrogen at an LSR velocity of about -105 km s-1 is most likely due to the effect of the SNR, suggesting that the SNR is expanding at a velocity of about 15 km s -1, it is at a kinematic distance of 12 kpc, and its largest angular extent is of the order of 200 pc. These parameters are shown to be consistent with a dynamical model in which the SNR is in a very late stage of its isothermal evolution, where the pressure inside the SNR is approaching the pressure of the ambient interstellar medium. We also describe the H II region Sh 141, which is about 20' north of G106.3+2.7.

Confirmation of three candidate Galactic supernova remnants from GMRT observations

We report 327-MHz observations of three large (>15 arcmin) candidate supernova remnants (SNRs) at high Galactic latitudes using the Giant Metrewave Radio Telescope (GMRT). These objects were proposed by Duncan et al. as candidate SNRs in the Parkes 2.4-GHz survey. We detect extended 327-MHz emission in all three fields. G356.2+4.5 has a well-resolved shell of emission. This field also contains a 32-mJy pulsar situated at a distance of 1.4 kpc as derived from the dispersion measure of the pulsar. The field of G358.0+3.8 is highly confused and, although we confirm the presence of a partial ring, it has a lower signal-to-noise ratio detection. G004.8+6.2 (formerly G4.5+6.2) is located approximately 40 arcmin away from Kepler's SNR. Apart from GMRT observations of this field, this SNR lies in the field of view of a Very Large Array (VLA) D-array observation at 325 MHz and we detect a 17×18 arcmin SNR, possibly of shell morphology. All these fields show clear, well-resolved shells in the National Radio Astronomy Observatory/VLA Sky Survey (NVSS) maps, which are also presented.

ROSAT HRI catalogue of X-ray sources in the LMC region

All 543 pointed observations of the ROSAT High Resolution Imager (HRI) with exposure times higher than 50 sec in a field of 10 deg x 10 deg covering the Large Magellanic Cloud (LMC) were analyzed. A catalogue was produced containing 397 X-ray sources with their properties measured by the HRI. The list was cross-correlated with the ROSAT Position Sensitive Propotional Counter (PSPC) source catalogue presented by Haberl & Pietsch (1999), the SIMBAD data base, and the TYCHO catalogue. 138 HRI sources are contained in the PSPC catalogue. The spatial resolution of the HRI was higher than that of the PSPC and the source position could be determined with errors mostly smaller than 15 arcsec which are dominated by systematic attitude errors. 94 HRI sources were identified with known objects based on their positional coincidence and X-ray properties. The catalogue contains 39 foreground stars, 24 supernova remnants (SNRs), five supersoft sources (SSSs), nine X-ray binaries (XBs), and nine AGN well known from literature. Another eight sources were identified with known candidates for these source classes. Additional 21 HRI sources are suggested in the present work as candidates for SNR, X-ray binary in the LMC, or background AGN because of their extent, hardness ratios, X-ray to optical flux ratio, or flux variability.

Discrete Radio Sources in the Spiral Galaxy NGC 6946

We analyze the 2''-resolution, high-sensitivity radio continuum observations of the nearby spiral galaxy NGC 6946 at 6 and 20 cm by Lacey, Duric, & Goss to derive a luminosity function at each wavelength for 43 H II region candidates and for 15 supernova remnant (SNR) candidates. The H II region radio luminosity functions are best fitted by power laws consistent with the shape of the optical luminosity function of Bonnarel, Boulsteix, & Marcelin. The SNR luminosity functions are fitted by considerably steeper power laws than the H II region luminosity functions. Extinctions to the H II regions are estimated from a comparison of radio flux densities with new optical fluxes derived from the Hα observations of Van Dyk. A dependence of extinction on galactocentric radius is investigated, but none is found.

A ROSAT PSPC catalogue of X-ray sources in the SMC region

We present a catalogue of 517 discrete X-ray sources in a field covering the Small Magellanic Cloud (SMC). The catalogue was derived from the pointed ROSAT PSPC observations performed between October 1991 and May 1994 and is complementary to the Large Magellanic Cloud (LMC) catalogue published by Haberl & Pietsch (1999). We followed the same identification scheme and used, among other information, X-ray hardness ratios and spatial extent to classify unknown sources as candidates for active galactic nuclei (AGN), foreground stars, supernova remnants (SNRs), supersoft sources (SSSs) and X-ray binaries. For 158 sources a likely source type is given, from which 46 sources are suggested as background AGN (including candidates resulting from a comparison of X-ray and radio images). Nearly all of the X-ray binaries known in the SMC were detected in ROSAT PSPC observations; most of them with luminosities below 1036 erg s-1 suggesting that the fraction of high luminosity X-ray binary systems in the Magellanic Clouds (MCs) is not significantly larger than in our galaxy. Seventeen X-ray sources are associated with SNRs found in earlier work and we suggest here two additional extended sources as SNR candidates. Three very soft sources are newly classified as SSSs from which one is identified with the symbiotic star LIN 358 in the SMC.

Supernova Remnants in the Fossil Starburst in M82

We report the discovery of 10 compact Hα-bright sources in the poststarburst region northeast of the center of M82, called M82 B. These objects have Hα luminosities and sizes consistent with Type II supernova remnants (SNRs). They fall on the same Hα surface brightness–diameter relation (Σ-D) defined by SNRs in other nearby star-forming galaxies, with the M82 candidates lying preferentially at the small-diameter end. These are the first candidates for optically visible SNRs in M82 outside the heavily obscured central starburst within about 250 pc from the galactic center. If these sources are SNRs, they set an upper limit to the end of the starburst in region B2, about 500 pc from the galaxy's core, of about 50 Myr. Region B1, about 1000 pc from the core, lacks good SNR candidates and is evidently somewhat older. This suggests that star formation in the galaxy has propagated inward toward the present-day intense starburst core.

Spectrophotometry of H [CSC]ii[/CSC] Regions, Diffuse Ionized Gas, and Supernova Remnants in M31: The Transition from Photoionization to Shock Ionization

We present results of Kitt Peak National Observatory (KPNO) 4 m optical spectroscopy of discrete emission-line nebulae and regions of diffuse ionized gas (DIG) in M31. Long-slit spectra of 16 positions in the northeast half of M31 were obtained over a 5–15 kpc range in radial distance from the center of the galaxy with special attention to the annulus of high star formation between 8 and 12 kpc. Slit positions were chosen such that several emission-line nebulae and large sections of diffuse gas could be studied without compromising the parallactic angle. The spectra have been used to confirm 16 supernova remnant (SNR) candidates from the Braun & Walterbos (1993) catalog. The slits also covered 46 H II regions that show significant differences among the various morphological types (center-brightened, diffuse, rings). Radial gradients in emission-line ratios such as [O III]/Hβ and [O II]/[O III] are observed most prominently in the center-brightened H II regions. These line-ratio trends are either much weaker or completely absent in the diffuse and ring nebulae. The line-ratio gradients seen previously in M31 SNRs by Blair, Kirshner, & Chevalier in 1981 and 1982 are reproduced well by our new data. The spectra of center-brightened H II regions and SNRs confirm previous determinations of the radial abundance gradient in M31. We use diagnostic diagrams that separate photoionized gas from shock-ionized gas to compare the spectral properties of H II regions, SNRs, and DIG. This analysis strengthens the earlier claims made by Greenawalt, Walterbos, & Braun in 1997 that the DIG in the disk of M31 is photoionized by a dilute radiation field.

A ROSAT PSPC catalogue of X-ray sources in the LMC region

We analyzed more than 200 ROSAT PSPC observations in a 10 by 10 degree field centered on the Large Magellanic Cloud (LMC) and performed between 1990 and 1994 to derive a catalogue of X-ray sources. The list contains 758 sources with their X-ray properties. From cross-correlations of the PSPC catalogue with the SIMBAD data base and literature searches we give likely identifications for 144 X-ray sources based on positional coincidence, but taking into account X-ray properties like hardness ratios and source extent. 46 known sources are associated with supernova remnants (SNRs) and candidates in the LMC, most of them already detected by previous X-ray missions. Including the new candidates from Haberl & Pietsch (1999) based on variability studies of the sources in our PSPC catalogue, the number of X-ray binaries in the LMC increased to 17 and that of the supersoft sources (SSSs) to 9. The remaining ~50% of the identified sources comprise mainly foreground stars (up to 57) and background extragalactic objects (up to 15). The often distinguished X-ray properties of the different source types were used for a first classification of new, unknown X-ray sources. Eight new PSPC sources are classified as SNRs from their hardness ratios and one promising new SNR candidate with extended X-ray emission is found further north than all known SNRs. Three soft X-ray sources have hardness ratios compatible to those of the known SSSs. A selection on hardness ratios and X-ray to optical flux ratio further suggests 27 foreground stars and 3 AGN.

Kinematics of the Ionized Gas in the Irregular Galaxies IC 10 and NGC 4449

We have searched for supernova remnants (SNRs) in the irregular galaxies IC 10 and NGC 4449 by means of Fabry-Perot interferometry in Hα and [SII] lines, complemented by radio continuum observations. We find several new SNR candidates in each galaxy in addition to the known ones. Although these galaxies have several common properties, the distributions of SNRs in each galaxy is different. This shows that the star formation mechanisms in these galaxies act in different ways, in favor of the formation of a large nebular HII complex in IC 10, similar to 30 Dor in the Large Magellanic Cloud. We are studying the conditions over which these different star formation histories take place.

Physical Structure of Small Wolf-Rayet Ring Nebulae

We have selected the seven most well-defined Wolf-Rayet (WR) ring nebulae in the Large Magellanic Cloud (LMC), Br 2, 10, 13, 40a, 48, 52, and 100, to study their physical nature and evolutionary stages. New CCD imaging and echelle observations have been obtained for five of these nebulae; previous photographic imaging and echelle observations are available for the remaining two nebulae. Using the nebular dynamics and abundances, we find that the Br 13 nebula is a circumstellar bubble, and that the Br 2 nebula may represent a circumstellar bubble merging with a fossil main-sequence interstellar bubble. The nebulae around Br 10, 52, and 100 all show influence of the ambient interstellar medium. Their regular expansion patterns suggest that they still contain significant amounts of circumstellar material. Their nebular abundances would be extremely interesting, as their central stars are WC5 and WN3–WN4 stars whose nebular abundances have not been derived previously. Intriguing and tantalizing implications are obtained from comparisons of the LMC WR ring nebulae with ring nebulae around Galactic WR stars, Galactic LBVs, LMC LBVs, and LMC BSGs; however, these implications may be limited by small-number statistics. A SNR candidate close to Br 2 is diagnosed by its large expansion velocity and nonthermal radio emission. There is no indication that Br 2's ring nebula interacts dynamically with this SNR candidate.

X-rays from supernova remnants

ROSAT has detected X-ray emission from more than 100 galactic supernova remnants with more than 100 SNR candidates, out of which a dozen sources have been confirmed by radio and optical measurements. Highlights of more than 7 years of ROSAT observations of SNRs include the discovery of progenitor fragments in the Vela SNR, the discovery of colliding SNR shells, high resolution images of shocked clouds in the Cygnus Loop, the studies of spatial and thermal structure, which favour an ambient medium of evaporating clouds. ASCA has added to this by high quality energy spectra including Doppler velocity maps of Cas-A in various emission lines, ionization non-equilibrium in the Cygnus Loop, the detection of non-thermal X-rays from sections of the SN 1006 shell and RX J1713.7-3946, an SNR discovered by ROSAT. These observations have broadened our understanding of SN explosions, the propagation of shock waves and the properties of the ambient medium as well as the acceleration of cosmic rays in SNR shells.

A New Optical Sample of Supernova Remnants in M33

We present a new and larger sample of supernova remnants in the nearby spiral galaxy M33. The sample is based upon CCD interference filter observations obtained with the Kitt Peak 4 m telescope and spectroscopic observations obtained with the Multiple Mirror Telescope. Using optical emission-line ratios, supplemented by a radio continuum map of M33 (Duric et al.; Gordon et al.), we have identified 98 supernova remnant (SNR) candidates, of which 53 were previously unknown. We have obtained spectra of 27 SNR candidates, bringing the total number of M33 SNRs for which spectra are available to 72. All the spectra show the characteristic signature of shock-heated gas, which leads us to believe that the rest of the candidates are also supernova remnants. The large sample provides a useful database to investigate the global properties of SNRs. In this paper, we present a new cumulative number-diameter [N(<D)] relation for SNRs. We find that the free expansion model is inconsistent with the observed N(<D) relation and that the Sedov-Taylor (ST) expansion model is a much better fit. The ST expansion model is used to derive a SN rate in M33 of one SN every 360 yr. The spectroscopic sample of SNRs is the largest of its kind in any galaxy. Our investigations show no relation between [N II]:Hα and [S II] λ6717:λ6731. We do, however, find weak trends between the SNR diameter and [N II]:Hα and [S II]:Hα. Further, we find a strong correlation between [N II]:Hα and the galactocentric distance (GCD) indicative of an abundance gradient. There is a large dispersion in the [N II]:Hα ratio at a given GCD that cannot be explained by evolutionary effects and that may be due to a larger abundance dispersion in the inner part of the galaxy.

A radio continuum study of the Magellanic Clouds

By comparing Parkes telescope radio surveys with the X-ray ROSAT All-Sky Survey (RASS) we have found 71 discrete sources of both radio and X-ray emission in the Large Magellanic Cloud (LMC). These 71 sources are mainly supernova remnants (SNRs) and SNR candi- dates (36), and background sources (27). For six of the sources we have no proposed identication and the other two are H ii regions. A source-intensity comparison of the radio and X-ray sources shows very little correlation, but we note that the strongest SNRs at both radio and X-ray frequencies are young SNRs from Population I. Six new LMC SNR candidates are proposed. From the radio flux density of the SNRs we have estimated the SNR birth rate to be one every 100 (20) yr and the star-formation rate (SFR) to be 0: 7( 0 :2) M yr 1 . A similar comparison was undertaken for the Small Magellanic Cloud (SMC), but instead of the RASS we used a roster of pointed observations made with the ROSAT Position Sensitive Proportional Counter (PSPC). This comparison resulted in 27 sources in common be- tween the Parkes radio and ROSAT PSPC surveys. Two

G359.87+0.18: A Young Supernova Remnant Candidate near the Galactic Center?

Subarcsecond radio continuum observations of the Galactic center region at λ = 6 and 2 cm reveal a 05 diameter source with a shell-like morphology. This source is linearly polarized at a level of 16% at 6 cm and has a steep nonthermal spectrum with a spectral index of 1.6 between 6 and 2 cm. The distance to this source is not known, but the large rotation measure value of 3000 rad m−2 suggests that G359.87+0.18 is likely to be located in the inner Galaxy or at an extragalactic distance. We discuss possible interpretations of this object as a recent supernova, a very young supernova remnant, a nova remnant, or an extragalactic source. All possibilities are highly problematic.

A radio continuum study of the Magellanic Clouds

By comparing Parkes telescope radio surveys with the X-ray ROSAT All-Sky Survey (RASS) we have found 71 discrete sources of both radio and X-ray emission in the Large Magellanic Cloud (LMC). These 71 sources are mainly supernova remnants (SNRs) and SNR candi- dates (36), and background sources (27). For six of the sources we have no proposed identication and the other two are H ii regions. A source-intensity comparison of the radio and X-ray sources shows very little correlation, but we note that the strongest SNRs at both radio and X-ray frequencies are young SNRs from Population I. Six new LMC SNR candidates are proposed. From the radio flux density of the SNRs we have estimated the SNR birth rate to be one every 100 (20) yr and the star-formation rate (SFR) to be 0: 7( 0 :2) M yr 1 . A similar comparison was undertaken for the Small Magellanic Cloud (SMC), but instead of the RASS we used a roster of pointed observations made with the ROSAT Position Sensitive Proportional Counter (PSPC). This comparison resulted in 27 sources in common be- tween the Parkes radio and ROSAT PSPC surveys. Two

X-Rays from Supernova Remnants

ROSAT has detected X-ray emission from more than 100 galactic supernova remnantswith more than 100 SNR candidates, out of which a dozen sources havebeen confirmed by radio and optical measurements. Highlights of more than 7years of ROSAT observations of SNRs include the discovery of progenitor fragmentsin the Vela SNR, the discovery of colliding SNR shells, high resolutionimages of shocked clouds in the Cygnus Loop, the studies of spatial and thermalstructure, which favour an ambient medium of evaporating clouds. ASCA hasadded to this by high quality energy spectra including Doppler velocity maps ofCas-A in various emission lines, ionization non-equilibrium in the Cygnus Loop,the detection of non-thermal x-rays from sections of the SN 1006 shell and RXJ1713.7-3946, an SNR discovered by ROSAT. These observations have broadenedour understanding of SN explosions, the propagation of shock waves andthe properties of the ambient medium as well as the acceleration of cosmic raysin SNR shells.

The Interstellar Environment of Filled-Center Supernova = +1.1

A multi-wavelength investigation of the candidate supernova remnant G63.7+1.1 and its surrounding interstellar medium is presented. On the basis of radio continuum data we conclude that the object is a filled-center supernova remnant, perhaps in the course of becoming a composite remnant. The morphology of the remnant, along with HI, 12CO and high resolution IRAS data, suggest that G63.7+1.1 is interacting directly with the ISM, and does not lie in a low density region of the ISM. This in turn strongly suggests that the detected nebula is not surrounded by an invisible halo of supernova ejecta. The association between the SNR and HI and CO features near the tangent point implies a kinematic distance for G63.7+1.1 of 3.8 +/- 1.5 kpc.

Supernova remnant candidates from the Parkes 2.4-GHz survey

25 supernova remnant (SNR) candidates are presented from the recent Parkes 2.4-GHz survey of the southern Galactic plane. Their angular diameters range from approximately 16 arcmin to 18°. Polarized emission is also detected from some of these candidates. The inclusion of these objects in current SNR catalogues will raise the number of large SNRs (diameters ≳20 arcmin) in the longitude range 270° ≤/≤ 360° to a value comparable to that of the northern Galactic plane (0° ≤/≤ 90°). An excess of SNR candidates around longitudes of 310°-330° is noted, suggesting that these objects, which are mainly of large angular diameter (greater than 2°), may form part of a nearby 'cluster' of remnants. Comparisons with the 4.85-GHz PMN survey are also given. Pulsar associations are considered, although only one possible candidate has been found.

UNCOVERING A SUPERNOVA REMNANT HIDDEN NEAR LMCX-1

ABSTRACT In a kinematic study of LMC \\hii\\ regions, we serendipitously found a fast-expanding (velocity offsets up to ∼200 \\kms) shell in the northern lobe of the luminous \\hii\\ region N159. The expansion pattern and the \\sii/\\ha\\ ratio of the shell are consistent with those seen in supernova remnants (SNRs). Confirmation of this SNR candidate in radio and X-ray wavelengths has been hampered by thermal emission of the luminous \\hii\\ region N159 itself and scattered X-rays from the nearby luminous source LMC X-1. Recent {ROSAT} observations finally resolved N159 from LMC X-1, thus confirming the identification of this as a newly identified SNR, which we designate as SNR 0540-697.