THE CHANDRA ACIS SURVEY OF M33: X-RAY, OPTICAL, AND RADIO PROPERTIES OF THE SUPERNOVA REMNANTS

Abstract

M33 contains a large number of emission nebulae identified as supernova remnants (SNRs) based on the high [S ii]:Hα ratios characteristic of shocked gas. Using Chandra data from the ChASeM33 survey with a 0.35-2 keV sensitivity of ∼2 × 10 34 erg s −1 , we have detected 82 of 137 SNR candidates, yielding confirmation of (or at least strongly support for) their SNR identifications. This provides the largest sample of remnants detected at optical and X-ray wavelengths in any galaxy, including the Milky Way. A spectral analysis of the seven X-ray brightest SNRs reveals that two, G98-31 and G98-35, have spectra that appear to indicate enrichment by ejecta from core-collapse supernova explosions. In general, the X-ray-detected SNRs have soft X-ray spectra compared to the vast majority of sources detected along the line of sight to M33. It is unlikely that there are any other undiscovered thermally dominated X-ray SNRs with luminosities in excess of ∼4 × 10 35 erg s −1 in the portions of M33 covered by the ChASeM33 survey. We have used a combination of new and archival optical and radio observations to attempt to better understand why some objects are detected as X-ray sources and others are not. We have also developed a morphological classification scheme for the optically identified SNRs and discussed the efficacy of this scheme as a predictor of X-ray detectability. Finally, we have compared the SNRs found in M33 to those that have been observed in the Galaxy and the Magellanic Clouds. There are no close analogs of Cas A, Kepler's SNR, Tycho's SNR, or the Crab Nebula in the regions of M33 surveyed, but we have found an X-ray source with a power-law spectrum coincident with a small-diameter radio source that may be the first pulsar-wind nebula recognized in M33.

The population of X-ray supernova remnants in the Large Magellanic Cloud

“…Towards the faint end, the M33 XLF flattens and diverges from the power law below 10 35 erg s −1 , indicating incompleteness. Long et al (2010) concluded that no SNR brighter than 4 × 10 35 erg s −1 was missed across the surveyed field. It is likely that they were over-conservative and that missing SNRs are only those which have luminosity below 10 35 erg s −1 .…”

“…They recovered most of the SNRs of Long et al (2010), except in the central region where source confusion is an issue for XMM-Newton, and detected or confirmed eight new SNRs, including three sources far in the outskirts of M33. We converted the unabsorbed 0.2−4.5 keV EPIC flux of the new inclusions of Williams et al (2015), obtained assuming a power-law spectrum, to the 0.3−8 keV luminosity for the same spectrum as the SNRs of Long et al (2010). The final, concatenated list of M33 SNRs thus comprises 90 objects.…”

“…26 objects were confirmed SNRs in Sasaki et al (2012), and another 20 were candidate SNRs. Long et al (2010) present a large catalogue of 82 confirmed SNRs in M33, based on the Chandra ACIS survey of M33 (ChASeM33, Tüllmann et al 2011). They give L X in the 0.35 keV−2 keV band, converted from ACIS count rates, assuming a thermal plasma at 0.5 solar, kT = 0.6 keV, and total N H = 10 21 cm −2 (i.e.…”

“…the spectrum found for the brightest source). We obtained the corresponding 0.3 keV−8 keV luminosity by scaling up the values of Long et al (2010) by 4%.…”

The deepXMM-NewtonSurvey of M 31

“…Similar criteria were used to select supernova remnant candidates in XMM-Newton observations of M 33 (Pietsch et al 2004;Misanovic et al 2006). Ghavamian et al (2005) and Long et al (2010) confirmed the supernova remnant nature of many of these candidates based on optical and radio follow-up observations. They also used a hardness ratio criterion to select supernova remnant candidates from Chandra data.…”