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Register a new userThe Chandra ACIS Survey of M33 (ChASeM33): Transient X-ray Sources Discovered in M33
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Added by
Benjamin F. Williams
Publication date
2008
fields
Physics
and research's language is
English
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The Chandra ACIS Survey of M33 (ChASeM33) has acquired 7 fields of ACIS data covering M33 with 200 ks of exposure in each field. A catalog from the first 10 months of data, along with archival Chandra observations dating back to the year 2000, is currently available. We have searched these data for transient sources that are measured to have a 0.35-8.0 keV unabsorbed luminosity of at least 4$times10^{35}$ erg s$^{-1}$ in one epoch and are not detected in another epoch. This set of the survey data has yielded seven such sources, including one previously-known supersoft source. We analyzed XMM-Newton data from the archive distributed over the years 2000 to 2003 to search for recurrent outbursts and to get a spectrum for the supersoft transient. We find only one recurrent transient in our sample. The X-ray spectra, light curves, and optical counterpart candidates of two of the other sources suggest that they are high-mass X-ray binaries. Archival Spitzer photometry and high X-ray absorption suggest that one of the sources is a highly variable background active galactic nucleus. The other three sources are more difficult to classify. The bright transient population of M33 appears to contain a large fraction of high-mass X-ray binaries compared with the transient populations of M31 and the Galaxy, reflecting the later morphology of M33.
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We present and interpret new X-ray data for M33SNR21, the brightest X-ray supernova remnant (SNR) in M33. The SNR is in seen projection against (and appears to be interacting with) the bright HII region NGC592. Data for this source were obtained as part of the Chandra ACIS Survey of M33 (ChASeM33) Very Large Project. The nearly on-axis Chandra data resolve the SNR into a ~5 diameter (20 pc at our assumed M33 distance of 817+/-58 kpc) slightly elliptical shell. The shell is brighter in the east, which suggests that it is encountering higher density material in that direction. The optical emission is coextensive with the X-ray shell in the north, but extends well beyond the X-ray rim in the southwest. Modeling the X-ray spectrum with an absorbed sedov model yields a shock temperature of 0.46(+0.01,-0.02) keV, an ionization timescale of n_e t = $2.1 (+0.2,-0.3) times 10^{12}$ cm$^{-3}$ s, and half-solar abundances (0.45 (+0.12, -0.09)). Assuming Sedov dynamics gives an average preshock H density of 1.7 +/- 0.3 cm$^{-3}$. The dynamical age estimate is 6500 +/- 600 yr, while the best fit $n_e t$ value and derived $n_e$ gives 8200 +/- 1700 yr; the weighted mean of the age estimates is 7600 +/- 600 yr. We estimate an X-ray luminosity (0.25-4.5 keV) of (1.2 +/- 0.2) times $10^{37}$ ergs s$^{-1}$ (absorbed), and (1.7 +/- 0.3) times $10^{37}$ ergs s$^{-1}$ (unabsorbed), in good agreement with the recent XMM-Newton determination. No significant excess hard emission was detected; the luminosity $le 1.2times 10^{35}$ ergs s$^{-1}$ (2-8 keV) for any hard point source.
We present an overview of the Chandra ACIS Survey of M33 (ChASeM33): A Deep Survey of the Nearest Face-on Spiral Galaxy. The 1.4 Ms survey covers the galaxy out to $R approx 18arcmin (approx 4$ kpc). These data provide the most intensive, high spatial resolution assessment of the X-ray source populations available for the confused inner regions of M33. Mosaic images of the ChASeM33 observations show several hundred individual X-ray sources as well as soft diffuse emission from the hot interstellar medium. Bright, extended emission surrounds the nucleus and is also seen from the giant hii regions NGC 604 and IC 131. Fainter extended emission and numerous individual sources appear to trace the inner spiral structure. The initial source catalog, arising from $sim$~2/3 of the expected survey data, includes 394 sources significant at the $3sigma$ confidence level or greater, down to a limiting luminosity (absorbed) of $sim$1.6ergs{35} (0.35 -- 8.0 keV). The hardness ratios of the sources separate those with soft, thermal spectra such as supernova remnants from those with hard, non-thermal spectra such as X-ray binaries and background active galactic nuclei. Emission extended beyond the Chandra point spread function is evident in 23 of the 394 sources. Cross-correlation of the ChASeM33 sources against previous catalogs of X-ray sources in M33 results in matches for the vast majority of the brighter sources and shows 28 ChASeM33 sources within 10arcsec of supernova remnants identified by prior optical and radio searches. This brings the total number of such associations to 31 out of 100 known supernova remnants in M33.
This study presents the final source catalog of the Chandra ACIS Survey of M33 (ChASeM33). With a total exposure time of 1.4 Ms, ChASeM33 covers ~70% of the D25 isophote (Rapprox4kpc) of M33 and provides the deepest, most complete, and detailed look at a spiral galaxy in X-rays. The source catalog includes 662 sources, reaches a limiting unabsorbed luminosity of ~2.4x10^(34) erg/s in the 0.35-8.0keV energy band, and contains source positions, source net counts, fluxes and significances in several energy bands, and information on source variability. The analysis challenges posed by ChASeM33 and the techniques adopted to address these challenges are discussed. To constrain the nature of the detected X-ray source, hardness ratios were constructed and spectra were fit for 254 sources, followup MMT spectra of 116 sources were acquired, and cross-correlations with previous X-ray catalogs and other multi-wavelength data were generated. Based on this effort, 183 of the 662 ChASeM33 sources could be identified. Finally, the luminosity function for the detected point sources as well as the one for the X-ray binaries in M33 is presented. The luminosity functions in the soft band (0.5-2.0 keV) and the hard band (2.0-8.0 keV) have a limiting luminosity at the 90% completeness limit of 4.0x10^(34) erg/s and 1.6x10^(35) erg/s (for D=817kpc), respectively, which is significantly lower than what was reported by previous X-ray binary population studies in galaxies more distant than M33. The resulting distribution is consistent with a dominant population of high mass X-ray binaries as would be expected for M33.
NGC604 is the largest HII-region in M33, second only within the Local Group to 30 Dor, and is important as a laboratory for understanding how massive young stellar clusters interact with the surrounding interstellar medium. Here, we present deep (300ks) X-ray imagery of NGC604 obtained as part of the Chandra ACIS Survey of M33 (ChASeM33), which show highly structured X-ray emission covering ~70% of the full Halpha extent of NGC604. The main bubbles and cavities in NGC604 are filled with hot (kT=0.5keV) X-ray emitting gas and X-ray spectra extracted from these regions indicate that the gas is thermal. For the western part of NGC604 we derive an X-ray gas mass of ~4300M_sol and an unabsorbed (0.35-2.5keV) X-ray luminosity of L_X = 9.3E35 erg/s. These values are onsistent with a stellar mass loss bubble entirely powered by about 200 OB-stars. This result is remarkable because the standard bubble model tends to underpredict the luminosity of X-ray bright bubbles and usually requires additional heating from SNRs. Given a cluster age of ~3Myr it is likely that the massive stars have not yet evolved into SNe. We detect two discrete spots of enhanced and harder X-ray emission, which we consider to be fingerprints from a reverse shock produced by a supersonic wind after it collided with the shell wall. In the eastern part of NGC604 the X-ray gas mass amounts to ~1750M_sol. However, mass loss from young stars cannot account for the unabsorbed X-ray luminosity of L_X = 4.8E35 erg/s. Off-center SNRs could produce the additional luminosity. The bubbles in the east seem to be much older and were most likely formed and powered by stars and SNe in the past.
M33 contains a large number of emission nebulae identified as supernova remnants (SNRs) based on the high [S II]:Ha ratios characteristic of shocked gas. Using Chandra data from the ChASeM33 survey with a 0.35-2 keV sensitivity of about 2 x 10**34 ergs/s, 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 about 4 x 10**35 ergs/s 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 discuss 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, Keplers SNR, Tychos 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.
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