A DEEP X-RAY IMAGE OF M33

摘要

A 50.4 ks ROSAT Position Sensitive Proportional Counter image of the nearby spiral galaxy M33 reveals 37 sources within 15′ of the nucleus brighter than 7 x 10~(35) ergs s~(-1). There are at least 13 additional sources farther from the nucleus, most of which are likely to be associated with M33 as well. Most of the bright sources that had been detected with Einstein in the same region are still visible. The bulk of the sources in the galaxy are associated with Population I tracers. Several of the sources are time variable. There are 10 sources in the image that lie within 20″ of optically identified supernova remnants (SNRs) in M33. The spectra of these sources are soft compared with most other sources of comparable brightness, and therefore it is likely that most of these X-ray sources are SNRs. Based on the identification of sources in M33, it appears likely that ROSAT hardness ratios of this type can be used to separate SNRs and compact sources in other nearby normal galaxies as well. The northern and the southern spiral arms of M33 appear as diffuse features in the X-ray image. There is additional diffuse (or unresolved source) emission throughout the inner portions of M33. The diffuse emission is softer than the faint point sources and the SNRs in the survey, and is well fitted in terms of a bremsstrahlung spectrum with kT ～0.4 keV and log N_H ～20.6.