Normal Galaxies

External galaxies, and in particular galaxies in the Local Group, are better laboratories to study global properties of X-ray binaries, supernova remnants, pulsars and stellar associations populations than our own galaxy. These studies are limited in the Galaxy because of the large line-of site absorption, which obscures parts of the Galaxy from view, and because of distance uncertainties for most sources. Due to their proximity, observations of the Magellanic Clouds (MC) and M31 with the past generation of X-ray instruments have allowed the first sensitive studies of the source distribution and luminosity function. It has been shown that the X-ray luminosities of galaxies can be dominated by different source population, for example the MC by population I and the bulge of M31 by population II sources.

image of x-ray pulsars in the SMC
The MECS BeppoSAX mosaic image of four of the newly discovered pulsars in the SMC. (Figure from the 1999 BeppoSAX calendar)

The ROSAT archive can be used to construct high spatial resolution (~ 2-5'') maps of the Large and Small Magellanic clouds. These maps allow studies of distance-limited samples of X-ray sources on spatial scales of less than 1 parsec. Recent mapping studies include a survey of 31 supernovae remnants in the LMC by Williams et al. (1999, ApJS, 123,467); a study of the LMC2 supershell (Chu et al. 1999, ApJ, 518, 298), which showed that this supershell is probably not expanding, with implications for understanding the total energy budget of similar supershells in other nearby galaxies; a survey of the LMC point source population by Points et al. (1999, AAS 194, #71.25), showing that most of the sources in the LMC are X-ray binaries with a sub-population that may be associated with star clusters; and a multi-wavelength (X-ray, UV, and optical) study of the HII complex N11 by MacLow et al. (1998, ApJ 493, 260), which showed that X-ray emission from compact objects is not a universal property of clusters of young massive stars.

During 1997-1998 more than 25 IAU Telegrams (from 6777 to 7101) appeared on X-ray pulsar discoveries within the SMC. The large area detector on RXTE triggered what turned out to be a "gold rush" of pulsars, with the detections of several periods from this one region of sky. BeppoSAX and ASCA joined the hunt but, because of the different position reconstruction capabilities of the three instruments, several iterations were required to sort out the competing discovery claims. Archival data from ROSAT and Einstein with higher imaging capability served to resolve the confusion. Furthermore, the long-term baseline obtained by combining all the data from Einstein with the latest observations by RXTE, ASCA, and BeppoSAX was a powerful tool for establishing or reinforcing the transient nature of these newly discovered pulsars. Prior to 1997, only three pulsars were known in the SMC, but with these new discoveries, 16 pulsars have now been identified in X-ray, with 14 most likely to be in binary systems (Yokogawa et al. 2000, ApJS in press, astro-ph/0002167).

M31 images from Einstein, EXOSAT, and ROSAT
From Einstein to ROSAT: the M31 central 10'.

At a distance of 690 kpc, M31 is the nearest large spiral galaxy. The known distance, low absorption (a hydrogen column of 6.7 x 1020cm-2), and favorable inclination let us sample the entire X-ray source population of this galaxy. The bulge of M31 has been the subject of several papers, which have either cataloged and identified X-ray sources, or derived the global source properties. Variability is one of the key characteristics used to classify sources. In figure 5 archival images of the central region of M31 from Einstein, EXOSAT and ROSAT are displayed; sources indicated with arrows do not have a counterpart in the more deeply exposed ROSAT HRI image (circles). A complete comparison of the Einstein and ROSAT HRI sources has shown that ~42% of the sources within 7'.5 of the nucleus are variable (Primini et al. 1993, ApJ,410, 615). ROSAT HRI archival data have been used to identify two of these as X-ray pulsators (Israel et al. 1995, IAU 6156). The recent Chandra observation of M31 discovered a new 1038 erg/s transient (Murray et al. 1999, IAU 7291) not present in the Einstein images.

The nature of these sources has also been investigated by searching for specific spectral signatures. The possible existence of a soft component in LMXBs is of particular interest for early-type galaxies that are very underluminous in X-rays. The bulge of M31 can be used as a proxy for such a galaxy. The X-ray emission in bright elliptical galaxies is predominantly attributed to hot (0.8 keV) gas, whereas X-ray faint galaxies appear to be lacking this component. The X-ray emission from the latter has been explained by a two-temperature model; the high temperature (5-10 keV) component is attributed to the integrated emission from LMXBs, while the low temperature component has been explained either as emission from a diffuse interstellar gas or as the integrated soft emission of LMXBs. To test this hypothesis Irwin and Bregman (1999, ApJ 527,125) used all the M31 archival data taken with ASCA and ROSAT to construct a synthetic spectrum of the inner 5' of the bulge of M31. They demonstrated that the LMXB component is a viable explanation for the soft emission in X-ray faint galaxies.

M31 offers the possibility of sampling the source population far from the bulge. ROSAT discovered a new class of X-ray sources, the supersoft sources, characterized by a blackbody temperature of a few tens of eV and a bolometric luminosity of ~1038 ergs/s. These sources are mostly detected below 0.5 keV, and so those in our Galaxy are mostly hidden by interstellar absorption. They are, however, clearly evident in external galaxies, and many have been discovered in the MC (Hasinger 1994, AIP 308, 611) and M31 (Supper 1994, AIP 308, 631). Several supersoft sources show transient outbursts which may be recurrent, and others are stable on time-scales of years. A notable example is RX J0045.4+4154 (White et al. 1995, ApJ 445, L125), the first recurrent supersoft transient found in M31, which was discovered during the cataloging effort of the ROSAT PSPC that yielded WGACAT.

Turning to more distant galaxies, Colbert & Mushotzky (1999, ApJ 519, 89) found compact X-ray sources in the center of 54% of nearby face-on spiral and elliptical galaxies using archived ROSAT HRI data. ROSAT X-ray luminosities (0.2--2.4 keV) of these compact X-ray sources are ~1037-1040 ergs s-1}, which means these sources are more luminous than individual binary star systems found in our own galaxy. These unexpected X-ray sources are generally found close to the galaxy nuclei, favoring their identification as accreting massive black hole systems. What was particularly surprising is that estimates of the mass of these systems place them in the range ~102-104 Msolar. No previous systems have ever been observed in this mass range, making these sources a new class of "middleweight" black holes, bridging the gap between the known stellar-mass systems and the 106 - 108 Msolar objects in AGN. Thus these observations have altered our understanding of the range of conditions under which accretion onto a black hole occurs.

A major new contribution is expected from observations with Chandra and XMM-Newton. The HEASARC archival data will provide the capability to examine the time history of these source populations over a two decade interval.

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