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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](https://webarchive.library.unt.edu/eot2008/20090514055915im_/http://heasarc.gsfc.nasa.gov/Images/heasarc/science/smc.gif)
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](https://webarchive.library.unt.edu/eot2008/20090514055915im_/http://heasarc.gsfc.nasa.gov/Images/heasarc/science/m31new.gif)
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 M . 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
M
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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Last modified: Monday, 19-Jun-2006 11:24:57 EDT
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