Raymond White
University of Alabama
ABSTRACT The X-ray emission from normal elliptical galaxies has two major components: soft (0.2-1 keV) emission from diffuse gas and harder (5-6 keV) emission from populations of accreting stellar X-ray binaries (XRB). The X-ray luminosity of the gaseous component has long been known to exhibit a very large range (factor of 100) for a given optical (stellar) luminosity. The X-ray luminosity of the XRB component was expected to be simply proportional to the optical luminosity of the galaxy, since it is so intimately tied to the stellar population. However, recent ASCA and Chandra observations show that the X-ray luminosity of the binary component exhibits significant scatter (a factor of 3-4) at a given optical luminosity. This scatter may reflect a range of evolutionary stages in the X-ray binary populations. If so, the XRB X-ray to (galactic) optical luminosity ratio may be used to date the last major bursts of star formation in ellipticals. However, there seems to be littleHowever, there seems to be little correlation between variations in the luminosity of XRB populations and optically-derived estimates of galaxy ages. A stronger correlation is found between XRB and globular cluster populations in ellipticals. This may indicate that most low mass XRB are formed in globular clusters.
Introduction
As stated in the Abstract, the X-ray emission from
normal elliptical galaxies has two
major components: soft (0.2-1 keV) emission from diffuse gas
and harder (5-6 keV) emission from populations of accreting
(low-mass) stellar X-ray binaries (XRB).
The hardness of the stellar XRB component (placing its
emission outside the most responsive parts of the
ROSAT and ASCA bandpasses)
and its spatial confusion with the softer gaseous
component have made it difficult to constrain the
global temperature and luminosity of the XRB population
in elliptical galaxies. Chandra observations
are now allowing us to resolve out individual XRB in
nearby ellipticals, making their composite spectral
analysis much easier. Meanwhile, as Chandra works its
way through nearby ellipticals,
there is a large database of long ASCA observations
which has more to yield ...
Determining the Variance in the Stellar XRB Component
Strong spectral constraints on the hard stellar XRB component in ellipticals can be made by simultaneously analyzing ASCA spectra from multiple ellipticals. ASCA GIS spectra from individual ellipticals were analyzed first, to see which ellipticals provided the strongest individual constraints on the hard XRB component. Mosttraints on the hard XRB component. Most ellipticals require both soft (gaseous) and hard (XRB) components. Subsequently, I simultaneously fit the spectra from the 5-7 best candidates, forcing the temperature of the XRB component to be the same for all galaxies; the temperatures of the soft components (if existent) were allowed to vary individually. The global spectrum of the XRB component is fit equally well by a
where 90% confidence limits are in parentheses. These are the tightest constraints to date on the spectral properties of the stellar XRB component in ellipticals and are consistent with the spectral character of many individual Low Mass XRBs in our Galaxy.
In comparing the X-ray fluxes deduced for the XRB component in these ellipticals to the total optical magnitudes for these galaxies, there is a factor of ~4 range in the X-ray to optical flux ratio (see Figures). Although this range is much smaller than that of the softer gaseous component (which has a factor of 100 range in X-ray/optical flux ratio), it is still larger than expected, since the XRB component is supposed to be slaved to the stellar component. What is the source of the variance?
Possible Sources of Variance in the XRB Component
If the evolution of an XRB population is tied to
the evolution of visible stars, variations in
the X-ray properties of the XRB component may be an
age indicator. The luminosity of the XRB component
(neglecting burst sources)
is expected to slowly decline after an initial
Gyr ramp-up
(N.E. White & P. Gosh 1998 ApJ 504 L31),
even as the XRB population is supplemented by
successively lower-mass companions continually evolving
off the main sequence
(R.E. White 2001, in prep).
Thus, relatively X-ray luminous XRB populations
(for a given optical luminosity) may be found
in younger ellipticals.
To test this, I compared recent age estimates
for elliptical galaxies
(from Trager et al 2000 AJ 120 165)
to the X-ray/optical flux ratio of
the XRB component versus the galaxies' optical magnitude.
The stellar age estimates
are derived from optical stellar spectral features of Fe, Mg and Hbeta.
The top Figure shows that there is no correlation between
optical stellar age estimates and the X-ray/optical ratio
of the XRB population.
Elliptical galaxies exhibit a wide range of globular cluster
populations for a given galaxian luminosity.
Furtions for a given galaxian luminosity.
Furthermore, stellar XRB are produced much more efficiently in
globular clusters than in the field.
It is therefore conceivable
that nearly ALL stellar XRB are formed in globular
clusters
(S. Kulkarni, private communication).
Low mass stellar XRB which are not now in globular clusters
may have been ejected from globulars by supernova kicks
immediately after the primary collapsed to a neutron star.
In this case, we might expect the X-ray luminosity of the
XRB component to be correlated with the globular cluster
population of a galaxy. Even if most XRBs have been
ejected from globular clusters, their numbers should still
be proportional to one another.
To test this, I plot in the lower Figure the specific frequency of
globular clusters
(from Kissler-Patig 1997 AA 319 83)
versus the
X-ray/optical flux ratio for the XRB populations in the elliptical
sample described above.
There appears to be a weak correlation, so XRB populations may
be controlled by globular cluster populations.
Conclusions
This preliminary study suggests that globular cluster populations
in ellipticals control the evolution of their populations of low mass
stellar X-ray binaries. At the present epoch, age differences
among the ellipticals in the small sample described above do not seem
to be correlated with do not seem
to be correlated with the variations in their X-ray binary populations.
As usual, more work is needed to enlarge the sample...