Chandra Observations of the Low Mass X-ray Binary Populations of
X-ray Faint Elliptical and S0 Galaxies
Craig L. Sarazin, Elizabeth L. Blanton (Unv. of Virginia), Jimmy A. Irwin, Joel N. Bregman (Univ. of Michigan)
[Contributed talk, 15min.]
Abstract
Chandra ACIS-S3 observations of the X-ray faint elliptical and S0 galaxies
NGC 1553, NGC 4365, NGC 4382, and NGC 4697 resolve much of the X-ray emission
into point sources, most of which appear to be Low Mass X-ray Binaries
(LMXBs) associated with these galaxies.
The dominance of LMXBs indicates that X-ray faint early-type galaxies
have lost much of their interstellar gas,
although there still is a small amount of X-ray emission from
interstellar gas at
keV.
Taken together, the LMXBs have a hard spectrum, which can be fit by
thermal bremsstrahlung at
keV or a power-law.
This means that the soft component in the spectra of early-type
galaxies discovered with ROSAT is mainly due to hot gas.
A few of the X-ray sources have very soft X-ray colors, and appear to
be supersoft sources.
A significant fraction of the LMXBs in these nearby ellipticals are in
globular clusters,
which indicates that globulars have a high probability of containing
X-ray binaries compared to the normal stellar population.
It is possible that most LMXBs in elliptical galaxies may have
been formed in globular clusters via stellar dynamical interactions.
The variation in the globular cluster specific abundance from galaxy
to galaxy and particularly along the Hubble sequence might explain the
variations in the X-ray-to-optical flux ratios of X-ray faint galaxies.
The X-ray luminosity functions of LMXBs in early-type galaxies generally
have a knee or break at
ergs/s,
which is approximately the Eddington luminosity for a
1.4
neutron star.
Thus, we suggest that the LMXBs with higher luminosities generally contain
accreting black holes.
This ``Eddington break'' luminosity might be used as a distance indicator.
In the bulges of late-type galaxies, there may be an upper cut-off
to the luminosity function at about the same luminosity.
The high luminosities of the brightest sources suggest that they contain
fairly massive black holes, if they are Eddington-limited.
The presence of this large population of NS and massive BH stellar remnants
in early-type galaxies shows that these galaxy, which now contain only low
mass optical stars, once contained a large population of massive
main sequence stars.
CATEGORY: NORMAL GALAXIES