X‐Ray Point Sources in the Sombrero Galaxy: Very Soft Sources, the Globular Cluster/Low‐Mass X‐Ray Binary Connection, and an Overview
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abstract
We report on the population of point sources discovered during an 18.5 ksec
Chandra ACIS-S observation of the Sombrero Galaxy. We present the luminosity
function, the spectra of the 6 brightest sources, consider correlations with
globular clusters (GCs) and with planetary nebulae (PNe), and study the
galaxy's population of SSSs. We detected 122 sources, 22 of them are identified
as luminous supersoft X-ray sources (SSSs). There is an over density of SSSs
within 1.5 kpc of the nucleus, which is itself the brightest X-ray source. SSSs
are also found in the disk and halo, with 1 SSS in a globular cluster (GC).
Several sources in Sombrero's halo are good candidates for SSS models in which
the accretor is a nuclear-burning white dwarf. In total, 32 X-ray sources are
associated with GCs. The majority of sources with luminosity > 1e38 erg/s are
in GCs. These results for M104, an Sa galaxy, are similar to what has been
found for elliptical galaxies and for the late-type spiral M31. We find that
those optically bright GCs with X-ray sources house only the brightest X-ray
sources. We find that, in common with other galaxies, there appears to be a
positive connection between young (metal-rich) GCs and X-ray sources, but that
the brightest X-ray sources are equally likely to be in metal-poor GCs. We
propose a model which can explain the trends seen in the data sets from the
Sombrero and other galaxies. Thermal-time scale mass transfer can occur in some
of the the younger clusters in which the turn-off mass is slightly greater than
$0.8 M_\odot$; multiplicity may play a role in some of the most massive
clusters; accretion from giant stars may be the dominant mechanism in some
older, less massive and less centrally concentrated clusters.
