X‐Ray Point Sources in the Sombrero Galaxy: Very Soft Sources, the Globular Cluster/Low‐Mass X‐Ray Binary Connection, and an Overview

Abstract

We report on the population of point sources discovered during an 18.5 ks Chandra ACIS-S observation of the Sombrero galaxy. We present the luminosity function and the spectra of the six brightest sources, consider correlations with globular clusters (GCs) and with planetary nebulae, and study the galaxy's population of very soft sources. We detected 122 sources. Twenty-two sources are identified as very soft; of these, five appear to be classical luminous supersoft X-ray sources (SSSs), while 17 may belong to the slightly harder class referred to as quasi-soft (QSSs). There is an overdensity of very soft sources within 2 kpc of the nucleus, which is itself the brightest X-ray source. Very soft sources are also found in the disk and halo, with one QSS in a globular cluster (GC). This source is somewhat harder than most SSSs; the energy distribution of its photons is consistent with what is expected from an accreting intermediate-mass black hole. Several sources in the 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 greater than 10 38 ergs s À1 are in GCs. These results for M104, an Sa galaxy, are similar to what has been found for elliptical galaxies and for the latetype 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. The luminosity function of X-ray sources in GCs has a cut-off near the Eddington luminosity for a 1.4 M object. We propose a model that can explain the trends seen in the data sets from the Sombrero and other galaxies. Thermal timescale mass transfer can occur in some of the younger clusters in which the turnoff mass is slightly greater than 0.8 M ; 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. Key elements of the model can be tested.

Globular Clusters and X‐Ray Point Sources in Centaurus A (NGC 5128)

“…This trend already can be seen in the small Milky Way sample of LMXBs, but becomes much more evident in larger galaxies (e.g., Angelini et al 2001;Kundu et al 2002Kundu et al , 2007Di Stefano et al 2003;Kim et al 2006;Sivakoff et al 2007;Posson-Brown et al 2006). The physical mechanisms underlying this observational result are still unknown, although several possibilities have been put forward, including metallicity-dependent variations in the initial mass function (Grindlay 1987), the link between metallicity and the outer convective zones of stars (Ivanova 2006), and the effects of metallicity on radiation-induced stellar winds ( Maccarone et al 2004).…”

“…Moreover, Fabbiano et al (2007) have recently shown that GCs in NGC 3379 lack low-luminosity LMXBs when compared to the field population, implying that the latter cannot have originated from GCs (see also Voss & Gilfanov 2007). To date, large populations of LMXBs have been found in X-ray observations of E/S0 galaxies over a wide range of luminosities (e.g., Sivakoff et al 2007;Kundu et al 2007) and in large disk galaxies such as M31, NGC 3115, and NGC 4594 (Fan et al 2005;Kundu et al 2003;Di Stefano et al 2003).…”

Probing the Formation of Low‐Mass X‐Ray Binaries in Globular Clusters and the Field

“…Some aspects of the LMXB properties in several of these galaxies have been analyzed in previous studies Di Stefano et al 2003;Sarazin et al 2003;Randall et al 2004;Kim et al 2006b). The X-ray properties of our sample are consistent with these studies in the areas that these analyses overlap.…”

“…We find only one GC that is marginally in the blue population of NGC 4594. Given the higher contamination rate in the ground-based GC sample of Di Stefano et al (2003), it is possible that some of the ''blue'' GC LMXB candidates in their sample are contaminating background objects such as AGNs (also see x 3.5).…”

The globular cluster mass/low mass X-ray binary correlation: implications for kick velocity distributions from supernovae

“…We take the distances to these galaxies from Tonry et al (2001). Partial results for all of these galaxies have been presented previously (ALM; Kundu et al 2003Kundu et al , 2004KMZ;Di Stefano et al 2003), and a detailed analysis with full source catalogs will be presented in future work (Kundu et al, in prep. ) In all, our sample includes 98 X-ray sources in 2276 globular clusters.…”