Bright X‐Ray Sources in M31 Globular Clusters

Abstract

We have conducted Chandra observations of $2560 arcmin 2 ($131 kpc 2 ) of M31 and find that the most luminous X-ray sources in most of our fields are in globular clusters. Of the 28 globular cluster X-ray sources in our fields, 15 are newly discovered. Approximately one-third of all the sources have L X ([0.5-7] keV Þ > 10 37 ergs s À1 , and approximately one-tenth of all the sources have L X ([0.5-7] keV) close to or above 10 38 ergs s À1 . The most luminous source, in the globular cluster Bo 375, is consistently observed to have L X greater than 2 Â 10 38 ergs s À1 . (1) We present data on the spectra and/or light curves of the five most luminous M31 globular cluster sources. (2) We explore possible explanations for the high X-ray luminosities of the brightest sources. These include that the X-ray sources may be composites, the radiation we receive may be beamed, metallicity effects could be at work, or the sources may be accreting black holes. We weigh each of these possibilities against the data. In addition, we introduce a neutron star model in which mass transfer proceeds on the thermal timescale of the donor star. Our model can produce luminosities of several times 10 38 ergs s À1 and leads to a set of well-defined predictions. (3) We compute the X-ray luminosity function and the distribution of counts in wavebands that span the range of energies to which Chandra is sensitive. We find the peak X-ray luminosity is higher and that systems with L X > 10 37 ergs s À1 constitute a larger fraction of all GC sources than in our Galaxy. (4) We study the possible reasons for this difference between M31 and Galactic globular cluster X-ray sources and identify three promising explanations.

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

“…There also seems to be a lack of matches that are both bright and blue. We therefore find a result similar to others reported for NGC 4472 (Maccarone, Kundu, & Zepf 2003;Kundu, Maccarone, & Zepf 2002) and for the Milky Way and M31 (Bellazzini et al 1995; but see also Di Stefano et al 2002), i.e., that X-ray sources are preferentially found in GCs that are metal-rich. (See Fig.…”

The deepXMM-NewtonSurvey of M 31

“…These blocks will be found by the SAS detection tools and appear as sources with extremely soft spectrum (so called supersoft sources). To reduce the number of false detections in this source 2 See also http://xmm2.esac.esa.int/docs/documents/ CAL-TN-0050-1-0.ps.gz Primini et al (1993, PFJ93) R (HRI) 86 > ∼ 1.8 × 10 36 bulge region 18 sources variable; ∼3 transients (0.2-4 keV) Supper et al (1997Supper et al ( , 2001 R (PSPC) 560 5 × 10 35 -5 × 10 38 whole galaxy two deep surveys (SPH97, SHL2001) (0.1-2.4 keV) 491 sources not detected with Einstein 11 sources variable, 7 transients compared to Einstein 34 sources variable, 8 transients between ROSAT surveys Osborne et al (2001) X 116 > ∼ 6 × 10 35 centre examined the ∼60 brightest sources for variability (0.3-12 keV) Kong et al (2002b) C (ACIS-I) 204 > ∼ 2 × 10 35 central ∼17 × 17 observations between 1999 and 2001 (0.3-7 keV) ∼50% of the sources are variable, 13 transients Kaaret (2002) C (HRC) 142 2 × 10 35 -2 × 10 38 centre one 47 ks observation; 46 ± 26% of the sources (0.1-10 keV) with L X > 5 × 10 36 erg s −1 are variable Di Stefano et al (2002) C ( A C I S -I /S) 28 5 × 10 35 -3 × 10 38 3 disc fields bright X-ray binaries (0.3-7 keV) Di Stefano et al (2004) C (ACIS-S S3) 33 3 disc fields + centre supersoft sources and quasisoft sources Williams et al (2004) C (HRC) 166 1.4 × 10 36 -5 × 10 38 major axis + centre > ∼ 25% showed significant variability (0.1-10 keV) Trudolyubov & Priedhorsky (2004) C , X 4 3 ∼10 35 -∼10 39 major axis + centre globular cluster study (0.3-10 keV) Pietsch et al (2005b X 856 4.4 × 10 34 -2.8 × 10 38 major axis + centre source catalogue (0.2-4.5 keV) Pietsch et al (2005a C, R, X 21 ∼10 35 -∼10 38 centre correlations with optical novae (0.2-1 keV) Orio (2006) C , X 4 2 6 × 10 35 -∼10 39 major axis + centre supersoft sources and quasisoft sources (0.2-2 keV) (0.3-10 keV) Pietsch et al (2007 C, X 46 ∼10 35 -∼10 38 centre correlations with optical novae (0.2-1 keV) Voss & Gilfanov (2007) C 263 5 × 10 33 -1.5 × 10 38 bulge region low mass X-ray binary study (0.5-8 keV) Stiele et al (2008 X 39 7 × 10 34 -6 × 10 37 centre re-analysis of archival and new 2004 observations 300 4.4 × 10 34 -2.8 × 10 38 time variability analysis; 149 sources with a significance (0.2-4.5 keV)…”

“…Of the 28 bright X-ray sources located in globular clusters (Di Stefano et al 2002), two were not found in the XMM-Newton data (see Table 12). They are also not included in the source catalogue of PFH2005 and SPH2008.…”

“…From the numerous studies of X-ray globular cluster counterparts in M 31 based on Chandra observations (Kong et al 2002b;Di Stefano et al 2002;Williams et al 2004 Figure 34 shows the position of these six sources (in red) and the sources of the XMM LP-total catalogue (in yellow). If the brightness of the six sources had not changed between the Chandra and XMM-Newton observations, they would be in principle bright enough to be detected by XMM-Newton in the merged observations of the central field, which have in total an exposure > ∼ 100 ks.…”

“…Three different M 31 disc fields, consisting of different stellar population mixtures, were observed by Chandra. Di Stefano et al (2002) investigated bright X-ray binaries (XRBs) in these fields, while Di Stefano et al (2004) examined the populations of supersoft sources (SSSs) and quasisoft sources (QSSs), including observations of the central field. Using Chandra HRC observations, Williams et al (2004) measured the mean fluxes and long-term time variability of 166 sources detected in these data.…”