A second black hole candidate in a M31 globular cluster is identified with XMM-Newton
aa r X i v : . [ a s t r o - ph . H E ] M a y Mon. Not. R. Astron. Soc. , 1–4 (2009) Printed 8 October 2018 (MN L A TEX style file v2.2)
A second black hole candidate in a M31 globular cluster isidentified with XMM-Newton
R. Barnard and U. Kolb Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
ABSTRACT
We use arguments developed in previous work to identify a second black hole candidateassociated with a M31 globular cluster, Bo 144, on the basis of X-ray spectral andtiming properties. The 2002 XMM-Newton observation of the associated X-ray source(hereafter XBo 144) revealed behaviour that is common to all low-mass X-ray binaries(LMXBs) in the low-hard state. Studies have shown that neutron star LMXBs exhibitthis behaviour at 0.01–1000 keV luminosities
10% of the Eddington limit ( L Edd ).However, the unabsorbed 0.3–10 keV of XBo 144 luminosity was ∼ L Edd for a1.4 M ⊙ neutron star, and the 0.01–1000 keV luminosity is expected to be & Key words:
X-rays: general – X-rays: binaries – Galaxies: individual: M31 – blackhole physics
In Barnard et al. (2008), we identified the first genuine blackhole candidate using our X-ray classification method, whichwas developed to the fullest in Barnard et al. (2008). TheX-ray source associated with the globular cluster Bo 45(hereafter known as XBo 45) exhibited behaviour associatedwith all low mass X-ray binaries (LMXBs) in the low state:hard power law emission, and high r.m.s variability (see e.g.van der Klis 1994 1995). A recent survey of Galactic neu-tron star LMXBs has shown that the low state is observedat 0.01–1000 keV luminosities . L Edd for systems thattrace a diagonal transition in colour-colour space from lowstate to high state, and . L Edd for systems that ex-hibit a vertical transition (Gladstone et al. 2007); L Edd isthe Eddington luminosity. However, XBo 45 exhibited thisbehaviour at a 0.3–10 keV luminosity ∼
120 % Eddingtonfor a 1.4 M ⊙ neutron star, hence we identified it as a blackhole candidate.XBo 45 is particularly interesting because there hasbeen a distinct lack of black holes in globular clusters (GCs),and most black hole LMXBs are thought to be ejected fromthe cluster. The theoretical work of Kalogera et al. (2004)showed that one possible formation channel for black holeLMXBs is the tidal capture of a main sequence star; indeed,it is the most likely channel for dense clusters, such as thosewith collapsed cores. Kalogera et al. (2004) predicted suchsystems to be persistently bright, and inferred from the ab-sence of such systems that the prospective donor is disrupted during capture. XBo 45 has appeared in all X-ray observa-tions covering that region of sky, spanning ∼
30 years, andhence is consistent with theoretical predictions for a systemformed by tidal capture.In this paper, we use the same arguments to identifyXBo 144 (r2-5 in Kong et al. 2002, α = 00:42:59.803 δ =41:16:06.01) as another black hole candidate. Furthermore,XBo 144 is located in a confirmed M31 GC (see the revisedBologna Catalogue V.3.5, March 2008; Galleti et al. 20042005 2006 2007).We present detailed analysis of the 2002 June XMM-Newton observation of XBo 144, the deepest of four expo-sures made between 2000 and 2002. Results from the otherobservations are consistent with this one. In Section 2 we de-scribe the observations and data reduction, then present theresults of our analysis in Section 3. We discuss our findingsin Section 4, and draw conclusions in Section 5. We rebuilt the data products for the 2002 June 26 XMM-Newton observation of the central region of M31 using ver-sion 7.1 of the Science Analysis Software suite (SAS). Inorder to screen for background flares, we followed the rec-ommended procedure, finding a small flare near the startof the observation; this was removed. We then synchronisedthe pn and MOS lightcurves as described in Barnard et al.(2007). c (cid:13) R. Barnard, and U. Kolb
We extracted pn (Str¨uder et al. 2001), and MOS(MOS1 and MOS2 Turner et al. 2001) data from from a cir-cular source region with radius 20 ′′ , with a 20 ′′ backgroundregion at a similar off-axis angle, with no point sources andon the same CCD chip. A larger background is desirable,but was prevented by crowding. Source and backgroundlightcurves were constructed from the pn and MOS datain 0.3–2.5, 2.5–10 and 0.3–10 keV bands. The lightcurveswere background subtracted. Source and background spec-tra were then created, along with corresponding responsematrices and ancillary response files. The pn image yielded5542 net source photons, while the data from the two MOScameras were combined, yielding 4893 net source photons. The pn and combined MOS spectra were modeled simul-taneously, using XSPEC ver 11.3. Each model consisted ofan emission model, with line-of-sight absorption and a nor-malisation constant that accounts for differences in the pnand MOS callibrations, after setting the pn normalisationto 1. The parameters of the emission models were forcedto be the same for the pn and MOS spectra, but free tovary. Three emission models were initially chosen: black-body (BB), bremsstrahlung (BR), and power law (PL). Theblackbody and bremsstrahlung models were characterisedby k T , where T is the temperature and k is the Boltzmannconstant, while power law emission is characterised by pho-ton index, Γ. Absorption is expressed in terms of N H , theequivalent absorption by neutral hydrogen.We first modelled the XBo 144 spectra with single com-ponent emission models : BB, BR and PL. We found the pnand MOS spectra to be best fitted by the PL model, with N H = 8.1 ± × atom cm − and Γ = 1.48 ± ∼ L Edd in the 0.01–1000 keV band (Barnard et al. 2008). The 0.3–10keV luminosity for our best fit PL model is 5.33 ± × erg s − , which is 0.29 L Edd for a 1.4 M ⊙ neutron star, and0.20 L Edd for the most massive observed neutron star (2.1M ⊙ , see e.g. Nice et al. 2005). This is sufficently bright forXBo 144 to be a plausible black hole candidate.We then modelled the XBo 144 spectra with a two com-ponent (BB+PL) emission model. The favoured blackbodytemperature is 0.0082 ± Figure 1.
Unfolded 0.3–10 keV pn spectrum of XBo 144 withbest fit PL emission model.
Figure 2.
Unfolded 0.3–10 keV pn spectrum of XBo 144 withbest fit BB+PL emission model. The PL component is repre-sented by a solid line, while the BB component is represented bya dashed line.
The combined pn+MOS, background-subtracted, 0.3–10keV lightcurve of XBo144 is shown in Fig. 3, with 400 sbinning. The intensity appears to vary in the manner of lowstate LMXBs; however, the uncertainties are large, and thebest fit line of constant intensity yields χ /dof = 630/601,i.e. the lightcurve is consistent with being constant. The frac-tional r.m.s. variability is 6 ±
4% on time-scales longer than100 s; this is consistent with a LMXB in the low state, butadds no extra strength to our argument. Without the defi-nite variability exhibited by XBo 45 (Barnard et al. 2008),the case for XBo 144 being a black hole X-ray binary is some-what weaker, and rests on its association with the globularcluster Bo 144.
The X-ray source associated with the M31 GC Bo 144, XBo144, exhibits an emission spectrum that is characteristic of a c (cid:13) , 1–4 lack hole binary in M31 globular cluster Table 1.
Best fits to XBo 144 pn and MOS spectra with various emission models. Blackbody (BB), bremsstrahlung (BR) and powerlaw (PL) emission models were used, all suffering line-of-sight absorption. For each model, we provide the absorption ( N H / 10 atomcm − ), temperature (k T / keV), photon index (Γ), and MOS normalisation constant ( n MOS ). Numbers in parentheses indicate 90%confidence limits. We then provide the χ /dof. No uncertainties were calculated for the BB model, as χ /dof = 8.5.Model N H k T Γ n MOS χ /dofBB 7 0.68 . . . . . . . . . . . . . . . . . . . . . Figure 3.
Combined, background-subtracted pn+MOSlightcurve for XBo 144 in the 0.3–10 keV band. The lightcurve isbinned to 400 s.
LMXB in the low state: the 0.3–10 keV emission is describedby a pure power law, with Γ = 1.48 ± .
10% of the Ed-dington limit; the 0.3–10 keV luminosity of XBo 144 exceeds0.20 L Edd for all known neutron stars (2.1 M ⊙ or less).Estimating the 0.01–1000 keV luminosity of XBo 144 isnot as simple as extrapolating the power law to 1000 keV.Low state spectra are thought to be caused by unsaturatedinverse Compton scattering of cool photons on hot electronsin a corona, resulting in a power law for energies lower thanthe electron temperature, and a Wien spectrum for higherenergies (Sunyaev & Titarchuk 1980). The coronae in blackhole X-ray binaries tend to have temperatures of ∼ ∼ > L Edd for a neutron star withmass 2.1 M ⊙ . Hence, we propose XBo 144 as a candidateGC black hole LMXB.Without the strong time-variability observed in XBo45, it is possible that XBo 144 is simply an active galactic nucleus (AGN), since Γ ∼ × − and 5.69 × − erg cm − s − , respec-tively). From the XLFs of Moretti et al. (2003), we expect3.8 ± × − AGN per square arcsec to exhibit 1–2 keVfluxes as bright as XBo 144, and only 8 ± × − AGN persquare arcsec to exhibit 2–10 keV fluxes as bright as XBo144. Kong et al. (2002) found the Chandra position of XBo144 to be 2.2 ′′ from the GC position. There are 20 GCswithin the field of view of the XMM-Newton observation weare interested in; hence, the probability for a chance coin-cidence between a GC and an AGN as bright as Bo 144 inthe 2–10 keV band within 2.2 ′′ is 2.4 ± × − .The 0.3–10 keV lightcurve of XBo 144 is consistent witha low state LMXB, but also is consistent with being con-stant. We note that XBo 144 is ∼ ∼
30 year period of observation. Weinfer that XBo 144 and XBo 45 are the same class of ob-ject: black hole LMXBs, likely produced by tidal capture ofa main sequence star.We now consider the influence of metallicity onthe probability of finding black hole LMXBs in GCs.Bellazzini et al. (1995) discovered a novel mechanism to pro-mote tidal capture at higher metallicities, in addition to thepreviously suspected correlation between metallicity and ini-tial mass function (IMF). They assume that for a fixed clus-ter density, the rate of tidal capture depends on the mass andradius of the capturing star. They find that higher metallic-ity stars have larger masses and radii; therefore, they expectthe tidal capture rate to increase with metallicity. Further-more, a metal-rich star will more easily fill its Roche lobe.They conclude that this effect alone could explain the ob-served ratio between frequencies of X-ray sources in metal-rich and metal-poor clusters, although there is no reason toexclude the effects of metallicity on the IMF.Fan et al. (2008) have produced the most comprehen- c (cid:13) , 1–4 R. Barnard, and U. Kolb sive survey of metallicities of M31 GCs yet made. They com-bine spectroscopically-derived metallicities for 295 GCs andGC candidates with colour-derived metallicities for 209 GCsand candidates. Fan et al. (2008) found a mean [Fe/H] of − ± − ± − ± ∼ − ± ∼
60% more metal rich than the mean, with pos-sible metallicity range 80–300% of the M31 mean. The highmetallicity of Bo 144, and the inferred increase in tidal cap-ture rate, support the plausibility of XBo 144 being blackhole LMXB.
We have identified a second black hole candidate in a M31GC; such a system would most likely be formed by tidalcapture. Kalogera et al. (2004) found tidal capture to be thedominant mechanism for LMXB formation in collapsed coreGCs, or even denser clusters such as M15. A survey of Galac-tic clusters (Harris 1996) showed ∼
20% of clusters to be corecollapsed; therefore, we expect no more than 20% of GC X-ray sources to harbour black holes. Traditional methods ofidentifying a black hole involve obtaining a mass functionfrom the radial velocity curves of the donor, and are quiteunsuitable for identifying black holes in GC LMXBs becausethe donor is near-impossible to identify. Our X-ray identi-fication of black holes is not so hindered, but does identifyonly that subset of black hole LMXBs in the low state at anobserved luminosity & × erg s − . Applying our X-raytechnique to other known, bright X-ray sources in globularclusters may reveal further candidate black hole systems. ACKNOWLEGMENTS
We thank the anonymous referee for their constructive com-ments. This work is based on observations with XMM-Newton, an ESA science mission with instruments and con-tributions directly funded by ESA member states and theUS (NASA). Astronomy research at the Open University isfunded by an STFC Rolling Grant.
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