XMM-Newton and Chandra observations of the M31 globular cluster black hole candidate XB135: a heavyweight contender cut down to size

0.35 MB·English

by  R. Barnard

Draftversion January27,2015
PreprinttypesetusingLATEXstyleemulateapjv.08/22/09
XMM-NEWTON AND CHANDRA OBSERVATIONS OF THE M31 GLOBULAR CLUSTER BLACK HOLE
CANDIDATE XB135: A HEAVYWEIGHT CONTENDER CUT DOWN TO SIZE
R. Barnard and F. Primini and M. R. Garcia
Harvard-SmithsonianCenterforAstrophysics(CFA),CambridgeMA02138
and
U. C. Kolb
5 TheOpenUniversity,MiltonKeynes,UK
1 and
0
S. S. Murray
2
Johns HopkinsUniversity,Baltimore,Maryland;CFA

n Draft version January 27, 2015
a
J ABSTRACT

6 CXOM31J004252.030+413107.87isoneofthebrightestX-raysourceswithintheD regionofM31,
25
2 andassociatedwithaglobularcluster(GC)knownasB135;wethereforecallthisX-raysourceXB135.
XB135 is a low mass X-ray binary (LMXB) that apparently exhibited hard state characteristics at
E] 0.3–10keVluminosities4–6×1038ergs−1,andthehardstateisonlyobservedbelow∼10%Eddington.
H Iftrue,theaccretorwouldbeahighmassblackhole(BH)(&50M⊙); suchaBHmaybeformedfrom
directcollapseofametal-poor,highmassstar,andthe verylowmetalicityofB135(0.015Z⊙)makes
. such a scenario plausible. We have obtained new XMM-Newton and Chandra HRC observations to
h
p shed light on the nature of this object. We find from the HRC observation that XB135 is a single
- point source located close to the center of B135. The new XMM-Newton spectrum is consistent with
o arapidlyspinning∼10–20M⊙ BHinthesteeppowerlaworthermaldominantstate,butinconsistent
r withthe hardstatethatwepreviouslyassumed. Wecannotformallyrejectthreecomponentemission
t
s models that have been associated with high luminosity neutron star LMXBs (known as Z-sources);
a
however, we prefer a BH accretor. We note that deeper observation of XB135 could discriminate
[
against a neutron star accretor.

1 Subject headings: x-rays: general—x-rays: binaries—globularclusters: general— globularclusters:
v individual
9
5
5 1. INTRODUCTION Newton spectra of XB135 with neutron star (NS) and
6 OneofthebrightestX-rayobjectsinM31iscoincident BH XB emission models; additionally, we compare our
0 XB135 spectrum with the XMM-Newton pn spectrum
with the globular cluster (GC) known as B135, follow-
. fromthebrightNSXBLMCX-2. Weobtainanaccurate
1 ing the Revised Bologna Catalog (RBC) v 3.4 (Galleti
position for XB135 from our Chandra HRC data.
0 et al., 2004, 2006, 2009). We refer to this X-ray source
5 as XB135. In our ∼13year Chandrasurvey,it exhibited 2. OBSERVATIONSANDDATAANALYSIS
1 0.3–10 keV luminosities ∼4–6×1038 erg s−1, assuming
2.1. XMM-Newton analysis
: power law emission; the best fit photon index (Γ) was
v
∼1.8throughout(Barnard et al.2014a). The highlumi- XB135 was observed by XMM-Newton on 2012 June
i
X nosity and hard spectrum of XB135 led us to propose 26–27(ObsID0690600401,PIR.Barnard). Weanalyzed
that the accretor was a black hole (BH) with mass ∼50 the data using SAS version 13.0. A substantial portion
r
a M⊙,whichismadepossiblebytheverylowmetalicityof oftheobservationwascontaminatedbybackgroundflar-
XB135(seeBarnard et al.2013a,andreferenceswithin). ing. We screened out high background intervals in the
We note that persistently brightBH binaries in GCs are pn instrument by creating a lightcurve filtered with the
consistentwithpopulationsynthesistheoryfortidalcap- expression “(PATTERN==0)&&(FLAG==0)&&(PI in
ture of a main sequence star (Kalogera et al. 2004, al- [10000:12000])” and binned to 400 s; we rejected inter-
though the companion might be disrupted), or with the vals with 10–12keV rates >0.4 count s−1. This resulted
formation of an ultracompact binary with a degenerate in 75 ks of good time.
donor from a triple system (Ivanova et al. 2010). Circular source and backgroundregions on the pn im-
We were awarded a 120 ks XMM-Newton observation agewerechosen,andoptimizedbythesoftware. Spectra
of XB135, along with a 5 ks Chandra HRC observation. were extracted from these regions using the expression
The XMM-Newton observation was designed to test the “(PATTERN<=4)&&(FLAG==0)” and the good time
possibility that XB135 was formed in a low-metalicity interval. WedidnotuseanydatafromtheMOScameras
environment. The purpose of the HRC observation was becausetheysufferedsignificantpileup. Inourproposal
to confirmthat XB135 consistedof a single point source for this observation, we made a case for using the RGS
thatisconsistentwithbeinglocatedinB135;XB135had spectra to help constrain the metalicity. However since
previously been observedat high off-axis angles, making muchoftheobservationwaslosttoflaring,thegoodtime
its position uncertain. In this work, we fit the XMM- RGS spectra were not of sufficient quality to constrain
2 Barnard et al.
the fits.
We estimated our uncertainties on spectral fits as fol-
lows. We first obtained the best fits to the pn spectra.
We then simulated 1000 sets of spectra from this best
fit model using the multifake command in XSPEC;
variations in the simulated spectra were drawn from the
statistical properties of the observed spectra. Each sim-
ulated spectrum was modeled, obtaining the best fit pa-
rametervaluesandthe0.3–10keVunabsorbedflux(2–20
keV flux in some cases,see below). Once all1000 sets of
spectra were analyzed, each parameter was sorted into
ascending order, and the 1σ confidence limits were ob-
th th
tainedfromthe16 and84 percentile. Theparameter
values appear to be Gaussian distributed.
2.2. Chandra analysis Figure 1. B band image of the GC B135, from Field 5 of the
LGGSsurveyofM31(Masseyetal.2006),overlaidwithanellipse
Our Chandra HRC-I observation was performed on thatshowsthe3σpositionuncertaintyforXB135,usingall4GCs
2013 February 24 (Obs ID 14400, PI R. Barnard). We for registration. The uncertainties from the 3 GC solution are
registeredthisobservationtotheBbandField5M31im- entirelycontained withinthisellipse.
ageprovidedbytheLocalGroupGalaxySurvey(LGGS, the expectation that the at least some BHs migrate to
Massey et al. 2006), using three X-ray bright GCs, and the center because of mass segregation (Morscher et al.
followingour procedure setout in Barnardet al.(2012). 2013),possiblyallofthem(Spitzer1969). Figure1shows
This involved using the iraf task imcentroid to find the GC B135 in the Field 5 B band image provided by
the center coordinates for each GC and its correspond- Massey et al.(2006),overlayedwithanellipserepresent-
ing X-ray source, and using ccmap to match the sky ing the 3σ position uncertainties for XB135 using all 4
coordinates of the GCs in the Field 5 image to the X- X-raybrightGCs;the3σpositionuncertaintiesobtained
ray positions in the Chandra image. We used PC-IRAF fromthe3GCsolutionareentirelycontainedwithinthis
Revision 2.14.1. ellipse.
This procedure assumes that the X-ray sources are at We have confined a BHC to the center of a GC for
the centers of their host GCs. Our assumption appears the first time. We therefore calculated the probability
to be justified because the r.m.s. offset when aligning that XB135 is an unassociated bright X-ray source that
the X-ray and optical positions of 27 X-ray bright GCs happens to be within 0.2′′ of the GC center (∼10% of
in the M31 center was just 0.11′′ in R.A. and 0.09′′ in the GC radius). The XMM-Newton pipeline found 8
Decl. (Barnard et al. 2012), or ∼0.3 ACIS pixels. sources with fluxes >1.4×10−12 erg s−1 in our XMM-
Newton observation, equivalent to luminosities & 1038
3. RESULTS ergs−1atthedistanceofM31. TheEPICpnfieldofview
3.1. Locating XB135 may be approximated by a circle with 14′ radius, hence
The native HRC pixel size is 0.13175′′, and our image the number density of bright X-ray sources is ∼4×10−6
was binned by a factor 4 to ∼0.5′′ pixels. The HRC-I per square arc second. The probability of finding one of
these bright X-ray sources coincidentally within 0.2′′ of
observationrevealedthatXB135isasinglebrightsource,
thecenterofanyofthe80confirmedGCsobservedinthe
rather than a blend of multiple sources. Using the iraf
region (Peacock et al. 2010) is ∼4×10−5. We therefore
tool imcentroid, we found the X-ray centroid with 1σ
conclude that XB135 is indeed associated with the GC
position uncertainties of 0.07 image pixels in RA and
Decl., i.e. 0.04′′. B135. WeinferfromthisthattheX-raysourcesinB045,
B116,and B091Dare alsolocated at the centers of their
We foundatotaloffourX-raybrightGCs inthe field-
respective GCs, as suggested by our previous work.
of-view: B045,B116,B135,andB091D.Weinitiallyreg-
istered the image using B045, B116, and B091D; this
3.2. Lightcurves
placed XB135 at the center of the GC B135, but the
uncertainty in registration was negligible, perhaps due We present short-term and long-term lightcurves of
to our using the minimum number of sources required XB135 in Figures 2 and 3 respectively. All lu-
for the transformation. This resulted in a position of minosities assume a distance to M31 of 780 kpc
00:42:51.985 +41:31:08.32, 0.08′′ from the GC center in (Stanek & Garnavich1998).
the Field 5 image. Figure 2 shows the 0.3–10 keV pn intensity lightcurve
Additionally, we tried re-registering the image includ- of XB135 and the background for our 120 ks XMM-
ing XB135, to estimate the uncertainty in registration Newton observation. The lightcurve is binned to 400
withanextrasource;theresultingr.m.s. deviationswere s and periods of high background flaring are removed.
0.03′′ in R.A. and0.003′′ in Decl. The positionobtained XB135 exhibited some variability that is not apparent
from this solution was 00:42:51.982 +41:31:08.31, 0.04′′ in the background lightcurve; the r.m.s. variability is
from the Field 5 GC center; the combined 1σ uncertain- 10.1±0.6% on time-scales longer than 400 s. The hard-
ties(centroidpositionandregistration)were0.05′′inRA ness (2.0–10 keV intensity divided by 0.3–2.0 keV in-
and 0.04′′ in Decl. tensity) was consistent with being constant (χ2/dof =
For either solution, XB135 is confined to the central 253/216), from which we infer that this variation is
region of the GC at the 3σ level; this is consistent with energy-independentandlikelyduetothestochasticvari-
XMM-Newton and Chandra study XB135 3
Bin time: 400.0 s
XB135 60
1 g s−1 0
er 4
I / count s−10.3−10 keV 0.5 L / 10 370.3−10 keV 20
0
0 1000 2000 3000 4000 5000
MJD−51450 / day
0
0 5×104 105
Figure 3. Long term unabsorbed 0.3–10 keV luminosity
Time (s)
lightcurve for XB135. Circles represent Chandra ACIS observa-
Start Time 16104 6:33:03:182 Stop Time 16105 11:33:03:182 tions,whilecrossesrepresentXMM-Newtonobservations.
Figure 2. Source (black) and background (gray) 0.3–10 keV pn
lightcurves for XB135 from our 120 ks, 2012 June 26–27 XMM- s−1. We present this fit to the unfolded pn spectrum in
Newton observation. Periods of intense background flares were Figure 4. The 2–20 keV disk contribution to the flux
removed. XB135 appears to have been somewhat variable, since was 69±11% (1σ), meaning that XB135 was not in a
it exhibited variation not seen in the background lightcurve; the
r.m.s. variabilitywas10.1±0.6%. hardstate(c.f. Remillard&McClintock,2006). Instead,
our spectrum of XB135 was consistent with either the
ations in accretion rate. thermallydominatedstateorthe steeppowerlaw(SPL)
Figure 3 shows a 0.3–10 keV, unabsorbed luminosity state. Theinnerdisktemperatureissubstantiallyhigher
lightcurve of XB135, using Chandra/ACIS and XMM- thanformostGalacticBHXBs;howeverGRS1915+105
Newton observations, represented by circles and crosses has exhibited similar spectra, because the high BH spin
respectively; the earlier XMM-Newton observation was results in a smaller ISCO and increased energy output
analyzed in Barnard,Clark & Kolb (2008). The Chan- (Remillard & McClintock 2006).
draACISluminositieswerederivedfromabsorbedpower We note that the SPL state is only observed during
law models (Barnard et al. 2012), while the XMM- outbursts of transient BH XBs in our own galaxy. How-
Newton luminosities were derived from two-component ever, the two persistently bright, dynamically-confirmed
models. The XMM-Newton observations appear to have blackhole+Wolf-RayetbinariesIC10X-1andNGC300
somewhat lower luminosities than the ACIS observa- X-1 both exhibit emisson spectra consistent with the
tions; it is possible that the ACIS luminosities are over- SPL state, as does the persistent BH XB LMC X-1; in
estimated. However,theclosestACISobservationtoour Barnard,Clark & Kolb (2008) we suggested that such
120 ks XMM-Newton observation occurred 15 days ear- spectraareemittedbyBHXBswithstablecoronae,sim-
lier, and has a luminosity that is consistent within 3σ. ilar to the spectra observed from persistently bright NS
Furthermore, the variation between the archival XMM- XBs.
Newton observation and the nearest neighbor (a fac- OnepossibledifferencebetweenthespectrumofXB135
tor ∼1.4 over 73 days) is consistent with variability we andtheSPLstateisthatthespectrumofXB135exhibits
have seen in bright X-ray binaries (Barnard et al. 2012, significant curvature at energies above ∼2 keV. In this
2014a). way it is similar to the high quality spectra observed
from ultra-luminous X-ray sources (ULXs) studied by
3.3. Simple spectral modeling Gladstone et al. (2009). Fitting our XB135 spectrum
witha brokenpowerlaw model(bknpower inXSPEC)
Since our XMM-Newton lightcurve exhibited substan-
hadΓ change from 1.53±0.02to 2.52±0.09at 4.64±0.13
tial variability, we accumulated spectra using only the
keV; these parameters are consistent with several ULXs
persistent emission (0.3-10 keV intensity <1 count s−1);
analyzedbyGladstone et al.(2009). Theyproposedthat
this spectrum contained 53500 counts, with 2% coming
ULXs exist in a special ultra-luminous state, and this
from the background.
may be true because the luminosities of the ULXs are
Werebinnedthespectrumtoincludeatleast20counts
considerably higher than that of XB135. However, it is
per bin. An absorbed power law fit to the spectrum
possiblethatULXshavesimilarspectratootherBHXBs
(tbabs*po) was rejected (χ2/dof = 1273/1033,good fit
(disk blackbody + power law) and the curved spectrum
probability, g.f.p., = 4×10−7). An absorbed disk black-
is caused by a dominant thermal component.
body fit (tbabs*diskbb) was unsuccessful also (χ2/dof We note that some authors including Gladstone et al.
= 1185/1033,g.f.p. = 7×10−4). (2009) consider fits to be unphysical when the low en-
Fitting a disk blackbody + power law yielded ab- ergyspectrumisdominatedbytheComptonizedcompo-
sorption (NH) = 2.7±0.3×1021 atom cm−2, kTDBB = nent. However,spectralanalysisofultra-luminousX-ray
2.15±0.08 keV, and Γ = 2.2±0.3; all uncertainties are sourcesinNGC253andthe confirmedBH+ Wolf-Rayet
quoted at the 1σ level; χ2/dof = 984/1031 (g.f.p. = binary IC10 X-1 with XMM-Newton strongly suggests
0.85). The 0.3–10 keV luminosity was 4.1±0.3×1038 erg thatthissoftexcessisreal;fittingthesespectrawiththe
4 Barnard et al.
Unfolded Spectrum around a spinning BH (following Steiner et al. 2009).
0−4 XB135 TBABS*(DISKBB+PO) The variables for the emission model are: the fraction
1
× of disk emission that is Comptonized (f ), the BH spin
5 C
parameter (a), the disk inclination (i), the BH mass
V)−1 2×10−4 (gMs−BH1))., aTnhdethKeEaRcRcrBetBiomnoradteel a(Mls˙o, rneoqrumiraelsizeaddtiosta1n0c1e8
e
m s k−2−1 10−4 (sRaeBlsfs-Buirmrmaedodidatetoliowbneeflr7ea8gk0eakpnptdca)li;tmttbhheedsadprekefceatnurilantlgshfleatartgdineognfsi.tnhgeFfoKarcEttoRhre-,
ns c 10−5 Comptonized emission, we assumed Γ = 2.4, as is typi-
o ×
ot 5 cal for the steep power law state. In the end, the results
h
keV (P ×10−5 prDoveecdreraastihnegrtihneseMns3i1tivmeettoaliΓc.ity resultedin hotterspec-
2 tra,translatingaslowerBHmasses. Thesolarmetalicity
10−5 0.5 1 2 5 tfiatlyNield=ed1t.h6e0±hi0g.h0e4s×t1B0H21Mataosms:c2m1.−82±,0f.8 M=⊙0,.5w7±ith0.0to3-,
H C
Energy (keV) a = 0.998±0.005, i = 65.7±0.7◦, M˙ = 8.93±0.05×1017
Figure 4. UnfoldedpnspectrumfromourXMM-Newtonobserva- g s−1, and χ2/dof = 1008/1030. Assuming an absorbed
tionofXB135,withthebestfitabsorbeddiskblackbody+power
lawmodel. Dotted anddashed linesrepresentthe diskblackbody metalicity of 0.015 Z⊙ outside our Galaxy yielded a BH
and blackbody components respectively. The spectrum is multi- mass of 16±3 M⊙, with NHM31 = 4.9±0.4×1021 atom
plied by channel energy, in order to indicate the power at each cm2, f = 0.34±0.10, a = 0.95±0.02, i = 78±3◦, M˙ =
energy. The data recieved further binning in this image for the C
sakeofclarity. 2.5±0.5×1018 g s−1 and χ2/dof = 977/1030. All uncer-
tainties are quoted at the 1σ level.
SIMPL convolution model for Comptonization created We caution that these results should not be treated
by Steiner et al. (2009) results in rejection by NGC253 as accurate, since we make assumptions in our modeling
ULX3 and IC10X-1, andresults in ∆χ2 = +12overthe that may not apply. As such, we find that the mod-
two componentmodel fits with the same degrees of free- els suggest a ∼10–20 M⊙ BH with spin ∼1, somewhat
dom for NGC253 ULX1 and NGC253 ULX2 (Barnard like the Galactic BH LMXB GRS 1915+105 which has
2010). a spin >0.98 (McClintock et al. 2006). They only con-
Barnard (2010) proposed that this soft excess is due sideredspectrawithluminosities<30%Eddingtonwhen
to an extended corona, and the evolution exhibited by estimating the spin, to ensure that the disk is thin. Our
IC10X-1during “eclipse”providedverystrongevidence Comptonized Kerr BH models yielded 0.3–10 keV lumi-
for a corona radius & 1011 cm (Barnard et al. 2014c), nosities ∼3×1038 erg s−1, ∼12–24% Eddington for BH
consistent with with the empirical relation between 1– masses ∼10–20 M⊙, which is in the range considered by
30 keV luminosity and corona radius observed in Galac- McClintock et al. (2006); our disk blackbody + power
tic high inclination XBs (Church & Bal ucin´ska-Church law fit yielded a luminosity ∼4×1038, which is 16–32%
2004). A coplanar, extended corona may feed from Eddington for a 10–20 M⊙ BH. Unlike GRS 1915+105,
cool photons from the outer disk while heating the XB135 appears to have a strong Comptonized compo-
disk surface and stimulating production of cool photons nent, which may complicate our interpretation.
(Haardt & Maraschi 1993). WealsonotethatMcClintock et al.(2006)identifieda
weakness in the kerrbb model: the spectral hardening
3.4. BH emission models
parameter (color temperature divided by effective tem-
We decided to estimate the BH parameters for XB135 perature)isassumedtobeaconstant. Instead,theyused
(assuming a BH accretor)by adopting a model that has a tabular model kerrbb2 where the hardening is com-
been used to describe Galactic BH binaries. Studies of puted. However, the hardening factor for the range of
the metalicity in the M31 bulge (Jablonka & Sarajedini parametersthat we areinterestedinis likelyto be ∼1.7,
2005) and part of the halo (Durrell, Harris & Pritchet thevalueassumedinourmodel(J.Steiner,privatecom-
2001) show that the metalicity varies over 2 orders of munication). Hencethedifferencesbetweenkerrbband
magnitude in each region. While it is unlikely that the kerrbb2modelsarenotexpectedtobegreatforXB135.
hostGCB135(Z ∼0.015Z⊙)harborsmuchmaterial,we 3.5. Comparison with Galactic NS binaries
obtained best fit spectra for models where the absorber
metalicity Z = 0.1–1.0 Z⊙ as well as Z = 0.015. To do GalacticNSXBsexhibitawiderangeofbehaviors;re-
this,wealteredthemetalicitywiththeabundcommand centlyLin et al.(2007,2009,2012)parameterisedthefull
in XSPEC. gamut by examining thousands of RXTE spectra from
The XSPEC model used was persistentandtransientXBsexhibitingallspectralstates
WABS*TBABS*SIMPL*KERRBB, described in using the same disk blackbody + blackbody emission
the following paragraphs. For absorption, we included model. They found that their model described all NS
7×1020 atom cm−2 at Solar metalicity to account XBspectrawell,withtwoexceptions: lowaccretionrate
for Galactic absorption (WABS); we also included an states where the emission dominated by Comptoniza-
absorber with variable metalicity (TBABS, with the tion (Lin et al. 2007, 2009); and the so-called Horizon-
abundance controlled by the abund command). tal Branches of the highest luminosity NS XBs called Z-
We modelled the emission as SIMPL*KERRBB, de- sources (Hasinger & van der Klis 1989) where a Comp-
scribing the partially Comptonized emission from a disk tonized component is required in addition to the disk
XMM-Newton and Chandra study XB135 5
blackbody and blackbody components (Lin et al. 2009, thethreecomponentmodelaswellasthetwocomponent
2012). Lin et al.(2010)successfully appliedtheir double model.
thermal model to Beppo-SAX and Suzaku spectra of a
3.6. Comparison with an XMM-Newton observation of
persistent NS XB, with energy ranges 1.0–150 keV and
the Z-source LMC X-2
1.2–40 keV respectively; hence the apparent success of
the double thermal model is not confined to the RXTE LMC X-2 is a NS XB residing in the LargeMagellanic
band. Wenotethatthisapproachisnotnecessarilyreal- Cloud which exhibits spectral and timing behavior rem-
istic,buttheapproachisvaluablebecauseitallowsusto iniscent of a Sco-like Z-source (Smale & Kuulkers 2000;
assess the full gamut of NS LMXB behavior in a single Smale, Homan, & Kuulkers 2003). It is also extremely
parameter space. bright for a NS XB, ∼0.5–2 LEdd where LEdd is the Ed-
Wehaveidentified50BHCsinM31bycomparingdisk dington limit. It has a known counterpart, and a mass
blackbody + blackbody fits to their spectra with the ratio ≤0.4 (Cornelisse et al. 2007).
range of parameters exhibited by Galactic NS XBs (see Unlike the Galactic Z-sources, LMC X-2 is suf-
Barnard et al. 2014b, and references within). For each ficiently faint for observation with XMM-Newton;
BHC, we calculated the probability that the disk black- Lavagetto et al.(2008)presenttheiranalysisofthe 2003
body temperature,blackbodytemperature,andthe disk April 21 XMM-Newton observation of LMC X-2, which
blackbody contribution were consistent with the NS XB wasconductedinSmallWindowmodewiththeMedium
parameter space (P , P and P respectively). Mul- filter, allowing pn spectra that were free from pile-up.
DBB BB f
tiplying these probabilities together yielded the proba- They usedSAS version7.0andXSPEC version11.3.2ad
bility that the BHC was consistent with being a NS XB to analyze the pn and RGS spectra. After establish-
(P ). Since the only Galactic NS XBs to exhibit lumi- ing that the background was stable, and that the spec-
NS
nosities comparable with XB135 are Z-sources, we com- trum varied little over time, Lavagetto et al. (2008) ex-
pared our spectrum with the Z-source states studied by tracted source and background spectra from the whole
Lin et al.(2009,2012);theyfoundthatkT >1.3keV, observation. They found a large discrepancy between
DBB
kT >2.2 keV, and f >0.66. the pn and RGS spectra for energies below 1.3 keV,
BB D
and attributed this to poor pn calibration for bright X-
3.5.1. Comparison with disk blackbody + blackbody emission raysources (Boirin & Parmar2003); they therefore only
models used pn data above 1.3 keV.
Lin et al. (2009; 2012) found that the major- Lavagetto et al. (2008) fitted the pn and RGS spec-
ity of spectra were well described by disk blackbody tra with various models, including the disk blackbody
+ blackbody emission models. We created mod- + blackbody model adopted by Lin et al. (2007, 2009,
els with Z = 0.015–1.0Z⊙ using the XSPEC model 2012). Their best fits yieldeda disk blackbodytempera-
WABS*TBABS*(CFLUX*DISKBB+CFLUX*BB); the tureof0.815±0.002keV,andablackbodytemperatureof
fluxes were obtained in the 2–20 keV range for better 1.543±0.009keV,which aresignificantly coolerthan the
comparison with the results of Lin et al. (2009; 2012). parameters obtained from the Z-source spectra of XTE
As the metalicity of the M31 absorber increased, J1701−462andGX 17+2(Lin et al.2009, 2012)and re-
kT , kT , and f systematically decreased; kT markablysimilarto the parametersthatwe obtainedfor
DBB BB D DBB
fell from 1.15±0.10 keV to 0.87±0.05 keV, kT from XB135. We therefore decided to investigate the XMM-
BB
1.84±0.15keVto1.57±0.05keV,andf from0.41±0.08 Newton observation of LMC X-2 ourselves.
D
to 0.26±0.03. P ranged from 10−5.1 to 10−93. Using SAS version 13.0 and XSPEC version 12.8.1p,
NS
we modeled the pn spectrum above 1.3 keV using a
3.5.2. Comparison with disk blackbody + blackbody + power disk blackbody + blackbody model (XSPEC model
law emission models WABS*(DISKBB+BB) for consistency with Lin et al.
Lin et al. (2009; 2012) found that the Horizon- (2007, 2009, 2012)). The spectrum contained ∼750,000
tal branch states of XTE J1701462 and GX 17+2 re- counts, with the source contributing ∼96%. We binned
quired a Comptonized component, and found that the the spectra to a minimum of 20 counts per bin. The ab-
combined flux contribution of the disk blackbody and sorption was unconstrained, so we adopted the best fit
Comptonized component (f ) was >76% of the column density obtained by Lavagetto et al. (2008) for
DBB+PL
total. Hence we also made a series of models with the disk blackbody + blackbody model, 4×1020 atom
WABS*TBABS*(CFLUX*DISKBB+CFLUX*BB+ cm−2.
CFLUX*PO); we fixed the photon index of the Comp- Our best fit disk blackbody and blackbody tempera-
tonized component to 2.4, as before. tures for LMC X-2 were 1.36±0.04 keV and 2.42±0.08
For these models kT and kT systematically keV respectively with χ2/dof = 1812/1737, very differ-
DBB BB
increased with increasing metalicity: kT from enttotheresultsobtainedbyLavagetto et al.(2008)and
DBB
1.14±0.10keVto1.4±0.4keV,andkT from1.83±0.15 consistent with the parameters observed in Galactic Z-
BB
keVto2.4±0.8keV,withf risingfrom0.35±0.05 sources (Lin et al. 2009, 2012). When the temperatures
DBB+PL
to 0.6±0.3. We find that for Z &0.4, P >0.01, and were frozen at the values obtained by Lavagetto et al.
NS
XB135 is consistent with NS XB spectra fitted with this (2008), χ2/dof = 3002/1739.
emission model at the 3σ level. However, we note that We present our best fit disk blackbody + blackbody
theuncertaintiesinallparametersaresignificantlylarger fits to 0.3–10 keV and 1.3–10 keV pn spectra for LMC
for this three-component emission model than for the X-2 in the left and right panels of Figure 5 respectively.
previous two-component model; hence deeper observa- We also present residuals. The 0.3–10 keV fit yielded
tions of XB135, or with a broader energy range, could unaccepatable results, with strong systematic variations
potentially separate XB135 from Galactic Z-sources for in the residuals. The null hypothesis probability for the
6 Barnard et al.
Unfolded Spectrum Unfolded Spectrum
LMC X−2 XMM−Newton pn 0.3−10 keV χ2/dof = 3002/1739 LMC X−2 XMM−Newton pn 1.3−10 keV χ2/dof = 1812/1737
0.05 0.05
m s keV)−2−1−1 0.02 m s keV)−2−1−1 0.02
keV (Photons c 5×01.00−13 keV (Photons c 5×01.00−13
2×10−3 2×10−3
normalized counts s keV−1−1 −−0242 normalized counts s keV−1−1 −0121
0.5 1 2 5 0.5 1 2 5 10
Energy (keV) Energy (keV)
Figure 5. Best fit diskblackbody + blackbody models forthe 0.3–10 keV pn spectrum (left) and 1.3–10 keV pnspectrum (right) from
the Z-source LMC X-2. The dotted and dashed lines represent the disk blackbody and blackbody components respectively. The lower
panel shows the χ2 residuals. For the leftpanel, wesee considerable oscillationinthe residuals,indicating abad fit fromthe 0.3–10 keV
spectrum. For the 1.3–10 keV fit, there are no systematic residuals, but some channels show large deviations that are probably due to
uncertainties incalibrationinthis750,000countspectrum.
1.3–10 keV fit (i.e. that differences between model and Stiele et al. (2011) found 1897 X-ray sources within
data are due to random variations in the spectrum) is the M31 D region at luminosities > 1035 erg s−1;
25
0.10, making this an acceptable fit even though χ2 ∼5– around 60% of these sources were either classified only
10forsomechannels;thesearelikelyduetouncertainties asHard,orunclassified,andStiele et al.(2011)expected
in calibration, which are only significant for very bright these to be AGN. However, the Chandra survey of the
sources. M31 center (within ∼20′ or 4 kpc of M31*) revealed
The fact that our new parameters for LMC X-2 are over 200 X-ray sources with luminosities >1035 erg s−1
consistent with the Galactic Z-sources suggests that the that were missed by XMM-Newton, thanks to Chan-
results obtained by Lavagetto et al. (2008) were proba- dra’slowerbackgroundandsuperiorPSF(Barnard et al.
bly affected by calibration errors, especially since they 2014a). Furthermore, we identified ∼200 new X-ray bi-
reported the discrepency between RGS and pn fits be- naries,and foundstrongevidence that the populationof
low 1.3 keV yet chose to fit the RGS spectra. We note X-ray sources consistent with AGN contains a number
that Lavagetto et al. (2008) reported >5400 degrees of ofunidentifiedXBs. Hence therearemoreX-raysources
freedom in their spectra, and most of these come from andfewerAGNwithintheM31D regionthanexpected
25
low-weighted RGS bins; hence it is possible that their by Stiele et al. (2011).
RGS+pn fit would be rejected if they had only consid- In Barnard et al. (2014b), we present analysis of 50
ered the pn data. BHCsin M31,adding 13transientsand2 GC XBs stud-
Fitting an absorbed power law model to the 1.3–10 ied with XMM-Newton to our existing sample. By com-
keV pn spectrum of LMC X-2 allows us to compare the paringthe peristentandtransientXB populationwithin
general shape of its spectrum with that of XB135. We 6′ of M31∗, we found that >40% of XBs >1035 erg
were unable to constrain the absorption due to the 1.3 s−1 are expected to contain BH primaries. Remarkably,
keV cut-off; however, Lavagetto et al. (2008) found NH BHC XBs represent >90% of those XBs with luminosi-
to be ∼4–10×1020 atom cm−2, depending on the emis- ties >1038 erg s−1; this is very different to the Milky
sion model. For N = 1.0×1021, Γ = 1.373±0.003,with Way, where the majority of XBs >1038 erg s−1 contain
H
χ2/dof = 4546/1739, and lower values of N yielded NS primaries.
H
harder Γ; such a spectrum is significantly harder than Of the 15 BHCs associated with GCs, 12 exhibit disk
any BH spectra (Remillard & McClintock 2006). Sim- blackbody+blackbodyspectrathatdifferfromNSspec-
ilarly, fitting XB135 with the best fit temperatures of traby>5σ (Barnard et al.2014b). While∼30M31GCs
LMC X-2 results in χ2/dof = 999/1033, whereas the containX-raysourcesthathaveexceeded1037 ergs−1 at
equivalent free fit yielded χ2/dof = 973/1031; F-testing some point in the last 13 years, only one Galactic GC
shows that the probability that this improvement is due has ever been observed to exceed this luminosity. Hence
to random variations is 1.2×10−6, and the spectra of the lack of confirmed BH XBs in Galactic GCs does not
XB135 and LMC X-2 differ at a >4.8σ level. The spec- hinder our classification of XB 135 as a BHC.
trum of XB135 is significantly softer than that of LMC We know of two M31 X-ray sources that exceed 1038
X-2 when observed with the same instrument, strength- erg s−1 in the 0.3–10 keV band, and most likely con-
ening our case for XB135 containing a BH accretor. tain NS accretors. First is XB158, a high inclination
X-ray binary that exhibits repeated intensity variations
3.7. Comparison with M31 XBs on a ∼2.8 hr period (Trudolyubov et al. 2002) as well
as a superorbital period of ∼5.8 days that was recently
While it is instructive to compare the X-ray popula-
revealed by daily Swift observations over 30 days (R.
tions of M31 and the Milky Way, this does not provide
Barnard et al. 2015, ApJ submitted). Its 2002 XMM-
the full picture because the X-ray populations of these
Newton spectrum is consistent with disk blackbody +
two galaxies are very different.
XMM-Newton and Chandra study XB135 7
Z−source χ2/dof = 381/111 for constant Γ [Prob 2E−31] XB135 χ2/dof = 27/12 for constant Γ [Prob 7E−3]
0 0
8 8
g s−1 60 g s−1 60
I / 10 er370.3−10 2040 I / 10 er370.3−10 2040
0 0
2 2
Γ 1.5 Γ 1.5
1 1
0.5 0.5
0 0 2000 4000 0 0 2000 4000
MJD − 51450 / day MJD − 51450 / day
Figure 6. ChandraACIS0.3–10keVluminositylightcurves(top)andspectralhistories(bottom),fortheM31Z-sourceRXJ0042.6+4115
(left)andXB135(right),presentedwithequalscaling. Wefoundthebestfitabsorbedpowerlawmodelforeachobservation,andexamined
thespectralvariabilitybyfittingalineofconstant photonindex;XB135isconsistentwithnospectralvariationatthe3σ level,whilethe
Z-sourceRXJ0042.6+4115 exhibitedsubstantial spectralvariability.
blackbody + power law fits to Galactic NS XBs; how- 4. DISCUSSIONANDCONCLUSIONS
ever, a simple power law fit yields Γ 0.57±0.09, con-
We obtained new XMM-Newton and Chandra obser-
siderably harder than any BH spectrum (Barnard et al.
vations of the BH candidate associated with the M31
2015). Second is RX J0042.6+4115,which exhibited tri-
globular cluster B135. Previous results were suggestive
m(Boadranlasrpdeecttraall.e2v0o0lu3)t.ioPnoawnearlalagwoufisttsotGoaoluarct∼ic1Z0-0soCuhracnes- oXfMaMn -eNxterwemtoenlyyiheilgdhedBH∼5m0,a0s0s0onfe∼t5s0ouMrc⊙e.cTouhnets12f0rokms
dra ACIS observations of RX J0042.6+4115 yielded a
intervals free from background flares and source varia-
mean Γ of 0.856±0.008(Barnard et al. 2014a), although
tion. A high luminosity, hard state fit to such a high
the spectrum varied considerably, as discussed below.
quality spectrum would have been extremely compelling
While these power law emission models are clearly not
evidenceforsuchamassiveaccretor;unfortunately,such
realistic,theydemonstratethatthesetwobrightM31NS
ahardstatewasruledout. Fitting ablackhole emission
XBs are significantly harder than XB135.
modeltoourXB135spectrumsuggestsablackholemass
InFigure6wecomparethe0.3–10keVlightcurvesand ∼10–20M⊙, considerablysmaller than the ∼50M⊙ BH
spectral evolution from XB135 and the M31 Z-source
previously predicted.
candidate RX J00421.6+41152 over 13 years’ worth of
Our Chandra HRC observation provided the first ob-
Chandra (see Barnard et al. 2014a, for main survey
servation of XB135 where the PSF was not greatly en-
results). Z-sources evolved over time-scales of days to
largedduetobeingobservedatahighoff-axisangle. We
weeks (Muno et al. 2002), hence if XB135 were a Z-
find that XB135 is a single point source, located at or
source, we should expect substantial spectral variabil-
very near the center of the GC B135 with a 3σ position
ity. We present the 0.3–10 keV lightcurve in the up- uncertainty of 0.15′′ in R.A. and 0.12′′ in Decl.
per panel, and fit photon index for the best fit absorbed
We prefer a BH primary for reasons stated earlier in
power law emission model in the lower panel; the scales
the paper. We expect >90%of M31 X-ray binaries with
are identical. We fitted each object with the best fit 0.3–10 keV luminosities >1038 erg s−1 to contain BH
line of constant Γ. For XB135, χ2/dof = 27/12, with accretors(Barnard et al.2014b),and∼90%ofthosevery
probability 7×10−3, meaning that Γ is consistent with bright X-ray sources are located in GCs. Also, the 0.3–
being constant at the 3σ level. For the Z-source RX 10 keV spectrum of XB135 is significantly softer than
J0042.6+4115, the best fit line of constant Γ yielded the similarly luminous Z-source LMC X-2 when studied
χ2/dof = 381/112, with a probability of 2×10−31. We with the same instrument (XMM-Newton pn), as well
note that the ACIS observations of RX J0042.6+4115 as RX J0042.6+4115 and XB158. Furthermore, XB135
didresultinpile-up; however,one wouldexpect Γ tode- exhibits less spectral variability than the M31 Z-source
crease for a piled-up source as the intensity increased, RX J0042.6+4115. Fitting the XB135 spectrum with a
yet Γ for RX J0042.6+4115 substantially increased, so disk blackbody + power law model yields results that
thetruespectralvariationofRXJ0042.6+115waslikely are consistent with the steep power law state when the
to be greater than observed. We present this as evi- uncertainties are considered; while the steep power law
dence in support of a BH accretor for XB135. We note state is only exhibited by transient BH XBs in our own
that RX J0042.6+4115 exhibited flares during the 2002 galaxy, persistent BH XBs IC10 X-1, NGC300 X-1, and
XMM-Newton observation similar to those observed in LMC X-1 all exhibit spectra consistent with the steep
Figure2;however,theflaresobservedinRXJ0042.6+15 power law state.
wereclearlyenergydependentwithhigheramplitudesat However, some aspects of the behavior observed from
higher energies (Barnard et al. 2003), in contrast with XB135 may contradict our BH assessment. We note
the energy-independent flares observed from XB135. that the best fit disk blackbody + power law param-
eters for XB135 are not obviously attributable to any
8 Barnard et al.
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