Blue Supergiant X-ray Binaries in the Nearby Dwarf Galaxy IC 10

0.98 MB·

by  Silas G. T. Laycock

Draft version January 17, 2017
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
BLUE SUPERGIANT X-RAY BINARIES IN THE NEARBY DWARF GALAXY IC 10
Silas G. T. Laycock 1, Dimitris M. Christodoulou 1, Benjamin F. Williams 2, Breanna Binder 3, and Andrea
Prestwich4
Draft version January 17, 2017
ABSTRACT
In young starburst galaxies, the X-ray population is expected to be dominated by the relics of the
most massive and short-lived stars, black-hole and neutron-star high mass X-ray binaries (XRBs). In
the closest such galaxy, IC 10, we have made a multi-wavelength census of these objects. Employing
7
a novel statistical correlation technique, we have matched our list of 110 X-ray point sources, derived
1
from a decade of Chandra observations, against published photometric data. We report an 8σ corre-
0
lation between the celestial coordinates of the two catalogs, with 42 X-ray sources having an optical
2
counterpart. Applyinganopticalcolor-magnitudeselectiontoisolatebluesupergiant(SG)starsinIC
n 10, we find 16 matches. Both cases show a statistically significant overabundance versus the expec-
a
tation value for chance alignments. The blue objects also exhibit systematically higher f /f ratios
J x v
than other stars in the same magnitude range. Blue SG-XRBs include a major class of progenitors
3 of double-degenerate binaries, hence their numbers are an important factor in modeling the rate of
1
gravitational wave sources. We suggest that the anomalous features of the IC 10 stellar population

areexplainediftheageoftheIC10starburstisclosetothetimeofthepeakofinteractionformassive
]
E binaries.
H Subjectheadings:galaxies: individual(IC10)—pulsars: general—stars: neutron—stars: supergiants—
surveys—X-rays: binaries
.
h
p
- 1. INTRODUCTIONANDMOTIVATION these identifications, otherwise the logN −logS relation
o will be heavily biased by foreground Galactic interlop-
The dwarf starburst galaxy IC 10 is remarkable for its
r ers. We have adopted a multi-wavelength approach in
t young stellar population (Massey et al. 2007), high den-
s order to narrow down the probable nature of the X-ray
sity of Wolf-Rayet (WR) stars (Crowther et al. 2003),
a population in IC 10.
[ andmassivestarsingeneral(Massey&Holmes2002). It
hosts the WR+BH (black hole) binary IC10 X-1 (Prest- Compact and dwarf galaxies present the opportunity

1 wich et al. 2007), the blue-supergiant (BSG) transient for “whole galaxy” population studies, and can thus
v X-raybinaryIC10X-2(Laycocketal.2014),andanex- probe a wide parameter space in age, mass, and metal-
3 licity. The production rate of high mass X-ray binaries
cess of X-ray point sources concentrated within the op-
0 (HMXBs) is linked to the star formation rate (SFR) by
tical outline of the galaxy (Wang et al. 2005). We have
8 a scaling law (Grimm et al. 2003), the universality of
identified more than a dozen other transient and vari-
3 which raises interesting questions. Using a large sample
ableobjectsamong110pointsourcesdetectedinaseries
0 of blue compact galaxies, Brorby et al. (2014) revealed
of 10 Chandra observations spanning 2003-10 (Laycock
. a significantly higher abundance of HMXBs for a given
1 et al. 2016). The ongoing starburst has produced a rich
0 populationofexoticcompact-objectbinaries. Giventhat SFR than in spiral galaxies. Additionally, the slope of
7 the age of the starburst is only ∼6 My, these must in- theXLFisfoundtobesteeperinthebluecompactsam-
1 volve progenitors of extreme masses and short lifetimes. ple, in the sense that the XRB population is skewed to
v: Identifying the nature of each individual source is a slow higherLX valuesinadditiontobeingmorenumerouson
aper-SFRunitbasis. Prestwichetal.(2013)foundfrom
i and difficult process, but ultimately necessary if we are
X toobtainacompletecensusofX-raybinaries(XRBs)for their analysis of a large sample of galaxies that ultralu-
minous X-ray sources (ULXs) (L > 1039erg s−1) are
r comparison with other galaxies, for example the Magel- X
a lanic Clouds (Coe & Kirk 2015; Antoniou et al. 2010) preferentially formed at low metalicities.
The inference preferred by many authors is that low
and the local volume survey (Binder et al. 2015a). Even
metallicities drive higher compact-object masses, as for
to obtain the X-ray luminosity function (XLF) requires
example in the direct-collapse scenario of Mapelli et al.
(2010). Indeed,stellarevolutioncalculationsalsopredict
1Lowell Center for Space Science and Technology, Uni-
versity of Massachusetts Lowell, 600 Suffolk Street, Low- sucharelationship(Belczynskietal.2010). Acounterar-
ell, MA 01854, USA. Email: silas [email protected], dim- gumenthoweverwouldbethatthestellarcompanionsin
itris [email protected] these systems are of a different nature (or have different
2University of Washington, Department of Astron-
binaryparameters)suchthatsystematicallyhighermass-
omy, Box 351580, Seattle, WA 98195, USA. Email:
[email protected],[email protected] accretion rates are delivered. In support of this idea, we
3Department of Physics and Astronomy, California State point to recent studies of bright extragalactic XRBs and
Polytechnic University, 3801 West Temple Ave, Pomona, CA ULXs such as M82 X-2 (Bachetti et al. 2014), NGC 300
91768
4Harvard-Smithsonian Center for Astrophysics, 60 Gar- X-1(Binder et al. 2015b), and IC 10 X-1 (Laycock et al.
den Street, Cambridge, MA, 02138, USA. Email: aprest- 2015; Steiner et al. 2016). These cases are actively re-
[email protected] casting the paradigm of ULXs as extreme binaries host-
2 Laycock et al.
ing black holes of >100 M , as instead representing the action. IC 10 can be seen as a laboratory for obtaining
(cid:12)
upperendofcontinuousXLFsforneutron-star(NS)and improved inputs to these models.
BH binaries; perhaps the same skewed XLF discovered In this paper, we focus on the statistical correlation
by Brorby et al. (2014). between X-ray and optical catalogs and on the resulting
Testing these scenarios requires nearby galaxies where photometric properties of the sample of positional coun-
the stellar companions can be resolved and optical spec- terparts. We begin by matching source lists, combining
tracanbeobtained. IC10istheclosesttobeinganideal the optical and X-ray photometry, then make a series of
laboratory for such a study since its O and B stars are selections on the observable source properties. Finally,
within range of ground-based telescopes, yet its angular we assess the correlations that emerge between these
size (<8 arcmin) still fits within a single Chandra ACIS- subsets.
S3 field.
IC 10 is a unique galaxy and reconciling the strange
features of its stellar population and structure will pro- 2. OPTICALCOUNTERPARTSANDCOLOR-MAGNITUDE
vide further insight into XRB formation. Researchers SELECTIONFORBLUESUPERGIANTS
using different SFR indicators have reported wildly dif- Following the procedures detailed in Laycock et al.
fering results for IC 10 (Chomiuk & Povitch 2011). The (2014), we matched our Chandra point-source catalog
“brightest radio SNR” method yields the highest rate against the Massey et al. (2007) photometric catalog
whichtiesinwiththeabundanceofmassivestars. Leroy which has a limiting magnitude of V≈24. A total of 42
et al. (2006) suggests that the starburst has not yet X-raysources(40%)haveapositionalcounterpartwithin
peaked, and since its WR stars each represent a future their95%confidenceradius(inquadraturewitha1(cid:48)(cid:48) tol-
type II supernova, IC 10 has produced only a small frac- erance to account for systematic error). The remaining
tionofitseventualsupernovaremnantsandcompactob- 69 sources have no counterpart to down to V=23.8 (the
jects. TheworkofEldridgeetal.(2013)ontheevolution magnitude of the faintest counterpart found). In Fig-
of massive binaries points to interaction as an under- ure 1, we show the color-magnitude diagram (CMD) for
estimated driver of core-collapse supernovae (SNe). In these candidate counterparts. At the distance and red-
essence,mass-transferbetweenthetwostarsnarrows(or dening of IC 10 (µ = 24 , A = 3, E = 0.85), the
V B−V
even reverses) their mass-ratio causing the initially less mainsequenceisnotvisiblemuchbeyondB0Vinground-
massive star to undergo an accelerated evolution. Al- based images under normal seeing conditions. BSGs are
though many scenarios are possible depending on the among the most luminous stars and will be above the
initial mass ratio and separation, the main result that crowing limit, nonetheless this is a limitation for our
emergesisagreatlyenhancedpopulationofblueandyel- HMXB study since the literature points to most HMXB
lowsupergiantbinariesatthepre-andpost-core-collapse counterpartsbeinghotterthanB3withthespectraltype
stages. Enhanced tidal stripping of the hydrogen en- distributionpeakingatB0(Antoniouetal.2009). There-
velopes also leads to elevated production of WR stars. fore we estimate the catalog to be about 50% complete
Interestingly, the Massey et al. (2007) HR diagram for forHMXBcounterparts. ThelineofsighttoIC10passes
IC10showsalargepopulationofblueandyellowsuper- though the outer part of the Galactic Plane, thus fore-
giant stars, in addition to the abundance of WR stars ground stars lie in the field. Fortunately, the redden-
already noted. ing vector is such that the main sequence defined by the
HMXBs containing blue supergiants (incl. WR stars) Galacticstarsiswellseparatedincolor-magnitudespace
are expected to be a leading channel for production from the IC10 BSG branch. This fact was analyzed in
of double-degenerate binaries (DDs) of the BH+BH, detail by Massey et al. (2007) and we follow their Fig-
BH+NS,NS+NSspeciesBulik, Belczynski, &Prestwich ure 14 (of which our Figure 1 is a subset of the same
(2011); Bogomazov (2014). If a massive binary system data) to annotate the CMD highlighting the locus of IC
survives the first supernova and the remaining star is 10 BSGs and the foreground sequence.
sufficiently massive, it too may undergo supernova, with Having selected our sample of BSG-XRB candidates,
someprobabilityofeachstarendingaseitherBHorNS, we examined their photometric properties in Figure 2
while remaining gravitationally bound. A wide range which is the color-color diagram (B-V, V-R). All coun-
of evolutionary paths have been studied, some involving terparts are plotted and the BSG subset is highlighted.
mass transfer, common envelope, or other binary inter- We see the expected linear sequence defined by the ma-
action. As in the Mandel & de Mink (2016) mechanism jority of the optical counterparts. Interestingly, many
forcreatingdoubleblackholesbyhomogeneouschemical of the blue objects in B-V are significantly redder in V-
evolution driven by rotational mixing of the stars’ inte- R compared to the blackbody locus. This effect cannot
riors. Secure DD discoveries include the Hulse-Taylor be interstellar absorption because the stars are already
pulsar (NS+NS), the WD+NS pulsar of Antoniadis et bluerinB-Vthanmostoftheforegroundpopulationand
al. (2013), and some 10s of binary WDs Gianninas et al. absorption is stronger at shorter wavelengths. Similarly,
(2015). The recent advent of gravitational-wave astron- sincetheBSGsarethebluestobjectsinB-V,theyshould
omy has revolutionized this field, piquing interest in the also be the bluest in V-R if their continua follow an ap-
production channels for DDs, and event rates for their proximate blackbody spectral energy distribution. This
mergers, which are now observable as GW signals Ab- red excess is possibly related to outflows from the com-
bott et al. (2016). Production and merger rates for DDs panions and circumstellar material intrinsic to the bina-
have been modeled by several groups (e.g. Dominik et ries. Alternatively AGN could be present in the sample
al. 2015; Abbott et al. 2016; Eldridge & Stanway 2016; and occupy similar regions of the CMD. Spectroscopic
Belczynskietal.2016)consideringawidevarietyofevo- and radio followup are needed to explore these possibli-
lutionary paths, some highly modified by binary inter- ities.
Blue Supergiant XRBs in IC 10 3
positionalaccuracy. Thisisadangerousassumptionand
a needless one, since a quantitative solution to the prob-
6
1 lem does exist: the peak-up test (Laycock et al. 2005;
Foreground Field
Zhao et al. 2005) can be used to deal with the case of
Chandra observations of crowded fields where chance
8
1 alignment is non-negligible and each X-ray source has a
different positional uncertainty.
Inthe peak-uptest, thetwoinputcatalogs arerepeat-
0
V 2 IC 10 Blue edly matched against each other, introducing a small
Supergiants offset in the coordinate system at each iteration, and
recordingthelistofmatchingobjectsateachoffset. The
2
2 offsets are arranged on a regular grid whose spacing is
finer than the positional accuracy of the input catalogs
and extends to several times the radius of the largest er-
4
2 ror circles. In this way, the distribution of chance align-
IC 10 YSGs
mentsisobtainedfreeofanyrealones. Theoutputtable
can then be filtered according to any desired properties
0.0 0.5 1.0 1.5 2.0 2.5 of the input catalogs. Our code uses tools from the unix
B-V relational database package starbase5 for the matching
and filtering and R6 for analysis and visualization of the
Fig. 1.— Color-Magnitude diagram for positional counterparts
resultingpeak-uptable. Ifanexcessofrealcounterparts
toChandraX-raysourcesintheIC10field. Ovalsindicatethelocii
of BSGs in IC 10 and the foreground Galactic population which arepresentabovethemediannumberexpectedbychance
lies atop the yellow SG track for IC 10 (comparison with Massey (n¯),apeakofheightn isseeninthe2Dmap,asinFig-
x
etal.2007,Figure14). Afewobjectshavemultiplecounterparts;
ure 3, hence the name “peak-up” test. The significance
then the one at the smallest offset is retained and the others are
of the peak is assessed by computing the standard devi-
indicatedbyasmallerdotsymbol. Dottedlinesshowtheselection
B-V<1.5,V>19.5. ation (σ) of the distribution of chance alignments after
excisingtheregionwithinthemaximummatchingradius
of the zero-offset position. The significance of a positive
correlation peak can be expressed as S = (n −n¯)/σ.
x
ForourChandracatalogversusMasseyetal.(2007),us-
0 (cid:112)
2. ing a matching radius for each source of r = r2 +12
95
(arcsec), we obtained n =42, n¯=16, σ=3.1, and S=8.4
x
Since the candidate counterparts depicted in Figure 1
5 fall into distinct regions of the CMD, identified by
1. Massey et al. (2007) as BSGs and foreground main se-
quence stars, we applied a color-magnitude filter to the
output of the peak-up test. Adopting B−V <1.5, and
R
V- 1.0 V < 19.5 (see dotted lines in Figure 1, based on the
photometric properties of the IC 10 population and the
opticalcounterpartofIC10X-2,weobtainn =16,n¯=9,
x
σ=2.16, and S=3.24. This result is shown graphically in
5
0. Figure 4, where the surface defined by the peak-up test
has been azimuthally averaged using a gaussian kernel
(bandwidth=0.1 arcsec) to obtain the radial profile of
0 the distribution. We can further assess the significance
0. of the peak using the fluctuations of the resulting radial
distribution. We find σ = 0.97, S = 24 for all counter-
r r
0.0 0.5 1.0 1.5 2.0 2.5 parts, and σ = 0.56, S = 8 for BSGs.
r r
B-V Confidenceregionsforthepeak-uptestareconstructed
Fig. 2.—Color-colordiagramforpositionalopticalcounterparts. usingbootstrapsamplinginR.Ateachiterationwedraw
ObjectsselectedasBSGcandidatesinFigure1arehighlightedas halfoftheoffsetpositionsinthepeak-uptableatrandom
filledbluesymbols. Asafewobjectshave2counterparts,theone andcomputetheradialprofile(asabove). Theprocessis
atthesmallestoffsetisretainedandtheothersareindicatedbya repeated (104 trials) to accumulate the probability dis-
smaller dot symbol. Several of the BSG stars are red in R-I, in-
dicativeofaninfraredexcessthatcouldberelatedtocircumstellar tribution, from which confidence regions are extracted
material(outflows,disks,etc.). using the quantile function.
4. DISCUSSION
3. PEAK-UPTESTFORPOSITIONALCORRELATION 4.1. Evidence for blue supergiant X-ray binaries
Optical counterpart identification in multi-wavelength First we must establish that our sample of X-ray se-
surveys is a problem encountered frequently in astron- lected BSGs are accreting compact-object binaries since
omy. Oftenresearchersassumethatpositionalalignment
implies physical association, especially when the objects 5 http://hopper.si.edu/wiki/mmti/Starbase
being searched for are rare and both catalogs have good 6 https://www.r-project.org
4 Laycock et al.
All Stars
40 Blue Supergiants
6
1
e)
g
era 35
av
al
muth 0 14
azi 3
es (
h
Matc 25 2
of 1
er
b
m
Nu 20
0
1
5
1
0 2 4 6 8 10
Radial Offset (arcsec)
Fig. 4.—Cross-correlatonbetweenourChandracatalogandthe
opticalcatalogofMassey2007. Inordertoaccumulatetheplotted
radialprofileandthestatisticaldistributionofchancealignments,
thecatalogswererepeatedlymatchedonalargegridofpositional
Fig. 3.—Peak-uptestsurfaceplotshowingthecross-correlaton offsetsasinFigure3. Radialprofilesareplottedforallstars(black,
betweenourChandraX-raycatalogandthe Masseyetal.(2007) left-hand scale) and the blue supergiant subset (blue, right-hand
optical catalog. The surface denotes the number of sources with scale). 95% confidence regions determined by bootstrap sampling
an optical counterpart, generated by matching the two catalogs withakernelbandwidthof0.25(cid:48)(cid:48) areshownasshadedareas. The
over a grid of positional offsets. A sharp peak appears at the narrownessofthecorrelationpeak(<2(cid:48)(cid:48))indicatesthatcrowding
correctalignment,demonstratingthattheopticalcounterpartsare isonlymoderateinthemagnituderangebeingprobed,supported
real. Thevariationsinthesurface,maptherandomfluctuationsin bythefactthatmorethanhalfoftheX-raysourceshavenocoun-
the number of chance alignments. The distribution of the surface terpartdowntoV=24.
heights yields σ and hence the correlation significance. For all
stars,thecorrelationpeakis8.4σ;fortheBSGsalone,itis3.24σ.
Five of the BSGs are X-ray variables as reported by
Laycock et al. (2016) (source numbers 1, 8, 20, 29, 46).
all BSGs are strong sources of X-rays (Berghoefer et The f /f points in Figure 5 are the maximum values
x v
al. 1997). Wind-origin X-rays are seen in both isolated obtained from the highest observed flux in each case.
and binary systems, however for the X-ray luminosities Variability ranges from a factor of a few to ∼100 (#46),
visible at the 660 kpc distance of IC 10, the only vi- corresponding to changes in flux ratio ∆f /f of 0.3-
x v
able power-source is accretion onto a compact object. 2. Optical spectra have been published for source #20
For B3 – O5 spectral types, the intrinsic wind-origin (WR+BH;IC10X-1)and#46(IC10X-2). Thevariabil-
Lx = 1031 − 1033 erg s−1, which is below the (cid:38) 1035 ity properties of these sources were discussed in Laycock
ergs−1 limitoftheChandraobservationscomprisingour et al. (2016).
catalog. Massey et al. (2007) made a search for LBVs in IC
In order to put this comparison on a quantitative ba- 10 using their broad-band photometry (Massey et al.
sis, we computed the distance independent X-ray to Op- 2007) (the same catalog that we have employed in this
tical flux ratio log(fx/fv) = log(fx)+V/2.5+5.37 (fx paper) augmented with narrow-band filter observations
in the energy band: 0.3-8 keV) for all Chandra sources (Hα,SII,OIII)inordertoconstructamulti-indexbased
with counterparts in Massey et al. (2007), as well as up- selection for intrinsically blue emission line stars. Their
per limits for sources with no counterpart in the cata- resulting sample contained 96 candidates, only one of
log. Since the faintest optical counterpart is V=23.8, whichhasanX-raycounterpart(IC10X-2)inourChan-
we adopted this limiting magnitude in the upper-limit dra catalog. This odd result is in line with those au-
calculation. Figure 5 shows the optical magnitude ver- thors’ puzzlement at the small number of WR stars in
sus maximum broad-band (0.3-8 keV) flux from the 10 the emission-line sample. They explained the anomaly
ACIS-S observations, assuming an absorbed power law as an unintended consequence of applying selection cri-
(nH = 5×1021cm−2, photon index Γ = 1.5). Contours teria that were based on M31. In particular, Figure 3 of
of equal fx/fv are shown and sources with BSG coun- Masseyetal.(2007)showsthatthecutonOIIIemission
terparts are flagged. Notably, the highest 5 fx/fv values designedtoweedoutHIIregionshadtheeffectofexclud-
belongtoBSGs,asare7ofthetop9. Forallstarsinthe ing14/17knownWRstarsinthefield,alongwithama-
same magnitude cut (V >19.5), the BSGs have system- jorityoftheemission-lineobjects. Wealsonotethatonly
aticallyhigherfx/fv valueswhichwouldnotbeexpected oneX-raysource(IC10X-1)hasapositionmatchinthe
duetochance. TheMWW7testshowsasignificantoffset WRcatalogofCrowtheretal.(2003);Clark&Crowther
(p=0.01) for BSGs versus all counterparts. (2004). A new emission-line catalog with relaxed crite-
ria would be useful, however spectroscopic follow-up of
7 Mann-Whitney-WilcoxontestimplementedinR. all theoptical counterparts willstill beessential, inview
Blue Supergiant XRBs in IC 10 5
of possible contamination by AGN in the f /f selected conspires to drive both stars toward the blue. This ef-
x v
sample. fectissupportedbythefactthattheprogenitorsofcore-
collapseSNespanawiderregionoftheHRdiagramthan
4.2. Estimating the underlying population do isolated massive stars (Eldridge et al. 2013). In the
caseofSN1993J,theprogenitorstarandthecompanion
If we assume that the n = 16 BSG X-ray objects
x were resolved in pre- and post- explosion HST images
in Figure 2 are all HMXBs, we can estimate their pro-
(Fox et al. 2014) showing just such a situation—the sec-
duction rate and the expected yield of precursor double-
ondaryhavingaccretedtheprimary’shydrogenenvelope
degenerate binaries n as follows:
PDD prior to SN. Eldridge et al. (2013) show that the excess
of BSGs is generated along with a population of yellow
(a) If IC10 has produced n precursor double-
PDD supergiant binaries containing what are effectively sub-
degenerate binaries over the past T ≈6 Myr, then
luminous, weak-lined WR analogs. The former binaries
thedutycycleduringwhichtheLBVsaredonating
would naturally fall into the BSG region of the CMD
mass to their compact companions is D = t /T,
m (Figure 1), whereas the latter would reside into the area
where t is the mean lifetime of the LBVs taken
m inconveniently occupied by the intersection of the IC 10
here to be t =0.4 Myr for a typical LBV mass of
m and foreground populations. The ratio of the two WR
M =30M (Maeder&Meynet2001;Fragosetal.
2 (cid:12) subtypes, WC and WN, has a strong metalicity depen-
2015;Masseyetal.2016). Inthiscase,weestimate
dence Eldridge & Vink (2006); the proportion of WN
thatD =0.067andthatn =n /D =240pro-
PDD x stars at SMC metallicity being an order of magnitude
genitors over the duration of the starburst.
higher than at solar metallicity (data from M31). Ac-
cording to Kangas et al. (2016), the spatial distribution
(b) We require a high mass ratio q = M /M , where
2 1 of WN stars with masses (cid:38)20 M in nearby galaxies
M is the mass of the progenitor of the compact (cid:12)
1 matches that of SNe Ic. These are the most powerful
objectandM >M sothattheprogenitorwillin-
1 2 type of core-collapse SNe and they are believed to be a
deedcollapsefirst. Moe&DiStefano(2013)found
privileged channel of BH production. On this basis, IC
that the close binary fraction of O/B stars with
10 is likely to be producing BH + massive star binaries
P < 20 days and q > 0.1 in the low-metallicity
orb and these would be among the X-ray selected objects in
Magellanic Clouds is F = 0.22(M /10M )0.4,
cl 1 (cid:12) our sample.
from which we find using M = 35 M that
1 (cid:12) The subset that we have assumed to be foreground
F =0.36. Furthermore,Moe&DiStefano(2013)
cl objects could also contain possible progenitor systems
findthattheproportionF of“twin”systemswith
tw of Thorne-Zytkow objects (TZOs) in IC 10, long specu-
q > 0.9 is in the range of 0.06-0.16. We adopt a
lated to result from the merger of a NS with a compan-
typical value of 0.1 for q >0.9, but we double this
ion via common envelope evolution or supernova kick.
estimate to account for additional binaries whose
With its overabundance of SG stars, IC 10 may be a
massratiofallsintherangeof,e.g.,q =0.5−0.9;so
good candidate for producing TZOs. The strongest by
below we use an estimate of F =0.2 for q >0.5.
tw far TZO candidate to date was found recently in the
Then we find that n = n /(F F ) = 222
PDD x cl tw SMC by Levesque et al. (2014); Worley et al. (2016),
progenitors over the duration of the starburst.
who point out that the key observational difference be-
tween a TZO and a normal RSG is the unique elemental
Thus,itappearsthatthe16observedBSGX-raybina-
abundance profile. Tout et al. (2014) and Maccarone
ries in IC10 indicate an underlying population of ∼ 200
& de Mink (2016) provide alternative scenarios for pro-
double-degenerate precursors and a production rate of
ducing intriguing TZO mimics, respectively in the form
200/T ∼30−40Myr−1 overthepast6Myr. Theseesti-
of super-AGB stars, and Galactic S-star + WD (or NS)
matescouldbedoubledifincase(b)onechoosestodou-
binaries. Yetmorestrongmotivationforopticalspectro-
ble the lowest twin fraction observed in the Magellanic
scopic surveys in local-group galaxies.
Clouds(0.06;Moe&DiStefano2013)andusesF ≈0.1
tw
insteadof0.2. ThelifetimeoftheX-raybrightaccreting 5. CONCLUSIONS
phase is also subject to some uncertainty, but is in the
Thispaperdemonstratesthatweareseeingadifferent
rangeof0.4Myrfrombinarysynthesiscalculations(Fra-
XRB population in IC 10 than either the Milky Way
gos et al. 2015). We caution that n =16 is an imperfect
x or the Magellanic Clouds with the systems identified
measurementofthenumberofBSGXRBsinIC10since
so far in IC 10 being dominated by BSG counterparts
the excess above chance n −n¯ = 7 and the true num-
X rather than Be stars. This contrast between galaxies
ber lies between those values. On the other hand, the
is due to their different ages, metal contents, and star-
optical catalog is about 50% complete for HMXB sec-
formation histories. Of the 16 X-ray selected BSG can-
ondaries in general (see 2), which correction goes in the
didates reported here, five are X-ray variables (Laycock
oppositedirection. Untilspectroscopicidentificationsare
et al. 2016), of which two are known HMXBs. Spectro-
completed, we treat n =16 as a working value.
x scopicfollowupandphotometricmonitoringarerequired
to confirm the nature (spectral types, binary periods) of
4.3. Binarity and massive star evolution
thisnewsampleofextragalacticHMXBsinaveryyoung
Binarityaffectstheevolutionofbothstarsinaclosebi- starburst environment.
nary,withtheoutcomedependingonthepointintimeat Direct observations of the timescales for HMXB pro-
whichthetwostarsinteract. Strippingoftheenvelopeof duction have now been made in several nearby galaxies:
the primary occurs as it enters the giant phase, and the the Magellanic Clouds (Antoniou et al. 2010; Antoniou
coincident accretion of this matter onto the secondary &Zezas2016),NGC2403,andNGC300(Williamsetal.
6 Laycock et al.
The identified mass-donor stars span several magni-
tudes in the visible spectrum. We can further consider
thepossibilitythatHMXBsareamongtheorphanX-ray
sub-sample without any optical counterpart to V (cid:38) 24.
+2
ThislimitingmagnitudecorrespondstoM ∼−3which
2 V
1 would make Be stars later than B0V too faint to be well
e- +1
1 represented in the catalog of Massey et al. (2007).
Our estimate of the production rate of BSG-HMXB is
log(fx/fv)=0
3 ∼200 over 6 Myr, the duration of the starburst, given
1
21−s) 2e- -1 emsatismsartaetsioosf.LTBhVelipfertoidmuectainond tohfebdinisatrriiebsutaisonaoffubnicntaiorny
−m 4 of spectral class remains an active research topic. For
c 1
g e- earlyBtypestarsinparticular,Moe&DiStefano(2013)
er 5
f (X fmouasnsdesaofcl1o0seMbin)aarnydfrnaocteivoindeonfce22fo±r m5%eta(lficoirtyprdiempaerny-
4 (cid:12)
e-1 -2 dence in either mass ratio or orbital period distribution.
1 Other researchers have however identified strong metal-
icity effects in stellar rotation (de Mink et al. 2013) and
5 mass-loss rates (Vink, de Koter, & Lamers 2001). We
1
e- hope that this paper will motivate researchers to carry
2
out the observations and the model calculations needed
18 20 22 24 to improve upon the assumptions in our chain of reason-
ing. As an example, recent population synthesis calcula-
V (mag)
tions by Eldridge & Stanway (2016) show that mergers
begin a minimum of 3 Myr after the onset of star forma-
Fig. 5.—X-rayfluxversusVmagnitudeforX-rayselectedstars
tion and the merger rate rapidly rises to a peak of ∼0.1
inIC10. BSGsareshownasfilledsymbols,othercounterpartsas
opensymbols. OrphanX-raysources(withoutanopticalcounter- merger per Myr per 106 M(cid:12) at a time of about 10 Myr
part) are plotted at the limiting magnitude of the catalog. Con- before gradually declining. The initial “waiting period”
toursofconstantX-raytoopticalfluxratiolog(fx/fv)areplotted isessentiallythetimetakenforstellarevolutiontoreach
for integer values from −2 to +2. Error bars for V values are
the point when mass-transfer begins in massive binaries.
<0.1mag,whilethefluxratiosarethemaximumvaluesobtained
fromthebrightestX-raydetections. Theemergingparadigmisthatbinaryinteractionplays
a central role in the evolution of massive stars and the
production of SNe. We conclude that the age of the IC
2013). Most of these galaxies show a general agreement 10 starburst is fortuitously such that massive binary in-
on a timescale of 40-70 Myr for the peak age of HMXB teraction is approaching its peak. The picture painted
production. A similar estimate comes from the popula- in Prestwich et al. (2015) and Brorby et al. (2016) ad-
tion synthesis modeling of Fragos et al. (2015) for the vances the hope that the observable parameters can be
progenitor of M82 X-2. However the Large Magellanic organized intoa “fundamental plane”in SFR,XLF, and
Cloud shows two younger subpopulations in the range metalicity. Populating such a plane requires deep multi-
of 6-25 Myr, whose confirmed SG-HMXBs peak at ≤10 wavelength observations of star forming galaxies across
Myr; whether this younger subpopulation contains Be age and metalicity space and IC 10 is the youngest and
stars is still unclear. If little star formation has occurred most accessible case study.
inIC10priortothecurrentburst(whichisnotthecase Eldridge et al. (2013) and collaborators have estab-
fortheLargeMagellanicCloudwithmultiplesubpopula- lished the existence of a expanded region in the HR di-
tions of ages ranging up to >100 Myr), then not enough agram populated by massive binaries. The IC 10 red-
time has elapsed for the Be phenomenon to develop. To giant branch intersects the foreground Galactic main se-
test this hypothesis and show that no Be systems are quence in the vicinity of B −V = 1.5. It is therefore
presentinIC10,oneneedstosearchsubstantiallyfainter worth considering whether any of the X-ray selected op-
(V(cid:39)26) in order to reach B3V, the latest spectral type tical counterparts in the overlapping region of the CMD
known to occur in HMXBs. are yellow supergiants and hence an additional compo-
Very young HMXBs may have substantially different nentoftheHMXBpopulation. Opticalspectroscopycan
counterparts, for example the supernova impostor SN potentiallytestthishypothesissinceradialvelocitymea-
2010dahasanageof<5Myr,andit’squiteunclearwhat surements will provide definitive answers.
the donor star is (Lau et al. 2016). Our recent Gemini
spectroscopyappearsthatitmaybeinatransitionstate, 6. ACKNOWLEDGEMENTS
asitexhibitsbothBe-SGandLBVbehavior. Thisisone ThisprojectwasmadepossiblebythesupportofSAO
recentexampleofanextremelyyoungHMXBwithsome grant NAS8-03060 and the Physics Department of Uni-
sort of BSG companion (Binder et al. 2016). versity of Massachusetts Lowell.
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