Variability in Ultra-luminous X-ray Sources

0.27 MB·

by  Natalie Webb

Publications of the Astronomical Society of Australia(PASA)
(cid:13)c AstronomicalSocietyofAustralia2014;publishedbyCambridgeUniversityPress.
doi:10.1017/pas.2014.xxx.
Variability in Ultra-luminous X-ray Sources
N. A. Webb1,2, D. Cseh3 and F. Kirsten4
1Universit´edeToulouse;UPS-OMP;IRAP;Toulouse,France
2CNRS;IRAP;9av.ColonelRoche,BP44346, F-31028Toulousecedex4,France
4 3DepartmentofAstrophysics/IMAPP,RadboudUniversityNijmegen,P.O.Box9010, 6500GLNijmegen,TheNetherlands
1 4MaxPlanckInstitutfu¨rRadioastronomie(MPIfR),AufdemHu¨gel69,D-53121,Bonn,Germany
0
2
n Abstract
a Many upcoming surveys, particularly in the radio and optical domains, are designed to probe either the
J temporaland/orthespatialvariabilityofarangeofastronomicalobjects.Inthelightofthesehighresolution

8 surveys, we review the subject of ultra-luminous X-ray (ULX) sources, which are thought to be accreting
black holes for the most part. We also discuss the sub-class of ULXs known as the hyper-luminous X-ray

] sources, which may be accreting intermediate mass black holes. We focus on some of the open questions
E
that will be addressed with the new facilities, such as the mass of the black hole in ULXs, their temporal
H
variability and the nature of the state changes, their surrounding nebulae and the nature of the region in
. which ULXsreside.
h
p
- Keywords: stars: black holes – accretion, accretion discs – X-rays:binaries
o
r
t
s
a 1 INTRODUCTION Eddington accretion onto smaller mass BHs could al-
[ lowthemtogrowintosupermassiveblackholes,viathe
Ultra-luminousX-raysources(ULXs)areX-raysources
IMBHstate(Kawaguchiet al. 2004).HowIMBHsform
1 locatedoutsidethenucleusoftheirhostgalaxy,withlu-
v andwheretheyarelocatedaretwoofanumberofopen
minositiesthatexceedtheEddingtonluminosity(LEdd)
8 questions concerning IMBH (Miller & Colbert 2004).
2 of a stellar mass black hole (BH), where LEdd = 1.3 × They could be formed through the accretion of gas or
7 1038(M/M⊙) erg s−1. In the literature, an off-nuclear
1 sourceisoftenconsideredtobeaULXifL>1.3×1039 smaller stars and therefore reside in the centres of old
densestellarclusters(globularclusters)ortheymaybe
1. erg s−1. These enigmatic sources were first discovered
formeddirectlyinyoungstarformingregionse.g.Miller
0 with the Einstein observatory between 1978 and 1981
& Colbert(2004).IMBHs arethereforethe missing BH
4 (Fabbiano 1989) and their nature was unclear. Fabi-
1 link. Further, they are interesting for a diverse range
anno (1989) did state that if these sources were indeed
: offundamentalphysicsandastrophysicalsubjects.Itis
v accreting compact objects, they would indicate mas-
hasbeenproposedthattheymayinjectessentialenergy
i sive (stellar mass) black holes. The high luminosities,
X intoglobularclusterstoslowdowntheeventualcorecol-
in conjunction with observations that showed various
r lapse, e.g. Hut et al. (1992), that dark matter may be
a ULXs in different spectral states, along with the fact
relativelysimpletodetectaroundIMBH,e.g.Fornasa&
that some ULXs appeared to show spectral state tran-
Bertone (2008), that IMBHs may have played a role in
sitions (Kubota et al. 2001), supported the idea that
thecosmologicalionisation(Madau et al. 2004),orthat
most ULXs are accreting black holes.
theymaybestrongsourcesofgravitationalwavesifthe
ThemassesoftheULXblackholesarestillsomewhat
IMBH has a compact object companion in an elliptical
under debate. If we assume that the maximum radia-
orbit around it (Miller & Colbert 2004) and that gas
tiveluminosityfromthesphericalaccretionofmatteris
accretion onto IMBHs should have made a significant
in indeed at the Eddington limit, the high luminosities
contribution to the UV background(Kawaguchi 2003).
observedfromULXswouldimplythatthesesourcesare
However, not all ULXs can host black holes of such
the previously unobserved and long sought after inter-
mediate mass black holes (IMBH, ∼102 to ∼105 M⊙). a high mass. For instance, King (2004) showed that
the population of ULXs in the cartwheel galaxy is
Such sources are believed to exist as it is thought that
difficult to explain if they are all IMBHs, as much
mergersofIMBHscouldbeattheoriginofsupermassive
of the star forming mass would then end up as
black holes e.g. Madau & Rees (2001), or that super-
IMBHs. Alternatively, if the ULX emission is colli-
1
2 Webb et al.
mated, possibly through a geometrically thick accre- jority of ULXs show a more extreme version of the
tion disc (King et al. 2001) or via relativistic boost- GBHB states, going beyond the high state and into
ing(K¨ording et al. 2002),theemissioncouldsimplyap- super-Eddingtonaccretionregimes,knownastheultra-
pear to exceed the Eddington limit. Indeed a num- luminous accretion state (Gladstone et al. 2009b), see
ber of Galactic black hole binaries are seen to surpass also Sec. 2. Observing these states, and transitions be-
such a limit, i.e. V4641 Sgr (Hjellming et al. 2000) and tween them, could provide important information on
GRS 1915+105 (Done, Wardzin´ski & Gierlin´ski 2004). how the Eddington limit can be broken, which may re-
Luminosities up to ∼1041 erg s−1 can be plausibly ex- quire new physics.Super Eddingtonaccretionhas been
plained through beaming effects (Poutanen et al. 2007; invoked to explain how the earliest supermassive black
Freeland et al. 2006) and/or hyper-accretion onto stel- holes were formed e.g. Kajava et al (2009).
lar mass BHs (Poutanen et al. 2007; Begelman 2002). SomeULXs,however,havebeenshowntoexhibitthe
Therefore, the majority of ULXs are thought to host canonicalX-raysstatesobservedfromGBHBs,thefirst
BHs with masses close to those of stellar mass black of these being HLX-1, see Sec. 2.1. This may indicate
holes (Roberts 2007). However, a rare class of ULXs thatULXscanbedividedintosub-classes,namelywith
– the hyper-luminous X-ray sources (HLX) – emit X- suband super-Eddingtonemission.The implications of
raysatluminosities>1041ergs−1(Gao et al. 2003)and this are discussed further in Sec. 2.1.
require increasingly complicated scenarios to explain
them without invoking the presence of an IMBH.
Few candidate HLX have been observed,but one ex-
cellent candidate is a 2XMM X-ray catalogue source 1.2 ULX nebulae
(Watson et al. 2009) 2XMM J011028.1460421, discov-
Some ULXs show optical emission-line nebulae, due
ered as an off-nuclear X-ray source seemingly associ-
to shock-ionised driven jets, outflows or disc winds
atedwith the galaxyESO243-49,95 Mpc (z = 0.0224)
and/or because of photo-ionisation from the X-ray
away (Farrell et al. 2009). The optical counterpart lo-
and UV emission around the black hole. Some of
cated by Soria et al. (2010) using the excellent X-ray
these also show a radio counterpart. These nebu-
positionderivedfromChandradata(Webb et al. 2010),
lae can be used as a calorimeter to infer the total
shows an H emission line commensurate with the
α intrinsic power of the ULX, (Pakull & Mirioni 2002;
galaxy distance, thus confirming the association with
Pakull & Mirioni 2003), and also to understand how
ESO 243-49 (Wiersema et al. 2010). Taking the max-
outflowsandphoto-ionisationcanplay aroleinthe be-
imum X-ray luminosity of 1.1 × 1042 erg s−1 (0.2-
haviour of the ULX and on its surrounding environ-
10.0 keV, unabsorbed) and assuming that this value
ment (Pakull & Mirioni 2002; Pakull & Mirioni 2003;
exceeds the Eddington limit by a factor of 10, implies
Roberts et al. 2003; Kaaret et al. 2004).
a minimum mass of 500 M⊙ (Farrell et al. 2009). Mod-
elling and Eddington scaling of the X-ray data imply a
massof∼104 M⊙ (Godet et al. 2012;Davis et al. 2011;
Servillat et al. 2011)andrecentradioobservationsyield
1.3 Spatial and temporal variability with
anupper mass limit of9 × 104 M⊙, supposingEdding- upcoming instrumentation
ton scaling (Webb et al. 2012). This source goes under
the name of ESO 243-49 HLX-1 or HLX-1 for short. With the wide range of new and forthcoming obser-
vatories and surveys over the next few years, there
will be a huge amount of data available to study
1.1 ULXs and their different states
open questions surrounding ULXs. The new hard X-
Many ULXs are seen to be variable, particularly in the ray observatory, Nuclear Spectroscopic Telescope Ar-
X-ray domain, e.g. Kubota et al. (2001) and Kajava ray (NuSTAR), launched on 2012 June 13 operates in
& Poutanen (2009). In addition, the fact that most the band3-79keV(Harrison et al. 2013). Comparedto
ULXs are accreting black holes means that they are other X-ray satellites suchas XMM-Newton and Chan-
frequently likenedto Galactic blackhole X-raybinaries dra,thatonlyprobethespectrumupto∼12keV,NuS-
(GBHB), albeit extreme versions. GBHBs show both TAR’s extended X-ray spectral band allows us, for the
longandshorttermX-rayvariability.Onthelongterm, firsttime,toprobeULXspectraabove10keVwithhigh
they exhibit well defined X-ray states: the high/soft signaltonoise.Thisshouldallowustoconstrainmodels
(or thermal), low/hard (or hard) and very high (or describing the ultra-luminous state in the near future.
steep power law) states, see for instance Remillard & It will also allow us to uncover new ULXs (e.g. Walton
McClintock (2006). Much work has gone into under- etal.2013b),thankstothelargefieldofview(FOV,10’
standing if ULXs behave in a similar way to GBHBs at 10 keV, 50% response), that were previously veiled
(Gladstone & Roberts 2009; Kajava & Poutanen 2009; bystronginterstellarabsorption,mayberevealinganew
Roberts et al. 2012).Suchstudiesindicatethatthema- shrouded population of ULXs, for example.
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Variability in Ultra-luminous X-ray Sources 3
In the optical, future surveys such as the Large Syn- ULXs that we should get a handle on the number of
opticSurveyTelescope(LSST)1willobservetheskyus- compact object binaries that we will detect with the
ing an 8.4 m telescope. The observations will be made new gravitationalwave detectors.
using 6 bands (0.3-1.1 micron) and have a 3.5◦ FOV. Intherestofthisreviewwewilllookatsomespecific
It is expected to be operating in 2022 and will regu- aspects of ULX variability and how the new facilities
larly scan the sky, so it will be ideal for picking out coming on line will enable us to respond to questions
new and monitoring known transient ULXs. This will surrounding ULXs.
help to identify ULXs under-going state transition, in
ordertounderstandhowtheseprocessesrelatetothose
2 Variability due to state changes
in GBHBs, see Sec. 2.
Atlongerwavelengthsstill,thenewradioarraysthat X-ray spectral changes in conjunction with X-ray lu-
are coming on line in the near future will allow us to minosity changes have been observed in a number
probe the same region of the sky repeatedly, to depths of ULXs, e.g. Kubota et al. (2001). However, con-
that equalthe most sensitive radio telescopes currently trary to the GBHBs, the harder state is often as-
in use. The Hunt for Dynamic and Explosive Radio sociated with the higher luminosity and the softer
Transients with Meerkat (Thunderkat) project will de- state with the lower luminosity. This lead to the
vote 3000 h of time between 2016 and 2020 on the 64 fourth GBHB state being proposed, the apparently
dishesthatcompriseMeerkat (0.9-1.7GHz)2.Thishigh standard regime (Kubota et al. 2004) or ultra-luminous
sensitivity observatory will allow a 5 σ detection of a state (Gladstone et al. 2009b), when the BH accretes
faintradiosource(14µJy)injust8hours.Thunderkat at or above the Eddington limit. However, the physics
will be devotedto followingup transientphenomena as of this state are not yet understood e.g. Walton et al.
acomplimenttotheregularMeerkat program.Spectral (2013).Variabilityofafactorofabout10ormoreinX-
changes along with spatial and temporal variability in rays can be seen during these state changes. Recently
theradionebulaearoundULXswillbeprobedandnew Soriaetal.(2012b)observedapreviouslyundetected(L
radio ULXs will be discovered with such a program. <1036ergs−1)objectentertheULXstate(L∼4×1039
Similarly, VAST, An ASKAP Survey for Variables and erg s−1), an X-ray flux change of more than three or-
Slow Transients, (Murphy et al. 2013) will have a root dersofmagnitude.Morerecently,anewX-raybinaryin
mean square (RMS) continuum sensitivity of 47 µJy M 31 was discovered (Henze et al. 2012), which peaks
beam−1 injust 1 hr,thanks to its 36Australian Square atL >1×1039 ergs−1,qualifyingitasaULX.ItsX-
x
Kilometre Array Pathfinder (ASKAP) dishes. It will ray emission shows typicalULX states, but radio emis-
have a 30◦ FOV and will conduct broad and deep ob- sionvaryingonminute timescaleshasbeen observedat
servations to achieve similar goals, starting as early as the mJy level (Middleton et al. 2013). Another source
2012. also showing a transition to the ULX state, and with
Almost all of the above cited projects rely on detect- a variability of at least a factor 100, has been found
ing variability,ontemporalorspatialscales,to identify in M 94 (Lin et al. 2013). How and why this transition
newULXsandeventuallyrespondtotheopenquestions occurs is unclear, but it may be that super-Eddington
cited above. Identifying more ULXs is essential to un- discsremainslimandaccelerateasignificantwindwith
derstand their demographics, especially those that are a ’thick disc’ geometry, unlike in the standard scenario
thoughttocontainIMBH,ashowtheyformandevolve whereaccretiondiscsbecomethicke.g.Dotan&Shaviv
andwhethertheyexistinglobularclustersarestillvery (2011).
muchopenquestions,seeSecs.1and3.Further,ifULXs Itshouldbenoted,however,thatanX-rayvariability
are black holes accreting from a high mass companion, of a factor of a few can be seen with no spectral state
many will evolve to form compact object binaries e.g. change, see e.g. Soria et al. (2009). The variability and
Mandeletal.(2008).Whilstitwillbepossibletodetect the spectral changes are therefore poorly understood
such binaries using the electromagnetic spectrum, the and further observations of known and new sources are
new gravitational wave detectors Advanced LIGO and essential to understand the behaviour of ULXs, espe-
Advanced Virgo should allow us to detect the inspiral cially in the ultra-luminous regime.
of these binaries for which the BH has a mass greater
than 50 M⊙ and up to 10000 M⊙ (Mandel et al. 2008; 2.1 Canonical black hole states in ULXs
Gair et al. 2011). Many ofthe currentstudies aimedat
identifying the numbers of such binaries use the esti- The first ULX to show all three GBHB X-ray
mated population of ULXs e.g. Mandel et al. (2008). states, was HLX-1. The source has shown five fast
It is therefore through determining the population of rise and exponential decay (FRED) type X-ray out-
bursts, characterised by an increase in the count rate
1http://www.lsst.org/lsst/ by a factor 40 (Godet et al. 2012; Godet et al. 2009;
2http://www.ast.uct.ac.za/thunderkat/ThunderKAT.html Lasota et al. 2011) and see Fig. 1. From simple spec-
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4 Webb et al.
Figure 1.TheSwiftX-raylightcurveofESO243-49HLX-1from2008untiltheendof2013
tral fitting, the unabsorbed luminosities range from flares around this transition, e.g. (Fender et al. 2009;
1.9 × 1040 to 1.3 × 1042 erg s−1. At high luminosi- Corbel et al. 2004).Thesejetsareassociatedwithejec-
ties, the X-ray spectrum shows a thermal state dom- tionevents,where,forexample,thejetisexpelledwhich
inated by a disc component with temperatures ≤0.26 canleadtoradioflaringwhenthehighervelocityejecta
keV. At low luminosities the spectrum is dominated may collide with the lower-velocity material produced
by a hard power law with 1.4 ≤Γ≤ 2.1, consistent by the steady jet or the local environment. This rein-
with a hard state. The source has also been observed forces the similarity between HLX-1 and the GBHBs.
with a power law spectrum with Γ=3.3±0.2, consis- Also, Webb et al. (2014) showed that the V-band op-
tentwiththesteeppowerlawstate(Farrell et al. 2009). tical emission increases from before the X-ray outburst
When HLX-1 is in the high/soft state, the luminosity and through the X-ray outburst by 2 magnitudes. This
of the disc component appears to scale with the in- concurrent increase is similar to that observed in GB-
ner disc temperature to the power of four, which sup- HBs and opens up the possibility that optical surveys
ports the presence of an optically thick, geometrically will detect spectral state changes in ULXs.
thin accretion disc. The observed spectral changes and Other objects have been identified with X-ray lu-
long-term variability are not consistent with variations minosities >1040 erg s−1 that show hard power law
in the beaming of the emission, nor are they consis- spectra, e.g. Zolotukhin et al. (2013), and some with
tent with the source being in a super-Eddington accre- significant RMS variability (Sutton et al. 2012), typi-
tion state (Servillat et al. 2011). Indeed, through mod- cal of the low/hard state. These objects are not for-
elling the X-ray spectra with the Kawaguchi (2003) mallyHLXs.However,iftheyareinthelow/hardstate,
disc model and assuming a non-spinning black hole which is thought to occur at ∼1-2% Eddington lumi-
and an inclination of 0◦, the accretion flow was deter- nosity (Maccarone 2003), and then they switch to the
mined to be in the sub-Eddington regime for all obser- high state (i.e. close to the Eddington luminosity) they
vations (Godet et al. 2012), radiating close to the Ed- willhaveluminosities ofthe orderof∼1042 ergs−1 and
dington limit at the peak of the outburst. The disc so become HLXs. Such high luminosities indicate that
radiation efficiency was shown to be η = 0.11 ± 0.03 they may contain IMBHs, see Sec. 1. However, so far,
(Godet et al. 2012). nosuchstatechangehasbeenobservedintheseobjects
In addition, radio flares have been detected from andfurtherobservationswillberequiredtoconfirmthe
HLX-1 during the low/hard to high/soft state transi- IMBH nature. If confirmed, this would provide an ob-
tions in 2010 and 2011 (Webb et al. 2012), in a similar servational constraint for identifying ULXs containing
waytoGBHBs,whichareknowntoregularlyemitradio IMBH.
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Variability in Ultra-luminous X-ray Sources 5
2.2 The Ultra-luminous state Shih et al. 2010; Brassington et al. 2012;
Roberts et al. 2012). Whilst this may seem to support
RecentworkwithNuSTARhasfocusedonunderstand-
the idea that ULXs are found in metal poor regions,
ing the underlying mechanism of the ultra-luminous
Maccarone et al. (2011) found that the five globular
state. NuSTAR observations of the two ULXs NGC
cluster ULXs known at the time, were found in metal
1313 X-1 and X-2, show a clear cutoff of the X-1’s
rich clusters.
spectrum above 10 keV, which was only marginallyde-
No ULXs have so far been detected in Galactic glob-
tectable with previous X-ray observations. This cut-
ular clusters, but Strader et al. (2012) did discover the
off rules out the interpretation of X-1 as a BH in a
first stellar mass black holes using radio observations.
standard low/hard state, or in a reflection-dominated
These black holes may well be slightly more massive
state. The cutoff is alsofound to differ fromthat which
thanotherGalacticstellarmassblackholes,butfurther
is predicted by a single-temperature Comptonisation
observations will be requiredto confirm this possibility
model (Bachetti et al. 2013). This is also the case for
(Strader et al. 2012). Why no ULXs exist in Galactic
thenewULX,CircinusULX5,discoveredwithNuSTAR
globular clusters is so far unexplained, but it may be
(Walton et al. 2013b). A spectral transition in NGC
related to the metallicity (Mapelli & Bressan 2013) or
1313X-2,froma state with highluminosity andstrong
simply because the Galactic globular cluster system is
fractional variability that increases with energy to a
small and thus no such objects may be expected.
lower-luminosity state with no detectable variability,
With regards to the HLX population, Farrell et al.
wasexplainedbyatransitionfromasuper-Eddingtonto
(2012) showed through modelling the X-ray and near
the sub-Eddington regime (Bachetti et al. 2013). This
UV to near IR light that HLX-1 may reside in an
type of transition has been observed in other objects,
old stellar population, akin to a globular cluster, al-
e.g.Burkeetal.(2013).Suttonetal.(2013)showedthat
though the preferred solution is a young stellar popu-
high levels of fractional variability are seen in ULXs
lation (Farrell et al. 2013). None of the other proposed
with soft ultra-luminous spectra as well, likewise for
HLXs have been reliably associated with a host stellar
a couple of the broadened disc sources. Furthermore,
population.
they observe variability that is strongest at high ener-
Black hole mass scaling relations e.g. Lu¨tzgendorf
gies. They propose that such properties are consistent
et al. (2013) and stellar velocity dispersion measure-
withmodelsofsuper-Eddingtonemission,whereamas-
ments in globular clusters e.g. Anderson & van der
sive, radiatively driven wind forms a funnel-like geom-
Marel (2010), suggest the existence of IMBHs near or
etry around the central regions of the accretion flow.
exactly at the potential centre of globular clusters. If
Future observations with NuSTAR should help to con-
these objects are accreting material, they should ap-
firm or refute such a hypothesis.
pearasULXs/HLXs(dependingontheaccretionrate).
Searches for such objects in Galactic globular clus-
ters using X-ray observations have not revealed such
3 Where do ULXs reside?
a sourcee.g.Pooley&Rappaport(2006)andPooleyet
ULXs are found preferentially in star forming re- al.(2003).If,however,theBHwasaccretingfromgasin
gions e.g. Grimm et al. (2003), but there is also the globular cluster, the emission of photons might be
evidence to show that ULXs form in low metallic- inefficient in the X-ray regime. BHs would then appear
ity regions e.g. Pakull & Mirioni (2002), Mapelli et as compact radio sources co-moving with the globular
al. (2009) and Prestwich et al. (2013). Indeed two cluster.SearchesforsufficientquantitiesofgasinGalac-
of the recently discovered ULXs, see Sec. 2, appear tic globular clusters have been fruitful. Despite the low
to have old stellar counterparts (Soria et al. 2012b; gasdensity,Freireetal.(2001)detectedanon-negligible
Middleton et al. 2013), associating them with old, low amount of ionized gas in 47 Tuc through pulsar tim-
metallicity stellar populations. ing observations. M 15 also shows evidence for dust
If ULXs are simply BHs accreting from intermediate (Boyer et al. 2006) and only a small fraction (∼10%,
or high mass donor stars with the X-ray emission Maccarone (2004)) is required to be accreted by a pu-
beamed towards us, see Sec. 1, Irwin et al. (2004) tativeIMBHforittobecomevisibleintheX-ray/radio
argued that ULXs should not be found in old stellar domain.
populations. Irwin et al. (2004) showed that there Radioflaresthatcanaccompanystatechangesshould
are indeed very few ULXs associated with early-type also be detectable in long term radio observations. To
galaxies.However,Maccaroneetal.(2007)discovereda date,neitheracontinuousnoraflaringradiosourcehas
ULX ina globularcluster (typically an oldpopulation) been detected down to the sensitivity limits of current
in the giant elliptical galaxy NGC 4472, thanks to its instruments e.g. Maccarone& Servillat (2008), Kirsten
strong X-ray variability. Since then, at least five more & Vlemmings (2012) and Strader et al. (2012). This
ULXs have been proposed in extra-galactic globular might imply IMBHs simply do not exist in globular
clusters (Maccarone et al. 2011; Irwin et al. 2010; clusters or that gas produced by AGB stars in globular
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6 Webb et al.
clustersdoes notsurvivelongenoughto be accretedby (FWHM>20 ˚A), including He II 4686 ˚A, HeI 5876 ˚A
thecentralsource,henceprohibitingtheformationofan and He I 6679 ˚A, He I 4471 ˚A, He I 4922 ˚A and He II
accretion disc and relativistic jets (Boyer et al. 2006). 5411 ˚A lines and NIII 4640 ˚A lines. The He I 5876/He
II 5411˚A equivalent width ratio suggests a Wolf-Rayet
star. These lines have allowed Liu et al. (2013) to de-
4 ULXs in future surveys
terminethe massofthe Wolf-Rayetstar(17.5M⊙)and
The X-ray variability allows new ULXs to be identi- theradialvelocity,whichrevealsablackholemasscon-
fied using X-ray catalogues such as the 2XMM cata- straint of >5 M⊙, but more likely in the range 20-30
logueofserendipitouslydetectedXMM-Newtonsources M⊙.
(Watson et al. 2009), e.g. Lin, Webb & Barret (2012). The radio emission variability detected from HLX-1
Therecentlyreleased3XMMcatalogueincludes372728 (see Sec. 2.1) and the M 31 X-ray binary/ULX (see
unique X-ray sources distributed across 650 square de- Sec. 2) opens up the possibility of discovering new,
grees (∼1.5%) of sky, almost50% more sources than in nearby, low-luminosity ULXs with the upcoming radio
the previous version. Over 123800 of these sources are surveys. The new radio surveys have very low detec-
providedwithbothspectraandlightcurves.3XMMalso tion limits (see Sec. 1). Given that all the firm ra-
has the added value that it includes complete uniform dio ULX counterparts have a total flux density be-
reprocessing of all of the data, that exploits the most low ∼1 mJy and sizes below 2-15” at Mpc distances
recentprocessingandcalibrationimprovements.Thisis (Cseh et al. 2012; Webb et al. 2012), the forthcoming
then a fantastic resource for searching for new ULXs. facilities should break the silence in systematic radio
However, it is likely that within the next ten years surveys looking for ULXs.
theexistingsensitiveX-raysatellites(e.g.Swift, XMM- Some currentsurveys,e.g. Legacye-MERLIN Multi-
Newton,Chandra)willnolongerbeinoperation.Inthe bandImagingofNearbyGalaxiesSurvey(LeMMINGs),
2020’s, it is possible that there will be no new big X- also search for extended radio emission from ULXs.
ray telescope to take their place before the launch of However,lowerresolutionfacilitiesmaybeabletomea-
Athena+ at the end of the 2020’s (Nandra et al. 2013). sure flux variations of some of the already known or
X-ray telescopes have historically been used to detect newly discovered radio bubbles around ULXs. Such
and identify new ULXs, often through the variability variability is expected on the grounds of recent discov-
due to state changes, see Sec. 2. Without X-ray tele- eries (Mezcua et al. 2013; Cseh et al. 2013b), where a
scopessurveyingthesky,theopticaltelescopeswilltake working jet is embedded in the radio bubble, heavily
over this role, searchingfor the optical variability asso- perturbing it.
ciated with state change and the intrinsic optical vari- However, detecting distant (e.g. >100 Mpc) ULXs
ability, both described in Sec. 2. could still be challenging even with the new surveys.
Identifying new ULXs through optical observations This is mainly due to the faint expected flux density of
should eventually leadto the discoveryof opticalcoun- these sources and due to the natural confusion limits;
terparts that are sufficiently bright that short spectro- see e.g. the SKA (Jackson 2004). So, variability from
scopic exposures will achieve optical spectra with high future detections will have to trigger follow-up, high-
enough signal to noise to spectrally identify the com- resolution radio observations with VLBI.
panion and to carry out a radial velocity studies, to With regardsto spatialvariability,measuringproper
measure the mass function. This will allow an accu- motions of ULXs (Kirsten & Vlemmings 2012) may be
ratemassdeterminationofthe blackholetofinallyset- a future possibility to independently infer the masses
tle the debate on the mass of the back holes in these of these objects. SeveralULXs are seen to be displaced
objects. Previous attempts to detect a spectrum from fromnearby starclusters (Kaaret et al. 2004) andthey
the companion have revealed blue, almost featureless may have received natal velocity kicks that might be
spectra (Zampieri et al. 2004; Kaaret & Corbel 2009; up to hundreds of km s−1 for a 10 M⊙ black hole
Roberts et al. 2011; Gris´e et al. 2011; Cseh et al. 2011; (Mapelli & Bressan 2013; Repetto et al. 2012). To de-
Gris´e et al. 2012) which are probably dominated by tect such displacements would not just require better
the accretion disc and in some cases show evidence than milli-arcsecond resolution, but also perhaps, the
of the surrounding nebula. Some attempts to use disc detection of self-absorbed compact jets that are typi-
lines (Cseh et al. 2013; Motch et al. 2011) revealed no cally associated with hard, inefficient accretion states.
mass constraints. This may be because of inadequate Thismightbe challenginginthe lightofthe NuStarre-
time sampling of the radial velocity curve, too low sults, where it was shown that some ULXs are not in a
spectral resolution resulting in blurred narrow and canonical low/hard state, even though the X-ray spec-
broad components, contrasting viewing geometries of trum is hard (see Sec. 2.2). None the less, recent VLBI
the system, or that the He II line is not emitted from observations of the ULX N5457-X9 in the low/hard
the disc (Cseh et al. 2013). M101 ULX-1 has however state have revealed radio emission which is thought to
been shown to have a number of broad emission lines
PASA(2014)
doi:10.1017/pas.2014.xxx
Variability in Ultra-luminous X-ray Sources 7
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