1) A new ultraluminous X-ray source (ULX) was detected in the galaxy M31 with a peak X-ray luminosity exceeding 10^39 erg s^-1. 2) Radio observations found highly variable radio emission on timescales of minutes to days, indicating an extremely compact emission region. 3) The X-ray and radio properties of this source are consistent with stellar-mass black hole accretion near or above the Eddington limit, powered by a relativistic jet as seen in Galactic microquasars.

1. LETTER doi:10.1038/nature11697
Bright radio emission from an ultraluminous
stellar-mass microquasar in M 31
Matthew J. Middleton1,2, James C. A. Miller-Jones3, Sera Markoff2, Rob Fender4, Martin Henze5, Natasha Hurley-Walker3,
Anna M. M. Scaife4, Timothy P. Roberts1, Dominic Walton6,7, John Carpenter7, Jean-Pierre Macquart3,8, Geoffrey C. Bower9,
Mark Gurwell10, Wolfgang Pietsch5, Frank Haberl5, Jonathan Harris1, Michael Daniel1, Junayd Miah1, Chris Done1, John S. Morgan3,
Hugh Dickinson11, Phil Charles4,12, Vadim Burwitz5, Massimo Della Valle13,14, Michael Freyberg5, Jochen Greiner5,
Margarita Hernanz15, Dieter H. Hartmann16, Despina Hatzidimitriou17, Arno Riffeser18, Gloria Sala19, Stella Seitz18, Pablo Reig20,
Arne Rau5, Marina Orio21, David Titterington22 & Keith Grainge22
A subset of ultraluminous X-ray sources (those with luminosities by a second, weaker, thermal component at higher energies, possibly
of less than 1040 erg s21; ref. 1) are thought to be powered by the ac- due to Compton up-scattering of disk photons in a wind or pho-
cretion of gas onto black holes with masses of 5–20M[, probably tosphere. These two components are also required to fit high-quality
by means of an accretion disk2,3. The X-ray and radio emission are spectra of nearby ‘low-luminosity’ ULXs3,15, implying that similar accre-
coupled in such Galactic sources; the radio emission originates in a tion processes are taking place. Although the intrinsic emission can
relativistic jet thought to be launched from the innermost regions potentially be amplified through geometrical beaming16, this is thought
near the black hole4,5, with the most powerful emission occurring to be important only for ULXs above 1040 erg s21.
when the rate of infalling matter approaches a theoretical maxi- After the X-ray outburst, the source was monitored by the Swift and
mum (the Eddington limit). Only four such maximal sources are Chandra missions for more than 8 weeks (see Fig. 1 and Supplemen-
known in the Milky Way6, and the absorption of soft X-rays in the tary Table 1), until the source was no longer observable because of the
interstellar medium hinders the determination of the causal Sun angle. The X-ray luminosity decreased slightly to ,1039 erg s21,
sequence of events that leads to the ejection of the jet. Here we with the spectrum evolving to be fully described by emission from a
report radio and X-ray observations of a bright new X-ray source standard accretion disk2 with a peak temperature characteristic of
in the nearby galaxy M 31, whose peak luminosity exceeded high-mass-accretion-rate Galactic BHXRBs9. Both the timescale for
1039 erg s21. The radio luminosity is extremely high and shows the X-ray spectral variations and the temperature of the disk compo-
variability on a timescale of tens of minutes, arguing that the source nent rule out an interpretation as a background active galaxy.
is highly compact and powered by accretion close to the Eddington A subsequent period of X-ray monitoring showed the source to
limit onto a black hole of stellar mass. Continued radio and X-ray remain disk-dominated while decaying to a luminosity of ,7 3 1037
monitoring of such sources should reveal the causal relationship erg s21. The correlation of luminosity with disk temperature as L / T4
between the accretion flow and the powerful jet emission. is a well-established observational tracer of sub-Eddington accretion.
XMM-Newton first detected XMMU J004243.61412519 on 15 We find that the best-fitting spectral parameters (see Supplementary
January 2012 (ref. 7) at an X-ray luminosity of 2 3 1038 erg s21 (for a Table 2) deviated strongly from this correlation when the source was
distance to M 31 of 0.78 Mpc (ref. 8)), with an X-ray spectrum that brightest: the temperature decreased with luminosity (Supplementary
could be fully described by a hard power-law, characteristic of sub- Fig. 2). Such behaviour is well documented in ULXs17, strengthening
Eddington accretion (mass accretion rates ,70% of the Eddington our identification of this object as a member of this class, albeit a low-
limit9,10). The source then rose to .1039 erg s21 in two subsequent luminosity one.
detections, fulfilling the traditional definition of an ultraluminous The X-ray detection of this nearby bright source motivated a series
X-ray source (ULX; although other definitions exist, the term ULX is of radio observations by the Karl G. Jansky Very Large Array (VLA; see
numerical rather than physically motivated)11 and significantly above Supplementary Table 3), Very Long Baseline Array (VLBA) and
the cutoff luminosity of the X-ray luminosity function of M 31 (ref. 12), Arcminute Microkelvin Imager Large Array (AMI-LA). The radio
which shows no sources more luminous than 2 3 1038 erg s21. light curves are shown in Fig. 2 (observational details are provided
At the peak luminosity of (1.26 6 0.01) 3 1039 erg s21, the X-ray in Supplementary Information). The source was initially detected by
spectrum seemed similar to that of Galactic black-hole X-ray binaries the VLA at a highly significant level (.40s), with a 4–5s AMI-LA
(BHXRBs) at mass accretion rates close to or above the Eddington detection the following day. The spectral index (defined by Sn / na)
limit13. In such cases, the emission is dominated by an optically thick ac- measured by the VLA was slightly inverted, at a 5 0.27 6 0.16.
cretion disk2 whose spectrum may appear broadened because the accre- The AMI-LA monitoring demonstrated that the radio emission was
tion process is no longer radiatively efficient14. This can be accompanied variable on timescales of days, as confirmed by a detection of strongly
1
Physics Department, University of Durham, Durham DH1 3LE, UK. 2Astronomical Institute Anton Pannekoek, Science Park 904, 1098 XH Amsterdam, The Netherlands. 3International Centre for Radio
Astronomy Research, Curtin University, GPO Box U1987, Perth, WA 6845, Australia. 4School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK. 5Max-Planck-Institut fur ¨
extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany. 6Institute of Astronomy, Cambridge University, Madingley Road, Cambridge CB3 0HA, UK. 7Astronomy Department, California
Institute of Technology, MC 249-17, 1200 East California Boulevard, Pasadena, California 91125, USA. 8ARC Centre of Excellence for All-Sky Astrophysics (CAASTRO), Curtin University, GPO Box U1987,
Perth, WA 6845, Australia. 9Astronomy Department, B-20 Hearst Field, Annex no. 3411, University of California at Berkeley, Berkeley, California 94720-3411, USA. 10Harvard-Smithsonian Center for
Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA. 11Stockholm University, Oskar Klein Centre, AlbaNova, SE-106 91, Stockholm, Sweden. 12Department of Astronomy, University of
Cape Town, Private Bag X3, Rondebosch 7701, Republic of South Africa. 13Osservatorio Astronomico di Capodimonte, INAF, Salita Moiariello 16, 80131 Napoli, Italy. 14International Centre for Relativistic
Astrophysics, Piazzale della Repubblica 2, 65122 Pescara, Italy. 15Institute of Space Sciences (CSIC-IEEC), Campus UAB, Facultade de Ciencies, Torre C5 parell 2, 08193 Bellaterra, Barcelona, Spain.
`
16
Physics and Astronomy Department, 118 Kinard Laboratory, Clemson University, Clemson, South Carolina 29631-0978, USA. 17Department of Astrophysics, Astronomy and Mechanics, Faculty of
Physics, National and Kapodistrian University of Athens, Panepistimiopolis, 157 84 Zografou, Athens, Greece. 18University Observatory Munich, Ludwig-Maximilians-Universitat, Scheinerstrasse 1,
¨
81679 Munchen, Germany. 19Department of Physics and Nuclear Engineering, EUETIB (UPC-IEEC), calle Comte d’Urgell 187, 08036 Barcelona, Spain. 20Foundation for Research and Technology-Hellas,
¨
Nikolaou Plastira 100, Vassilika Vouton, 71110 Heraklion, Crete, Greece. 21Osservatorio Astronomico di Padova, Vicolo Osservatorio 5, 35122 Padova, Italy. 22Astrophysics Group, Cavendish Laboratory,
Cambridge University, JJ Thomson Avenue, Cambridge CB3 0HE, UK.
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2. RESEARCH LETTER
100 from hard to soft X-ray spectral states5. Indeed, scaling the radio and
a
log[L (1038 erg s–1)]
c
X-ray flux from this source to be within our Galaxy would give fluxes
d
10 similar to soft, bright, radio-flaring BHXRBs such as GRS 19151105
b and Cygnus X-3 (ref. 21). The observed X-ray and radio emission of
e
1 f this source is therefore fully consistent with analogous behaviour, and
the radio emission is probably associated with shock acceleration of
particles within ejections moving away from the black hole22 with a
0 50 100 150 200 high Lorentz factor (C $ 2; ref. 5).
Time since first detection (days)
The AMI-LA light curve (Fig. 2) suggests multiple ejection events,
10–2 although the observed inverted, optically thick, spectrum is at odds
EFE (keV cm–2 s–1)
b c d
with the optically thin emission expected from the later stages of an
10–3 expanding synchrotron-emitting plasma. Either the VLA observed the
early, self-absorbed stage of a flare or there is additional, free-free ab-
10–4
sorbing material in the environment of the source.
10–5 A search for shorter-timescale fluctuations in the VLA data revealed
1 10 1 10 1 10
significant variability in the first epoch, on a characteristic timescale of
10–2 several minutes (Fig. 2). Regardless of whether we attribute this to
EFE (keV cm–2 s–1)
e f intrinsic variability or to scintillation (see Supplementary Informa-
10–3 tion), this timescale implies a source size of only ,5 AU (a few micro-
arcseconds at the distance of M 31), constraining the emitting region
10–4 to be highly compact.
There are clear similarities between the behaviour of XMMU
10–5 1 10 1 10 J004243.61412519 and the few ‘super-Eddington’ Galactic BHXRBs.
Energy (keV) Of these, the canonical microquasar GRS 19151105 is the only one
Figure 1 | Evolution of the X-ray luminosity and spectrum over time. whose luminosity regularly exceeds 1039 erg s21 and that would some-
a, Evolution of the source brightness over ,200 days together with the time of the times appear as a ULX to an extragalactic observer23. Multiwavelength
first VLA detection (blue vertical dashed line). The unabsorbed 0.3-10 keV studies of the disk–jet coupling in GRS 19151105 (ref. 24) have iden-
luminosities (and 90% error bars) are derived from the best-fitting models to the tified both ‘plateau’ states with steady jet emission at lower mass accre-
observations with XMM-Newton (red), Chandra (blue) and Swift (green) (see tion rates and ‘flaring’ states with rapid radio oscillations at accretion
Supplementary Tables 1 and 2). An upper limit from an XMM-Newton rates around the Eddington limit, accompanied by an extremely soft
observation taken a week before the source brightened is also included.
X-ray spectrum. The latter, minute-timescale25 radio flaring, has been
b–f, Spectral data (black points with 90% error bars) and best-fitting models (in
red; see Supplementary Information and Supplementary Table 2) from which the observed to occur immediately after a major radio flare, accompanied
unabsorbed luminosities are calculated. b, The first detection before the peak by X-ray brightness changes that are dominated by variability above
brightness shows a spectrum that can be fully described by a hard power-law. c, As 3 keV. If the short-timescale radio variability we observe is intrinsic (see
the source brightens to a peak luminosity of 1.26 3 1039 erg s21, the spectrum Supplementary Information), it could be an analogue of this behaviour,
evolves to one requiring emission from an optically thick accretion disk although we would not expect to detect the X-ray variability in our
(dot–dashed blue line) and a second, weaker thermal component (dot–dashed red observations because the flux above 3 keV is extremely low as a result of
line) that may be related to a wind or photosphere. Such spectra are also seen in the greater source distance. Although GRS 19151105 shows clearly
high-mass-accretion-rate BHXRBs13 and low-luminosity ULXs3,15. d, As the analogous behaviour, when scaled to a similar distance our source is
source dims to ,1039 erg s21, the spectrum becomes fully dominated by the disk
approximately an order of magnitude more luminous in the radio band,
component. e, The spectrum remains disk-dominated but the luminosity
decreases, similar to BHXRBs decaying after an outburst. f, The most recent with a variability brightness temperature for the flares of ,7 3 1010 K
XMM-Newton observation demonstrates that the source remains disk-dominated (see Supplementary Information). Such a discrepancy can readily be
down to ,7 3 1037 erg s21, which places an upper limit on the mass10 of ,17M[. explained by differences in the inclination angle of the jets to our
line of sight; GRS19151105 is highly inclined and thus Doppler-
reduced emission by the VLA (Fig. 2) and a non-detection by the Com- deboosted, whereas we may be looking closer to the jet axis in
bined Array for Research in Millimeter-wave Astronomy (CARMA). XMMU J004243.61412519. This physical picture is supported by
Previous, well-characterized, radio-luminous ULXs have been iden- comparison with the Galactic BHXRBs, Cygnus X-3 and V4641 Sgr,
tified with emission from an optically thin, diffuse nebula believed to both of which have reached similarly high radio luminosities and pro-
be powered by the bright accretion flow18. The radio variability from bably have low angles to the line of sight26,27. Our source seems par-
XMMU J004243.61412519 demonstrates that the emission cannot be ticularly well matched to V4641 Sgr, which reached ,0.5 Jy in its 1999
nebular in origin. Furthermore, the unresolved VLBA detection shown outburst28, and has also shown similarly high brightness temperatures.
in Fig. 2 constrains the size of the emitting region to be ,1,500 AU. The coupled radio and X-ray behaviour in this source is fully con-
This size limit, together with the variability on a timescale of days, sistent with a picture of a BHXRB rising above the Eddington limit in
confirms that the radio emission must originate in a relatively com- outburst then dimming to a bright sub-Eddington state, with radio
pact, relativistically outflowing region with a brightness temperature flaring analogous to that seen in GRS 19151105 and other BHXRBs5.
.6 3 106 K. We can therefore make a secure identification of the compact object in
BHXRBs accreting at sub-Eddington rates produce powerful, per- this source as a black hole of stellar mass (,70M[).
sistent radio jets; the radio and X-ray emission are strongly correlated Because the source remains in a disk-dominated state down to
and are linked to black-hole mass through the ‘fundamental plane of ,7 3 1037 erg s21, and the lower limit for such a state is ,3% of the
black hole activity’19,20. However, the thermal, disk-dominated X-ray Eddington limit10, this constrains the mass of the black hole to ,17M[.
spectrum after the peak of the outburst implies accretion at a relatively However, because the joint radio and X-ray behaviour implies
high fraction of the Eddington limit10 (3–100%). The fundamental Eddington-rate accretion5 at the peak luminosity of ,1.3 3 1039 erg s21,
plane relation does not hold for sources accreting at such high mass we favour a more conservative mass estimate of ,10M[.
accretion rates and so cannot be used to constrain the mass of the black We can also constrain the nature of the companion star from archival
hole. At these high accretion rates, the radio emission from BHXRBs is optical data29. The field does not contain a source within 3 arcsec (where
instead observed to undergo flaring periods, associated with transitions the positional accuracy from the VLBA observations is extremely high;
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3. LETTER RESEARCH
–100 0 100 200
0.6 b 15
VLA 5.26 GHz
d
Flux density (mJy)
VLA 7.45 GHz
0.5 10
0.4
5
Milliarcsec
0.3
0
0.2
Spectral index
c –5
1
0 –10
–1
–15
0 10 20 30 40 15 10 5 0 –5 –10 –15
Time (min) Milliarcsec
1.0
a AMI-LA 5-sigma
AMI-LA 3-sigma
AMI-LA 15 GHz
VLA 5.26 GHz
0.8
VLA 7.45 GHz
VLBA 8.4 GHz
CARMA 92 GHz
0.6
Flux density (mJy)
0.4
0.2
0
–0.2
40 50 60 70 80 90 100
Time since first XMM detection on MJD 55941 (days)
Figure 2 | Summary of the radio observations. a, Multifrequency radio light overall flux scale. We are unable to determine conclusively whether the observed
curves of the ULX from our AMI-LA, VLA, VLBA and CARMA data. For the variability is intrinsic to the source or whether it arises from scintillation
AMI-LA data we plot the flux density from the pixel value at the source in the interstellar medium. Regardless of the mechanism involved, this
location, because of dynamic range limitations from the bright nearby constrains the source size to #6 mas. c, Corresponding spectral indices derived
confusing source, 5C 3.111 (Supplementary Fig. 1). The resulting high noise level from the two VLA observing bands centred at 5.256 and 7.45 GHz. d, VLBA
close to the ULX hindered the standard image-plane source fitting methods used image of the source. Colour scale is in mJy per beam, and the contours are set at
for the VLA and VLBA data. To estimate the significance of the AMI points, we levels of 6(!2)n times the root mean squared noise of 21 mJy per beam, where
have plotted both the 3-sigma and 5-sigma noise levels (dot–dashed and dotted n 5 3,4,5. The source is unresolved down to a beam size of 2.0 3 1.2 mas2 in
lines, respectively), showing both a standard and a (favoured) more conservative position angle 226u east of north. The (J2000) source position, measured
detection threshold. b, Radio light curves binned to an interval of 6 min (the relative to J003814137 and before performing any self-calibration, is RA
phase referencing cycle time), showing the short-timescale variability during the 00 h 42 min 43.674360(9) s, dec. 41u 259 18.7037(1)0. Error bars in all panels
first VLA observation. There is an additional 1% systematic uncertainty on the show the statistical uncertainties at the 1s level.
see Supplementary Information) down to V < 25 and B < 26, ruling including how outflows from quasars redistributed matter and energy in
out an identification of the companion as an OB star and implying that the early Universe.
the system is accreting from a star of lower mass, similar to other Assuming that the properties of this source are representative of the
transient, low-luminosity ULXs3,30. This also provides further evidence larger population, we can consider the possibility of detecting similar
that the emission is not associated with a background active galaxy, events in future. The sensitivity of the VLA would allow us to detect a
which would be substantially brighter. similarly relativistically beamed event out to 4 Mpc, or 0.5 Mpc if
Because the X-ray properties of the source are typical of the class3,15,17, unbeamed. The X-ray monitoring cadence for M 31 has yielded two
we confidently extend the identification of Eddington-rate accretion candidates in 10 years; although the monitoring was not constant, this
onto a black hole of stellar mass in XMMU J004243.61412519, to the is roughly consistent with the observed rate of Galactic BHXRBs with
larger population of low-luminosity ULXs (a substantial component low-mass companions whose outbursts reach the Eddington lumino-
of the overall population, with up to 80% having luminosities of sity. It is therefore a realistic prospect (see Supplementary Informa-
(1–5) 3 1039 erg s21)1. As a result, this implies that we have a large tion) that future observations of transient ULX systems in nearby
population of sources from which to develop models for Eddington galaxies, using sensitive radio telescopes, will permit detailed disk–
accretion. In our own Galaxy we are limited to a very small number jet coupling studies and in doing so will significantly expand our un-
of such sources, and the large absorbing column through the plane of derstanding of Eddington accretion and associated phenomena.
our Galaxy prevents a clear view of the Eddington-limited accretion
Received 10 May; accepted 22 October 2012.
flow. However, this is not the case for the vast majority of extragalactic
Published online 12 December 2012.
sources, allowing the properties of the infalling matter to be accurately de-
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16. King, A. Masses, beaming and Eddington ratios in ultraluminous X-ray sources. Author Contributions M.J.M. wrote the manuscript with comments from all authors.
Mon. Not. R. Astron. Soc. 393, L41–L44 (2009). J.C.A.M.-J. designed and analysed the VLA and VLBA observations. N.H.-W. and
17. Feng, H. & Kaaret, P. Spectral states and evolution of ultraluminous X-ray sources. A.M.M.S. analysed the AMI-LA observations. J.-P.M. and J.C.A.M.-J. conducted the
Astrophys. J. 696, 1712–1726 (2009). scintillation analysis. S.M., R.F. and M. Henze made significant contributions to the
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19. Merloni, A., Heinz, S. & di Matteo, T. A fundamental plane of black hole activity. Mon. various aspects of the science case or are contributing members of the M 31 group.
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20. ¨rding, E. & Anderson, S. Using the fundamental
Plotkin, R., Markoff, S., Kelly, B., Ko Author Information Reprints and permissions information is available at
plane of black hole activity to distinguish X-ray processes from weakly accreting www.nature.com/reprints. The authors declare no competing financial interests.
black holes. Mon. Not. R. Astron. Soc. 419, 267–286 (2012). Readers are welcome to comment on the online version of the paper. Correspondence
21. Gallo, E., Fender, R. & Pooley, G. A universal radio-X-ray correlation in low/hard and requests for materials should be addressed to M.J.M.
state black hole binaries. Mon. Not. R. Astron. Soc. 344, 60–72 (2003). (m.j.middleton@durham.ac.uk).
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