arXiv:astro-ph/0210224v1 10 Oct 2002
Large-scale, Decelerating, Relativistic
X-ray Jets from the Microquasar
S. Corbel1, R.P. Fender2, A.K. Tzioumis3, J.A. Tomsick4,
J.A. Orosz5, J.M. Miller6, R. Wijnands6, P. Kaaret7
1Universit´ e Paris VII and Service d’Astrophysique, CEA, CE-Saclay,
91191 Gif sur Yvette, France
2Astronomical Institute ‘Anton Pannekoek’, University of Amsterdam, and Center for High
Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
3Australia Telescope National Facility, CSIRO, P.O. Box 76, Epping NSW 1710, Australia
4Center for Astrophysics and Space Sciences, University of California at San Diego,
MS 0424, La Jolla, CA 92093, USA
5Astronomical Institute, Utrecht University, Postbus 80000, 3508 TA Utrecht, The Netherlands
6Center for Space Research, MIT, NE80-6055, 77 Massachusetts Avenue,
Cambridge, MA 02139-4307, USA
7Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
We have discovered at x-ray and radio wavelengths large-scale moving jets
from the microquasar XTE J1550−564. Plasma ejected from near the black
hole traveled at relativistic velocities for at least four years. We present di-
rect evidence for gradual deceleration in a relativistic jet. The broadband
spectrum of the jets is consistent with synchrotron emission from high energy
(up to 10 TeV) particles accelerated in shock waves formed within the rela-
tivistic ejecta or by the interaction of the jets with the interstellar medium.
XTE J1550−564 offers a unique opportunity to study the dynamical evolu-
tion of relativistic jets on time scales inaccessible for active galactic nuclei jets,
with implications for our understanding of relativistic jets from Galactic x-ray
binaries and active galactic nuclei.
Collimated relativistic jets produced by active galactic nuclei (1) (AGN) and by accretion-
powered stellar compact objects in sources called microquasars (2) are commonly observed at
radio wavelengths. Such jets are produced close to black holes (supermassive ones in AGN and
stellar-mass ones in microquasars) and may help probe the dynamics of matter being accreted
in intense gravitational fields. The unprecedented sub-arc second resolution of the Chandra
x-ray observatory has recently allowed the detection of x-ray jets in many AGNs. Whereas
the radio emission of AGN jets is thought to originate from synchrotron emission, the nature
of the x-ray emission is still under debate, but synchrotron or inverse Compton radiation are
likely involved (3). Jets produced by Galactic black holes, such as XTE J1550−564 should
evolvemuch morerapidly than AGN jets and, therefore, could provideinsightsto thedynamical
evolution of relativistic outflows and also to the processes of particle acceleration. Here, we
present the first detection of large-scale, moving, relativistic jets ejected from a Galactic black
The x-ray transient XTE J1550−564 (Galactic longitude and latitude l = 325.88◦, b = –
1.83◦) was discovered by the All-Sky Monitor (ASM) aboard the Rossi X-ray Timing Explorer
(RXTE) on 7 September 1998 (4). Shortly after its discovery, a strong and brief (about one day)
x-ray flare was observed on 20 September 1998 (5,6) and radio jets with apparent superluminal
velocities (> 2 c, where c is the speed of light) were observed starting on 24 September 1998
(7). Subsequent optical observations showed that the dynamical mass of the compact object is
10.5 ± 1.0 M⊙, indicating that the compact object in XTE J1550−564 is a black hole, revealed
the binary companion to be a low mass star, and led to a distance estimate of about 5.3 kpc (8).
Following the re-appearance of x-ray emission from XTE J1550−564 in early 2002 (9), we
initiated a series of radio observations with the Australia Telescope Compact Array (ATCA).
black hole binary (10). These observations also revealed a new radio source ∼ 22 arc sec to
the west of the black hole binary. ATCA observations performed on 29 January 2002 (Fig. 1),
in an array configuration allowing higher spatial resolution imaging, showed that the western
source had a possible extension pointed toward XTE J1550−564. The position angle of this ra-
dio source relative to XTE J1550−564 was –85.8◦± 1.0◦, which is consistent with the position
angle (–86.1◦± 0.8◦, (11)) of the western component of the superluminal jet observed during
the September 1998 radio flare with long baseline interferometry (7).
we searched archival data from Chandra taken in 2000 for x-ray sources located along the jet
axis of XTE J1550−564. The field of view of XTE J1550−564 was imaged by Chandra on
9 June, 21 August and 11 September 2000. Examination of the 0.3–8 keV images (Fig. 2) re-
vealed an x-ray source ∼ 23 arc sec to the east of XTE J1550−564 at a position angle of 93.8◦±
0.9◦from XTE J1550−564, lying along the axis of the eastern components of the radio super-
luminal jets (7) (at a position angle of 93.9◦± 0.8◦; (11)). The angular separation between this
eastern source and XTE J1550−564 increased from 21.3 ± 0.5 arc sec on June 9 to 23.4 ± 0.5
arc sec on September 11, implying that the eastern source moved with an average proper mo-
tion of 21.2 ± 7.2 mas day−1between these two observations. This marks the first time that
an X-ray jet proper motion measurement has been obtained for any accretion powered Galactic
or extra-galactic source. Our radio observations (Fig. 1) performed with ATCA between April
2000 and February 2001 showed a weak, decaying, and moving radio source consistent with
the position of the eastern x-ray source. It was not detected in February 2002 (Fig. 1) with a
three sigma upper limit of 0.18 mJy at 3.5 cm.
With the discovery of the western radio source in early 2002, we obtained a 30 ks Chandra
observation on 11 March 2002. In the resulting 0.3–8 keV image (Fig. 2), three sources were
detected along the axis of the jet: the x-ray binary XTE J1550−564, an extended x-ray source
at the position of the western radio source, and a faint source that is 29.0 ± 0.5 arc sec east of
XTE J1550−564. This weak x-ray source was the eastern source that had smoothly decayed
and moved by 5.7 ± 0.7 arc sec since September 2000. The eastern source was active during a
period of at least two years (from April 2000 to March 2002).
The most remarkable feature of this Chandra observation is the discovery of x-ray emission
associated with the western radio source. Both the radio and x-ray emission of the western
the two wavelengths. Most (70%) of the x-ray emission was concentrated in the leading peak
which has a full width at half maximum (FWHM) of 1.2 arc sec. A trailing tail, pointed back
towards XTE J1550−564, gave a full width at 10% of maximum intensity of 5 arc sec.
The alignment of the eastern and western sources with the axis of the jet observed on 24
September 1998 (7), as well as the proper motion of the eastern source, imply that both new
sources are related to thejets of XTE J1550−564. In addition, both sources are likely connected
with the apparently superluminal ejecta from the brief and intense flare of 20 September 1998
(7). Indeed, large scale ejections of relativistic plasma (from XTE J1550−564) have been
observed and resolved only during this occasion; radio emission when detected at other epochs
has been associated with the compact jet of the low-hard x-ray spectral state (12). Also, the
RXTE/ASM has not detected any other x-ray flares similar to the large flare of 20 September
1998 in subsequent monitoring. The fact that the eastern source apparently moves faster (see
below) than the western source is consistent with the interpretation in which the eastern source
constitutes the jet that is pointing toward Earth (the approaching jet) and the western source the
With the positions of the eastern (and approaching) jet on 9 June 2000 and that of the west-
ern (and receding) jet on 16 January 2002, we find average proper motions of 32.9 ± 0.7 mas
day−1and 18.3 ± 0.7 mas day−1, respectively. At a distance of 5.3 kpc (8), this corresponds
to average apparent velocities on the plane of the sky of 1.0 c and 0.6 c for the eastern and
western jets, respectively. The proper motion of 21.2 ± 7.2 mas day−1measured by Chandra
for the eastern jet between 9 June 2000 and 11 September 2000 is significantly smaller than
its corresponding average proper motion, which indicates that the ejecta decelerated after the
ejection. This is confirmed by the Chandra detection of the eastern jet in March 2002, implying
an average proper motion of 10.4 ± 0.9 mas day−1between 11 September 2000 and 11 March
2002. The relativistic plasma was originally ejected at greater velocities, as the relative velocity
was initially greater than 2 c (the initial proper motion was greater than 57 mas day−1for the
approaching jet, (7)). These observations provide the first direct evidence for gradual decelera-
tion of relativistic materials in a jet. Previous observations of other microquasars are consistent
with purely ballistic motions (e.g. (2,13)) except for the system called XTE J1748−288, where,
after ballistic ejections, the jet was observed to stop suddenly, presumably following a collision
with environmental material (14,15).
of 29 arc sec and 23 arc sec, respectively, which correspond to projected physical separations
of 0.75 pc and 0.59 pc, respectively, for a distance of 5.3 kpc (8). These are large distances
for moving relativistic ejecta (in GRS 1915+105, the ejecta have been observed to travel up
to projected distance of 0.08 pc and on a maximum time scale of four months, (2)). Persis-
tent large scale (1-3 pc) jets have previously been observed only at radio wavelengths, e.g.
1E 1740.7−2942 and GRS 1758−258 (16, 17), but without indication of associated moving
ejecta. Our observations reveal that the relativistic ejecta of a Galactic black hole have been
able to travel over parsec scale distances at relativistic velocities during several years. An im-
portant aspect of our discovery is that it provides the first direct evidence for a large-scale,
moving x-ray jet from any black hole (Galactic or in AGN).
SS 433 is the only other x-ray binary for which large scale (up to ∼ 40 arc min, i.e. several
tens of parsecs), non-thermal x-ray emission has been previously observed, probably associated
with interactions of the jets with the interstellar medium (ISM) (18,19,20,21,22). The helical
pattern observed in the lobes, at radio wavelengths, indicates a connection between the lobes
and the corkscrew pattern associated with plasma ejection close to (on arc sec scale) the core of
the SS 433/W50 system (23). However, relativisticmotion at large scales has not been observed
in SS 433. We note that thermal X-ray emissionarising from movingrelativisticejecta, but only
out to ∼ 0.05 pc from the compact object, has been reported in SS 433 (24,25).
Our results demonstrate that the emission from relativisticejecta of Galactic black holes can
be observed at wavelengths extending up to x-rays. Future sensitive, high-resolution observa-
tions of other Galactic black hole jets in the infrared (26), optical, and x-rays bands may reveal
that broadband emission from relativistic ejecta of Galactic black holes is more common than
previously thought and offer an exciting way to probe the physics of the jets. AGN jets, which
were previously detected at radio and optical wavelengths, are now known, with the advent of
the Chandra observatory, to often produce x-rays. Whereas the radio emission of AGN jets is
thought to originate from synchrotron emission, the nature of the x-ray emission has not always
been clearly identified. Although it is thoughtto be non-thermal, it is not always knownwhether
synchrotron or inverse Compton radiation predominates for a particular object (3,27,28,29).
The nature of the physical mechanism producing the emission from the relativistic jets of
XTE J1550−564 can be understood by looking at the broadband spectrum, e.g. for the western
jet on 11 March 2002 (Fig. 3). The position and morphology of the radio and x-ray counterparts
of the western jet are consistent with each other (Figs 1 and 2) and the spectral energy distribu-
tion is consistent with a single power law (of spectral index –0.660 ± 0.005). These facts favor
a scenario in which the broadband emission from the jets is synchrotron emission from high
energy particles. Similar conclusions could be drawn for the eastern jet in 2000, because the
overall radio flux was also consistent with an extrapolation of the x-ray spectrum with a spectral
index of –0.6. Detection of x-ray synchrotron emission would imply a large Lorentz factor, of
the order of 2 × 107(corresponding to an energy of ∼ 10 TeV), for the x-ray emitting electrons
(under the equipartition assumption giving a magnetic field of ∼ 0.3 mG).
Acceleration in a shock wave is the most likely origin for the very high energies required.
Shock waves could be produced by internal instabilities (30) or by varying flow speeds within
the jet, as proposed to occur in some models of gamma-ray bursts or AGNs (31,32). If several
relativistic plasmoids were ejected around 24 September 1998 (7) and their velocities were
slightly different, then it would have taken several months (maybe years) for them to collide.
Such a collision would have produced shock waves, particle acceleration, and the brightening
of the jets.
A more plausible alternative is that the shock waves are produced when the jet material
moving with bulk relativistic speed interacts with the ISM (i.e. an external shock). In fact,
the gradual deceleration we observed for the eastern jet would be easily explained by such in-
teractions. Inhomogeneities in the ISM could also be at the origin of the brightening of the
eastern and western jet at different epochs. The origin of the western jet is less clear as no
proper motion has been yet observed. Future observations will show whether or not the western
jet is still moving and together with high spatial resolution observations and broadband spectra
will be important in deciding between the models (internal or external shocks). Also, regular
observation of the jets of XTE J1550−564 would map the propagation of the shocks and al-
low estimation of the energy dissipated in the jets while decelerating in the ISM. Therefore,
XTE J1550−564 offers a unique opportunity to study the dynamical evolution of relativistic
jets on time scales inaccessible for AGN jets, and has implications not only for the study of jets
from Galactic x-ray binaries, but also for our understanding of relativistic jets from AGNs.
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35. SC and JAT acknowledge useful conversations with A. Celotti, S. Heinz and V. Dhawan.
PK acknowledges useful discussions with H. Falcke and D. Harris. SC thanks C. Bailyn, S.
Chaty, D. Hannikainen and D. Hunstead for providing information before publication and
F. Mirabel for a careful reading of this manuscript. SC would like to thank R. Ekers, D.
McConnell, R. Norris, B. Sault and the ATCA TAC for allowing the radio observations. We
thank H. Tananbaum for granting Director’s Discretionary Time for the Chandra observa-
tions and J. Nichols for rapid processing of the data. We have made use of observations
performed with ESO Melipal Telescope at the Paranal Observatory under Director’s Discre-
tionary Time programme 268.D-5771. The Australia Telescope is funded by the Common-
wealth of Australia for operation as a National Facility managed by CSIRO.
XTE J1550-564Western jet
Eastern JetXTE J1550-564
Axis of VLBI jet
Axis of VLBI jet
2000 June 1
2002 January 29
Figure 1: Uniform weighted maps of the field of view around the black hole candidate
XTE J1550−564 on 1 June 2000 (top) and 29 January 2002 (bottom) showing the radio coun-
terpart to the eastern and western jets (when detected). The stationary black hole binary
XTE J1550−564 is at the center of the image, and has a radio spectrum typical of the self-
absorbed compact jet (33,34) that is observed during the x-ray low/hard spectral state (ref 12).
(A) (1 June 2000): Map at 4800 MHz (6 cm). The position of the eastern radio jet is α(J2000)
= 15h 51m 01s.30 and δ(J2000) = –56◦28′36.9′′with a total uncertainty of 0.3 arc sec, i.e. at a
position angle of 93.8◦± 0.9◦from XTE J1550−564. The synthesized beam (in the lower right
corner) is 5.5 × 2.1 arc sec with the major axis in position angle of 63.1◦. The peak brightness
in the image is 1.1 mJy per beam. Contours are plotted at –3, 3, 4, 5, 6, 9 times the r.m.s. noise
equal to 0.1 mJy per beam. The cross marks the position of the western jet, as measured on 29
January 2002. (B) (29 January 2002): Map at 8640 MHz (3.5 cm). The position of the western
radio jet is α(J2000) = 15h 50m 55s.94 and δ(J2000) = –56◦28′33.5′′with a total uncertainty
of 0.3 arc sec. The synthesized beam is 2.4 × 1.3 arc sec with the major axis in position angle
of –54.6◦. The peak brightness in the image is 1.79 mJy per beam. Contours are plotted at –3,
3, 4, 5, 6, 9, 15, 20 30 times the r.m.s. noise equal to 0.05 mJy per beam. The cross marks the
position of the eastern jet, as measured on 11 March 2002 during the Chandra observation.
11 March 2002
21 Aug. 2000
11 Sept. 2000
9 June 2000
Eastern X-ray Jet
Western X-ray jetXTE J1550-564
Figure 2: Chandra 0.3–8 keV images (using the Advanced CCD Imaging Spectrometer spec-
troscopy array ACIS-S), which show the black hole binary XTE J1550–564 (center), the ap-
proaching eastern x-ray jet (left) and the receding western x-ray jet (right). The observations
are ordered chronologically from top to bottom, and each image is labeled with the observation
date. These filled contour plots have been produced by convolving the original Chandra image
with a 2-dimensional Gaussian with a width of 2 pixels in both directions. In 2000 (A), there
are 11 contours between the lowest contour of 1.33 × 10−3count s−1pixel−1and the highest
contour of 8.16 × 10−3count s−1pixel−1. The same contour levels are used in all three 2000
images, but it should be noted that the flux levels for 9 June 2000 are not directly comparable
to those for the other two observations because a grating was inserted for the June 9 observa-
tion. For the 2002 image (B), there are 11 contours between 0.33 ×10−3count s−1pixel−1and
8.16 × 10−3count s−1pixel−1. The dashed lines mark the positions of XTE J1550-564 and
the eastern x-ray jet on September 11 when the sources were separated by 23′′.4. The proper
motion of the x-ray jet is 21.2 ± 7.2 mas day−1between 9 June 2000 and 11 September 2000
and averages 10.4±0.9 mas day−1between 11 September 2000 and 11 March 2002, indicating
deceleration of the jet. Assuming a power-law spectral shape with a photon index of 1.7 and
Radio Download full-text
Figure 3: The spectral energy distribution of the western jet around 2002 March 11. The radio
points near 1010Hz are ACTA measurements from March 6. The radio flux density were 5.7
± 0.3, 3.60 ± 0.08 and 2.55 ± 0.07 mJy at 2496, 4800, 8640 MHz respectively, giving a
spectral index of –0.63 ± 0.05 in the radio range. The x-ray measurement near 5 × 1017Hz
is the Chandra measurement from March 11. X-ray spectral fitting of the Chandra data for the
western source assuminga powerlaw form with interstellar absorption fixed to the total Galactic
HI column density (NH = 9.0 × 1021cm−2) gives a spectral index of –0.70 ± 0.15 (90 %
confidence level). The spectrum may be somewhat steeper if there is additional absorption near
thesource. Theunabsorbed 0.3–8keV flux is 3.8× 10−13ergs cm−2s−1(i.e. 18 nJyat 2.2 keV).
The optical upper limits (99% confidence level) in between are derived from deep observations
carried out with the 8.2 metre Unit 3 telescope at the European Southern Observatory, Paranal.
The source was observed with the FORS1 instrumentin the Bessel V and R filters on March 10,
with limiting magnitudes for point sources of 25.2 and 25.5 mag., respectively, and on March
18 with FORS1 and the Bessel I filter, with a limiting magnitude for point sources of 25.5
mag. We assumed an optical extinction of AV = 4.75 mag. (8). The broadband spectral energy
distribution is consistent with a single powerlaw of spectral index –0.660 ± 0.005.