- Email: [email protected]
Large-scale X-ray jets from Galactic black holes
Nuclear Physics B (Proc. Suppl.) 132 (2004) 354–362 www.elsevierphysics.com
Large-scale X-ray jets from Galactic black holes P. Kaareta, S. Corbelb , J.A. Tomsickc, Y. Butta , R.P. Fenderd , J. Lazendica , J.M. Millera , J.A. Orosze, A.K. Tzioumisf , Rudy Wijnandsg a
Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
b
Universit´e Paris VII and Service d’Astrophysique, CEA, CE-Saclay. 91191 Gif sur Yvette, France
c
Center for Astrophysics and Space Sciences, University of California at San Diego, La Jolla, CA 92093-0424, USA d
Astronomical Institute ‘Anton Pannekoek’, University of Amsterdam and Center for High Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
e Department of Astronomy, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182, USA f
Australia Telescope National Facility, CSIRO, P.O. Box 76, Epping, NSW 1710, Australia
g
School of Physics and Astronomy, University of Saint Andrews, North Haugh, Saint Andrews, Fife KY16 9SS, Scotland, UK.
Observations of jets from stellar-mass sources located in our Galaxy offer a unique opportunity to study the dynamical evolution of relativistic jets on time scales inaccessible for active galactic nuclei jets, with implications for our understanding of the dynamics and energetics of relativistic jets from Galactic x-ray binaries and active galactic nuclei. We review recent observations of X-ray jets from Galactic black hole candidates. Spatially resolved X-ray spectra from SS 433 have provided evidence for re-heating in a hadronic jet and may offer an observational probe of jet collimation. A large-scale jet from the now quiescent transient 4U 1755–33 appears to indicate continual jet formation over a period of 10–30 years. Detection of a jet from XTE J1550–564 has provided the first direct measurement of gradual deceleration of a jet from a black hole and strong evidence for the re-energization of jet particles to energies up to 10 TeV at sites far from the jet origin.
1. WHY STUDY JETS? Relativistic jets are a striking manifestation of the efficient energy release from matter accreted onto compact objects observed from X-ray binaries and active galactic nuclei (AGN). The mechanisms of jet formation and propagation remain mysterious and new observations will be needed to understand them. The evolution time scales for jets produced by X-ray binaries with stellarmass compact objects is millions of times faster than for AGN jets. Observations of jets from Xray binaries hold great promise for revealing jet dynamics. The first discovery of jets from Galactic X-ray 0920-5632/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2004.04.063
binaries was of stationary, large-scale lobes in the X-rays [1] and radio ([2] for a review). Closer in, near the compact object, jets take on two distinct incarnations. One is compact jets found in the hard X-ray spectral states. The compact jets tend to be steady, at least to the sensitivity of current observations, and, as the name would imply, compact, with angular sizes of tens of milliarcseconds. The other incarnation of jets from Galactic black hole candidates are large scale jets. Such jets are formed in high mass accretion rate states often, and perhaps always, at state transitions. Often these jets are impulsive events where dis-
P. Kaaret et al. / Nuclear Physics B (Proc. Suppl.) 132 (2004) 354–362
tinct blobs from individual injection events can be resolved and their motion can be measured. We will concentrate on this class of jet in this review. Why should we study jets? • Outflows may be important in the structure of accretion flows. • Jets may dissipate a large fraction of the total accretion energy. • Jets may be a source of light element nucleosynthesis and input energy to the interstellar medium. • They look cool. The first two items are the primary motivation for studying jets. The kinetic energy of the outflows in jets appears to be comparable to the total energy released by accretion in many systems [3,4]. Clearly, this implies that the outflow must have a major impact on the structure of the accretion flow. Realistic models of the accretion flow must necessarily include outflows and the production of jets. The study of jets, including obtaining an understanding of how accretion power is divided between radiation luminosity and kinetic energy in outflows, is essential in fully understanding accretion flows onto black holes. Beyond interest in compact objects and their accretion flows, jets may also have a significant impact on the Galactic environment. It has been found that the companion stars to some jet sources show unusually high abundances of light elements, particularly lithium [5]. It has been suggested that this may be due to the impact of a hadronic jet on the companion star [6]. The jet is essentially a beam of high energy particles. Interaction of this beam with the stellar material may cause spallation, opening new channels for light element synthesis. Jets may also contribute, at the few percent level, to the energy budget of the interstellar medium. Some of the major questions in understanding the physical nature of jets and their relation to the overall accretion flow are: • Are jets electron/proton tron/positron?
or
elec-
355
• Are jets purely accretion powered or can jets extract energy from the rotation of a black hole? • How are jets formed and collimated and what is the role of magnetic fields? • What fraction of the total power output of compact objects is in jets versus radiated energy? • Can jets be re-energized at large distances from their origin? The tools we have available to answer these questions are imaging and spectroscopy. A multiwavelength approach is essential in attempting to understand the physical nature of jets and, as we will see below, multiwavelength spectra place the most direct constraints on the jet emission mechanism. An understanding of the emission mechanism is essential to estimating the energetics of jets. However, the most exciting new advance comes from imaging. Repeated imaging of large scale jets from one Galactic black candidate has allowed us to measure the motion of the jets and thereby constrain the jet kinematics. This provides an exciting opportunity to study the jet dynamics and measure the total kinetic energy of the jets. 2. XTE J1550-564 XTE J1550-564 is an X-ray transient which exhibited a major outburst in September 1998 which produced a superluminal jet ejection event detected with radio VLBI with a separation speed greater than 2c [7]. The source exhibited subsequent outbursts of much lower amplitude in 2000, late 2001/early 2002, and 2003. The repeated smaller outbursts lead to a series of radio monitoring observations. In 2002, Corbel and collaborators using the Australia Telescope Compact Array (ATCA) discovered a radio transient to the West of the source [8] which lay along the superluminal jet axis [9]. A re-analysis of archival radio and Chandra data showed a jet, also along the superluminal jet axis, to the East of the source in data taken in 2000 [9,10]. Motion of the jet away
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ASM counts/second
600 500 400 300 200 100 0 1998
1999
2001 2000 Time (Year)
2002
2003
Figure 1. X-ray light curve of XTE J1550-564 in the 2–12 keV band obtained with the All-Sky Monitor on the Rossi X-Ray Timing Explorer.
from the source was detected in the X-ray data. Because the Eastern jet has a larger angular separation from the source at a given time (in our reference frame), we identify it as the approaching jet and identify the Western jet as receding. X-ray observations made after the discovery of the jets permitted the motion of the receding jet to be tracked [11]. A composite image of six observations (we now have a total of seven, with an eighth scheduled for October 2003), is shown in Fig. 2. These images are the first time series showing motion of a relativistic jet in X-ray images. To perform quantitative analysis of the jet motion, we use the position of the central source, which is XTE J1550-564 to adjust the relative astrometry of each image. This provides an significant improvement over the absolute Chandra astrometry provided by the star trackers. Due to the limited statistics available for the Eastern jet, we calculated the jet position from the centroid of the events. For the Western jet, we used a one-dimensional KS test to compare the distributions of event positions along the jet axis. We found the relative shift of the jet between different observations which gave the best match in profiles according to the KS test. This technique assumes that the jet profile remains constant as
Figure 2. Images from Chandra observations of XTE J1550-564. The central source in each image is XTE J1550-564. The source on the left is the Eastern and approaching jet, on the right is the Western and receding jet.
the jet moves. This assumption appears to be reasonable, since the KS test values show that the X-ray profiles are very similar except for the overall shift in position, see Fig. 3. The jet morphology shows a strong peak at the head (leading edge) of the jet with a lower amplitude tail trailing behind. This ‘comet-like’ morphology is suggestive of an interaction with the interstellar medium. Throughout 2002, the jet morphology appears very similar in X-rays and radio. The motion of the jets is shown in Fig. 4. We plot the observed separation of each jet from XTE J1550-564. There is a point on the plot for each detection of each jet with crosses for the approaching (Eastern) jet and diamonds for the receding (Western) jet. The motion of the jets is inconsistent with a linear trend, which would be expected if the jets were moving at constant speed. Therefore, the jets must be decelerating. These observations are the first direct evidence
120
Counts/bin
100 80 60 40 20 0 30 25 20 15 10 Displacement along jet axis (arcsec)
Figure 3. Profiles of the receding jet from XTE J1550-564 . The plots are a histogram of events in the jet versus position along the jet axis. The solid curve is for data from March 2002, the first X-ray detection of the jet. The dashed curve is data from January 2003 scaled to match the peak of the March 2002 histogram.
for gradual deceleration of jets from a black hole. Extrapolation of the first three data points for the approaching jet would indicate a time of origin for the jet before September 1998. Because there is no evidence for any X-ray activity from XTE J1550-564 in RXTE ASM data from before September 1998 and because relativistic jets were observed in the radio at this time, we assume that both jets were created in a single injection episode in September 1998. This allows us to fix the time of zero separation for both jets. We take the distance to XTE J1550–564 to be in the range 2.8– 7.6 kpc [12]. To model the jet motion, we assume the jet has decelerated with an exponential dependence of speed (in the reference frame of XTE J1550564) versus time. We then calculate the observed position versus time in our reference frame for
Angular seperation (arcsec)
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35 30 25 20 15 10 5 0 500 1000 1500 2000 0 Time after X-ray flare (days)
Figure 4. Motion of the jets of XTE J1550-564. Crosses indicate the position of the approaching jet versus time and diamonds the receding jet. The upper solid line is a fit to the motion of the approaching jet described in the text. The lower solid line indicates the corresponding observed motion expected for the receding jet if its motion is identical to that of the approaching jet under a spatial reflection about the position of XTE J1550-564.
each jet taking into account the light travel time delays. The upper curve in Fig. 4 represents a fit assuming a distance to XTE J1550-564 of 4 kpc. The free parameters of the fit and their best fit values are: an initial jet speed of 0.91c, an exponential decay time of 1095 days, and a jet axis relative to our line of sight of 61◦ . Note that the fitted values depend on the assumed distance and the ad-hoc assumption of exponential decay in speed. However, the curve appears to provide a reasonable fit to the data and the shape of the best fit curve varies little when the source distance is varied over the allowed range. The lower curve represents the motion expected for the receding jet if its motion is identical
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Flux (erg cm-2 s-1)
10-12
10-13
10-14
10-15 500 1000 1500 2000 0 Time after X-ray flare (days)
Figure 5. Flux evolution of the jets of XTE J1550-564. The fluxes for the approaching jet are marked as crosses and those for receding jet as diamonds. The dashed lines are fits of exponential decays to the data.
to that of the approaching jet except spatially reflected through the position of XTE J1550564. The difference between the upper and lower curves arises from light travel time delay effects. We note that none of the data for the receding jet were used in the calculation of the lower (or upper) curve. Interestingly, the first data point for the receding jet lies on the curve, while the last two points lie below and the final point is significantly below. This suggests that the receding jet had a motion similar to that of the approaching jet before early 2002 and then suffered significantly larger deceleration beginning in early 2002. The flux evolution of the jets is shown in Fig. 5. There is a point on the plot for each detection of each jet with crosses for the approaching (Eastern) jet and diamonds for the receding (Western) jet as in Fig. 4. We fit an exponential decay to the data points for each jet. The flux decay time
scale is 260±25 days for the approaching jet and 206±20 days for the receding jet. These two values are consistent within the errors. We note that the receding jet was not detected in 2000. The upper limit plotted around day 700 was obtained by combining all of the 2000 data and used a large extraction region covering the full likely range of jet positions. The large brightening of the receding jet suggests that it was re-energized between 2000 and early 2002. Note that after the turnon of the receding jet, it appears significantly brighter than the approaching jet. This is exactly the opposite from that expected if the approaching and receding jets are identical in their intrinsic properties and differ in their observed properties only because of relativistic Doppler boosting. Therefore, the intrinsic luminosity of the approaching and receding jets must differ. The motion and flux data are consistent with a scenario in which the receding jet had a motion similar to that of the approaching jet before early 2002 and then suffered significantly larger deceleration beginning in early 2002. The deceleration re-energized the jet causing it to significantly brighten in the radio and X-rays. Fig. 6 shows the broadband spectrum of the approaching jet using data from the receding jet in the radio, optical, and X-rays all taken in March 2002. All of the data are consistent with a single powerlaw with an energy index α = −0.660 ± 0.005 extending from the radio to the X-ray band. The X-ray spectrum shows no evidence for line emission, but the constraints are weak. The multiwavelength spectrum, taken together with the fact that the radio and X-ray emission arise from the same region and have very similar morphology, is strong evidence that the emission is synchrotron with a single population of energetic particles (with energies up to 10 TeV, i.e. electrons with γ ∼ 107 ) producing the radiation across the entire spectrum. Given that the emission mechanism is synchrotron radiation and deriving an upper limit on the jet volume from the observed size (we assume that the jet is symmetric about the jet axis and take account of the allowed range of distances) a lower limit on the magnetic field can be derived, B > ∼ 150 µG. The rate of flux decay places an
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359
10-2
Flux density (Jy)
10-3 10-4 10-5 10-6 10-7 10-8 108
1010
1012 1014 1016 Frequency (Hz)
1018
Figure 6. Multiwavelength spectrum of the receding jet of XTE J1550-564 following [9].
upper limit on the magnetic field, B < ∼ 2500 µG. At this field strength the observed decay in Xrays would be entirely due to synchrotron losses. Similar considerations allow us to make an order of magnitude estimate of the total jet mass m ∼ 1021 g. Because the electrons are highly energetic, the total mass is similar for either a electron-proton or electron-positron jet. If we take the model used above for the motion of the jet, then we can calculate the deceleration as a function of time and therefore the kinetic energy loss rate E˙ K . We find that E˙ K ∼ 1034 erg s−1 for the approaching jet in June 2000. The approaching jet luminosity at the same epoch is L ∼ 1032 erg s−1 . Thus an efficiency for conversion of jet kinetic energy into luminosity of order 10−2 is required. This value is reasonable in the context of relativistic shock acceleration models. To conclude this section, we summarize our results. These observations of XTE J1550-564 provide the first direct measurement of deceleration in a black hole jet. The gradual jet deceleration,
Figure 7. XMM-Newton image of 4U 1755– 33 following [13]. The image is the summed MOS1+MOS2+pn data in the 0.5–10 keV band and has been smoothed. The arrow indicates the position of 4U 1755–33. The jet extends to the NW (up and to the right) and SE (down and to the left) from 4U 1755–33.
the jet morphology, and the brightening of the receding jet several years after its ejection suggest an interaction with the interstellar medium and provide evidence for in-situ particle acceleration powered by bulk deceleration. Measurement of the kinematics can help constrain the energetics of the jet. 3. OTHER SOURCES 3.1. Fossil Jet from 4U 1755-33 Recently, Angelini and White discovered a large scale X-ray jet from the long-term X-ray transient and black hole candidate 4U 1755–33 using XMM-Newton [13]. As shown in Fig. 7, the jet is a linear structure which is roughly symmetric about the position of 4U 1755–33, a double sided jet, extending about 3 to the north-west and to the south-east of the black hole candidate.
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There appear to be multiple knots in the jets. For estimated distance of 4–9 kpc, this corresponds to a jet length of 3–8 pc. Therefore, the jet must have taken at least 10-30 years to form. 4U 1755– 33 was active from its discovery with Uhuru in 1970 (it may have been active at earlier times) until it was found in an off state in 1996. It has not been found in an active state since 1996. The source was active for at least 25 years which appears sufficient to have formed the jet. The primary questions concerning the jet are: what is the X-ray emission mechanism (thermal bremsstrahlung or non-thermal synchrotron or inverse Compton radiation), how is the jet emission powered (internal shocks or interaction of the jet with the interstellar medium), and what is the total energy in the jet. A key discriminate between thermal and nonthermal emission is that thermal emission should produce line emission. We extracted a spectrum of the jet emission from 4U 1755–33 to search for line emission. No line emission is apparent. The upper bounds on line emission depend on the assumed line parameters. For line parameters similar to those found for SS433 (see below), we find 90% confidence upper bounds on the equivalent widths of Fe-K emission lines of 0.9-2 keV. These are in the range of the equivalent width measured for the inner jets of SS433 and are not constraining. We fit the continuum spectrum and found adequate fits with an absorbed powerlaw with a photon index of 1.7 ± 0.3 or a MEKAL thermal emission model with solar abundances and kT > 4.0 keV, similar to the result of [13]. In both cases, we fixed the absorption to the line of sight value NH = 3 × 1021 cm−2 . The unabsorbed flux is 6 × 10−14 erg cm−2 s−1 in the 0.5-2 keV band and 3×10−13 erg cm−2 s−1 in the 0.5-10 keV band. We analyzed archival VLA data for 4U 1755–33 taken in 1981 at 20 cm and in 1990 at 6 cm. We find no radio counterpart to the X-ray jet in either image. The rms noise level in the images is about 0.1 mJy. However, these VLA observations were taken in configurations which provide high angular resolution and are not sensitive to structure on arcminute scales. Therefore, large scale structures could have higher fluxes and still not have been detected.
If the X-ray emission is thermal bremsstrahlung, then the total mass and energy of the jet can be estimated from its observed luminosity, temperature, and volume. For the volume, we assume that the jet occupies a roughly cylindrical volume with a diameter perpendicular to the direction of motion of 0.2 and a linear dimension of 6 multiplied by a filling factor of 0.2. The volume is then ∼ 1054 cm3 for an assumed distance of 4 kpc. From the flux and temperature (we take kT = 4 keV) quoted above, we find that the density of the jet material is n ∼ 5 cm−3 , the total energy of the jet is ∼ 1047 erg and the total mass in the jet is ∼ 1031 g. If the jet were fed by the outflow from a mass accretion rate of 1019 g/s corresponding to the Eddington rate for a 10M compact object, at least 104 years would be required to accumulate the needed mass. Given the linear structure of the jet (i.e. it does not appear to be dominated by the termination of an energetic jet stopping in a dense region of the ISM), it is difficult to construct a model containing this amount of mass. If X-ray emission is synchrotron, then the equipartition magnetic field can be calculated. Using the photon index quoted above and depending on the distance and the lower frequency cutoff in the spectrum, we find magnetic fields of order 100 µG. The radiative lifetime of the electrons producing the X-ray emission (which must have high Lorentz factors ∼ 107 ) are of order 100 years. The number of electrons needed to produce the observed radiation is of order 1047 . Assuming that the jet is composed of normal matter (i.e. roughly one proton per electron), then the required mass is ∼ 1023 g. This mass estimate is rather uncertain, due to the lack of a multiwavelength spectrum for the jet, but it is appears that the material required for a synchrotron emitting jet could accumulated in less than the estimated lifetime of the jet. The available data on the jet of 4U 1755–33 are consistent with the X-rays being synchrotron emission. Determining the X-ray emission mechanism is the essential first step in understanding the physical properties of the jet and is critical to using the jet from 4U 1755–33 to address the fundamental questions concerning the physics of jets raised
P. Kaaret et al. / Nuclear Physics B (Proc. Suppl.) 132 (2004) 354–362
above. If the X-ray emission is really synchrotron, then the jet should be detectable in the radio. Contemporaneous XMM-Newton and deep radio observations are needed. New X-ray data should also permit measurement of the decay rate of the jet and therefore an estimate of its overall energetics. This will be of great interest to compare the total energy in the jet versus the energy produced over the X-ray active lifetime of the source and to begin to constrain the energy input to the ISM from relativistic jets.
3.2. SS 433 The unusual X-ray binary SS 433 has largescale lobes extending to scales of 45 (60 pc) which were the first discovered Galactic X-ray jets [1]. The outer X-ray lobes of SS433 are similar to the large-scale jets of XTE J1550-564 and have a power-law spectrum extending up to 100 keV indicating that the emission is non-thermal [14] and do not show line emission [15]. The central X-ray source shows red and blueshifted Fe emission lines [16,15]. Recently, the inner jets have been resolved. They show extent up to 6 (0.14 pc) from the compact object and prominent X-ray line emission [17]. The line structure is complex with several elements detected: Ne, Mg, Si, S, Ca, and Fe. The red and blue shifted Fe line emission is spatially separated [18] and the velocity dispersion is higher for Fe than for Si [19]. The detection of line emission indicates that the jet contains baryons and that the emission is thermal. The spatially resolved spectra give strong evidence that jet material is reheated far from the point of injection. The difference in velocity dispersion between Fe and Si is inconsistent with that expected from thermal broadening. It has been suggested that the difference in velocity dispersion is a geometrical effect due to differing jet geometry between the regions where the Fe and Si emission is produced and that this is evidence that the jets narrow with distance from the origin [19]. If correct, these observations may provide information on the mechanisms of collimation in relativistic jets.
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4. CONCLUSIONS New observations of X-ray jets from Galactic black hole candidates offer a new probe of black hole jets which may provide new insights to the dynamics of jets which cannot be obtained from observations of AGN jets due to the long evolution time scales of jets from supermassive black holes. The large-scale jets from XTE J1550-564 may represent an intermediate stage between the compact jets which have been resolved with VLBI observations and the stationary lobes previously found in the radio. New observations of these jets have provided the first direct measurement of deceleration in black hole jets. The jets from XTE J1550-564 and also those from SS 433 have provided evidence for in-situ particle acceleration and re-energization of jets far from their origin. Study of the dynamics of the XTE J1550-564 jets have led to some initial, crude constraints on the kinetic energy content and evolution of relativistic jets. The inner jets in SS 433 are clearly baryonic. Whether this is true for all Galactic (and AGN) jets remains an open question, particularly because the jet speed in SS 433 is 0.26c which is significantly lower than the values near 0.9c found in most black hole candidate X-ray transients. The inner jets of SS 433 may also show observational signatures of jet collimation. If confirmed, this may provide a very exciting means to provide observational constraints on collimation mechanisms. In conclusion, new observations of X-ray jets have already provided exciting results and are a promising new area for future work. PK thanks the scientific organizing committee for inviting him to the conference and acknowledges support from NASA grant NAG5-7405 and Chandra grant number G03-4043X. REFERENCES 1. Seward, F. et al. 1980, Nature, 287, 806 2. Mirabel, I.F. & Rodriguez, L.F. 1999, ARA&A, 37, 409
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Large-scale X-ray jets from Galactic black holes P. Kaareta, S. Corbelb , J.A. Tomsickc, Y. Butta , R.P. Fenderd , J. Lazendica , J.M. Millera , J.A. Orosze, A.K. Tzioumisf , Rudy Wijnandsg a
Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
b
Universit´e Paris VII and Service d’Astrophysique, CEA, CE-Saclay. 91191 Gif sur Yvette, France
c
Center for Astrophysics and Space Sciences, University of California at San Diego, La Jolla, CA 92093-0424, USA d
Astronomical Institute ‘Anton Pannekoek’, University of Amsterdam and Center for High Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
e Department of Astronomy, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182, USA f
Australia Telescope National Facility, CSIRO, P.O. Box 76, Epping, NSW 1710, Australia
g
School of Physics and Astronomy, University of Saint Andrews, North Haugh, Saint Andrews, Fife KY16 9SS, Scotland, UK.
Observations of jets from stellar-mass sources located in our Galaxy offer a unique opportunity to study the dynamical evolution of relativistic jets on time scales inaccessible for active galactic nuclei jets, with implications for our understanding of the dynamics and energetics of relativistic jets from Galactic x-ray binaries and active galactic nuclei. We review recent observations of X-ray jets from Galactic black hole candidates. Spatially resolved X-ray spectra from SS 433 have provided evidence for re-heating in a hadronic jet and may offer an observational probe of jet collimation. A large-scale jet from the now quiescent transient 4U 1755–33 appears to indicate continual jet formation over a period of 10–30 years. Detection of a jet from XTE J1550–564 has provided the first direct measurement of gradual deceleration of a jet from a black hole and strong evidence for the re-energization of jet particles to energies up to 10 TeV at sites far from the jet origin.
1. WHY STUDY JETS? Relativistic jets are a striking manifestation of the efficient energy release from matter accreted onto compact objects observed from X-ray binaries and active galactic nuclei (AGN). The mechanisms of jet formation and propagation remain mysterious and new observations will be needed to understand them. The evolution time scales for jets produced by X-ray binaries with stellarmass compact objects is millions of times faster than for AGN jets. Observations of jets from Xray binaries hold great promise for revealing jet dynamics. The first discovery of jets from Galactic X-ray 0920-5632/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2004.04.063
binaries was of stationary, large-scale lobes in the X-rays [1] and radio ([2] for a review). Closer in, near the compact object, jets take on two distinct incarnations. One is compact jets found in the hard X-ray spectral states. The compact jets tend to be steady, at least to the sensitivity of current observations, and, as the name would imply, compact, with angular sizes of tens of milliarcseconds. The other incarnation of jets from Galactic black hole candidates are large scale jets. Such jets are formed in high mass accretion rate states often, and perhaps always, at state transitions. Often these jets are impulsive events where dis-
P. Kaaret et al. / Nuclear Physics B (Proc. Suppl.) 132 (2004) 354–362
tinct blobs from individual injection events can be resolved and their motion can be measured. We will concentrate on this class of jet in this review. Why should we study jets? • Outflows may be important in the structure of accretion flows. • Jets may dissipate a large fraction of the total accretion energy. • Jets may be a source of light element nucleosynthesis and input energy to the interstellar medium. • They look cool. The first two items are the primary motivation for studying jets. The kinetic energy of the outflows in jets appears to be comparable to the total energy released by accretion in many systems [3,4]. Clearly, this implies that the outflow must have a major impact on the structure of the accretion flow. Realistic models of the accretion flow must necessarily include outflows and the production of jets. The study of jets, including obtaining an understanding of how accretion power is divided between radiation luminosity and kinetic energy in outflows, is essential in fully understanding accretion flows onto black holes. Beyond interest in compact objects and their accretion flows, jets may also have a significant impact on the Galactic environment. It has been found that the companion stars to some jet sources show unusually high abundances of light elements, particularly lithium [5]. It has been suggested that this may be due to the impact of a hadronic jet on the companion star [6]. The jet is essentially a beam of high energy particles. Interaction of this beam with the stellar material may cause spallation, opening new channels for light element synthesis. Jets may also contribute, at the few percent level, to the energy budget of the interstellar medium. Some of the major questions in understanding the physical nature of jets and their relation to the overall accretion flow are: • Are jets electron/proton tron/positron?
or
elec-
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• Are jets purely accretion powered or can jets extract energy from the rotation of a black hole? • How are jets formed and collimated and what is the role of magnetic fields? • What fraction of the total power output of compact objects is in jets versus radiated energy? • Can jets be re-energized at large distances from their origin? The tools we have available to answer these questions are imaging and spectroscopy. A multiwavelength approach is essential in attempting to understand the physical nature of jets and, as we will see below, multiwavelength spectra place the most direct constraints on the jet emission mechanism. An understanding of the emission mechanism is essential to estimating the energetics of jets. However, the most exciting new advance comes from imaging. Repeated imaging of large scale jets from one Galactic black candidate has allowed us to measure the motion of the jets and thereby constrain the jet kinematics. This provides an exciting opportunity to study the jet dynamics and measure the total kinetic energy of the jets. 2. XTE J1550-564 XTE J1550-564 is an X-ray transient which exhibited a major outburst in September 1998 which produced a superluminal jet ejection event detected with radio VLBI with a separation speed greater than 2c [7]. The source exhibited subsequent outbursts of much lower amplitude in 2000, late 2001/early 2002, and 2003. The repeated smaller outbursts lead to a series of radio monitoring observations. In 2002, Corbel and collaborators using the Australia Telescope Compact Array (ATCA) discovered a radio transient to the West of the source [8] which lay along the superluminal jet axis [9]. A re-analysis of archival radio and Chandra data showed a jet, also along the superluminal jet axis, to the East of the source in data taken in 2000 [9,10]. Motion of the jet away
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ASM counts/second
600 500 400 300 200 100 0 1998
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Figure 1. X-ray light curve of XTE J1550-564 in the 2–12 keV band obtained with the All-Sky Monitor on the Rossi X-Ray Timing Explorer.
from the source was detected in the X-ray data. Because the Eastern jet has a larger angular separation from the source at a given time (in our reference frame), we identify it as the approaching jet and identify the Western jet as receding. X-ray observations made after the discovery of the jets permitted the motion of the receding jet to be tracked [11]. A composite image of six observations (we now have a total of seven, with an eighth scheduled for October 2003), is shown in Fig. 2. These images are the first time series showing motion of a relativistic jet in X-ray images. To perform quantitative analysis of the jet motion, we use the position of the central source, which is XTE J1550-564 to adjust the relative astrometry of each image. This provides an significant improvement over the absolute Chandra astrometry provided by the star trackers. Due to the limited statistics available for the Eastern jet, we calculated the jet position from the centroid of the events. For the Western jet, we used a one-dimensional KS test to compare the distributions of event positions along the jet axis. We found the relative shift of the jet between different observations which gave the best match in profiles according to the KS test. This technique assumes that the jet profile remains constant as
Figure 2. Images from Chandra observations of XTE J1550-564. The central source in each image is XTE J1550-564. The source on the left is the Eastern and approaching jet, on the right is the Western and receding jet.
the jet moves. This assumption appears to be reasonable, since the KS test values show that the X-ray profiles are very similar except for the overall shift in position, see Fig. 3. The jet morphology shows a strong peak at the head (leading edge) of the jet with a lower amplitude tail trailing behind. This ‘comet-like’ morphology is suggestive of an interaction with the interstellar medium. Throughout 2002, the jet morphology appears very similar in X-rays and radio. The motion of the jets is shown in Fig. 4. We plot the observed separation of each jet from XTE J1550-564. There is a point on the plot for each detection of each jet with crosses for the approaching (Eastern) jet and diamonds for the receding (Western) jet. The motion of the jets is inconsistent with a linear trend, which would be expected if the jets were moving at constant speed. Therefore, the jets must be decelerating. These observations are the first direct evidence
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100 80 60 40 20 0 30 25 20 15 10 Displacement along jet axis (arcsec)
Figure 3. Profiles of the receding jet from XTE J1550-564 . The plots are a histogram of events in the jet versus position along the jet axis. The solid curve is for data from March 2002, the first X-ray detection of the jet. The dashed curve is data from January 2003 scaled to match the peak of the March 2002 histogram.
for gradual deceleration of jets from a black hole. Extrapolation of the first three data points for the approaching jet would indicate a time of origin for the jet before September 1998. Because there is no evidence for any X-ray activity from XTE J1550-564 in RXTE ASM data from before September 1998 and because relativistic jets were observed in the radio at this time, we assume that both jets were created in a single injection episode in September 1998. This allows us to fix the time of zero separation for both jets. We take the distance to XTE J1550–564 to be in the range 2.8– 7.6 kpc [12]. To model the jet motion, we assume the jet has decelerated with an exponential dependence of speed (in the reference frame of XTE J1550564) versus time. We then calculate the observed position versus time in our reference frame for
Angular seperation (arcsec)
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35 30 25 20 15 10 5 0 500 1000 1500 2000 0 Time after X-ray flare (days)
Figure 4. Motion of the jets of XTE J1550-564. Crosses indicate the position of the approaching jet versus time and diamonds the receding jet. The upper solid line is a fit to the motion of the approaching jet described in the text. The lower solid line indicates the corresponding observed motion expected for the receding jet if its motion is identical to that of the approaching jet under a spatial reflection about the position of XTE J1550-564.
each jet taking into account the light travel time delays. The upper curve in Fig. 4 represents a fit assuming a distance to XTE J1550-564 of 4 kpc. The free parameters of the fit and their best fit values are: an initial jet speed of 0.91c, an exponential decay time of 1095 days, and a jet axis relative to our line of sight of 61◦ . Note that the fitted values depend on the assumed distance and the ad-hoc assumption of exponential decay in speed. However, the curve appears to provide a reasonable fit to the data and the shape of the best fit curve varies little when the source distance is varied over the allowed range. The lower curve represents the motion expected for the receding jet if its motion is identical
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Flux (erg cm-2 s-1)
10-12
10-13
10-14
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Figure 5. Flux evolution of the jets of XTE J1550-564. The fluxes for the approaching jet are marked as crosses and those for receding jet as diamonds. The dashed lines are fits of exponential decays to the data.
to that of the approaching jet except spatially reflected through the position of XTE J1550564. The difference between the upper and lower curves arises from light travel time delay effects. We note that none of the data for the receding jet were used in the calculation of the lower (or upper) curve. Interestingly, the first data point for the receding jet lies on the curve, while the last two points lie below and the final point is significantly below. This suggests that the receding jet had a motion similar to that of the approaching jet before early 2002 and then suffered significantly larger deceleration beginning in early 2002. The flux evolution of the jets is shown in Fig. 5. There is a point on the plot for each detection of each jet with crosses for the approaching (Eastern) jet and diamonds for the receding (Western) jet as in Fig. 4. We fit an exponential decay to the data points for each jet. The flux decay time
scale is 260±25 days for the approaching jet and 206±20 days for the receding jet. These two values are consistent within the errors. We note that the receding jet was not detected in 2000. The upper limit plotted around day 700 was obtained by combining all of the 2000 data and used a large extraction region covering the full likely range of jet positions. The large brightening of the receding jet suggests that it was re-energized between 2000 and early 2002. Note that after the turnon of the receding jet, it appears significantly brighter than the approaching jet. This is exactly the opposite from that expected if the approaching and receding jets are identical in their intrinsic properties and differ in their observed properties only because of relativistic Doppler boosting. Therefore, the intrinsic luminosity of the approaching and receding jets must differ. The motion and flux data are consistent with a scenario in which the receding jet had a motion similar to that of the approaching jet before early 2002 and then suffered significantly larger deceleration beginning in early 2002. The deceleration re-energized the jet causing it to significantly brighten in the radio and X-rays. Fig. 6 shows the broadband spectrum of the approaching jet using data from the receding jet in the radio, optical, and X-rays all taken in March 2002. All of the data are consistent with a single powerlaw with an energy index α = −0.660 ± 0.005 extending from the radio to the X-ray band. The X-ray spectrum shows no evidence for line emission, but the constraints are weak. The multiwavelength spectrum, taken together with the fact that the radio and X-ray emission arise from the same region and have very similar morphology, is strong evidence that the emission is synchrotron with a single population of energetic particles (with energies up to 10 TeV, i.e. electrons with γ ∼ 107 ) producing the radiation across the entire spectrum. Given that the emission mechanism is synchrotron radiation and deriving an upper limit on the jet volume from the observed size (we assume that the jet is symmetric about the jet axis and take account of the allowed range of distances) a lower limit on the magnetic field can be derived, B > ∼ 150 µG. The rate of flux decay places an
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10-2
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Figure 6. Multiwavelength spectrum of the receding jet of XTE J1550-564 following [9].
upper limit on the magnetic field, B < ∼ 2500 µG. At this field strength the observed decay in Xrays would be entirely due to synchrotron losses. Similar considerations allow us to make an order of magnitude estimate of the total jet mass m ∼ 1021 g. Because the electrons are highly energetic, the total mass is similar for either a electron-proton or electron-positron jet. If we take the model used above for the motion of the jet, then we can calculate the deceleration as a function of time and therefore the kinetic energy loss rate E˙ K . We find that E˙ K ∼ 1034 erg s−1 for the approaching jet in June 2000. The approaching jet luminosity at the same epoch is L ∼ 1032 erg s−1 . Thus an efficiency for conversion of jet kinetic energy into luminosity of order 10−2 is required. This value is reasonable in the context of relativistic shock acceleration models. To conclude this section, we summarize our results. These observations of XTE J1550-564 provide the first direct measurement of deceleration in a black hole jet. The gradual jet deceleration,
Figure 7. XMM-Newton image of 4U 1755– 33 following [13]. The image is the summed MOS1+MOS2+pn data in the 0.5–10 keV band and has been smoothed. The arrow indicates the position of 4U 1755–33. The jet extends to the NW (up and to the right) and SE (down and to the left) from 4U 1755–33.
the jet morphology, and the brightening of the receding jet several years after its ejection suggest an interaction with the interstellar medium and provide evidence for in-situ particle acceleration powered by bulk deceleration. Measurement of the kinematics can help constrain the energetics of the jet. 3. OTHER SOURCES 3.1. Fossil Jet from 4U 1755-33 Recently, Angelini and White discovered a large scale X-ray jet from the long-term X-ray transient and black hole candidate 4U 1755–33 using XMM-Newton [13]. As shown in Fig. 7, the jet is a linear structure which is roughly symmetric about the position of 4U 1755–33, a double sided jet, extending about 3 to the north-west and to the south-east of the black hole candidate.
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There appear to be multiple knots in the jets. For estimated distance of 4–9 kpc, this corresponds to a jet length of 3–8 pc. Therefore, the jet must have taken at least 10-30 years to form. 4U 1755– 33 was active from its discovery with Uhuru in 1970 (it may have been active at earlier times) until it was found in an off state in 1996. It has not been found in an active state since 1996. The source was active for at least 25 years which appears sufficient to have formed the jet. The primary questions concerning the jet are: what is the X-ray emission mechanism (thermal bremsstrahlung or non-thermal synchrotron or inverse Compton radiation), how is the jet emission powered (internal shocks or interaction of the jet with the interstellar medium), and what is the total energy in the jet. A key discriminate between thermal and nonthermal emission is that thermal emission should produce line emission. We extracted a spectrum of the jet emission from 4U 1755–33 to search for line emission. No line emission is apparent. The upper bounds on line emission depend on the assumed line parameters. For line parameters similar to those found for SS433 (see below), we find 90% confidence upper bounds on the equivalent widths of Fe-K emission lines of 0.9-2 keV. These are in the range of the equivalent width measured for the inner jets of SS433 and are not constraining. We fit the continuum spectrum and found adequate fits with an absorbed powerlaw with a photon index of 1.7 ± 0.3 or a MEKAL thermal emission model with solar abundances and kT > 4.0 keV, similar to the result of [13]. In both cases, we fixed the absorption to the line of sight value NH = 3 × 1021 cm−2 . The unabsorbed flux is 6 × 10−14 erg cm−2 s−1 in the 0.5-2 keV band and 3×10−13 erg cm−2 s−1 in the 0.5-10 keV band. We analyzed archival VLA data for 4U 1755–33 taken in 1981 at 20 cm and in 1990 at 6 cm. We find no radio counterpart to the X-ray jet in either image. The rms noise level in the images is about 0.1 mJy. However, these VLA observations were taken in configurations which provide high angular resolution and are not sensitive to structure on arcminute scales. Therefore, large scale structures could have higher fluxes and still not have been detected.
If the X-ray emission is thermal bremsstrahlung, then the total mass and energy of the jet can be estimated from its observed luminosity, temperature, and volume. For the volume, we assume that the jet occupies a roughly cylindrical volume with a diameter perpendicular to the direction of motion of 0.2 and a linear dimension of 6 multiplied by a filling factor of 0.2. The volume is then ∼ 1054 cm3 for an assumed distance of 4 kpc. From the flux and temperature (we take kT = 4 keV) quoted above, we find that the density of the jet material is n ∼ 5 cm−3 , the total energy of the jet is ∼ 1047 erg and the total mass in the jet is ∼ 1031 g. If the jet were fed by the outflow from a mass accretion rate of 1019 g/s corresponding to the Eddington rate for a 10M compact object, at least 104 years would be required to accumulate the needed mass. Given the linear structure of the jet (i.e. it does not appear to be dominated by the termination of an energetic jet stopping in a dense region of the ISM), it is difficult to construct a model containing this amount of mass. If X-ray emission is synchrotron, then the equipartition magnetic field can be calculated. Using the photon index quoted above and depending on the distance and the lower frequency cutoff in the spectrum, we find magnetic fields of order 100 µG. The radiative lifetime of the electrons producing the X-ray emission (which must have high Lorentz factors ∼ 107 ) are of order 100 years. The number of electrons needed to produce the observed radiation is of order 1047 . Assuming that the jet is composed of normal matter (i.e. roughly one proton per electron), then the required mass is ∼ 1023 g. This mass estimate is rather uncertain, due to the lack of a multiwavelength spectrum for the jet, but it is appears that the material required for a synchrotron emitting jet could accumulated in less than the estimated lifetime of the jet. The available data on the jet of 4U 1755–33 are consistent with the X-rays being synchrotron emission. Determining the X-ray emission mechanism is the essential first step in understanding the physical properties of the jet and is critical to using the jet from 4U 1755–33 to address the fundamental questions concerning the physics of jets raised
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above. If the X-ray emission is really synchrotron, then the jet should be detectable in the radio. Contemporaneous XMM-Newton and deep radio observations are needed. New X-ray data should also permit measurement of the decay rate of the jet and therefore an estimate of its overall energetics. This will be of great interest to compare the total energy in the jet versus the energy produced over the X-ray active lifetime of the source and to begin to constrain the energy input to the ISM from relativistic jets.
3.2. SS 433 The unusual X-ray binary SS 433 has largescale lobes extending to scales of 45 (60 pc) which were the first discovered Galactic X-ray jets [1]. The outer X-ray lobes of SS433 are similar to the large-scale jets of XTE J1550-564 and have a power-law spectrum extending up to 100 keV indicating that the emission is non-thermal [14] and do not show line emission [15]. The central X-ray source shows red and blueshifted Fe emission lines [16,15]. Recently, the inner jets have been resolved. They show extent up to 6 (0.14 pc) from the compact object and prominent X-ray line emission [17]. The line structure is complex with several elements detected: Ne, Mg, Si, S, Ca, and Fe. The red and blue shifted Fe line emission is spatially separated [18] and the velocity dispersion is higher for Fe than for Si [19]. The detection of line emission indicates that the jet contains baryons and that the emission is thermal. The spatially resolved spectra give strong evidence that jet material is reheated far from the point of injection. The difference in velocity dispersion between Fe and Si is inconsistent with that expected from thermal broadening. It has been suggested that the difference in velocity dispersion is a geometrical effect due to differing jet geometry between the regions where the Fe and Si emission is produced and that this is evidence that the jets narrow with distance from the origin [19]. If correct, these observations may provide information on the mechanisms of collimation in relativistic jets.
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4. CONCLUSIONS New observations of X-ray jets from Galactic black hole candidates offer a new probe of black hole jets which may provide new insights to the dynamics of jets which cannot be obtained from observations of AGN jets due to the long evolution time scales of jets from supermassive black holes. The large-scale jets from XTE J1550-564 may represent an intermediate stage between the compact jets which have been resolved with VLBI observations and the stationary lobes previously found in the radio. New observations of these jets have provided the first direct measurement of deceleration in black hole jets. The jets from XTE J1550-564 and also those from SS 433 have provided evidence for in-situ particle acceleration and re-energization of jets far from their origin. Study of the dynamics of the XTE J1550-564 jets have led to some initial, crude constraints on the kinetic energy content and evolution of relativistic jets. The inner jets in SS 433 are clearly baryonic. Whether this is true for all Galactic (and AGN) jets remains an open question, particularly because the jet speed in SS 433 is 0.26c which is significantly lower than the values near 0.9c found in most black hole candidate X-ray transients. The inner jets of SS 433 may also show observational signatures of jet collimation. If confirmed, this may provide a very exciting means to provide observational constraints on collimation mechanisms. In conclusion, new observations of X-ray jets have already provided exciting results and are a promising new area for future work. PK thanks the scientific organizing committee for inviting him to the conference and acknowledges support from NASA grant NAG5-7405 and Chandra grant number G03-4043X. REFERENCES 1. Seward, F. et al. 1980, Nature, 287, 806 2. Mirabel, I.F. & Rodriguez, L.F. 1999, ARA&A, 37, 409
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