- Email: [email protected]
Iron emission lines from extended, reheated, thermal X-ray jets in SS 433
New Astronomy Reviews 47 (2003) 481–485 www.elsevier.com / locate / newastrev
Iron emission lines from extended, reheated, thermal X-ray jets in SS 433 b ´ R. Fender a , *, S. Migliari a , M. Mendez a
University of Amsterdam, Astronomical Institute, Kruislaan 403, Amsterdam 1098 SJ, The Netherlands b SRON, National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
Abstract We present a deep Chandra ACIS-S observation of the X-ray binary jet source SS 433. Arcsec-scale X-ray jets are clearly resolved on either side of the core, aligned with the well-known radio jets and the major axis of the associated W50 remnant. We are able to extract X-ray spectra separately for each side of the jet, and find evidence for a hot continuum and Doppler-shifted emission lines. The evidence strongly indicates that material at least half a year downstream in the jet flow from SS 433 is reheating itself to high (.10 7 K) temperatures, presumably by conversion of some fraction of the jets’ enormous kinetic energy. This is the first direct evidence for ongoing heating of thermal plasma downstream in a jet from an X-ray binary system. These observations are seemingly in conflict with all existing models for X-ray and optical line production in the jets of SS 433, in which matter should be cooled down below X-ray-emitting temperatures within, at most, a few hours after being ejected. Possible physical interpretations of these results are briefly discussed. 2003 Published by Elsevier B.V.
1. Introduction SS 433 is an X-ray binary system first identified in 1978 as an unusual X-ray, radio, and optical emission-line source (Clark and Murdin, 1978). The system is also an eclipsing binary with an orbital period of |13 days. The most remarkable feature of the source is the presence of mildly relativistic, quasi-continuous jets. Despite some controversy about the nature of the accreting compact object (neutron star or black hole), SS 433 may be considered as the prototype of microquasars. Optical spectral analysis (Abell and Margon, 1979) and subsequent radio imaging on arcminute and ar*Corresponding author. Tel.: 131-20-525-7478; fax: 131-20525-7484. E-mail addresses: [email protected] (R. Fender), [email protected] (S. Migliari). 1387-6473 / 03 / $ – see front matter 2003 Published by Elsevier B.V. doi:10.1016 / S1387-6473(03)00076-9
csecond scales confirmed the presence of bipolar outflows coming from the system with a velocity of about 26% of the speed of light. SS433 is also the best example we have of precessing jets. In 1978, Margon et al. (1979) found that blue- and red-shifted Ha emission lines coming, respectively, from the east and west jet, change with time. Moreover a period of approximately 164 days was observed in these ‘moving lines’. In 1979, Abell and Margon (1979) were able to fit the observations with a simple precession model. The proposal was that narrow jets, with an opening angle & 5 degrees, precess in a 208 half-opening angle cone with a period of | 164 days. Since the beams rotate on a cone, the projection of the jets’ pattern onto the plane of the sky is a twisted trace (Fig. 1) (Hjellming and Johnston, 1981). The motions of the individual jet components are ballistic and the matter moves in a straight line away from the center. Extensive
482
R. Fender et al. / New Astronomy Reviews 47 (2003) 481–485
Fig. 1. Top: Chandra ACIS-S image of SS 433 with the twisted precession traces of the jets superimposed to it and the regions of the jet we have isolated and analysed. Bottom: the two spectra of the lobes, east (left) and west (right), with the fitting model (absorbed bremsstrahlung and Gaussian emission line).
observing campaigns at many frequencies have placed constraints on the emission physics, geometry, and scales associated with the jet. The model developed and generally accepted for the thermal component of the jet is the adiabatic expansion cooling model (Brinkmann et al., 1991; Kotani et al., 1996). This model assumes that outflowing matter is expanding adiabatically in a conical outflow and that temperature and matter density are only a function of the distance from the core. The matter is ejected with
a temperature of more than 10 8 K (kT . 10 keV). This temperature is comparable to that expected for the inner region of the accretion disc. Close to the binary core, at distances &10 11 cm, the jet emits soft X-rays thermally (Marshall et al., 2002), and it cools to temperatures less than 10 5 K at distances of 10 12 cm. Optical thermal emission lines originate at distances & 10 15 cm. The distance is constrained since there is no measurable offset between blue- and red-shifted beams, as would occur if the emission
R. Fender et al. / New Astronomy Reviews 47 (2003) 481–485
arose at larger distances. The absence of higher excitation lines such as HeII and CIII / NIII in this region implies temperatures less than 10 4 K (Shaham, 1981). In the adiabatic model the temperature decreases with distance from the core, R, as T ~ R 24 / 3 , thus at distances greater than |10 16 cm the temperature is expected to be less than 100 K, and the matter should be too cool to emit X-ray / optical radiation thermally. Besides this thermal radiation, there is also non-thermal synchrotron radiation observed in the radio band at distances between 10 15 and 10 17 cm. Further out the jet is not detected until it produces the termination shocks which deform the surrounding nebula W50, forming the so-called ‘ears’ a few degrees away from the central source (Brinkmann et al., 1996; Dubner et al., 1998). Marshall et al. (2002) have recently analysed the X-ray line emission from SS 433 with the Chandra High Energy Transmission Grating Spectrometer (HETGS). They confirmed the models of the jets described above and gave refinements of the previous observations limiting the thermal X-ray emission region to within |10 11 cm. They also discovered faint arcsec X-ray emission lobes at distances of |10 17 cm, but they were not able to measure any emission lines and concluded that the jet gas would be too cool at such a distance to emit thermal X-rays.
2. Data analysis and results We have analysed the SS 433 observation of 27 June 2000 with the Chandra Advanced CCD Imaging Spectrometer (ACIS)-S3, with an exposure time of | 9.6 ks. The reduced image (Fig. 2, top) resolves arcsec-scale X-ray jets approximately symmetrically placed on either side of a weaker core and aligned with both the sub-arcsec radio jets and degree-scale structures in W50. Analysis of the image indicates that photon pile-up in the core artificially reduced the measured count rate there, although we also note that according to the orbital ephemeris of Dolan et al. (1997) the orbital phase of the binary was 0.89. This implies that our observation was close to X-ray eclipse which may also have reduced the core flux. The precession phase of the jets, using the Hjellming
483
and Johnston convention (Hjellming and Johnston, 1981; Eikenberry et al., 2001), is 0.25. For the first time we were able to spatially isolate and take X-ray spectra of the two jets. In Fig. 2 (top) we show the twisted traces of the precessing jets superimposed on the ACIS-S image of SS 433. The two boxes show the regions we have isolated and analysed. We can distinguish the two bright jet lobes a few pixels (the ACIS-S pixel size is | 0.5 arcsec) from the core corresponding, at a distance of 5 kpc, to a physical scale of |10 17 cm. The X-ray luminosity of each of these lobes is 3–4310 33 erg / s, a few % of the average total luminosity of the source. In Fig. 2 (bottom) we show the spectra of the lobes in the range 0.8–10 keV. The data are well fitted by a bremsstrahlung continuum emission with a temperature of about 5 keV, and a Gaussian emission line at | 7.3 keV and | 6.4 keV for the approaching and receding jet, respectively. A hydrogen column density of |10 22 cm 22 was found for both lobes, consistent with the column density found by Marshall et al. (2002) for the core of SS 433. A power law with a photon index of 2.160.2 fits the continuum equally well, and based on our current data we cannot distinguish between a power law and bremsstrahlung for the continuum emission. The significantly different line centroid energies measured for the east and west jets suggest that they may originate in the moving jet material. To test this hypothesis we developed a model which sums the emission from ten equally-spaced phase bins around the precession cycle, calculating for each bin the Doppler shift d and Doppler boosting of the line peak intensity d 2 (assuming the jets are intrinsically symmetric about the precession axis, and east–west). A simulation based on this model for a 10 ksec observation, assuming a rest energy for the line of 7.06 keV, corresponding to Fe XXV Kb, a velocity of 0.26c, and using the same normalizations for the Gaussians as found in our observations, corresponds well to our data. The line energies ( | 7.4 keV for the east and | 6.4 keV for the west jet) and widths we obtain in our simulations are a good match to those measured in our observations. Thus the jet is emitting around the precession cone at a large distance from the core (see Fig. 2). While we cannot use our model to establish conclusively that the Doppler-shifted line is Fe XXV
484
R. Fender et al. / New Astronomy Reviews 47 (2003) 481–485
Fig. 2. Artist representation of SS 433, showing the blobs emitted in precession jets.
Kb, (actually a more likely solution, due to cross section arguments, would be a Fe XXV Ka ) we do need a transition of hot highly-ionized Fe in order to achieve a high enough energy for the line in the east jet. We cannot exclude an origin for the |6.4 keV line in the west jet from neutral Fe; however, we cannot envisage a plausible geometry whereby we are observing neutral Fe at 6.4 keV in the west jet and the same line extremely blue-shifted in the east jet. A further prediction of this model is that the east jet should be stronger than the west jet as a result of the overall larger Doppler boosting. The ratio of 2–10 keV fluxes from the east to west jets is |1.3, approximately what would be expected for the model considered here (note that the ratios of lines and continua are rather different but, given the relatively poor statistics, the overall flux is a better measurement). Although we cannot explicitly show that the source of the continuum emission arises in the moving jet gas, the measured bremsstrahlung temperature is consistent with an origin in the same plasma as the high-excitation Fe lines. We therefore conclude that we are observing lines, and probably also continuum, from a hot ( . 10 7 K) plasma which is moving with a similarly high velocity to that measured for the ‘inner’ jets, i.e., 0.26c, on scales of 10 17 cm from the binary core. This is contrary to the adiabatic cooling model (Brinkmann et al., 1991;
Kotani et al., 1996; Marshall et al., 2002), which at that distance predicts that the baryonic component will have cooled to the ambient interstellar medium temperature. Therefore the jet has to re-heat itself. The most plausible energy source is the kinetic energy (|10 38 erg / s (Marshall et al., 2002)). Alternatives to this scenario can be taken into account. The possibility of a non-thermal continuum seems unlikely, because of the evidence for thermal emission lines. Moreover, X-ray synchrotron emission would still require in-situ particle acceleration (the lifetime for X-ray synchrotron emitting electrons would be orders of magnitude shorter than the travel time in the jet). Reflection of the core emission does not seem possible, because the line flux we observe at 10 17 cm is only a factor of | 10 less than that observed to originate at 10 12 cm, and yet our images indicate it does not subtend a significantly larger solid angle than the jets on smaller scales. Furthermore, the line-to-continuum flux ratio we observe is an order of magnitude greater than in the core (Fline /Fcontinuum | 1 for our east jet, but is | 0.1 for the 7.3 keV line in Chandra HETGS observations Marshall et al., 2002). Photoionisation is not a likely origin for the X-ray emission lines we observe at such a distance (Marshall et al., 2002). Reheating of a leptonic (electron and possibly also positron) component of jet plasma, presumably by
R. Fender et al. / New Astronomy Reviews 47 (2003) 481–485
conversion of the jet kinetic energy, is commonly invoked to explain high-energy emission on large scales in the jets of AGNs. The reheating of the baryonic plasma that we have observed in SS 433 requires a similar tapping of the bulk kinetic energy of the flow, by processes which are able to act on atomic nuclei. In the internal shock model for blazars (Spada et al., 2001), a slightly varying jet speed results in shocks downstream in the jet flow. In the context of this model it is interesting to note that random discrepancies of about 15% in the velocity of the jets from SS 433, inferred from long-term optical monitoring (Eikenberry et al., 2001), would produce shocks a few hundred days downstream (for a precession period of 162 days and a mean jet velocity of 0.26c), comparable to what we observe. Indeed, assuming a velocity slightly higher than 0.26c we can find a good match of the lines with the more likely Fe XXV Ka. Under the assumption of comparable particle acceleration physics, comparison of the X-ray spectra from the arcsec-scale jets of SS 433 with those of AGNs may further constrain the particle, and hence energy, content of jets from supermassive black holes.
485
References Abell, G.O., Margon, B., 1979. Nature 279, 701. Brinkmann, W., Aschenbach, B., Kawai, N., 1996. A&A 312, 306. Brinkmann, W., Kawai, N., Matsuoka, M., Fink, H.H., 1991. A&A 241, 112. Clark, D.H., Murdin, P., 1978. Nature 276, 44. Dolan, J.F., Boyd, P.T., Fabrika, S. et al., 1997. ApJ 327, 648. Dubner, G.M., Holdaway, M., Goss, W.M., Mirabel, I.F., 1998. ApJ 116, 1842. Eikenberry, S.S., Cameron, P.B., Fierce, B.W. et al., 2001. ApJ 561, 1027. Hjellming, R.M., Johnston, K.J., 1981. Nature 290, 100. Kotani, T., Kawai, N., Matsuoka, M., Brinkmann, W., 1996. PASJ 48, 619. Margon, B., Stone, R.P.S., Klemola, A., Ford, H.C. et al., 1979. ApJ 230, L41. Marshall, H.L., Canizares, C.R., Norbert, S.S., 2002. ApJ 564, 941. Shaham, J., 1981. VA 25, 217. Spada, M., Ghisellini, G., Lazzati, D., Celotti, A., 2001. MNRAS 325, 1559.
Iron emission lines from extended, reheated, thermal X-ray jets in SS 433 b ´ R. Fender a , *, S. Migliari a , M. Mendez a
University of Amsterdam, Astronomical Institute, Kruislaan 403, Amsterdam 1098 SJ, The Netherlands b SRON, National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
Abstract We present a deep Chandra ACIS-S observation of the X-ray binary jet source SS 433. Arcsec-scale X-ray jets are clearly resolved on either side of the core, aligned with the well-known radio jets and the major axis of the associated W50 remnant. We are able to extract X-ray spectra separately for each side of the jet, and find evidence for a hot continuum and Doppler-shifted emission lines. The evidence strongly indicates that material at least half a year downstream in the jet flow from SS 433 is reheating itself to high (.10 7 K) temperatures, presumably by conversion of some fraction of the jets’ enormous kinetic energy. This is the first direct evidence for ongoing heating of thermal plasma downstream in a jet from an X-ray binary system. These observations are seemingly in conflict with all existing models for X-ray and optical line production in the jets of SS 433, in which matter should be cooled down below X-ray-emitting temperatures within, at most, a few hours after being ejected. Possible physical interpretations of these results are briefly discussed. 2003 Published by Elsevier B.V.
1. Introduction SS 433 is an X-ray binary system first identified in 1978 as an unusual X-ray, radio, and optical emission-line source (Clark and Murdin, 1978). The system is also an eclipsing binary with an orbital period of |13 days. The most remarkable feature of the source is the presence of mildly relativistic, quasi-continuous jets. Despite some controversy about the nature of the accreting compact object (neutron star or black hole), SS 433 may be considered as the prototype of microquasars. Optical spectral analysis (Abell and Margon, 1979) and subsequent radio imaging on arcminute and ar*Corresponding author. Tel.: 131-20-525-7478; fax: 131-20525-7484. E-mail addresses: [email protected] (R. Fender), [email protected] (S. Migliari). 1387-6473 / 03 / $ – see front matter 2003 Published by Elsevier B.V. doi:10.1016 / S1387-6473(03)00076-9
csecond scales confirmed the presence of bipolar outflows coming from the system with a velocity of about 26% of the speed of light. SS433 is also the best example we have of precessing jets. In 1978, Margon et al. (1979) found that blue- and red-shifted Ha emission lines coming, respectively, from the east and west jet, change with time. Moreover a period of approximately 164 days was observed in these ‘moving lines’. In 1979, Abell and Margon (1979) were able to fit the observations with a simple precession model. The proposal was that narrow jets, with an opening angle & 5 degrees, precess in a 208 half-opening angle cone with a period of | 164 days. Since the beams rotate on a cone, the projection of the jets’ pattern onto the plane of the sky is a twisted trace (Fig. 1) (Hjellming and Johnston, 1981). The motions of the individual jet components are ballistic and the matter moves in a straight line away from the center. Extensive
482
R. Fender et al. / New Astronomy Reviews 47 (2003) 481–485
Fig. 1. Top: Chandra ACIS-S image of SS 433 with the twisted precession traces of the jets superimposed to it and the regions of the jet we have isolated and analysed. Bottom: the two spectra of the lobes, east (left) and west (right), with the fitting model (absorbed bremsstrahlung and Gaussian emission line).
observing campaigns at many frequencies have placed constraints on the emission physics, geometry, and scales associated with the jet. The model developed and generally accepted for the thermal component of the jet is the adiabatic expansion cooling model (Brinkmann et al., 1991; Kotani et al., 1996). This model assumes that outflowing matter is expanding adiabatically in a conical outflow and that temperature and matter density are only a function of the distance from the core. The matter is ejected with
a temperature of more than 10 8 K (kT . 10 keV). This temperature is comparable to that expected for the inner region of the accretion disc. Close to the binary core, at distances &10 11 cm, the jet emits soft X-rays thermally (Marshall et al., 2002), and it cools to temperatures less than 10 5 K at distances of 10 12 cm. Optical thermal emission lines originate at distances & 10 15 cm. The distance is constrained since there is no measurable offset between blue- and red-shifted beams, as would occur if the emission
R. Fender et al. / New Astronomy Reviews 47 (2003) 481–485
arose at larger distances. The absence of higher excitation lines such as HeII and CIII / NIII in this region implies temperatures less than 10 4 K (Shaham, 1981). In the adiabatic model the temperature decreases with distance from the core, R, as T ~ R 24 / 3 , thus at distances greater than |10 16 cm the temperature is expected to be less than 100 K, and the matter should be too cool to emit X-ray / optical radiation thermally. Besides this thermal radiation, there is also non-thermal synchrotron radiation observed in the radio band at distances between 10 15 and 10 17 cm. Further out the jet is not detected until it produces the termination shocks which deform the surrounding nebula W50, forming the so-called ‘ears’ a few degrees away from the central source (Brinkmann et al., 1996; Dubner et al., 1998). Marshall et al. (2002) have recently analysed the X-ray line emission from SS 433 with the Chandra High Energy Transmission Grating Spectrometer (HETGS). They confirmed the models of the jets described above and gave refinements of the previous observations limiting the thermal X-ray emission region to within |10 11 cm. They also discovered faint arcsec X-ray emission lobes at distances of |10 17 cm, but they were not able to measure any emission lines and concluded that the jet gas would be too cool at such a distance to emit thermal X-rays.
2. Data analysis and results We have analysed the SS 433 observation of 27 June 2000 with the Chandra Advanced CCD Imaging Spectrometer (ACIS)-S3, with an exposure time of | 9.6 ks. The reduced image (Fig. 2, top) resolves arcsec-scale X-ray jets approximately symmetrically placed on either side of a weaker core and aligned with both the sub-arcsec radio jets and degree-scale structures in W50. Analysis of the image indicates that photon pile-up in the core artificially reduced the measured count rate there, although we also note that according to the orbital ephemeris of Dolan et al. (1997) the orbital phase of the binary was 0.89. This implies that our observation was close to X-ray eclipse which may also have reduced the core flux. The precession phase of the jets, using the Hjellming
483
and Johnston convention (Hjellming and Johnston, 1981; Eikenberry et al., 2001), is 0.25. For the first time we were able to spatially isolate and take X-ray spectra of the two jets. In Fig. 2 (top) we show the twisted traces of the precessing jets superimposed on the ACIS-S image of SS 433. The two boxes show the regions we have isolated and analysed. We can distinguish the two bright jet lobes a few pixels (the ACIS-S pixel size is | 0.5 arcsec) from the core corresponding, at a distance of 5 kpc, to a physical scale of |10 17 cm. The X-ray luminosity of each of these lobes is 3–4310 33 erg / s, a few % of the average total luminosity of the source. In Fig. 2 (bottom) we show the spectra of the lobes in the range 0.8–10 keV. The data are well fitted by a bremsstrahlung continuum emission with a temperature of about 5 keV, and a Gaussian emission line at | 7.3 keV and | 6.4 keV for the approaching and receding jet, respectively. A hydrogen column density of |10 22 cm 22 was found for both lobes, consistent with the column density found by Marshall et al. (2002) for the core of SS 433. A power law with a photon index of 2.160.2 fits the continuum equally well, and based on our current data we cannot distinguish between a power law and bremsstrahlung for the continuum emission. The significantly different line centroid energies measured for the east and west jets suggest that they may originate in the moving jet material. To test this hypothesis we developed a model which sums the emission from ten equally-spaced phase bins around the precession cycle, calculating for each bin the Doppler shift d and Doppler boosting of the line peak intensity d 2 (assuming the jets are intrinsically symmetric about the precession axis, and east–west). A simulation based on this model for a 10 ksec observation, assuming a rest energy for the line of 7.06 keV, corresponding to Fe XXV Kb, a velocity of 0.26c, and using the same normalizations for the Gaussians as found in our observations, corresponds well to our data. The line energies ( | 7.4 keV for the east and | 6.4 keV for the west jet) and widths we obtain in our simulations are a good match to those measured in our observations. Thus the jet is emitting around the precession cone at a large distance from the core (see Fig. 2). While we cannot use our model to establish conclusively that the Doppler-shifted line is Fe XXV
484
R. Fender et al. / New Astronomy Reviews 47 (2003) 481–485
Fig. 2. Artist representation of SS 433, showing the blobs emitted in precession jets.
Kb, (actually a more likely solution, due to cross section arguments, would be a Fe XXV Ka ) we do need a transition of hot highly-ionized Fe in order to achieve a high enough energy for the line in the east jet. We cannot exclude an origin for the |6.4 keV line in the west jet from neutral Fe; however, we cannot envisage a plausible geometry whereby we are observing neutral Fe at 6.4 keV in the west jet and the same line extremely blue-shifted in the east jet. A further prediction of this model is that the east jet should be stronger than the west jet as a result of the overall larger Doppler boosting. The ratio of 2–10 keV fluxes from the east to west jets is |1.3, approximately what would be expected for the model considered here (note that the ratios of lines and continua are rather different but, given the relatively poor statistics, the overall flux is a better measurement). Although we cannot explicitly show that the source of the continuum emission arises in the moving jet gas, the measured bremsstrahlung temperature is consistent with an origin in the same plasma as the high-excitation Fe lines. We therefore conclude that we are observing lines, and probably also continuum, from a hot ( . 10 7 K) plasma which is moving with a similarly high velocity to that measured for the ‘inner’ jets, i.e., 0.26c, on scales of 10 17 cm from the binary core. This is contrary to the adiabatic cooling model (Brinkmann et al., 1991;
Kotani et al., 1996; Marshall et al., 2002), which at that distance predicts that the baryonic component will have cooled to the ambient interstellar medium temperature. Therefore the jet has to re-heat itself. The most plausible energy source is the kinetic energy (|10 38 erg / s (Marshall et al., 2002)). Alternatives to this scenario can be taken into account. The possibility of a non-thermal continuum seems unlikely, because of the evidence for thermal emission lines. Moreover, X-ray synchrotron emission would still require in-situ particle acceleration (the lifetime for X-ray synchrotron emitting electrons would be orders of magnitude shorter than the travel time in the jet). Reflection of the core emission does not seem possible, because the line flux we observe at 10 17 cm is only a factor of | 10 less than that observed to originate at 10 12 cm, and yet our images indicate it does not subtend a significantly larger solid angle than the jets on smaller scales. Furthermore, the line-to-continuum flux ratio we observe is an order of magnitude greater than in the core (Fline /Fcontinuum | 1 for our east jet, but is | 0.1 for the 7.3 keV line in Chandra HETGS observations Marshall et al., 2002). Photoionisation is not a likely origin for the X-ray emission lines we observe at such a distance (Marshall et al., 2002). Reheating of a leptonic (electron and possibly also positron) component of jet plasma, presumably by
R. Fender et al. / New Astronomy Reviews 47 (2003) 481–485
conversion of the jet kinetic energy, is commonly invoked to explain high-energy emission on large scales in the jets of AGNs. The reheating of the baryonic plasma that we have observed in SS 433 requires a similar tapping of the bulk kinetic energy of the flow, by processes which are able to act on atomic nuclei. In the internal shock model for blazars (Spada et al., 2001), a slightly varying jet speed results in shocks downstream in the jet flow. In the context of this model it is interesting to note that random discrepancies of about 15% in the velocity of the jets from SS 433, inferred from long-term optical monitoring (Eikenberry et al., 2001), would produce shocks a few hundred days downstream (for a precession period of 162 days and a mean jet velocity of 0.26c), comparable to what we observe. Indeed, assuming a velocity slightly higher than 0.26c we can find a good match of the lines with the more likely Fe XXV Ka. Under the assumption of comparable particle acceleration physics, comparison of the X-ray spectra from the arcsec-scale jets of SS 433 with those of AGNs may further constrain the particle, and hence energy, content of jets from supermassive black holes.
485
References Abell, G.O., Margon, B., 1979. Nature 279, 701. Brinkmann, W., Aschenbach, B., Kawai, N., 1996. A&A 312, 306. Brinkmann, W., Kawai, N., Matsuoka, M., Fink, H.H., 1991. A&A 241, 112. Clark, D.H., Murdin, P., 1978. Nature 276, 44. Dolan, J.F., Boyd, P.T., Fabrika, S. et al., 1997. ApJ 327, 648. Dubner, G.M., Holdaway, M., Goss, W.M., Mirabel, I.F., 1998. ApJ 116, 1842. Eikenberry, S.S., Cameron, P.B., Fierce, B.W. et al., 2001. ApJ 561, 1027. Hjellming, R.M., Johnston, K.J., 1981. Nature 290, 100. Kotani, T., Kawai, N., Matsuoka, M., Brinkmann, W., 1996. PASJ 48, 619. Margon, B., Stone, R.P.S., Klemola, A., Ford, H.C. et al., 1979. ApJ 230, L41. Marshall, H.L., Canizares, C.R., Norbert, S.S., 2002. ApJ 564, 941. Shaham, J., 1981. VA 25, 217. Spada, M., Ghisellini, G., Lazzati, D., Celotti, A., 2001. MNRAS 325, 1559.