SS433, microquasars, and other transients
aa r X i v : . [ a s t r o - ph . H E ] O c t SS433, microquasars, and other transients
Zsolt Paragi ∗ Joint Institute for VLBI in Europe (JIVE), Postbus 2, 7990 AA Dwingeloo, The NetherlandsE-mail: [email protected]
René Vermeulen
Netherlands Institute for Radio Astronomy (ASTRON), Postbus 2, 7990 AA Dwingeloo, TheNetherlandsE-mail: [email protected]
Ralph Spencer † Jodrell Bank Centre for Astrophysics The University of Manchester Oxford Rd Manchester M139PL UKE-mail: [email protected]
X-ray binaries have been an important key in understanding the jet-disc symbiosis in accretingblack holes on all mass scales, from stellar-mass to supermassive black holes. SS433 was thefirst Galactic XRB that has been extensively studied in the radio regime. The radio properties,including the highest angular resolution data can now be better understood in the framework foraccretion disc state transitions that is observed in microquasars (black hole X-ray binary systems).SS433 remains unique in various ways to date, and there is still much to learn about black hole ac-cretion phenomena. In the meantime, the electronic very long baseline (e-VLBI) developments atthe European VLBI Network (EVN) has allowed us to study microquasars and other transients atmilliarcsecond resolutions more flexibly than was possible before. Even more new opportunitieswill arise as the SKA pathfinders become operational.
Resolving the Sky - Radio Interferometry: Past, Present and Future -RTS2012April 17-20, 2012Manchester, UK ∗ Speaker. † This paper is dedicated to Prof. István Fejes (1932-2011), enthusiastic researcher of SS433, who established radioastronomy (and especially VLBI) in Hungary. e-VLBI research infrastructure in Europe is supported by the EuropeanUnion’s Seventh Framework Programme (FP7/2007-2013) under grant agreement RI-261525 NEXPReS. The EVN is ajoint facility of European, Chinese, South African and other radio astronomy institutes funded by their national researchcouncils. c (cid:13) Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/
S433 and other transients
Zsolt Paragi
1. The first radio-jet X-ray binary
A very unusual star received great attention at the end of 1970s which showed peculiar H a emission lines: SS433, an eclipsing binary system that had counterparts in the X-ray and radiodomain as well. The spectral lines showed high and variable Doppler-shifts between -30000 and50000 km/s, and these were soon interpreted as signatures of jets precessing with a period of about164 days [for an early review, see ]. High resolution radio observation in 1979 with MERLINindeed showed an elongated structure on arcsecond scale [ ], and the same year, compact structureon milliarcsecond scales was reported following the first VLBI detection with a 3-station arrayof Effelsberg, Onsala, and the Westerbork Synthesis Radio Telescope [ ]. Just in 1979, a dozenof Nature papers were published on the source, and the excitement about this fascinating objectcertainly played a great role in the forming of the European VLBI Network (EVN) in 1980.And there was a lot to discover. The highest angular resolution images of the precessingradio beams allowed an accurate measurement of the source distance to 5 ± ] Dueto the variable jet viewing angle and mildly relativistic speeds of 0.26c, it became possible totest the Doppler-beaming paradigm for relativistic jets as well [ ]. The source showed flaringepisodes every ∼
400 days. There were two types of flares identified, one showing a delay withobserving frequency, typical of synchrotron self-absorbed ejecta, the other peaked simultaneouslyat all frequencies –the origin of the flares were not known [ ]. Besides, there was a drop in fluxdensity observed before the flares. There were two VLBI monitoring campaigns organized in 1985and 1987, both lasting 6 days, with 2 day separation in between the observations. These werereally monster VLBI campaigns at the time, using practically the whole supplies of EVN recordingtapes, weighing over a tonne. But the results were spectacular. There were a series of movingejecta detected that originated in an elongated core, and later brightened up at an angular distanceof 40–60 mas (a brightening zone) – these two regions were responsible for the two different typesof flares. The 1987 campaign coincided with a major flare by chance, and a series of spectacularimages were produced. The Doppler-beaming effect was confirmed both in the permanent core, aswell as the in the moving ejecta, in agreement with the measured proper motion of 0.26c [ ].
2. The compact jets
Multifrequency imaging with the VLBA provided new insights to the nature of the core region[ ]. The brightest peaks in the approaching and receding jets have increasingly larger separationat lower frequencies. This is due to a change in synchrotron opacity: the distance of the t ∼ ]. Indeed, the compact object in SS433 lies about midway between the eastern andwestern peaks of emission at 1.6 GHz (in the centre of images in Fig.1), not in the brightest "radiocore" component. This was established by fitting the kinematic model to the moving ejecta inthe system. While this core-jet region has been observed most of the time in the system, there isevidence for supression of the inner jet during (at least in some of the) flares [ ]; the temporal Note that measuring the distances of transient Galactic radio sources remains a big challenge to date; the uncertaindistances and jet viewing angles still limit our ability to determine the jet Lorentz-factors accurately in black hole XRB. S433 and other transients
Zsolt Paragi
Figure 1:
VLBA 1.6 GHz (top), 5 GHz (bottom) and 15 GHz (bottom) maps of of SS433 on the samescale, demonstrating the multifrequency properties of the self-absorbed compact jets. The compact object islocated near the centre of these maps (found by fitting the kinematic model to the ejecta). The gap betweenthe observed "radio cores" closes at higher frequencies, this effect is known in AGN as frequency depen-dent core-shift. The brightness assymetry at 1.6 GHz is consistent with Doppler-beaming; the increasingassymetry at higher frequencies is likely due to additional free-free absorption close to the central engine. S433 and other transients
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Figure 2: disappearance of the continuous jets before the flaring episode starts can be therefore the cause ofthe flux density dips mentioned above (see Fig.2).Compact jets are now frequently observed in microquasars in the low-hard X-ray state, al-though resolved VLBI images are only available for a few of these. The activity cycle of blackhole XRB is understood as follows [ ]. The X-rays originate in a comptonized corona at the jetbase in the hard state. The accretion rate is a few percent Eddington, the jet Lorentz-factor is small( < S433 and other transients
Zsolt Paragi
3. The equatorial outflow in SS433 and other black holes: driven by an accretiondisk wind?
Besides the steady inner jets and moving ejecta, there was another type of emission seen indeep radio images, mainly at 1.6 GHz. This emitting region surrounds the central source within ∼
100 mas: some parts are occasionally detected as distinct features in VLBI images [ ], whileadding shorter spacings reveals its more diffuse nature [ ]. Comparing multi-epoch detectionsthere is evidence for proper motion in this region outwards from the centre, indicating an equatorialoutflow [ ]. Similar outflows in microquasars and AGN were not known at the time. However,there is now evidence in the X-ray observations for accretion disc driven ultra-fast outflows in bothradio-loud [ ] and radio-quiet [ ] AGN. These may have significant contribution to the AGNfeedback. In stellar-mass black holes similar outflows were found in the soft states [ ]; thesewinds may carry sufficient mass and energy to quench the jet radio emission as observed duringstate transitions. The massive equatorial outflow in SS433 may help us to understand anotherphenomenon as well. In 2001 it was proposed that X-ray emission in the supercritical accretiondisc is channeled towards to the polar regions. When looked at face-on, SS433-like systems mayappear as ultra-luminous X-ray sources (ULX) in nearby galaxies [ ]. Recently, new evidencewas found for highly scattered supercritical disc funnel radiation [ ].
4. Transients and the e-EVN
During the last 8 years, the real-time e-VLBI developments made the EVN more capable ofobserving transient radio sources. This is because observations are no longer limited by recordingmedia, the performance of the telescopes can be checked immediately in ad-hoc observations, andthe scheduling has become more flexible as well. Responses to flaring activity is now possiblein dedicated target of opportunity observations. For example, e-EVN ToO observations of SS433[ ] further confirm that discrete ejecta during flares travel with the same speed as measured inthe regular jet, therefore there is no sign of an increasing Lorentz-factor in the jet during accretiondisc state transitions. The e-EVN has provided rapid imaging results on several well-known mi-croquasars and other variable or transient sources in recent years. It has proven to be especiallyuseful to combine the e-EVN and VLBA for denser monitoring observations, in which the for-mer can provide initial detections, improved coordinates for new transients, and pre-select nearbysecondary calibrator sources for more reliable imaging [e.g. ]. Most recent results include theidentification of a new gamma-ray binary stellar system [ ], and the possible localization of theorigin of gamma-ray flaring activity in the Crab Nebula [ ].The EVN and MERLIN played an important role in the very beginning of black hole XRB andtransient research with the observations of SS433 at the end of 1970s and in the 1980s, and thisis the case still today. With the electronic upgrades to e-EVN and e-MERLIN, new opportunitiesarise: a combination of these two networks will provide a unique range of uv-spacings to imagetransient sources from mas to arcsecond scales as they expand. In the next few years a great numberof new transients will likely be detected by SKA pathfinder instruments and precursors like LOFARor WSRT APERTIF. The great sensitivity and high resolving power provided by the EVN will beessential to localize and image these at milliarcsecond resolution.5 S433 and other transients
Zsolt Paragi
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