In this paper we study the relation of radio emission to X-ray spectral and variability properties for a large sample of black hole X-ray binary systems. This is done to test, refine and extend -- notably into the timing properties -- the previously published `unified model' for the coupling of accretion and ejection in such sources. In 14 outbursts from 11 different sources we find that in every case the peak radio flux, on occasion directly resolved into discrete relativistic ejections, is associated with the bright hard to soft state transition near the peak of the outburst. We also note the association of the radio flaring with periods of X-ray flaring during this transition in most, but not all, of the systems. In the soft state, radio emission is in nearly all cases either undetectable or optically thin, consistent with the suppression of the core jet in these states and `relic' radio emission from interactions of previously ejected material and the ambient medium. However, these data cannot rule out an intermittent, optically thin, jet in the soft state. In attempting to associate X-ray timing properties with the ejection events we find a close, but not exact, correspondence between phases of very low integrated X-ray variability and such ejections. In summary we find no strong evidence against the originally proposed model, confirming and extending some aspects of it with a much larger sample, but note that several aspects remain poorly tested. (ABRIDGED)
Deep Dive into Jets from black hole X-ray binaries: testing, refining and extending empirical models for the coupling to X-rays.
In this paper we study the relation of radio emission to X-ray spectral and variability properties for a large sample of black hole X-ray binary systems. This is done to test, refine and extend – notably into the timing properties – the previously published unified model' for the coupling of accretion and ejection in such sources. In 14 outbursts from 11 different sources we find that in every case the peak radio flux, on occasion directly resolved into discrete relativistic ejections, is associated with the bright hard to soft state transition near the peak of the outburst. We also note the association of the radio flaring with periods of X-ray flaring during this transition in most, but not all, of the systems. In the soft state, radio emission is in nearly all cases either undetectable or optically thin, consistent with the suppression of the core jet in these states and relic’ radio emission from interactions of previously ejected material and the ambient medium. However, these
Accreting stellar-mass black holes in binary systems display different accretion 'states', characterised primarily by different X-ray spectral and variability properties (e.g. Nowak 1995; Homan et al. 2001;Homan & Belloni 2005, hereafter HB05;Remillard & Mc-Clintock 2006;Klein-Wolt & van der Klis 2007;Done, Gierlinski & Kubota 2007;Belloni 2009). Understanding empirically the relation of these states to the formation of powerful relativistic outflows, or jets, is a key goal for more detailed theoretical understanding of the coupled accretion:jet formation processes (e.g. Meier 2001;Livio, Pringle & King 2003;Ferreira et al. 2006). It also allows us to estimate the kinetic feedback to the ambient medium (e.g. Heinz & Sunyaev 2002;Fender, Maccarone & van Kesteren ⋆ email:rpf@phys.soton.ac.uk 2005), and to make direct comparisons with supermassive black holes in active galactic nuclei (e.g. Falcke & Biermann 2001;Merloni, Heinz & di Matteo 2003;Falcke, Körding and Markoff 2004;Körding, Jester & Fender 2007;Fender 2008).
In Fender, Belloni & Gallo (2004, hereafter FBG04; see also Fender & Belloni 2004) we outlined a unified picture for the disc:jet coupling in black hole X-ray binaries, where ‘disc:jet’ should be taken as shorthand for the relation between the infall and outflow of matter around the compact object, in all its various proposed geometries (and ‘disc’ not taken to simply mean the optically thick, geometrically thin, variety of accretion flow). This was based primarily on the relation between X-ray and radio emission in four well-studied systems. In very brief summary, although we recommend the interested reader to read FBG04, the model’s major components are:
• A steady, powerful, relatively low bulk velocity (bulk Lorentz factor Γ ≤ 2) jet is always present in the canonical hard X-ray state.
• As a source makes a hard → soft X-ray state transition, the jet becomes initially unstable and then produces a major flare which in some cases has been directly resolved into a major relativistic ejection event (with Γ ≥ 2). The point at which this occurs in the X-ray hardness-intensity diagram (see below) is referred to as the ‘jet line’. The inferred origin for the flaring is internal shocking as this faster, transient jet runs into the pre-existing slower jet from the hard state.
• Subsequently in the soft X-ray state the core jet is off, or at least much weaker.
• In the soft → hard transition in the latter phases of the outburst the steady hard state jet reforms without a major flare (no slower jet in front to run into).
• In trying to understand these observational conclusions, it was suggested that the ‘jet line’ corresponded to the point at which an inwards-moving inner disc edge reached the innermost stable circular orbit (ISCO).
Empirical aspects of this model were confirmed for a different system in Corbel et al. (2004). In this paper we take a far larger sample of black hole X-ray binary outbursts and use them to test and refine the model of FBG04 in the context of X-ray spectral states. In addition we extend the model by making for the first time a comprehensive attempt to include the X-ray short-timescale variability properties of the systems.
The X-ray data presented in this paper were obtained with the Proportional Counter Array (PCA) onboard the Rossi X-ray Timing Explorer (RXTE). In our analysis we make extensive use of the hardness-intensity diagram (HID) as an indicator of the X-ray spectral state of a black hole binary in outburst. The HIDs presented here were taken from Homan et al. (in prep.). They were constructed from standard2 mode data from the PCA. These data were corrected for background, but not for dead time (typically a few percent). Averaged count rates were extracted for each observation in three bands: channel 20-40, 3-10, and 1-129, roughly corresponding to 9.5-17.0 keV, 2.8-5.5 keV and 2-60 keV, respectively. The ratio of the count rates in first two bands was used as an indicator of the spectral hardness and the third band as the broadband intensity. In case fast intensity or hardness variations were observed within an observation, the observation was split into two or more parts and hardness and intensity were calculated for each individual part.
In addition to HIDs we also studied the X-ray variability properties of all sources. Again, these data were taken from Homan et al. (in prep.). Power spectra were constructed from high-timereolution data from the Proportional Counter Array (PCA), using most of the PCA energy range and following standard fast-fourier techniques (see e.g. Homan et al. 2005 for a detailed description). As a measure of the strength of the X-ray variability we extracted the rms-normalized power from the 0.01-64 Hz frequency range, for each observation (or observation segment).
Radio data presented in this paper were gathered exclusively from the literature, and references are provided in the brief descriptions for each outburst in section
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