[abridged] The black hole X-ray binary XTE J1550-564 was monitored extensively at X-ray, optical and infrared wavelengths throughout its outburst in 2000. We show that it is possible to separate the optical/near-infrared (OIR) jet emission from the OIR disc emission. Focussing on the jet component, we find that as the source fades in the X-ray hard state, the OIR jet emission has a spectral index consistent with optically thin synchrotron emission (alpha ~ -0.6 to -0.7, where F_nu \propto nu^alpha). This jet emission is tightly and linearly correlated with the X-ray flux; L_OIR,jet \propto L_X^(0.98 +- 0.08) suggesting a common origin. This is supported by the OIR, X-ray and OIR to X-ray spectral indices being consistent with a single power law (alpha = -0.73). Ostensibly the compact, synchrotron jet could therefore account for ~ 100 % of the X-ray flux at low luminosities in the hard state. At the same time, (i) an excess is seen over the power law decay of the X-ray flux at the point in which the jet would start to dominate, (ii) the X-ray spectrum slightly softens, which seems to be due to a high energy cut-off or break shifting to a lower energy, and (iii) the X-ray rms variability increases. This may be the strongest evidence to date of synchrotron emission from the compact, steady jet dominating the X-ray flux of an X-ray binary. For XTE J1550-564, this is likely to occur within the luminosity range ~ (2 e-4 - 2 e-3) L_Edd on the hard state decline of this outburst. However, on the hard state rise of the outburst and initially on the hard state decline, the synchrotron jet can only provide a small fraction (~ a few per cent) of the X-ray flux. Both thermal Comptonization and the synchrotron jet can therefore produce the hard X-ray power law in accreting black holes.
Numerous efforts have been made in recent years to identify the emission from jets in X-ray binary systems. These jets, produced close to accreting black holes and neutron stars, are known to (at least in some cases) carry a signifi-⋆ E-mail: d.m.russell@uva.nl cant fraction of the accretion energy away from the binary, in the form of relativistic flows (e.g. Mirabel et al. 1992;Gallo, Fender & Pooley 2003;Gallo et al. 2005). Steady, continuously replenished 'compact' jets are seen in the hard X-ray state (e.g. Fender 2006). Like some jets produced by supermassive black holes in Active Galactic Nuclei (AGN), the emission is assumed to originate from synchrotron ra-diation produced by electrons or positrons (leptons) in the stratified jet.
From radio through infrared the observed radiation is a ∼ flat self-absorbed optically thick synchrotron spectrum with spectral index α ≈ 0.0 to +0.2 (where Fν ∝ ν α ), composed of a superposition of synchrotron-emitting particle distributions (Blandford & Konigl 1979). The higher energy synchrotron emission originates in a small, dense region of the jet, close to the location where the jets are launched near the compact object (Blandford & Konigl 1979;Kaiser 2005). Since α ∼ > 0, the bulk of the radiative power of the jet resides in this higher energy emission; at some frequency (likely in the infrared; e.g. Corbel & Fender 2002;Russell et al. 2006) there is a break in the jet spectrum from one which is ∼ flat (α ≈ 0.0 to +0.2) to optically thin (with α ≈ -0.7). In addition there is a cut-off in the jet spectrum at higher energies (likely in the X-ray regime; see e.g. Markoff, Falcke & Fender 2001;Maitra et al. 2009). Emission from the star and outer accretion disc often dominate the optical/infrared light, so the frequency of the aforementioned optically thick-thin jet break is hard to identify. Similarly, the inner accretion disc and hot inner flow/‘corona’ produces the majority of the X-rays (for reviews see Charles & Coe 2006;Gilfanov 2009). However, in some cases emission from compact jets has been successfully isolated in the infrared, optical (e.g. Corbel & Fender 2002;Buxton & Bailyn 2004;Migliari et al. 2006) and possibly X-ray (e.g. Hynes et al. 2003) regimes. Moreover, it was shown (Markoff, Nowak & Wilms 2005) that the base of the jet and the ‘corona’ could be synonymous and the hard X-rays could arise from inverse Compton emission at the jet base. In addition, the optical emission from the jet is correlated with that of inflowing matter, providing information about how the two are coupled, or how the disc feeds the jet (e.g. Kanbach et al. 2001;Malzac, Merloni & Fabian 2004).
An empirical unification of jet-disc coupling in black hole X-ray binaries (BHXBs) has been proposed. A BHXB usually traces out a hysteretical pattern in the X-ray hardness-intensity diagram (HID; a similar hysteresis has been noted in accreting neutron stars and even white dwarfs; Maccarone & Coppi 2003;Körding et al. 2008), and the broadband behaviour is correlated with the evolution through the HID (Done & Gierliński 2003;Fender, Belloni & Gallo 2004;Done, Gierliński & Kubota 2007;Dunn et al. 2008Dunn et al. , 2009;;Fender et al. 2009;Cabanac et al. 2009;Belloni 2009). Generally, the steady, compact jets exist in the hard state and are suppressed in the soft state (Gallo et al. 2003;Fender et al. 2004Fender et al. , 2009)).
Since its discovery, the BHXB XTE J1550-564 has performed outbursts or re-brightenings in 1998-99, 2000, 2001, 2002and 2003(Orosz et al. 2002;;Belloni et al. 2002;Aref’ev et al. 2004;Dunn et al. 2009). The compact object was found dynamically to be a black hole of mass ∼ 8-12 M⊙ (Orosz et al. 2002). The system is most famous for its arcmin-scale radio and X-ray jet ‘blobs’ which were seen to decelerate several years after the jets were launched, due to jet-ISM interactions (Corbel et al. 2002). The outburst of XTE J1550-564 in 2000 had very well-sampled optical, near-infrared (NIR) and Xray monitoring throughout the entire outburst (Jain et al. 2001;Tomsick, Corbel & Kaaret 2001;Reilly et al. 2001;Rodriguez, Corbel & Tomsick 2003). Here we analyse the light curves and spectral energy distributions (SEDs) and show that for this outburst it is possible to isolate the disc and jet components of the optical/NIR (OIR) emission. We use this to correlate changes in the broadband spectrum of the jet with evolution of the X-ray hysteresis. In Section 2 we describe the data collection. In Section 3 the multiwavelength light curves and spectral evolution are analysed. The OIR jet emission is isolated and the broadband evolution of the jets are discussed. We constrain the contribution of the jet to the X-ray flux and plot the broadband SEDs. The results and implications are discussed in Section 4 and the conclusions are summarised in Section 5.
Data from the Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array (PCA), High Energy X-ray Timing Experiment (HEXTE) and
This content is AI-processed based on open access ArXiv data.
