On the Origin of Radio Emission in the X-ray States of XTE J1650-500 during the 2001-2002 Outburst

We report on simultaneous radio and X-ray observations of the black hole candidate XTE J1650-500 during the course of its 2001-2002 outburst. The scheduling of the observations allowed us to sample the properties of XTE J1650-50 in different X-ray spectral states, namely the hard state, the steep power-law state and the thermal dominant state, according to the recent spectral classification of McClintock & Remillard. The hard state is consistent with a compact jet dominating the spectral energy distribution at radio frequencies; however, the current data suggest that its contribution as direct synchrotron emission at higher energies may not be significant. In that case, XTE J1650-50 may be dominated by Compton processes (either inverse Comptonization of thermal disk photons and/or SSC from the base of the compact jet) in the X-ray regime. We, surprisingly, detect a faint level of radio emission in the thermal dominant state that may be consistent with the emission of previously ejected material interacting with the interstellar medium, similar (but on a smaller angular scale) to what was observed in XTE J1550-564 by Corbel and co-workers. Based on the properties of radio emission in the steep power-law state of XTE J1650-50, and taking into account the behavior of other black hole candidates (namely GX 339-4, XTE J1550-564, and XTE J1859+226) while in the intermediate and steep power-law states, we are able to present a general pattern of behavior for the origin of radio emission in these two states that could be important for understanding the accretion-ejection coupling very close to the black hole event horizon.Comment: Accepted for publication in The Astrophysical Journal. 33 pages, 10 figure

The variable radio counterpart and possible large-scale jet of the new Z-source XTE J1701-462

We report radio observations, made with the Australia Telescope Compact Array, of the X-ray transient XTE J1701-462. This system has been classified as a new `Z' source, displaying characteristic patterns of behaviour probably associated with accretion onto a low magnetic field neutron star at close to the Eddington limit. The radio counterpart is highly variable, and was detected in six of sixteen observations over the period 2006 January -- April. The coupling of radio emission to X-ray state, despite limited sampling, appears to be similar to that of other `Z' sources, in that there is no radio emission on the flaring branch. The mean radio and X-ray luminosities are consistent with the other Z sources for a distance of 5--15 kpc. The radio spectrum is unusually flat, or even inverted, in contrast to the related sources, Sco X-1 and Cir X-1, which usually display an optically thin radio spectrum. Deep wide-field observations indicate an extended structure three arcminutes to the south which is aligned with the X-ray binary. This seems to represent a significant overdensity of radio sources for the field and so, although a background source remains a strong possibility, we consider it plausible that this is a large-scale jet associated with XTE J1701-462.Comment: Accepted for publication as a Letter in MNRA

Modeling the X-ray Contribution of X-ray Binary Jets

Astrophysical jets exist in both XRBs and AGN, and seem to share common features, particularly in the radio. While AGN jets are known to emit X-rays, the situation for XRB jets is not so clear. Radio jets have been resolved in several XRBs in the low/hard state, establishing that some form of outflow is routinely present in this state. Interestingly, the flat-to-inverted radio synchrotron emission associated with these outflows strongly correlates with the X-ray emission in several sources, suggesting that the jet plasma plays a role at higher frequencies. In this same state, there is increasing evidence for a turnover in the IR/optical where the flat-to-inverted spectrum seems to connect to an optically thin component extending into the X-rays. We discuss how jet synchrotron emission is likely to contribute to the X-rays, in addition to inverse Compton up-scattering, providing a natural explanation for these correlations and the turnover in the IR/optical band. We present model parameters for fits to several sources, and address some common misconceptions about the jet model.Comment: 4 pages, 1 Table, conference proceedings for "The Physics of Relativistic Jets in the Chandra and XMM Era, Bologna, 2002", Eds. G. Brunetti, D. E. Harris, R. M. Sambruna & G. Sett

Near-infrared synchrotron emission from the compact jet of GX339-4

We have compiled contemporaneous broadband observations of the black hole candidate X-ray binary GX 339-4 when in the low/hard X-ray state in 1981 and 1997. The data clearly reveal the presence of two spectral components, with thermal and non-thermal spectra, overlapping in the optical -- near-infrared bands. The non-thermal component lies on an extrapolation of the radio spectrum of the source, and we interpret it as optically thin synchrotron emission from the powerful, compact jet in the system. Detection of this break from self-absorbed to optically thin synchrotron emission from the jet allows us to place a firm lower limit on the ratio of jet (synchrotron) to X-ray luminosities of $\geq 5$%. We further note that extrapolation of the optically thin synchrotron component from the near-infrared to higher frequencies coincides with the observed X-ray spectrum, supporting models in which the X-rays could originate via optically thin synchrotron emission from the jet (possibly instead of Comptonisation).Comment: Accepted for publication in ApJ Lette

Spectral evolution of the microquasar XTE J1550-564 over its entire 2000 outburst

We report on RXTE observations of the microquasar XTE J1550-564 during a ~70 day outburst in April-June 2000. We study the evolution of the PCA+HEXTE spectra over the outburst. The source transited from an initial Low Hard State (LS), to an Intermediate State (IS), and then back to the LS. The source shows an hysteresis effect similar to what is observed in other sources, favoring a common origin for the state transitions in soft X-ray transients. The first transition occurs at a ~ constant 2-200 keV flux, which probably indicates a change in the relative importance of the emitting media. The second transition is more likely driven by a drop in the mass accretion rate. In both LS, the spectra are characterized by the presence of a strong power-law tail (Compton corona) with a variable high energy cut-off. During the IS, the spectra show the presence of a ~0.8 keV thermal component (accretion disk). We discuss the apparently independent evolution of the two media, and show that right after the X-ray maximum on MJD 51662, the decrease of the source luminosity is due to a decrease of the power-law luminosity, at a constant disk luminosity. This, together with the detection of radio emission (with a spectrum typical of optically thin synchrotron emission), may suggest that the corona is ejected and further detected as a discrete radio ejection.Comment: Accepted for publication in ApJ. 9 pages, 4 figures, abstract abridge

Revisiting the radio/X-ray flux correlation in the black hole V404 Cyg: from outburst to quiescence

We report results of Chandra X-ray and VLA radio observations of the Galactic accreting black hole V404 Cyg (GS 2023+338) in its quiescent state. V404 Cyg is detected at its faintest level of radio and X-ray emission with a 0.5-10 keV unabsorbed luminosity of 8.3 x 10^32 (d/3.5 kpc)^2 erg/s. The X-ray spectrum fit with an absorbed power-law model yields a photon index of 2.17 +/- 0.13. Contrary to previous findings, this clearly indicates that V404 Cyg undergoes - like most black holes in quiescence - a softening of its X-ray spectrum at very low luminosity compared to the standard hard state. The quiescent radio emission is consistent with the presence of self-absorbed compact jets. We have also reanalyzed archival data from the decay of the 1989 outburst of V404 Cyg in order to quantify more precisely the correlation between radio and X-ray emission in the hard state of V404 Cyg. We show that this correlation extends over five decades in X-ray flux and holds down to the quiescent state of V404 Cyg. The index of this correlation (~0.5) may suggest that synchrotron self-Compton emission is the dominant physical process at high energy in V404 Cyg. However, this index is also consistent with scale invariant jet models coupled to an inefficiently radiating accretion disc. We discuss the properties of the quiescent state of black holes and highlight the fact that some of their properties are different from the standard hard state.Comment: 7 pages, Accepted for publication in MNRA

Is the `IR Coincidence' Just That?

(Abridged) Motch (1985) suggested that in the hard state of GX 339-4 the soft X-ray power-law extrapolated backward in energy agrees with the IR flux. Corbel & Fender (2002) showed that the hard state radio power-law extrapolated forward in energy meets the extrapolated X-ray power-law at an IR break, which was explicitly observed twice in GX 339-4. This `IR coincidence' has been cited as further evidence that a jet might make a significant contribution to the X-rays in hard state systems. We explore this hypothesis with a series of simultaneous radio/X-ray observations of GX 339-4, taken during its 1997, 1999, and 2002 hard states. We fit these spectra, in detector space, with a simple, but remarkably successful, doubly broken power-law that requires an IR spectral break. For these observations, the break position and the integrated radio/IR flux have stronger dependences upon the X-rays than the simplest jet predictions. If one allows for a softening of the X-ray power law with increasing flux, then the jet model agrees with the correlation. We also find evidence that the radio/X-ray fcorrelation previously observed in GX 339-4 shows a `parallel track' for the 2002 hard state. The slope of the 2002 correlation is consistent with prior observations; however, the radio amplitude is reduced. We then examine the correlation in Cyg X-1 through the use of radio data, obtained with the Ryle radio telescope, and RXTE data, from the ASM and pointed observations. We again find evidence of `parallel tracks', and here they are associated with `failed transitions' to the soft state. We also find that for Cyg X-1 the radio flux is more fundamentally correlated with the hard X-ray flux.Comment: To Appear in the July 2005 Astrophysical Journal; 9 Pages, uses emulateapj.st

Radio / X-ray correlation in the low/hard state of GX 339--4

We present the results of a long-term study of the black hole candidate GX 339-4 using simultaneous radio (from the Australia Telescope Compact Array) and X-ray (from the Rossi X-ray Timing Explorer and BeppoSAX) observations performed between 1997 and 2000. We find strong evidence for a correlation between these two emission regimes that extends over more than three decades in X-ray flux, down to the quiescence level of GX 339-4. This is the strongest evidence to date for such strong coupling between radio and X-ray emission. We discuss these results in light of a jet model that can explain the radio/X-ray correlation. This could indicate that a significant fraction of the X-ray flux that is observed in the low-hard state of black hole candidates may be due to optically thin synchrotron emission from the compact jet.Comment: 8 pages. Accepted for publication in Astronomy & Astrophysics, 200