The radio/X-ray correlation in Cyg X-3 and the nature of its hard spectral state

We study the radio/X-ray correlation in Cyg X-3. It has been known that the soft and hard X-ray fluxes in the hard spectral state are correlated positively and negatively, respectively, with the radio flux. We show that this implies that the observed $\sim$1--100 keV flux (which is a fair approximation to the bolometric flux) is completely uncorrelated with the radio flux. We can recover a positive correlation (seen in other sources and expected theoretically) if the soft X-rays are strongly absorbed by a local medium. Then, however, the intrinsic X-ray spectrum of Cyg X-3 in its hard state becomes relatively soft, similar to that of an intermediate spectral state of black-hole binaries, but not to their true hard state. We also find the radio spectra in the hard state of Cyg X-3 are hard on average, and the flux distributions of the radio emission and soft X-rays can be described by sums of two log-normal functions. We compare Cyg X-3 with other X-ray binaries using colour-colour, colour-Eddington ratio and Eddington ratio-radio flux diagrams. We find Cyg X-3 to be spectrally most similar to GRS 1915+105, except that Cyg X-3 is substantially more radio loud, which appears to be due to its jet emission enhanced by interaction with the powerful stellar wind from the Wolf-Rayet donor.Comment: An error in the BAT light curve shown in Fig. A2 corrected. Published in MNRAS + erratu

Correlated radio--X-ray variability of Galactic Black Holes: A radio--X-ray flare in Cygnus X-1

We report on the first detection of a quasi-simultaneous radio-X-ray flare of Cygnus X-1. The detection was made on 2005 April 16 with pointed observations by the Rossi X-ray Timing Explorer and the Ryle telescope, during a phase where the black hole candidate was close to a transition from the its soft into its hard state. The radio flare lagged the X-rays by approximately 7 minutes, peaking at 3:20 hours barycentric time (TDB 2453476.63864). We discuss this lag in the context of models explaining such flaring events as the ejection of electron bubbles emitting synchrotron radiation.Comment: 4 pages, 4 figure

HST/NICMOS Observations of Fast Infrared Flickering in the Microquasar GRS 1915+105

We report infrared observations of the microquasar GRS 1915+105 using the NICMOS instrument of the Hubble Space Telescope during 9 visits in April-June 2003. During epochs of high X-ray/radio activity near the beginning and end of this period, we find that the $1.87$\um infrared flux is generally low ($\sim 2$ mJy) and relatively steady. However, during the X-ray/radio ``plateau'' state between these epochs, we find that the infrared flux is significantly higher ($\sim 4-6$ mJy), and strongly variable. In particular, we find events with amplitudes $\sim 20-30$% occurring on timescales of $\sim 10-20$s (e-folding timescales of $\sim 30$s). These flickering timescales are several times faster than any previously-observed infrared variability in GRS 1915+105 and the IR variations exceed corresponding X-ray variations at the same ($\sim 8s$) timescale. These results suggest an entirely new type of infrared variability from this object. Based on the properties of this flickering, we conclude that it arises in the plateau-state jet outflow itself, at a distance $<2.5$ AU from the accretion disk. We discuss the implications of this work and the potential of further flickering observations for understanding jet formation around black holes.Comment: 19 pages, incl. 4 figures; accepted for publication in Ap

X-ray Spectral Analysis of the Steady States of GRS 1915+105

We report on the X-ray spectral behavior within the steady states of GRS 1915+105. Our work is based on the full data set on the source obtained using the Proportional Counter Array on the Rossi X-ray Timing Explorer and 15 GHz radio data obtained using the Ryle Telescope. The steady observations within the X-ray data set naturally separated into two regions in the color-color diagram and we refer to them as steady-soft and steady-hard. GRS 1915+105 displays significant curvature in the coronal component in both the soft and hard data within the {\it RXTE}/PCA bandpass. A majority of the steady-soft observations displays a roughly constant inner disk radius (R_in), while the steady-hard observations display an evolving disk truncation which is correlated to the mass accretion rate through the disk. The disk flux and coronal flux are strongly correlated in steady-hard observations and very weakly correlated in the steady-soft observations. Within the steady-hard observations we observe two particular circumstances when there are correlations between the coronal X-ray flux and the radio flux with log slopes \eta~0.68 +/- 0.35 and \eta ~ 1.12 +/- 0.13. They are consistent with the upper and lower tracks of Gallo et al. (2012), respectively. A comparison of model parameters to the state definitions show that almost all steady-soft observations match the criteria of either thermal or steep power law state, while a large portion of the steady-hard observations match the hard state criteria when the disk fraction constraint is neglected.Comment: 21 pages, 15 figures, 2 tables. Accepted for publication in Ap

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

Discovery of Radio Outbursts in the Active Nucleus of M81

The low-luminosity active galactic nucleus of M81 has been monitored at centimeter wavelengths since early 1993 as a by-product of radio programs to study the radio emission from Supernova 1993J. The extensive data sets reveal that the nucleus experienced several radio outbursts during the monitoring period. At 2 and 3.6 cm, the main outburst occurred roughly in the beginning of 1993 September and lasted for approximately three months; at longer wavelengths, the maximum flux density decreases, and the onset of the burst is delayed. These characteristics qualitatively resemble the standard model for adiabatically expanding radio sources, although certain discrepancies between the observations and the theoretical predictions suggest that the model is too simplistic. In addition to the large-amplitude, prolonged variations, we also detected milder changes in the flux density at 3.6 cm and possibly at 6 cm on short (less than 1 day) timescales. We discuss a possible association between the radio activity and an optical flare observed during the period that the nucleus was monitored at radio wavelengths.Comment: To appear in The Astronomical Journal. Latex, 18 pages including embedded figures and table