Many X-ray binaries remain undetected in the mid-infrared, a regime where emission from their compact jets is likely to dominate. Here, we report the detection of the black hole binary GX 339-4 with the Widefield Infrared Survey Explorer (WISE) during a very bright, hard accretion state in 2010. Combined with a rich contemporaneous multiwavelength dataset, clear spectral curvature is found in the infrared, associated with the peak flux density expected from the compact jet. An optically-thin slope of ∼-0.7 and a jet radiative power of >6×10 35 erg s −1 (d/8 kpc) 2 are measured. A ∼24 h WISE light curve shows dramatic variations in mid-infrared spectral slope on timescales at least as short as the satellite orbital period ∼95 mins. There is also significant change during one pair of observations spaced by only 11 s. These variations imply that the spectral break associated with the transition from self-absorbed to optically-thin jet synchrotron radiation must be varying across the full wavelength range of ∼3-22 µm that WISE is sensitive to, and more. Based on fourband simultaneous mid-infrared detections, the break is constrained to frequencies of ≈4.6 +3.5 −2.0 ×10 13 Hz in at least two epochs of observation, consistent with a magnetic field B≈1.5(±0.8)×10 4 G assuming a single-zone synchrotron emission region. The observed variability implies that either B, or the size of the acceleration zone above the jet base, are being modulated by factors of ∼10 on relatively-short timescales.
We present ultradeep radio observations with the Expanded Very Large Array of 4U 1957+11, a Galactic black hole candidate X-ray binary known to exist in a persistent soft X-ray state. We derive a stringent upper limit of 11.4 µJy beam −1 (3σ) at 5-7 GHz, which provides the most rigorous upper limit to date on the presence of jets in a soft state black hole X-ray binary. X-ray, UV and optical fluxes obtained within a few weeks of the radio data can be explained by thermal emission from the disk. At this X-ray luminosity, a hard state black hole X-ray binary that follows the established empirical radio-X-ray correlation would be at least 330-810 times brighter at radio frequencies, depending on the distance to 4U 1957+11. This jet quenching of > 2.5 orders of magnitude is greater than some models predict, and implies the jets are prevented from being launched altogether in the soft state. 4U 1957+11 is also more than one order of magnitude fainter than the faintest of the 'radio-quiet' population of hard state black holes. In addition, we show that on average, soft state stellar-mass BHs probably have fainter jets than most active galactic nuclei in a state equivalent to the soft state. These results have implications for the conditions required for powerful, relativistic jets to form, and provide a new empirical constraint for time-and accretion mode-dependent jet models, furthering our understanding of jet production and accretion onto BHs.
We report high-energy results obtained with INTEGRAL and Rossi-XTE on two microquasars: the persistent high-mass system Cygnus X-1 and the transient low-mass binary SWIFT J1753.5-0127. INTEGRAL observed Cygnus X-1 from 2002 to 2004: the spectral (5-1000 keV) properties of the source, seen at least in three distinct spectral states, show disc and corona changes. In 2003 June, a high-energy tail at several hundred keV in excess of the thermal Comptonization model was observed, suggesting the presence of an additional non-thermal component. At that time, we detected an unusual correlation between radio data and high-energy hardness. We also report and compare the results obtained with simultaneous observations of the transient source SWIFT J1753.5-0127 performed with Rossi-XTE, INTEGRAL, VLA, REM and NTT on 2005 August 10-12 near its hard X-ray outburst. Broad-band spectra and fast time-variability properties are derived on this source (probably located in the galactic halo) together with radio, IR and optical data. We build a spectral energy distribution of the source and derive interesting multiwavelength constraints. Significantly detected up to 600 keV in a typical Low/Hard State, the transient does not seem to follow the usual radio/X-ray correlation.
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