We investigated the relation between compact jet emission and X-ray variability properties of all black hole transients with multiwavelength coverage during their outburst decays. We studied the evolution of all power spectral components (including l
ow frequency quasi-periodic oscillations), and related this evolution to changes in jet properties tracked by radio and infrared observations. We grouped sources according to their tracks in radio/X-ray luminosity relation, and show that the standards show stronger broadband X-ray variability than outliers at a given X-ray luminosity when the compact jet turned on. This trend is consistent with the internal shock model and can be important for the understanding of the presence of tracks in the radio/X-ray luminosity relation. We also observed that the total and the QPO rms amplitudes increase together during the earlier part of the outburst decay, but after the compact jet turns either the QPO disappears or its rms amplitude decreases significantly while the total rms amplitudes remain high. We discuss these results with a scenario including a variable corona and a non-variable disk with a mechanism for the QPO separate from the mechanism that create broad components. Finally, we evaluated the timing predictions of the magnetically dominated accretion flow model which can explain the presence of tracks in the radio/X-ray luminosity relation.
Compact, steady jets are observed in the near infrared and radio bands in the hard state of Galactic black hole transients as their luminosity decreases and the source moves towards a quiescent state. Recent radio observations indicate that the jets
turn off completely in the soft state, therefore multiwavelength monitoring of black hole transients are essential to probe the formation of jets. In this work we conducted a systematic study of all black hole transients with near infrared and radio coverage during their outburst decays. We characterized the timescales of changes in X-ray spectral and temporal properties and also in near infrared and/or in radio emission. We confirmed that state transitions occur in black hole transients at a very similar fraction of their respective Eddington luminosities. We also found that the near infrared flux increase that could be due to the formation of a compact jet is delayed by a time period of days with respect to the formation of a corona. Finally, we found a threshold disk Eddington luminosity fraction for the compact jets to form. We explain these results with a model such that the increase in the near infrared flux corresponds to a transition from a patchy, small scale height corona along with an optically thin out flow to a large scale height corona that allows for collimation of a steady compact jet. We discuss the timescale of jet formation in terms of transport of magnetic fields from the outer parts of the disk, and also consider two alternative explanations for the multiwavelength emission: hot inner accretion flows and irradiation.
We report multiwavelength observations of the black hole transient GX 339-4 during its outburst decay in 2011 using the data from RXTE, Swift and SMARTS. Based on the X-ray spectral, temporal, and the optical/infrared (OIR) properties, the source evo
lved from the soft-intermediate to the hard state. Twelve days after the start of the transition towards the hard state, a rebrightening was observed simultaneously in the optical and the infrared bands. Spectral energy distributions (SED) were created from observations at the start, and close to the peak of the rebrightening. The excess OIR emission above the smooth exponential decay yields flat spectral slopes for these SEDs. Assuming that the excess is from a compact jet, we discuss the possible locations of the spectral break that mark the transition from optically thick to optically thin synchrotron components. Only during the rising part of the rebrightening, we detected fluctuations with the binary period of the system. We discuss a scenario that includes irradiation of the disk in the intermediate state, irradiation of the secondary star during OIR rise and jet emission dominating during the peak to explain the entire evolution of the OIR light curve.
