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Register a new userAn evolving compact jet in the black hole X-ray binary MAXI J1836-194
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We report striking changes in the broadband spectrum of the compact jet of the black hole transient MAXI J1836-194 over state transitions during its discovery outburst in 2011. A fading of the optical-infrared (IR) flux occurred as the source entered the hard-intermediate state, followed by a brightening as it returned to the hard state. The optical-IR spectrum was consistent with a power law from optically thin synchrotron emission, except when the X-ray spectrum was softest. By fitting the radio to optical spectra with a broken power law, we constrain the frequency and flux of the optically thick/thin break in the jet synchrotron spectrum. The break gradually shifted to higher frequencies as the source hardened at X-ray energies, from ~ 10^11 to ~ 4 x 10^13 Hz. The radiative jet luminosity integrated over the spectrum appeared to be greatest when the source entered the hard state during the outburst decay (although this is dependent on the high energy cooling break, which is not seen directly), even though the radio flux was fading at the time. The physical process responsible for suppressing and reactivating the jet (neither of which are instantaneous but occur on timescales of weeks) is uncertain, but could arise from the varying inner accretion disk radius regulating the fraction of accreting matter that is channeled into the jet. This provides an unprecedented insight into the connection between inflow and outflow, and has implications for the conditions required for jets to be produced, and hence their launching process.
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We present the results of our quasi-simultaneous radio, sub-mm, infrared, optical and X-ray study of the Galactic black hole candidate X-ray binary MAXI J1836-194 during its 2011 outburst. We consider the full multi-wavelength spectral evolution of the outburst, investigating whether the evolution of the jet spectral break (the transition between optically-thick and optically-thin synchrotron emission) is caused by any specific properties of the accretion flow. Our observations show that the break does not scale with the X-ray luminosity or with the inner radius of the accretion disk, and is instead likely to be set by much more complex processes. We find that the radius of the acceleration zone at the base of the jet decreases from ~10$^6$ gravitational radii during the hard intermediate state to ~10$^3$ gravitational radii as the outburst fades (assuming a black hole mass of 8 M$_{odot}$), demonstrating that the electrons are accelerated on much larger scales than the radius of the inner accretion disk and that the jet properties change significantly during outburst. From our broadband modelling and high-resolution optical spectra, we argue that early in the outburst, the high-energy synchrotron cooling break was located in the optical band, between $approx 3.2 times 10^{14}$ Hz and $4.5 times 10^{14}$ Hz. We calculate that the jet has a total radiative power of $approx 3.1 times 10^{36}$ ergs s$^{-1}$, which is ~6% of the bolometric radiative luminosity at this time. We discuss how this cooling break may evolve during the outburst, and how that evolution dictates the total jet radiative power. Assuming the source is a stellar-mass black hole with canonical state transitions, from the measured flux and peak temperature of the disk component we constrain the source distance to be 4-10 kpc.
The X-ray transient MAXI J1836-194 is a newly-identified Galactic black hole binary candidate. As most X-ray transients, it was discovered at the beginning of an X-ray outburst. After the initial canonical X-ray hard state, the outburst evolved into a hard intermediate state and then went back to the hard state. The existing RATAN-600 radio monitoring observations revealed that it was variable on a timescale of days and had a flat or inverted spectrum, consistent with optically thick synchrotron emission, possibly from a self-absorbed jet in the vicinity of the central compact object. We observed the transient in the hard state near the end of the X-ray outburst with the European VLBI Network (EVN) at 5 GHz and the Chinese VLBI Network (CVN) at 2.3 and 8.3 GHz. The 8.3 GHz observations were carried out at a recording rate of 2048 Mbps using the newly-developed Chinese VLBI data acquisition system (CDAS), twice higher than the recording rate used in the other observations. We successfully detected the low-declination source with a high confidence level in both observations. The source was unresolved (<=0.5 mas), which is in agreement with an AU-scale compact jet.
During outbursts, the observational properties of black hole X-ray binaries (BHXBs) vary on timescales of days to months. These relatively short timescales make these systems ideal laboratories to probe the coupling between accreting material and outflowing jets as a the accretion rate varies. In particular, the origin of the hard X-ray emission is poorly understood and highly debated. This spectral component, which has a power-law shape, is due to Comptonisation of photons near the black hole, but it is unclear whether it originates in the accretion flow itself, or at the base of the jet, or possibly the interface region between them. In this paper we explore the disk-jet connection by modelling the multi-wavelength emission of MAXI J1836-194 during its 2011 outburst. We combine radio through X-ray spectra, X-ray timing information, and a robust joint-fitting method to better isolate the jets physical properties. Our results demonstrate that the jet base can produce power-law hard X-ray emission in this system/outburst, provided that its base is fairly compact and that the temperatures of the emitting electrons are sub-relativistic. Because of energetic considerations, our model favours mildly pair-loaded jets carrying at least 20 pairs per proton. Finally, we find that the properties of the X-ray power spectrum are correlated with the jet properties, suggesting that an underlying physical process regulates both.
We present results from six epochs of quasi-simultaneous radio, (sub-)millimetre, infrared, optical, and X-ray observations of the black hole X-ray binary MAXI~J1535$-$571. These observations show that as the source transitioned through the hard-intermediate X-ray state towards the soft intermediate X-ray state, the jet underwent dramatic and rapid changes. We observed the frequency of the jet spectral break, which corresponds to the most compact region in the jet where particle acceleration begins (higher frequencies indicate closer to the black hole), evolve from the IR band into the radio band (decreasing by $approx$3 orders of magnitude) in less than a day. During one observational epoch, we found evidence of the jet spectral break evolving in frequency through the radio band. Estimating the magnetic field and size of the particle acceleration region shows that the rapid fading of the high-energy jet emission was not consistent with radiative cooling; instead the particle acceleration region seems to be moving away from the black hole on approximately dynamical timescales. This result suggests that the compact jet quenching is not caused by local changes to the particle acceleration, rather we are observing the acceleration region of the jet travelling away from the black hole with the jet flow. Spectral analysis of the X-ray emission show a gradual softening in the few days before the dramatic jet changes, followed by a more rapid softening $sim$1--2,days after the onset of the jet quenching.
We study the jet in the hard state of the accreting black-hole binary MAXI J1820+070. From the available radio-to-optical spectral and variability data, we put strong constraints on the jet parameters. We find while it is not possible to uniquely determine the jet Lorentz factor from the spectral and variability properties alone, we can estimate the jet opening angle ($1.5pm 1$ deg), the distance at which the jet starts emitting synchrotron radiation ($sim$3$times10^{10}$cm), the magnetic field strength there ($sim$10$^4$G), and the maximum Lorentz factor of the synchrotron-emitting electrons ($sim$110--150) with relatively low uncertainty, as they depend weakly on the bulk Lorentz factor. We find the breaks in the variability power spectra from radio to sub-mm are consistent with variability damping over the time scale equal to the travel time along the jet at any Lorentz factor. This factor can still be constrained by the electron-positron pair production rate within the jet base, which we calculate based on the observed X-ray/soft gamma-ray spectrum, and the jet power, required to be less than the accretion power. The minimum ($sim$1.5) and maximum ($sim$4.5) Lorentz factors correspond to the dominance of pairs and ions, and the minimum and maximum jet power, respectively. We estimate the magnetic flux threading the black hole and find the jet can be powered by the Blandford-Znajek mechanism in a magnetically-arrested flow accretion flow. We point out the similarity of our derived formalism to that of core shifts, observed in extragalactic radio sources.
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