V. Dhawan

The First Accurate Parallax Distance to a Black Hole

Using astrometric VLBI observations, we have determined the parallax of the black hole X-ray binary V404 Cyg to be 0.418 ± 0.024 mas, corresponding to a distance of 2.39 ± 0.14 kpc, significantly lower than the previously accepted value. This model-independent estimate is the most accurate distance to a Galactic stellar-mass black hole measured to date. With this new distance, we confirm that the source was not super-Eddington during its 1989 outburst. The fitted distance and proper motion imply that the black hole in this system likely formed in a supernova, with the peculiar velocity being consistent with a recoil (Blaauw) kick. The size of the quiescent jets inferred to exist in this system is <1.4 AU at 22 GHz. Astrometric observations of a larger sample of such systems would provide useful insights into the formation and properties of accreting stellar-mass black holes.

Disc-jet coupling in the 2009 outburst of the black hole candidate H1743−322

We present an intensive radio and X-ray monitoring campaign on the 2009 outburst of the Galactic black hole candidate X-ray binary H1743−322. With the high angular resolution of the Very Long Baseline Array, we resolve the jet ejection event and measure the proper motions of the jet ejecta relative to the position of the compact core jets detected at the beginning of the outburst. This allows us to accurately couple the moment when the jet ejection event occurred with X-ray spectral and timing signatures. We find that X-ray timing signatures are the best diagnostic of the jet ejection event in this outburst, which occurred as the X-ray variability began to decrease and the Type C quasi-periodic oscillations disappeared from the X-ray power density spectrum. However, this sequence of events does not appear to be replicated in all black hole X-ray binary outbursts, even within an individual source. In our observations of H1743−322, the ejection was contemporaneous with a quenching of the radio emission, prior to the start of the major radio flare. This contradicts previous assumptions that the onset of the radio flare marks the moment of ejection. The jet speed appears to vary between outbursts, with a possible positive correlation with outburst luminosity. The compact core radio jet reactivated on transition to the hard intermediate state at the end of the outburst, and not when the source reached the low hard spectral state. Comparison with the known near-infrared behaviour of the compact jets suggests a gradual evolution of the compact jet power over a few days near the beginning and end of an outburst.

A transient radio jet in an erupting dwarf nova

Astrophysical jets seem to occur in nearly all types of accreting objects, from supermassive black holes to young stellar objects. On the basis of x-ray binaries, a unified scenario describing the disc/jet coupling has evolved and been extended to many accreting objects. The only major exceptions are thought to be cataclysmic variables: Dwarf novae, weakly accreting white dwarfs, show similar outburst behavior to x-ray binaries, but no jet has yet been detected. Here we present radio observations of a dwarf nova in outburst showing variable flat-spectrum radio emission that is best explained as synchrotron emission originating in a transient jet. Both the inferred jet power and the relation to the outburst cycle are analogous to those seen in x-ray binaries, suggesting that the disc/jet coupling mechanism is ubiquitous.

A PARALLAX DISTANCE TO THE MICROQUASAR GRS 1915+105 AND A REVISED ESTIMATE OF ITS BLACK HOLE MASS

Using the Very Long Baseline Array, we have measured a trigonometric parallax for the micro quasar GRS 1915+105, which contains a black hole and a K-giant companion. This yields a direct distance estimate of 8.6 (+2.0,-1.6) kpc and a revised estimate for the mass of the black hole of 12.4 (+2.0,-1.8) Msun. GRS 1915+105 is at about the same distance as some HII regions and water masers associated with high-mass star formation in the Sagittarius spiral arm of the Galaxy. The absolute proper motion of GRS 1915+105 is -3.19 +/- 0.03 mas/y and -6.24 +/- 0.05 mas/y toward the east and north, respectively, which corresponds to a modest peculiar speed of 22 +/-24 km/s at the parallax distance, suggesting that the binary did not receive a large velocity kick when the black hole formed. On one observational epoch, GRS 1915+105 displayed superluminal motion along the direction of its approaching jet. Considering previous observations of jet motions, the jet in GRS 1915+105 can be modeled with a jet inclination to the line of sight of 60 +/- 5 deg and a variable flow speed between 0.65c and 0.81c, which possibly indicates deceleration of the jet at distances from the black hole >2000 AU. Finally, using our measurements of distance and estimates of black hole mass and inclination, we provisionally confirm our earlier result that the black hole is spinning very rapidly.

Radio Emission and the Timing Properties of the Hard X-Ray State of GRS 1915+105

We combine a complete sample of 113 pointed observations taken with the Rossi X-Ray Timing Explorer between 1996 and 1999, monitoring observations taken with the Ryle Telescope and the Green Bank Interferometer, and selected observations with the Very Large Array to study the radio and X-ray properties of GRS 1915+105 when its X-ray emission is hard and steady. We establish that radio emission always accompanies the hard-steady state of GRS 1915+105 but that the radio flux density at 15.2 GHz and the X-ray flux between 2 and 200 keV are not correlated. Therefore, we study the X-ray spectral and timing properties of GRS 1915+105 using three approaches: first by describing in detail the properties of three characteristic observations, then by displaying the time evolution of the timing properties during periods of both faint and bright radio emission, and lastly by plotting the timing properties as a function of the radio flux density. We find that as the radio emission becomes brighter and more optically thick, (1) the frequency of a ubiquitous 0.5-10 Hz quasi-periodic oscillation (QPO) decreases, (2) the Fourier phase lags between hard (11.5-60 keV) and soft (2-4.3 keV) in the frequency range of 0.01-10 Hz change sign from negative to positive, (3) the coherence between hard and soft photons at low frequencies decreases, and (4) the relative amount of low-frequency power in hard photons compared to soft photons decreases. We discuss how these results reflect upon basic models from the literature describing the accretion flow around black holes and the possible connection between Comptonizing electrons and compact radio jets.

An Evolving Compact Jet in the Black Hole X-Ray Binary MAXI J1836–194

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 × 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.

Simultaneous multi-wavelength observations of GRS 1915+105

We present the result of multi-wavelength observations of the microquasar GRS 1915+105 in a plateau state with a luminosity of

EVOLUTION OF THE RADIO-X-RAY COUPLING THROUGHOUT AN ENTIRE OUTBURST OF AQUILA X-1

{\sim}7.5\times10^{38}

Radio and X-ray observations during the outburst decay of the black hole candidate XTE J1908+094

 erg s -1 (~40%

Kinematics of black hole X-ray binary GRS 1915+105

L_\mathrm{Edd}