A deep radio survey of hard state and quiescent black hole X-ray binaries

TL;DR

This study conducted deep radio observations of black hole X-ray binaries in hard and quiescent states, detecting one hard-state system and setting stringent upper limits on quiescent emissions, questioning the universality of the radio/X-ray correlation.

Contribution

First deep radio survey of black hole X-ray binaries in quiescence with the Expanded VLA, providing the most stringent constraints on their radio emissions to date.

Findings

Detected Swift J1753.5-0127 at 0.5 mJy/beam in hard state.

No detections in three quiescent systems with noise levels as low as 2.6 microJy/beam.

Results challenge the universality of the radio/X-ray luminosity correlation in quiescence.

Abstract

We have conducted a deep radio survey of a sample of black hole X-ray binaries in the hard and quiescent states, to determine whether any systems were sufficiently bright for astrometric follow-up with high-sensitivity very long baseline interferometric (VLBI) arrays. The one hard-state system, Swift J1753.5-0127, was detected at a level of 0.5 mJy/beam. All eleven quiescent systems were not detected. In the three cases with the highest predicted quiescent radio brightnesses (GRO J0422+32, XTE J1118+480, and GRO J1655-40), the new capabilities of the Expanded Very Large Array were used to reach noise levels as low as 2.6 microJy/beam. None of the three sources were detected, to 3-sigma upper limits of 8.3, 7.8, and 14.2 microJy/beam, respectively. These observations represent the most stringent constraints to date on quiescent radio emission from black hole X-ray binaries. The uncertainties in the source distances, quiescent X-ray luminosities at the times of the observations, and in the power-law index of the empirical correlation between radio and X-ray luminosities, make it impossible to determine whether these three sources are significantly less luminous in the radio band than expected. Thus it is not clear whether that correlation holds all the way down to quiescence for all black hole X-ray binaries.