The radio/X-ray correlation in Swift J1753.5-0127

Abstract

Great effort has gone into trying to explain the two observed radio/X-ray correlation tracks seen in the low/hard state of black hole X-ray binaries in recent years. The original, "standard" correlation of the form LR ∝ [...], where b = 0.7 ± 0.1, is paired with a separate, lower correlation track with a steeper slope of ~ 1-1.4, at least at high luminosities. These outlier sources seem to show fainter radio emission than expected for a given X-ray luminosity, thus acquiring the term "radio-quiet". While most sources seem to maintain their intrinsic correlation slopes over decades in luminosity, a growing sample of sources have recently been reported to move from one correlation to the other. We present preliminary results from a coordinated radio/X-ray monitoring campaign of the radio-quiet black hole binary Swift J1753.5-0127, spanning nearly two years in time. Our observations add lower-luminosity coverage to an existing sample of observations, and we observe the radio-quiet track to proceed horizontally towards the standard correlation as the X-ray luminosity slowly starts to decrease. The source stays on the transition track for ~ 60 days, during which its X-ray luminosity is observed to drop by more than an order of magnitude while its radio luminosity stays constant. Time-averaged X-ray energy spectra show very little change during this phase, leaving no obvious parameters to explain the observed transition behaviour.