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

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 ∝ LbX, 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. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)