Evidence for quiescent synchrotron emission in the black hole X-ray transient Swift J1357.2-0933

TL;DR

This study presents high time-resolution optical and infrared observations of the black hole X-ray transient Swift J1357.2-0933 in quiescence, revealing dominant non-thermal synchrotron emission likely from a weak jet, with variability and flares distinct from other systems.

Contribution

It provides the first detailed characterization of the quiescent optical and infrared variability and spectral energy distribution of Swift J1357.2-0933, highlighting non-thermal synchrotron emission from a jet.

Findings

Optical and infrared variability dominated by flares and dips.

Spectral energy distribution consistent with optically thin synchrotron emission.

Distance estimated between 0.5 and 6.3 kpc based on secondary star analysis.

Abstract

We present high time-resolution ULTRACAM optical and NOTCam infrared observations of the edge-on black hole X-ray transient Swift J1357.2-0933. Our data taken in 2012 and 2013 show the system to be at its pre-outburst magnitude and so the system is in quiescence. In contrast to other X-ray transients, the quiescent light curves of Swift J1357.2-0933 do not show the secondary star's ellipsoidal modulation. The optical light curve is dominated by variability with an optical fractional rms of ~35 per cent, a factor of >3 larger than what is observed in other systems at similar time-resolution. Optical flare events lasting 2-10min with amplitudes of up to ~1.5 mag are seen as well as numerous rapid ~0.8 mag dip events which are similar to the optical dips seen in outburst. Similarly the infrared J-band light curve is dominated by variability with a fractional rms of ~21 per cent and flare events lasting 10--30 min with amplitudes of up to ~1.5 mag are observed. The quiescent optical to mid-infrared spectral energy distribution in quiescence is dominated by a non-thermal component with a power--law index of -1.4, (the broad-band rms SED has a similar index) which arises from optically thin synchrotron emission most likely originating in a weak jet; the lack of a peak in the spectral energy distribution rules out advection-dominated models. Using the outburst amplitude--period relation for X-ray transients we estimate the quiescent magnitude of the secondary star to lie in the range V_min=22.7 to 25.6, which when combined with the absolute magnitude of the expected M4.5 V secondary star allows us to constrain to the distance to lie in the range 0.5 to 6.3 kpc. (Abridged)