Clockwise evolution in the hardness-intensity diagram of the black hole X-ray binary Swift J1910.2-0546

TL;DR

This study analyzes optical and UV data from the 2012 outburst of the black hole candidate Swift J1910.2-0546, revealing spectral state changes, jet formation, and a potential superhump modulation, suggesting it has the shortest orbital period among known black hole X-ray binaries.

Contribution

It provides the first detailed optical analysis of Swift J1910.2-0546's outburst, identifying state-dependent emission processes and a possible superhump, and estimates the system's orbital period and distance.

Findings

Optical brightening during the hard state transition due to jet onset.

Optical/UV emission dominated by X-ray irradiated disk in other states.

Potential superhump modulation indicating a 2.25-2.47 hr orbital period.

Abstract

We present a detailed study of optical data from the 2012 outburst of the candidate black hole X-ray binary Swift J1910.2-0546 using the Faulkes Telescope and Las Cumbres Observatory (LCO). We analyse the peculiar spectral state changes of Swift J1910.2-0546 in different energy bands, and characterise how the optical and UV emission correlates with the unusual spectral state evolution. Using various diagnostic tools like the optical/X-ray correlation and spectral energy distributions, we disentangle the different emission processes contributing towards the optical flux of the system. When Swift J1910.2-0546 transitions to the pure hard state, we find significant optical brightening of the source along with a dramatic change in the optical colour due to the onset of a jet during the spectral state transition. For the rest of the spectral states, the optical/UV emission is mostly dominated by an X-ray irradiated disk. From our high cadence optical study, we have discovered a putative modulation. Assuming that this modulation arises from a superhump, we suggest Swift J1910.2-0546 to have an orbital period of 2.25-2.47 hr, which would make it the shortest orbital period black hole X-ray binary known to date. Finally, from the state transition luminosity of the source, we find that the distance to the source is likely to be ~4.5-20.8 kpc, which is also supported by the comparative position of the source in the global optical/X-ray correlation of a large sample of black hole and neutron star X-ray binaries.