Parallel tracks in infrared versus X-ray emission in black hole X-ray transient outbursts: a hysteresis effect?

TL;DR

This study uncovers a hysteresis effect in black hole X-ray binaries where near-infrared emission differs during the rise and decline of outbursts at similar X-ray luminosities, indicating complex jet and accretion dynamics.

Contribution

It reveals a new hysteresis effect in NIR versus X-ray flux in black hole transients and analyzes the differing correlations during outburst phases.

Findings

NIR emission is weaker during the low/hard state rise than decline at the same X-ray luminosity.

A correlation between NIR and X-ray luminosities exists with different slopes in rise and decline phases.

In the high/soft state, NIR emission is likely dominated by viscously heated accretion discs.

Abstract

We report the discovery of a new hysteresis effect in black hole X-ray binary state transitions, that of the near-infrared (NIR) flux (which most likely originates in the jets) versus X-ray flux. We find, looking at existing data sets, that the infrared emission of black hole X-ray transients appears to be weaker in the low/hard state rise of an outburst than the low/hard state decline of an outburst at a given X-ray luminosity. We discuss how this effect may be caused by a shift in the radiative efficiency of the inflowing or outflowing matter, or variations in the disc viscosity or the spectrum/power of the jet. In addition we show that there is a correlation (in slope but not in normalisation) between infrared and X-ray luminosities on the rise and decline, for all three low-mass black hole X-ray binaries with well-sampled infrared and X-ray coverage: L_NIR propto L_x^(0.5-0.7). In the high/soft state this slope is much shallower; L_NIR propto Lx^(0.1-0.2), and we find that the NIR emission in this state is most likely dominated by the viscously heated (as opposed to X-ray heated) accretion disc in all three sources.