RE J1034+396: The origin of the soft X-ray excess and QPO

TL;DR

This paper investigates the origin of the soft X-ray excess and QPO in RE J1034+396, proposing that the soft excess arises from low temperature Comptonization and that the QPO originates in the power law tail, linking these features to super-Eddington accretion.

Contribution

It demonstrates that the soft X-ray excess can be modeled by low temperature Comptonization and links the QPO to variability in the power law tail, suggesting a connection to super-Eddington accretion.

Findings

Soft excess fits models of low temperature Comptonization.

QPO variability is dominated by the power law tail.

Super-Eddington accretion may trigger the QPO.

Abstract

The X-ray quasi-periodic oscillation (QPO) seen in RE J1034+396 is so far unique amongst AGN. Here we look at the another unique feature of RE J1034+396, namely its huge soft X-ray excess, to see if this is related in any way to the detection of the QPO. We show that all potential models considered for the soft energy excess can fit the 0.3-10 keV X-ray spectrum, but that the energy dependence of the rapid variability (which is dominated by the QPO) strongly supports a spectral decomposition where the soft excess is from low temperature Comptonization of the disc emission and remains mostly constant, while the rapid variability is produced by the power law tail changing in normalization. The presence of the QPO in the tail rather than in the disc is a common feature in black hole binaries, but low temperature Comptonization of the disc spectrum is not generally seen in these systems. The main exception to this is GRS 1915+105, the only black hole binary which routinely shows super-Eddington luminosities. We speculate that super-Eddington accretion rates lead to a change in disc structure, and that this also triggers the X-ray QPO.