The QPO in REJ1034+396 originates in the hot corona

TL;DR

This study shows that the quasi-periodic oscillation in REJ1034+396 originates from the hot corona, with the variability driven by coronal fluctuations and the disk responding to coronal emission.

Contribution

It provides the first clear evidence that the QPO in REJ1034+396 is produced in the corona, clarifying the origin of QPOs in active galactic nuclei.

Findings

QPO detected mainly in the coronal (hard) X-ray band.

Variability at QPO frequencies is due to coronal fluctuations.

The disk responds to coronal emission with a ~2000 s soft lag.

Abstract

REJ1034+396 is one of the few active galactic nuclei with a significant quasi-periodic oscillation (QPO). The QPO has been observed in over 1 Ms of XMM-Newton observations spanning over a decade. We investigate the power spectral density function (PSD) of 7 long (~90 ks) XMM-Newton observations of the active galactic nucleus REJ1034+396 in two energy bands. The soft (0.3-0.5 keV) band targets emission from the disk, while the hard (2-7 keV) band isolates the primary X-ray continuum emission from the corona. The QPO is significantly detected in the hard band of 5 of the 7 observations. The best fitting models indicate that the QPO detection in both bands is entirely attributable to the coronal emission with no additional contribution from the disk. This explains the strong coherence between the hard and soft bands at the QPO frequency. The covariance spectrum is consistent with this picture as the variability at QPO frequencies is attributed solely to fluctuations in the hot corona. The time lag as a function of energy is well described by a ~2000 s intrinsic soft lag, resulting from the disk responding to emission from the corona, that undergoes phase wrapping at approximately the QPO frequency. By demonstrating that in this system the QPO arises in the corona, we provide new insights into the mechanisms generating QPOs.