Detection of Strong Short-Term Variability in NGC 6946 X-1

TL;DR

This study reveals that NGC 6946 X-1 exhibits extreme short-term X-ray variability, with potential quasi-periodic oscillations, suggesting it hosts an intermediate-mass black hole based on its spectral and timing properties.

Contribution

First detection of strong short-term variability and QPOs in NGC 6946 X-1, providing insights into its accretion state and black hole mass estimation.

Findings

60% fractional rms variability in 1-100 mHz range

Power spectral density breaks at ~3 mHz

Possible QPOs near 8.5 mHz

Abstract

Using two archival XMM-Newton observations, we identify strong X-ray flux variations in NGC 6946 X-1 indicating it is the most variable ultraluminous X-ray source (ULX) on mHz time scales known so far. The 1-10 keV lightcurve exhibits variability with a fractional rms amplitude of 60% integrated in the frequency range of 1-100 mHz. The power spectral density of the source shows a flat-topped spectrum that breaks at about 3 mHz with possible quasi-periodic oscillations (QPOs) near 8.5 mHz. Black hole binaries usually produce strong fast variability in the hard or intermediate state. The energy spectrum of NGC 6946 X-1 is dominated by two components, a 0.18~keV thermal disk and a power law with a photon index of ~2.2, which is consistent with the intermediate state. The characteristic time scales of the X-ray emission suggests that the ULX may contain a black hole with a mass on the order of 10^3.