The kilo-second variability of X-ray sources in nearby galaxies

TL;DR

This study analyzes the short-term X-ray variability of sources in nearby galaxies, finding that most ultra-luminous X-ray sources are not highly variable on kilosecond timescales, with some exceptions indicating different accretion behaviors.

Contribution

It provides the first systematic variability analysis of bright X-ray sources in nearby galaxies, highlighting the low variability of ULXs and potential links to accretion disk instabilities.

Findings

Most ULXs show less than 10% variability on ksec timescales.

Some ULXs, especially ultra-soft ones, exhibit significant variability.

Variability is not strongly correlated with spectral hardness.

Abstract

Chandra observations of 17 nearby galaxies were analysed and 166 bright sources with X-ray counts > 100, were chosen for temporal analysis. Fractional root mean square variability amplitudes were estimated for lightcurves binned at ~ 4 ksec and of length ~ 40 ksec. While there are nine ultra-luminous X-ray sources (ULXs) with unabsorbed luminosity (in 0.3-8.0 keV band) L > 10^39 erg/s in the sample for which the fractional r.m.s variability is constrained to be < 10%, only two of them show variability. One of the variable ULXs exhibits a secular transition and has a ultra-soft spectrum with temperature ~ 0.3 keV while the other is a rapidly varying source in NGC 0628, which has been previously compared to the Galactic micro-quasar GRS1915+105. These results seem to indicate that ULXs are typically not highly variable in ksec time-scales, except for some ultra-soft ones. Among the relatively low luminosity sources (L ~ 10^38 erg/s) we find five of them to be variable. Apart from an earlier known source in NGC 1569, we identify a source in NGC 2403, which exhibits persistent high amplitude fluctuations.The variability of the sources in general, do not seem to be correlated with hardness,which indicates that they may not be due to variations in any absorbing material, but instead could reflect inner accretion disk instabilities.