Vol 13, No 12 (2013) / Mandal

The kilosecond variability of X-ray sources in nearby galaxies

Soma Mandal, Ranjeev Misra, Gulab C. Dewangan

Abstract

Chandra observations of 17 nearby galaxies were analyzed and 166 bright sources with X-ray counts > 100 were chosen for temporal analysis. Fractional root mean square (rms) variability amplitudes were estimated for light curves, binned at ~ 4 kilosecond (ks), with length ~ 40 ks. While there are nine ultra-luminous X- ray sources (ULXs) with unabsorbed luminosity (in the 0.3–8.0 keV band) L > 1039 erg s−1 in the sample for which the fractional rms variability is constrained to be < 10%, only two of them show variability. One of the variable ULXs exhibits a secular transition and has an ultra-soft spectrum with temperature ~ 0.3 keV while the other is a rapidly varying source in NGC 0628, which has previously been compared to the Galactic microquasar GRS 1915+105. These results seem to indicate that ULXs are typically not highly variable on ks timescales, except for some ultra-soft ones. Among the relatively low luminosity sources (L ~ 1038 erg s−1), we find five of them to be variable. Apart from an earlier known source in NGC 1569, we identify a source in NGC 2403 that exhibits persistent high amplitude fluctuations. In general, the variability of the sources does 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 instabilities in the inner accretion disk.

Keywords


galaxies: general — X-rays: binaries

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