The Long-Term Variability of a Population of ULXs Monitored by Chandra

Abstract

We present preliminary results of a Chandra Large Program to monitor the ultraluminous x‐ray source (ULX) populations of three nearby, ULX‐rich galaxies over the course of a year, finding the ULX population to show a variety of long‐term variability behaviours. Of a sample of 36 ULXs, some show persistent or moderately variable flux, often with a significant relationship between hardness and luminosity, consistent with a supercritically accreting source with varying accretion rates. Six show very high‐amplitude variability with no strong relationship between luminosity and hardness, though not all of them show evidence of any long‐term periodicity, nor of the bimodal distribution indicative of the propeller effect. We find evidence of additional eclipses for two previously‐identified eclipsing ULXs. Additionally, many sources that were previously identified as ULXs in previous studies were not detected at ULX luminosities during our monitoring campaign, indicating a large number of transient ULXs.

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