The long-term variability of a population of ULXs monitored by Chandra

TL;DR

This study monitors ULX populations in three galaxies over a year, revealing diverse long-term variability behaviors, including persistent, variable, and transient sources, with some showing high-amplitude changes and eclipses.

Contribution

First long-term Chandra monitoring of ULX populations in multiple galaxies, uncovering diverse variability patterns and transient behaviors.

Findings

ULX populations exhibit various long-term variability behaviors.

Some ULXs show high-amplitude variability without clear periodicity.

Detection of additional eclipses in two ULXs.

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.