The long-term variability of the X-ray sources in M82

We investigate the long-term variability exhibited by the X-ray point sources in the starburst galaxy M82. By combining nine Chandra observations taken between 1999 and 2007, we detect 58 X-ray point sources within the D₂₅ isophote of M82 down to a luminosity of ∼10³⁷ erg s^-1. Of these 58 sources, we identify three supernova remnant candidates and one supersoft source. 26 sources in M82 exhibit long-term (i.e. days to years) flux variability and three show long-term spectral variability. Furthermore, we classify 26 sources as variables and 10 as persistent sources. Among the total 26 variables, 17 varied by a flux ratio of >3 and six are transient candidates. By comparing with other nearby galaxies, M82 shows extremely strong long-term X-ray variability that 47 per cent of the X-ray sources are variables with a flux ratio of >3. The strong X-ray variability of M82 suggests that the population is dominated by X-ray binaries.