Abstract
We investigate the long-term variability exhibited by the X-ray point sources in the starburst galaxy M82. By combining 9 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 ergs s-1. Of these 58 sources, we identify 3 supernova remnant candidates and one supersoft source. Twenty-nine sources in M82 exhibit long-term (i.e., days to years) flux variability and 3 show long-term spectral variability. Furthermore, we classify 29 sources as variables and 8 as persistent sources. Among the total 29 variables, 20 varied by a flux ratio of > 3 and 6 are transient candidates. By comparing with other nearby galaxies, M82 shows extremely strong long-term X-ray variability that 54% 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. Besides, one of the X-ray supernovae we found, SN 2004am, shows rising X-ray flux for several years after explosion. There are only two supernovae, SN 1996cr and SN 1987A, which have similar long-term X-ray light curve. By comparing SN 2004am with SN 1996cr and SN 1987A, we might be able to gain valuable information about the circumstellar matter density structure. Hence, confirming the nature of SN 2004am will add an important sample in studying late-time behaviors of Type II supernovae. New follow-up Chandra observations would be useful to constrain the nature. Therefore, we present a proposed Chandra observation to monitor SN 2004am.