The Long-Term Variability of the X-Ray Sources in NGC 6946 and NGC   4485/4490
  We analyze data from five Chandra observations of the spiral galaxy NGC 6946 and from three Chandra observations of the irregular/spiral interacting galaxy pair NGC 4485/4490, with an emphasis on investigating the long-term variability exhibited by the source populations. We detect 90 point sources coincident with NGC 6946 down to luminosities of a few times 10^36 erg/s, and 38 sources coincident with NGC 4485/90 down to a luminosity of ~1x10^37 erg/s. Twenty-five (15) sources in NGC 6946 (NGC 4485/90) exhibit long-term (i.e., weeks to years) variability in luminosity; 11 (4) are transient candidates. The single ultraluminous X-ray source (ULX) in NGC 6946 and all but one of the eight ULXs in NGC 4485/90 exhibit long-term flux variability. Two of the ULXs in NGC 4485/90 have not been identified before as ultraluminous sources. The widespread variability in both systems is indicative of the populations being dominated by X-ray binaries, and this is supported by the X-ray colors of the sources. The distribution of colors among the sources indicates a large fraction of high-mass X-ray binaries in both systems. The shapes of the X-ray luminosity functions of the galaxies do not change significantly between observations and can be described by power laws with cumulative slopes ~0.6-0.7 (NGC 6946) and ~0.4 (NGC 4485/90). 
Joel K. Fridriksson, Jeroen Homan, Walter H. G. Lewin, Albert K. H.   Kong and David Pooley