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We analyze data from five Chandra observations of the spiral galaxy NGC 6946and from three Chandra observations of the irregular/spiral interacting galaxypair NGC 4485/4490, with an emphasis on investigating the long-term variabilityexhibited by the source populations. We detect 90 point sources coincident withNGC 6946 down to luminosities of a few times 10^36 erg/s, and 38 sourcescoincident 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 singleultraluminous X-ray source (ULX) in NGC 6946 and all but one of the eight ULXsin NGC 4485/90 exhibit long-term flux variability. Two of the ULXs in NGC4485/90 have not been identified before as ultraluminous sources. Thewidespread variability in both systems is indicative of the populations beingdominated by X-ray binaries, and this is supported by the X-ray colors of thesources. The distribution of colors among the sources indicates a largefraction of high-mass X-ray binaries in both systems. The shapes of the X-rayluminosity functions of the galaxies do not change significantly betweenobservations and can be described by power laws with cumulative slopes ~0.6-0.7(NGC 6946) and ~0.4 (NGC 4485/90).

The Long-Term Variability of the X-Ray Sources in NGC 6946 and NGC 4485/4490