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

• Published in: The Astrophysical journal. Supplement series Vol. 177; no. 2; pp. 465 - 492
• Main Authors: Fridriksson, Joel K, Homan, Jeroen, Lewin, Walter H. G, Kong, Albert K. H, Pooley, David
• Format: Journal Article
• Language: English
• Published: IOP Publishing 01.08.2008
• ISSN: 0067-0049; 1538-4365
• DOI: 10.1086/588817

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 super(36) ergs s super(-1), and 38 sources coincident with NGC 4485/90 down to a luminosity of [image]1 x 10 super(37) ergs s super(-1). 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 [image]0.6-0.7 (NGC 6946) and [image]0.4 (NGC 4485/90).