Ultraluminous X-ray sources in seven edge-on spiral galaxies

We investigate a sample of seven edge-on spiral galaxies using Chandra observations. Edge-on spiral galaxies allow us to clearly separate source associated with their star-forming regions versus the outer edges of the system, offering a clear advantage over other systems. We uncover a number of X-ray point sources across these galaxies, and after eliminating contaminating foreground and background sources, we identify 12 candidate ultraluminous X-ray sources (ULXs). All of these sources are projected on to the central regions, implying that the majority of ULXs in this sample of spiral galaxies are disc/bulge, and thus not halo sources. This also includes two transient ULXs, which may be long-duration transients and low-mass X-ray binaries. This finding illustrates the need for further studies of transient ULXs.