A Deep ROSAT HRI Observation of NGC 1313

We describe a series of observations of NGC 1313 using the ROSAT HRI with a combined exposure time of 183.5 ks. The observations span an interval between 1992 and 1998; the purpose of observations since 1994 was to monitor the X-ray flux of SN 1978K, one of several luminous sources in the galaxy. No diffuse emission is detected in the galaxy to a level of ~1-2×10³⁷ ergs s^-1 arcmin^-2. A total of eight sources are detected in the summed image within the D₂₅ diameter of the galaxy. The luminosities of five of the eight range from ~6×10³⁷ to ~6×10³⁸ ergs s^-1 these sources are most likely accreting X-ray binaries, similar to sources observed in M31 and M33. The remaining three sources all emit above 10³⁹ ergs s^-1. We present light curves of the five brightest sources. Variability is detected at the 99.9% level in four of these. We identify one of the sources as an NGC 1313 counterpart of a Galactic X-ray source. The light curve, though crudely sampled, most closely resembles that of a Galactic black hole candidate such as GX 339-4 but with considerably higher peak X-ray luminosity. An additional seven sources lie outside the D₂₅ diameter and are either foreground stars or background active galactic nuclei.