ASCA observations of the nearby galaxies Dwingeloo 1 and Maffei 1

We present ASCA observations of the nearby galaxies Dwingeloo 1 (Dw1) and Maffei 1 (Mf1). X-ray sources are clearly detected within 3 arcmin of the optical nuclei of both galaxies. Despite the low galactic latitude of these fields (|b| < 1°) we conclude, on probability and spectral grounds, that the detected sources are intrinsic to these galaxies rather than foreground or background interlopers. The Dw1 source, designated Dw1-X1, is interpreted as being either a hyperluminous X-ray binary (with a 0.5-10 keV luminosity of more than 10³⁹ erg s^-1) or an X-ray-bright supernova. The Mf1 emission is hard and extended, suggesting that it originates from a population of X-ray binaries. Prompted by the Dw1-X1 results, we discuss the nature of hyperluminous X-ray binary systems. Such sources are commonly seen in nearby galaxies with a frequency of approximately one per galaxy. We present a possible connection between these luminous systems and Galactic superluminal sources.