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 lati tude of these fields (\b\ < 1 degrees) we conclude, on probability and spectral grounds, that the detected sources are intrinsic to these ga laxies rather than foreground or background interlopers. The Dw1 sourc e, designated Dw1-X1, is interpreted as being either a hyperluminous X -ray binary (with a 0.5-10 keV luminosity of more than 10(39) 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. P rompted by the Dw1-X1 results, we discuss the nature of hyperluminous X-ray binary systems. Such sources are commonly seen in nearby galaxie s with a frequency of approximately one per galaxy. We present a possi ble connection between these luminous systems and Galactic superlumina l sources.