THE NATURE OF THE COMPACT X-RAY SOURCE IN THE SUPERNOVA REMNANT G27.4+0.0

High-resolution X-ray imaging data obtained with ROSAT is used to cons train the nature of the central compact source in the supernova remnan t G27.4 + 0.0. Diffuse emission is seen from throughout the approximat ely 4' diameter radio shell, while the central source remains unresolv ed at approximately 3''. We combine archival data from the Einstein HR I, IPC, and MPC with the ROSAT HRI data to define the X-ray spectra of the diffuse and point-like emission. The bulk of the shell radiation is consistent with that of a approximately 10(7) K plasma, although a higher temperature component is also suggested by the data; coupled wi th the remnant's size and distance, we derive an age of between 500 an d 2500 yr. The point source has a substantially harder spectrum, with a power-law photon index less-than-or-similar-to 1. A search for perio dic modulation from the point source yields upper limits ranging from 10%-35% for periods between 0.025 and 1000 s, depending on the assumed pulse shape. No aperiodic variability on timescales of from 10(3) to 10(8) s is required, although a factor of approximately 2 change betwe en the Einstein and ROSAT eras is possible. We show that the point sou rce cannot represent thermal emission from the surface of a young neut ron star and is unlikely to be explained as nonthermal, Crab-like X-ra y pulses or a small synchrotron nebula. The most likely models involve accretion-powered systems-either a wind-fed neutron star with a massi ve companion or a low-mass X-ray binary. In all probability, this is t he youngest X-ray binary in the Galaxy.