ON THE NATURE OF THE X-RAY PULSAR NEAR LYNDS 1457

We discuss the nature of H0253+193, the hard X-ray source which lies in the direction of the nearest known molecular cloud, Lynds 1457. The startling report of X-ray pulsations at a period of 206 s excludes all previously suggested models for the nature of this object, and forces us to consider an intrinsically unlikely scenario: the rotation of a magnetized white dwarf or neutron star, accidentally located within or behind the molecular cloud. We calculate that all models of this type have a low a priori probability, because the positions are so remarkably coincident (within 1′), and because the objects involved (white dwarfs, neutron stars, molecular clouds) are not sufficiently common. We reject a neutron star accreting in a close binary because its likelihood is extremely small (10-5), and because the observed position, luminosity, and spectrum of H0253+193 are all quite atypical for X-ray binaries. The other two explanations we consider remain viable. The probability of an isolated white dwarf or neutron star drifting accidentally into the cloud and accreting sufficient interstellar gas to power the X-ray source is low, but not absurdly so (∼0.1%-1%). The probability of a white dwarf accreting in a close binary, i.e., a cataclysmic variable, is only slightly higher, but this hypothesis is particularly attractive because the observed X-ray properties are in fact quite ordinary for a known class of cataclysmic variable, the "DQ Herculis stars." We suggest further optical, infrared, and X-ray observations which can test this hypothesis.