A potential cyclotron line signature in low-luminosity X-ray sources

Estimates indicate there may be $simgreat 10^3$ low luminosity X-ray pulsars ($L simless 10^{34} om{erg;s^{-1}}$) in the Galaxy undergoing ``low-state'' wind accretion in Be/X-ray binary systems, and $sim 10^8-10^9$ isolated neutron stars which may be accreting directly from the interstellar medium. Despite their low effective temperatures ($kT_e simless 300 om{eV}$), low luminosity accreting neutron stars with magnetic fields $B sim (0.7-7) imes 10^{12}$ G could emit a substantial fraction ($0.5 - 5%$) of their total luminosity in a moderately broadened ($E/Delta E sim 2-4$) cyclotron {it emission} line which peaks in the energy range $ sim 5-20 om{keV}$. The bulk of the thermal emission from these stars will be in the extreme ultraviolet/soft X-ray regime, which is subject to strong interstellar absorption and would be difficult to distinguish from spectra of other types of objects. In sharp contrast, the {it nonthermal} cyclotron component predicted here will not be strongly absorbed, and consequently it may be the only distinguishing signature for the bulk of these low luminosity sources. We propose a search for this cyclotron emission feature in long pointed observations of the newly discovered candidate isolated neutron star MS0317.7-6477, and the Be/X-ray transientComment: Accepted by ApJLett, 10 pages, uuencoded tar-compressed postscript, CITA-94-4