A POTENTIAL CYCLOTRON LINE SIGNATURE IN LOW-LUMINOSITY X-RAY SOURCES

摘要

Estimates indicate there may be approx > 10~3 low-luminosity X-ray pulsars (L approx < 10~(34) ergs s~(-1)) in the Galaxy undergoing "low-state" wind accretion in Be/X-ray binary systems, and ～10~8-10~9 isolated neutron stars which may be accreting directly from the interstellar medium. Despite their low effective temperatures (kT_e approx < 300 eV), low-luminosity accreting neutron stars with magnetic fields B ～ (0.7-7) x 1012 G could emit a substantial fraction (0.5%-5%) of their total luminosity in a moderately broadened (E/ΔE ～ 2-4) cyclotron emission line which peaks in the energy range ～5-20 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 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 MS 0317.7—6477, and the Be/X-ray transient pulsar 4U 0115 + 63 in its quiescent state. We note that an emission-like feature similar to the one we predict here has been reported in the energy spectrum of the unusual X-ray pulsar 1E 2259 + 586.