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

Estimates indicate there may be 10³ low luminosity X-ray pulsars (L 10³⁴ erg\;s⁻¹) in the Galaxy undergoing ``low-state'' wind accretion in Be/X-ray binary systems, and ~ 10⁸-10⁹ isolated neutron stars which may be accreting directly from the interstellar medium. Despite their low effective temperatures (kTₑ 300 eV), low luminosity accreting neutron stars with magnetic fields B ~ (0.7-7) × 10¹² G could emit a substantial fraction ($0.5 - 5%$) of their total luminosity in a moderately broadened (E/Δ E ~ 2-4) cyclotron { emission} line which peaks in the energy range ~ 5-20keV. 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 MS0317.7-6477, and the Be/X-ray transient