A potential cyclotron line signature in low luminosity x-ray pulsars

Abstract

Simple estimates indicate there should 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), we predict the low luminosity accreting neutron stars with magnetic fields B∼(0.7−7)×1012 G should emit a substantial fraction (0.5–5%) of their total luminosity in a narrow (E/ΔE∼2–4) cyclotron emission line which peaks in the energy range ∼5–20 keV. In sharp contrast to the underlying thermal emission, this nonthermal cyclotron component will not be strongly absorbed by the intervening H i gas, and consequently it may be the only observable signature for the bulk of these low luminosity sources. We propose a search for this cyclotron emission feature in long pointed observations by ASCA of the Be/x‐ray transient pulsars V0331+53 and 4U0115+63 in their quiescent ‘‘low state’’.