NONTHERMAL CYCLOTRON EMISSION FROM LOW-LUMINOSITY ACCRETION ONTO MAGNETIC NEUTRON-STARS

The ROSAT, Astro D, and AXAF imaging surveys could detect large numbers of low-luminosity X-ray pulsars (L less than or similar 10(34) ergs s-1), undergoing ''low-state'' wind accretion in Be/X-ray transient systems, or possibly isolated neutron stars accreting directly from the interstellar medium. If these pulsars were purely thermal emitters with polar cap temperatures T(e) approximately 100(L/10(32) ergs s-1)1/4(A(cap)/10(12) cm2)-1/4 eV, only nearby sources could be detected because of strong UV absorption by the intervening H I gas. Here we show that if low-luminosity accretion occurs onto magnetic neutron stars (10(12) G less than or similar B < 10(13) G), approximately 10% or more of the total accretion luminosity should be emitted in hard, nonthermal X-rays just below the magnetic cyclotron energy E(B) = 11.6B(12) keV, a spectral regime that is unaffected by interstellar absorption. We calculate the energy deposition and production of collisionally induced cyclotron photons by ionized hydrogen gas accreting into a highly magnetized neutron star atmosphere at a rate well below the effective Eddington limit. When the available free-fall collision energy is much larger than E(B), most of the accretion energy goes into 0 --> n electron Landau excitations (n(max) = m(e)v(ff)2/2E(B) = 9B-12(-1)M1.4r6(-1)), which then radiatively decay to produce a highly nonthermal source of cyclotron photons. We show that a significant fraction of these cyclotron photons will escape the atmosphere without being thermalized by free-free absorption, leading to a nonthermal and partially Comptonized cyclotron component, possibly a broad emission line feature, superposed on the Wien tail of a much lower energy thermal spectrum (E(B) much greater than T(e)).