view Abstract Citations (60) References (69) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Nonthermal Cyclotron Emission from Low-Luminosity Accretion onto Magnetic Neutron Stars Nelson, Robert W. ; Salpeter, E. E. ; Wasserman, Ira Abstract The ROSAT, Astro D, and AXAF imaging surveys could detect large numbers of low-luminosity X-ray pulsars (L ≲ 1034 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 Te ∼ 100(L/1032 ergs s-1)1/4(Acap/1012 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 (1012 G ≲ B < 1013 G), ∼10% or more of the total accretion luminosity should be emitted in hard, nonthermal X-rays just below the magnetic cyclotron energy EB = 11.6B12 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 EB, most of the accretion energy goes into 0 → n electron Landau excitations (nmax ≡ meυ2ff/2EB = 9B-112M1.4r-16), 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 free4r& 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 (EB ≫ Te). Publication: The Astrophysical Journal Pub Date: December 1993 DOI: 10.1086/173445 Bibcode: 1993ApJ...418..874N Keywords: ACCRETION; ACCRETION DISKS; RADIATION MECHANISMS: MISCELLANEOUS; STARS: MAGNETIC FIELDS; STARS: NEUTRON; X-RAYS: STARS full text sources ADS | data products SIMBAD (1)