Nonthermal Cyclotron Emission from Low-Luminosity Accretion onto Magnetic Neutron Stars

Details Nonthermal Cyclotron Emission from Low-Luminosity Accretion onto Magnetic Neutron Stars Nonthermal Cyclotron Emission from Low-Luminosity Accretion onto Magnetic Neutron Stars

Dec 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993apj...418..874n&link_type=abstract

Astrophysical Journal v.418, p.874

Astronomy and Astrophysics

Astronomy

33

Accretion, Accretion Disks, Radiation Mechanisms: Miscellaneous, Stars: Magnetic Fields, Stars: Neutron, X-Rays: Stars

Scientific paper

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).

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