Proton Cyclotron Features in Thermal Spectra of Ultra-magnetized Neutron Stars

A great deal of interest has been recently raised in connection with the possibility that soft γ-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) contain magnetars, young neutron stars endowed with magnetic fields ∼> 10 14 G. In this paper we calculate thermal spectra from ultra-magnetized neutron stars for values of the luminosity and magnetic field believed to be relevant to SGRs and AXPs. Emergent spectra are found to be very close to a blackbody at the star effective temperature and exhibit a distinctive absorption feature at the proton cyclotron energy Ec,p ≃ 0.63(B/10 14 G) keV. The proton cyclotron features (PCFs) are conspicuous (equivalent width of up to many hundreds eV) and relatively broad (relative width of ∼ 0.05 − 0.2). The detection of the PCFs is well within the capabilities of present X-ray spectrometers, like the HETGS and METGS on board Chandra. Their observation might provide decisive evidence in favor of the existence of magnetars. Subject headings: radiative transfer — stars: neutron — X-ray: stars