Proton cyclotron features in thermal spectra of ultramagnetized neutron stars

A great deal of interest has been recently raised in connection with the possibility that soft gamma -ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) contain magnetars, young neutron stars endowed with magnetic fields greater than or similar to 10(14) G. In this paper we calculate thermal spectra from ultramagnetized 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 E-c,E-p similar or equal to 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 (DeltaE/E similar to0.05-0.2). 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.