Proton cyclotron features in thermal spectra of ultramagnetized neutron stars

A great deal of interest has been recently raised in connection with the po ssibility 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 sp ectra from ultramagnetized neutron stars for values of the luminosity and m agnetic field believed to be relevant to SGRs and AXPs. Emergent spectra ar e found to be very close to a blackbody at the star effective temperature a nd 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 feat ures (PCFs) are conspicuous (equivalent width of up to many hundreds eV) an d 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 H ETGS and METGS on board Chandra. Their observation might provide decisive e vidence in favor of the existence of magnetars.