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 $\gamma$-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) contain {\em magnetars}, young neutron stars endowed with magnetic fields $\gtrsim 10^{14}$ 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 $E_{c,p}\simeq 0.63 (B/10^{14} {\rm G})$ keV. The proton cyclotron features (PCFs) are conspicuous (equivalent width of up to many hundreds eV) and relatively broad ($\Delta E/E \sim 0.05-0.2$). 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.