2001ApJ...552..738C

We have discovered a ∼29 keV cyclotron resonance scattering feature (CRSF) in the X-ray spectrum of 4U 0352+309 (X Per) using observations taken with the Rossi X-Ray Timing Explorer. The source 4U 0352+309 is a persistent low-luminosity (L_X=4.2x10³⁴ ergs.s^–1) X-ray pulsar with a 837 s period, which accretes material from the Be star X Per. The X-ray spectrum, unusual when compared to brighter accreting pulsars, may be due to the low mass accretion rate and could be typical of the new class of persistent low-luminosity Be/X-ray binary pulsars. We attempted spectral fits with continuum models used historically for 4U 0352+309 and found that all were improved by the addition of a CRSF at ∼29 keV. The model that best fitted the observations is a combination of a 1.45±0.02 keV blackbody with a 5.4x10⁸ cm² area and a power law with a 1.83±0.03 photon index modified by the CRSF. In these fits the CRSF energy is 28.6^+1.5_–1.7 keV, implying a magnetic field strength of 2.5(1+z)x10¹² G in the scattering region (where z is the gravitational redshift). Phase-resolved analysis shows that the blackbody and cyclotron line energies are consistent with being constant through the pulse.