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We have discovered a ~29 keV Cyclotron Resonant Scattering Feature (CRSF) inthe X-Ray spectrum of 4U 0352+309 (X Per) using observations taken with theRossi X-Ray Timing Explorer. 4U 0352+309 is a persistent low luminosity (L_x =4.2x10^34 ergs/s) X-ray pulsar, with a 837 s period and which accretes materialfrom the Be star X Per. The X-Ray spectrum, unusual when compared to brighteraccreting pulsars, may be due to the low mass accretion rate and could betypical of the new class of persistent low luminosity Be/X-Ray binary pulsars.We attempted spectral fits with continuum models used historically for 4U0352+309, and found that all were improved by the addition of a CRSF at ~29keV. The model that best fit the observations is a combination of a 1.45+/-0.02keV blackbody with a 5.4x10^8 cm^2 area, and a power-law with a 1.83+/-0.03photon index modified by the CRSF. In these fits the CRSF energy is28.6+1.5-1.7 keV, implying a magnetic field strength of 2.5(1+z)x10^12 G in thescattering region (where z is the gravitational redshift). Phase resolvedanalysis shows that the blackbody and cyclotron line energies are consistentwith being constant through the pulse.Comment: 34 pages, 6 figures, 3 tables, Accepted by Ap

Discovery of a Cyclotron Resonant Scattering Feature in theRossi X‐Ray Timing ExplorerSpectrum of 4U 0352+309 (X Persei)