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[22.08] Discovery of a Cyclotron Line in the Spectrum of 4U~0352+309 (X~Per)

W. Coburn, W. A. Heindl, D. E. Gruber, R. E. Rothschild (UCSD), R. Staubert, J. Wilms, I. Kreykenbohm (IAAT)

We report on the discovery of a ~0\,keV Cyclotron Resonance Scattering Feature (CRSF) in the X-ray spectrum of 4U~0352+309 (X~Per). This is the result of a broad band spectral analysis using observations taken with the Rossi X-Ray Timing Explorer. 4U~0352+309 is a persistent low luminosity (L_X = 4.2\times10³⁴\,\mathrm{ergs\,s^-1}) X-ray pulsar, with a 837\,s period and which accretes material from the Be star X~Per. The X-Ray spectrum, unique among 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 ~0\,keV. The simplest, most physically reasonable model that fit the observations is a combination of a 1.45±.02\,keV blackbody with a 5.4\times10⁸\,\rm{cm}² area, and an unbroken power-law with a 1.83±.03 photon index modified by the CRSF. In these fits the CRSF energy is 28.6_-1.7^+1.5\,keV, implying a magnetic field strength of 2.5(1+z)\times10¹²\,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.