We have discovered a \sim 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 .
4U 0352+309 is a persistent low luminosity ( L _ { X } = 4.2 \times 10 ^ { 34 } { 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 , 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 \sim 29 keV .
The model that best fit the observations is a combination of a 1.45 \pm 0.02 keV blackbody with a 5.4 \times 10 ^ { 8 } cm ^ { 2 } area , and a power-law with a 1.83 \pm 0.03 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 ) \times 10 ^ { 12 } 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 .