Is the Lack of Pulsations in Low Mass X-Ray Binaries due to Comptonizing   Coronae?
  The spin periods of the neutron stars in most Low Mass X-ray Binary (LMXB) systems still remain undetected. One of the models to explain the absence of coherent pulsations has been the suppression of the beamed signal by Compton scattering of X-ray photons by electrons in a surrounding corona. We point out that simultaneously with wiping out the pulsation signal, such a corona will upscatter (pulsating or not) X-ray emission originating at and/or near the surface of the neutron star leading to appearance of a hard tail of Comptonized radiation in the source spectrum. We analyze the hard X-ray spectra of a selected set of LMXBs and demonstrate that the optical depth of the corona is not likely to be large enough to cause the pulsations to disappear. 
Ersin Gogus (Sabanci Univ. Istanbul), M. Ali Alpar (Sabanci Univ.   Istanbul), & Marat Gilfanov (MPA Garching, IKI Moscow)