Very Low Frequency QPO in 4U1700-37
  Very low frequency ({\nu} ~ 1.9 mHz) quasi-periodic oscillations (QPO) have been detected in 2 - 60 keV X-rays from 4U1700-37 using auto-correlation function analysis of RXTE archival data. No pulsar signal was detected in the data. Less than 0.5% of the flux from the source (3{\sigma} upper limit) is coherently pulsed with a period between 250 ms and 40 s. This upper limit corresponds to a pulsed flux <0.02 mJy. The existence of QPO in 4U1700-37 and the lack of a detected pulsar indicates that the X-ray source is more likely to be a black hole than a neutron star, consistent with recent mass estimates. 
J. F. Dolan
astro-ph.HE