Abstract
New observations of 4U 1907+09 with the Rossi X-Ray Timing Explorer show a kind of behavior never recognized before from this X-ray pulsar: intermittently, the X-ray intensity fades below the detection threshold during intervals ranging from a few minutes to perhaps 1.5 hours. The largest decrease of the 2-15 keV intensity is at least 98%. The observations span 70% of the orbit of the binary system of which the pulsar is a part. The dips were found to have a broad range of orbital phases. Given the strong variability of the source outside the dip periods with moderate changes in the spectrum, the occasional variable weak emission during dip periods with timescales and spectrum similar to that outside the dip periods, and the absence of strong enhancements in the column density of cold circumstellar matter (i.e., N-H remains below 10(23) Cm-2), it is suggested that the dips are caused by cessations of the mass accretion by the neutron star rather than occultations by circumstellar gas. An inhomogeneous wind from the companion star may be responsible.