On the enigmatic X-ray source V1408 Aquilae (=4U 1957+11)

Models for the characteristically soft X-ray spectrum of the compact X-ray source V1408 Aql (=4U 1957+11) have ranged from optically thick Comptonization to multicolor accretion disk models. We critically examine the X-ray spectrum of V1408 Aql via archival Advanced Satellite for Cosmology and Astrophysics (ASCA) data, archival Röntgensatellit data, and recent Rossi X-Ray Timing Explorer (RXTE) data. Although we are able to fit a variety of X-ray spectral models to these data, we favor an interpretation of the X-ray spectrum as being due to an accretion disk viewed at large inclination angles. Evidence for this hypothesis includes long-term (117, 235, 352 day) periodicities seen by the RXTE All Sky Monitor, which we interpret as being due to a warped precessing disk, and a 1 keV feature in the ASCA data, which we interpret as being the blend of L fluorescence features from a disk atmosphere or wind. We also present a timing analysis of the RXTE data and find upper limits of 4% for the rms variability between f = 10^-3 and 16 Hz. The situation of whether the compact object is a black hole or neutron star is still ambiguous; however, it now seems more likely that an X-ray-emitting, warped accretion disk is an important component of this system.