Transient X-Ray Phenomena in Astrophysics

An extensive study has been made of the optical counterpart of the X-ray transient GS2023 + 338 (V404 Cygni) during the decline from a major outburst. Furthermore, the results from a comprehensive search for heretofore undiscovered faint X-ray transients is reported. High-speed photometry of V404 Cygni acquired in 1989 September confirms low frequency flickering previously reported at X-ray, optical, and radio wavelengths. No evidences found in the light curve for a unique coherent period. However, transient broad features are observed in the power spectra at periods in the range of 3-10 minutes. The spectral evolution of V404 Cygni is followed over the period from 1989 July to 1990 May. The spectrum is also found to be characteristic of emission from an accretion disk in a close binary system. V404 Cyg displays a smooth continuum, lacking in stellar absorption features, with broad, double -peaked Balmer, He, and Fe emission lines whose velocity widths increase with time. Time resolved spectroscopy sets an upper limit on the semiamplitude of the projected velocity variation of the compact object of K_1 _sp{~}< 25 km s^{-1} for orbital periods <=6 hours. A classical mass determination is not possible, but if the emission lines originate in the outer disk, as their profiles suggest, then for the most likely range of the orbital period (4 -6 hours), the binary inclination i is _sp {~}<20^ circ, and the mass of the compact star M _1 _sp{~}> 1.4 M_odot. We have conducted a search for short timescale (~10 sec) faint X-ray flashes in the complete 230,000,000 count Einstein Observatory Imaging Proportional Counter (IPC) database. Forty-two events were detected which are consistent with the instrumental response function and not associated with any detector or spacecraft anomalies. Most of the events have rise times of ~1 s and have very soft spectra (with most counts below 0.7 keV). They are isotropically distributed in the sky and their ~2 ^' positions are not coincident with any catalogued class of objects. We present the results of a number of tests we have performed to test the astrophysical origin of these events and to characterize their properties. If we associate these events with gamma-ray bursts, they provide important constraints on the space density and repetition rate of the underlying source population.