Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993phdt........41g&link_type=abstract
PhD Dissertation, Columbia Univ. New York, NY United States
Astronomy and Astrophysics
Astrophysics
X Ray Stars, X Ray Astronomy, Spectral Emission, X Ray Binaries, Stellar Spectra, Astronomical Photometry, Power Spectra, Heao 2, Astrophysics, Gamma Ray Bursts
Scientific paper
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) is less than or equal to 25 km s-1 for orbital periods less than or equal to 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 less than or equal to 20 deg, and the mass of the compact star M, is greater than or equal to 1.4 M(solar). We have conducted a search for short timescale (approximately 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 approximately 1 s and have very soft spectra (with most counts below 0.7 keV). They are isotropically distributed in the sky and their approximately 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.
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