A Survey of Fast X-Ray Transients Using the HEAO A-1 Sky Survey Experiment.
Abstract
Data from the first complete pass of the sky by the Sky Survey experiment on the HEAO 1 X-ray satellite (launched 12 August 1977) were systematically searched for fast X-ray transient events of less than a few hours duration. Ten fast transients were found, ranging in intensity from 8 x 10('-11) to 4 x 10('-9) ergs cm('-2) s('-1). Properties investigated include variability characteristics and timescales, presence or absence of quiescent flux, spectral hardness, identifications, yearly event rate, and the number-flux relation. Identifications are proposed for four transients, all with active coronal sources: the dMe flare stars EV Lac, AT Mic and EQ Vir, and the RS CVn binary HD 8357. Although unidentified, the properties of the brightest transient, HO547-14, could be determined in considerable detail and are sufficient to cast doubt on the identification of HO547-14 with any currently known flaring X-ray classes. The hypothesis that HO547-14 represents X-ray emission from a faint gamma-ray burst results in very good agreement with the observations and we propose this as a possible new class of fast transient X-ray events. The properties of H1907-18 resemble those of HO547-14 in a number of ways, and it may be a second event of this type. The major transient events identified with EV Lac and HD 8357 both had a peak to quiescent luminosity ratio of (TURN) 50, the highest yet seen from either class of object. Both light curves show a second, smaller flare, as well as a distinctly variable quiescent flux. The ratio of the time averaged X-ray flaring luminosity to quiescent luminosity, calculated for the first time, was (TURN) 0.5 for both sources, establishing X-ray flaring as a major form of energy release on these stars. The data for the remaining transients are too sketchy for obvious classification, although a stellar coronal origin is consistent in most cases. The survey suggests a fast transient all-sky event rate of (TURN) 1500 per year and a number-flux relation for the A-1 transients (excluding HO547-14) of S('-1), or marginally, S('-3/2). For HO547-14 type events, however, the observed event rates exclude an S('-3/2), or isotropic, distribution.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1984
- Bibcode:
- 1984PhDT.........2A
- Keywords:
-
- Physics: Astronomy and Astrophysics;
- Sky Surveys (Astronomy);
- Stellar Radiation;
- Surges;
- X Ray Sources;
- Flux (Rate);
- Gamma Ray Bursts;
- Heao 1;
- Stellar Flares;
- Stellar Luminosity;
- Astronomy