4. Survey sensitivity and coverage
Three main X-ray surveys are considered that have discovered or co-discovered all of the known low mass BHB transients and that dominate the historical sky coverage. These are the Ariel 5 ASM survey, the Ginga ASM survey and the ongoing BATSE survey of the high energy sky, for which only discoveries before mid-1995 are considered. Estimating the effective sensitivities of these surveys for the discovery of isolated transients with sufficient significance that they received detailed study is quite difficult. Certainly this is well above the threshold for monitoring fluctuations or detecting low intensity recurrences of known sources. It is also well above the flux limit of the faintest transients that have been detected; in particular archival analysis of these sky survey data sets can reveal fainter outbursts. Further, for these satellites pointed observations were also able to detect sources at substantially lower fluxes - these however add little to the integrated sky coverage. Even for the sky survey components of these missions the on-time for coverage of the Galactic plane is not always 100%; e.g. for the Ginga ASM an effective coverage of 30% is estimated (Tanaka, private communication). However for bursts similar to the identified BH transients, the relatively long decay time (Table 1) makes it probable that the source can be caught in a high state and decreases the incompleteness due to the partial temporal coverage.
In this paper the sky survey sensitivity is estimated from the flux at the first reported detection of a number of new transients (eg. 1524-617, Kaluzienski et al. 1975; GS 2000+250, Makino 1988; GRO 1915+05, cf. Grindlay et al 1996). The epochs of the surveys, the modeled duration and the estimated threshold sensitivity referenced to the appropriate soft (2-6keV) or hard (20-300keV) band are listed below (Table 3). For Ariel 5, confusion in the inner Galaxy seriously decreased the source sensitivity (Warwick et al. 1981); the sensitivity for is decreased to .
Table 3. X-ray sky surveys
© European Southern Observatory (ESO) 1998
Online publication: April 20, 1998