The EXPOSURE Table specifies the fraction of the time that a burst at a particular point on the sky is detectable by BATSE, and accounts for earth blockage, SAA passages, times when the burst trigger is disabled, and times when a burst readout is in progress. It is a function of declination only. The burst is assumed bright enough to exceed the trigger threshold. A separate efficiency correction must also be included in analysis of burst rates as a function of peak flux. This efficiency is provided in a separate table; work is in progress to improve the efficiency calculation by including atmospheric scattering and handling of telemetry gaps.

Due to the failure of the GRO tape recorders and the presence of telemetry gaps in the post-1B data, a different algorithm was used in preparing this table than was used for the 1B catalog. The new algorithm not only handles telemetry gaps, but is also more accurate in computing SAA passages. In addition, the old algorithm excluded time intervals during which the trigger thresholds were not at their standard settings of 5.5 sigma in each trigger timescale, while the current algorithm does not. A separate table is provided that specifies the history of the trigger thresholds. The recomputed 1B exposure is higher (0.42 versus 0.38), due primarily to the inclusion of times when thresholds were not at 5.5 sigma. The galactic dipole moment of the sky exposure has changed by about 0.005, due primarily to the more accurate accounting for the SAA. The change in the galactic quadrupole moment is negligable.

In the exposure table below, the declination is given in degrees, and the exposure fraction at that declination is given for each of the cumulative catalogs (1B through 4B), and for each subset that was added to make the next catalog. In the previous exposure tables, the exposure was an average over a range of declinations of typically about 10 degrees. Here, the exposure applies to a specific declination. The 4B catalog, currently in preparation, ends on 29 August 1996.

Additional statistics are provided at the end of the table. The average exposure is the average over the sky of the declination-dependant exposure fraction. Total time is the time between the start and the end of the catalog subset, in units of 10^6 seconds. Total exposure, the product of the previous two numbers, is the total time, in units of 10^6 seconds and averaged over the sky, that BATSE was sensitive was sensitive to bursts for that catalog subset. The values of <cos(theta)> and <sin^2(b)> - 1/3 are the galactic dipole and quadrupole moments of the exposure. The values of <sin(dec)> and <sin^2(dec)> - 1/3 are the dipole and quadrupole moments in the equatorial coordinate system.

The uncertainty in the exposure fractions are roughly estimated at about 4%. Possible sources of systematic error in the moments are currently under study, but are expected to be less than 0.003 for the galactic moments.

The EXPOSURE Table is available for download.

Exposure

Exposure Statistics