Additional Swift Science
Survey
The BAT will produce the most sensitive hard X-ray survey ever made based on serendipitous GRB pointings. No all-sky survey is planned by INTEGRAL, so Swift's survey will be unmatched. Assuming uniform coverage to estimate sensitivities, the BAT instrument will provide an exposure of 6.5 x 1010 cm2 s for each sky pixel yielding a 5σ statistical sensitivity of 200 μCrab in the 10-100 keV band. In this energy range, such deep exposures are systematics limited. Although 2D coded apertures generally have better systematics than the alternatives (modulation collimators, fourier grids, etc.), we estimate the BAT detection sensitivity to be systematics limited at ~1 mCrab at high galactic latitude (>45°) and ~3 mCrabs when strong galactic sources are in the FOV. These levels correspond to 20 sigma - 70 σ detections in a statistical sense. We will be 17 times more sensitive than the best complete hard X-ray survey (HEAO A-4, Levine et al. 1984; complete to 17 mCrab) for 1/3 of the sky and 6 times more sensitive for the rest. The scientific payoff from such a survey will be tremendous, and it is clearly endorsed for the study of black holes in the NRC Thaddeus report (1997).
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AGN
Recent studies with ASCA, Ginga and BeppoSAX have shown the existence of a large population of highly absorbed Seyfert 2's with line-of-sight column densities > 1023 cm-2. The large column makes the nuclei of these objects essentially invisible at optical and soft X-ray wavelengths. Detailed models (Madau et al. 1994; Hasinger & Zamora 1997) show such a population of highly absorbed AGN is needed to produce the observed 30 keV bump in the hard X-ray background and that it comprises ~1/2 of all AGN. The only known method of detecting such objects is an unbiased sky survey in the E > 10 keV band of sufficient sensitivity to detect a large population.The number of AGN observed at > 1 mCrab in the 2-10 keV range is ~100 (Piccinotti et al. 1982), but AGN spectra are harder than the Crab, so this corresponds to 200-300 sources at >1 mCrab in the 10-100 keV band. The factor for highly absorbed AGN raises this total to 400-600 sources. These estimates are consistent with scaling from the 6 AGN in the HEAO A-4 survey. Using 400 sources, we expect Swift to detect >200 AGN brighter than 1 mCrab within 45° of the Galactic poles and >70 AGN brighter than 3 mCrab in the rest of the sky. More than half will not have been identified in the ROSAT survey. At this time, ~20 AGN have quality detections at energies >30 keV (see Dermer & Gehrels 1995).
Soft Gamma Repeaters
BAT is designed to discover rare phenomena from unpredictable parts of the sky. Soft gamma repeaters (SGRs) are such objects and BAT will be sensitive to them because of its excellent short-burst trigger. While the SGR bursts are shorter than the Swift slew time, the XRT and UVOT will perform sensitive searches immediately after the burst for X-ray and optical counterparts and will likely be on-target when following bursts occur.
Rapid Reaction Science
The rapid reaction capability of the Swift observatory, using the TDRSS uplink, will provide a unique ability to respond in a matter of minutes with sensitive gamma-ray, X-ray, UV and optical observations to most events on the sky. This includes targets of opportunity for X-ray transients, pulsar glitches, outbursts from dwarf novae and stellar flares. The highly variable black-hole binary GX 339-4 is a good example of the importance of rapid multiwavelength observations for understanding accretion physics (Smith, Filippenko & Leonard 1998). The BAT is >10 times more sensitive as a monitor than BATSE and will initiate many of the targets of opportunity. Triggers from external sources will also be possible. There has never been a facility that can provide such rapid multiwavelength follow up to unpredictable events. As with any new observational capability, the potential for serendipitous science return is high.
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