1. Obscured AGN and Accretion Luminosity of Universe.
The most numerous objects in the EXIST survey are expected to be active galaxies. EXIST should locate at least 30,000 of these objects, scaling from optical/X-ray Active Galactic Nuclei (AGN) distributions. Furthermore, ~75% of the AGN that EXIST will detect should be obscured, i.e., AGN in which the active nucleus is not detected in the optical or radio (and is not prominent in the IR against what is often a star-forming background). The EXIST survey will therefore yield the first unbiased census of hidden AGN, strongly constraining the total accretion luminosity (per unit mass) of the universe. Understanding the frequency of obscured AGN (which must be high: three of the four AGN nearest to us - Cen A, NGC 4945 and the Circinus Galaxy - are heavily obscured) will elucidate the correlation of BH mass to galaxy bulge mass and also constrain the effect of massive black holes on galaxy formation. EXIST is thus a candidate for Black Hole Finder Probe.

EXIST will provide a continual record of the spectral and temporal properties of both obscured and previously known (>0.5mCrab) AGN, constraining the nature and workings of the central source by measuring the poorly known hard X-ray spectral breaks (~100 - 300keV) for a large sample. ProtoEXIST could measure variations of the break energy of NGC 4151 such as measured with OSSE (Maisack et al 1993). Furthermore, recent TeV detections of several blazars have shown that they can be used to measure the poorly known diffuse cosmic IR background. Direct measurement of their hard X-ray spectral breaks with EXIST enables their TeV breaks simultaneously observed with GLAST and predicted from Compton scattering models to measure diffuse IR absorption and probe the total redshifted output of starlight and re-radiation by dust which measures the nuclear burning luminosity of the universe. Although relatively infrequent, ProtoEXIST could detect bright flares from blazars like Mkn501 measured by VERITAS

2. Gamma Ray Bursts at the Limit: Finding the First Stars
EXIST is an optimal Next Generation Gamma-ray Burst Observatory and trigger (Grindlay et al 2002, 2005). It will provide 10 - 50" locations for 2 - 3 GRBs a day, extend sensitivity to weak events by a factor >5 below Swift, and with its large instantaneous field of view it can study the brightest events most likely to be observed by gravitational wave (LISA/LIGO) and neutrino detectors (IceCube). Current observations suggest that many GRBs (at least those lasting longer than ~2 sec) originate from the collapse of a massive star to a black hole. The first generation of stars in the universe (Pop III stars) were almost certainly very massive, and likely end their short lives in a GRB event. With its high sensitivity, EXIST can detect GRBs at high redshift (z ~10 - 20), providing a means to search for this first generation of stars and, with afterglow spectroscopy from NGST, to map structure back to the "dark ages". The broad energy band coverage of EXIST and the very large collection area for optimum statistics, will enable "photometric redshifts" to be derived from the observed correlation between GRB time-lags (hard vs. soft bands) and absolute luminosity (Norris et al 2000). The observed correlation between the time-lags and luminosity may be understood as a kinematic effect from beaming. Application of this technique to GRBs at high z requires the very large collection area for optimum statistics as well as the broad energy band and good spectral resolution of EXIST.

EXIST could provide GRB locations and redshifts for "orphan afterglows" likely to be found by LSST. The recent suggestion (Furlanetto and Loeb 2002) that GRB remnants may be visible for ~3 x 10⁴y as 511keV annihilation line sources suggests one should be detectable to EXIST in the Galaxy. Although only ~0.1/day are expected in its FOV, ProtoEXIST could locate BATSE-flux GRBs or SAX/WFC-flux X-ray flashes (Heise et al 2002).

3. Matter at the Extreme: Galactic Black Holes & Neutron Stars
EXIST will extend the RXTE and BATSE studies of accretion powered compact binaries to a complete galactic census of the neutron star (NS) and black hole (BH) populations in our Galaxy, as well as studying the possible bright, transient intermediate mass BH in the Local Group. Pulsars, burst sources, microquasars and transients are natural targets for EXIST; all provide considerable time-varying emission in the hard X-ray band. Magnetic field strengths derived from cyclotron lines detected with EXIST and combined with continuous spin-luminosity measures, will constrain the moment of inertia for the entire NS population. EXIST's continuous sky coverage will detect the giant flares from Soft Gamma Repeaters out to Virgo, hence constraining the nature and formation of the strongest magnetic field NSs, the magnetars. And in pointed observations, EXIST will study the particular high frequency quasi-periodic oscillations recently discovered with RXTE at >13keV from black holes in binaries and microquasars, further constraining the accretion physics as well as the mass, spin and radii of these objects. For black holes, determining these parameters tests the fundamental physics of strong field gravity. The wide field of ProtoEXIST will extend coverage for the bi-weekly galactic plane survey planned for INTEGRAL. Given the short exposures possible with a 1d science flight (LDB science flights will be proposed to follow in year 4), most ProtoEXIST science will be for the study of bright (~20 - 50mCrab) galactic sources. The ~10mCrab transients being found with BeppoSAX/WFC in the galactic center region could be found in the wider latitude coverage than with INTEGRAL plane surveys.

4. Hidden Supernovae and Novae: Rates from Nuclear Lines
EXIST's survey will reveal supernova remnants obscured by dust in the Galactic plane by resolving hot spot emission sources in the 68 and 78 keV decay lines (resolved ) of 44Ti that is ejected in Type II supernovae explosions. Scaling from the recent BeppoSAX detection of the combined line emission from Cas A, EXIST will provide a measure of the galactic Type II supernova rate by detecting and locating all supernova remnants within ~8 kpc which have occurred within the past ~500 years. Stellar novae can be discovered by a flash of 511 keV emission from 18F, which emits positrons as it decays with a 158-min half-life. This transient 511 keV emission is only detectable with an all-sky imager like EXIST, and will provide new measures of the nova rate in the Galaxy as well as constraints on the origins of Type Ia supernovae. The enhanced energy resolution of ProtoEXIST vs. Swift or INTEGRAL makes even a 1d flight (~6h exposure) interesting for obscured SNR in nearby broad molecular cloud complexes (e.g. Orion).