EXIST's Gamma-Ray Burst Sensitivity

We use semi-analytic techniques to evaluate the burst sensitivity of designs for the EXIST hard X-ray survey mission. Applying these techniques to the mission design proposed for the Beyond Einstein program, we find that with its very large field-of-view and faint gamma-ray burst detection threshold, EXIST will detect and localize approximately two bursts per day, a large fraction of which may be at high redshift. We estimate that EXIST's maximum sensitivity will be ~4 times greater than that of Swift's Burst Alert Telescope. Bursts will be localized to better than 40 arcsec at threshold, with a burst position as good as a few arcsec for strong bursts. EXIST's combination of three different detector systems will provide spectra from 3 keV to more than 10 MeV. Thus, EXIST will enable a major leap in the understanding of bursts, their evolution, environment, and utility as cosmological probes.Comment: 25 pages, 10 figures, accepted by Ap

Study of Thick CZT Detectors for X-ray and Gamma-Ray Astronomy

CdZnTe (CZT) is a wide bandgap II-VI semiconductor developed for the spectroscopic detection of X-rays and {\gamma}-rays at room temperature. The Swift Burst Alert Telescope is using an 5240 cm2 array of 2 mm thick CZT detectors for the detection of 15-150 keV X-rays from Gamma-Ray Bursts. We report on the systematic tests of thicker (\geq 0.5 cm) CZT detectors with volumes between 2 cm3 and 4 cm3 which are potential detector choices for a number of future X-ray telescopes that operate in the 10 keV to a few MeV energy range. The detectors contacted in our laboratory achieve Full Width Half Maximum energy resolutions of 2.7 keV (4.5%) at 59 keV, 3 keV (2.5%) at 122 keV and 4 keV (0.6%) at 662 keV. The 59 keV and 122 keV energy resolutions are among the world-best results for \geq 0.5 cm thick CZT detectors. We use the data set to study trends of how the energy resolution depends on the detector thickness and on the pixel pitch. Unfortunately, we do not find clear trends, indicating that even for the extremely good energy resolutions reported here, the achievable energy resolutions are largely determined by the properties of individual crystals. Somewhat surprisingly, we achieve the reported results without applying a correction of the anode signals for the depth of the interaction. Measuring the interaction depths thus does not seem to be a pre-requisite for achieving sub-1% energy resolutions at 662 keV.Comment: 15 pages, 11 figure

Proper Functions are Proximal Functions

This paper argues that proper functions are proximal functions. In other words, it rejects the notion that there are distal biological functions – strictly speaking, distal functions are not functions at all, but simply beneficial effects normally associated with a trait performing its function. Once we rule out distal functions, two further positions become available: dysfunctions are simply failures of proper function, and pathological conditions are dysfunctions. Although elegant and seemingly intuitive, this simple view has had surprisingly little uptake in the literature. Indeed, our position departs from that of almost every theorist who has engaged with the issue at any depth. We start by presenting three arguments for the position that proper functions are proximal: one from the specificity of functions, one from their relation to intervention, and one from their relation to pathology. We then consider two case studies evidencing the trouble that accepting distal functions causes for philosophical reflection on the nature of pathological conditions. Finally, we anticipate and respond to three objections: that there can be failure of function without dysfunction; that our account is unacceptably revisionary in respect of normal function-talk; and that our thesis over-generalises from a narrow set of cases

Design and tests of the hard X-ray polarimeter X-Calibur

X-ray polarimetry promises to give qualitatively new information about high-energy astrophysical sources, such as binary black hole systems, micro-quasars, active galactic nuclei, and gamma-ray bursts. We designed, built and tested a hard X-ray polarimeter X-Calibur to be used in the focal plane of the InFOCuS grazing incidence hard X-ray telescope. X-Calibur combines a low-Z Compton scatterer with a CZT detector assembly to measure the polarization of 10-80 keV X-rays making use of the fact that polarized photons Compton scatter preferentially perpendicular to the electric field orientation. X-Calibur achieves a high detection efficiency of order unity.Comment: 9 pages, 5 figures, conference proceedings: SPIE 2011 (San Diego

The Proposed High Energy Telescope (HET) for EXIST

The hard X-ray sky now being studied by INTEGRAL and Swift and soon by NuSTAR is rich with energetic phenomena and highly variable non-thermal phenomena on a broad range of timescales. The High Energy Telescope (HET) on the proposed Energetic X-ray Imaging Survey Telescope (EXIST) mission will repeatedly survey the full sky for rare and luminous hard X-ray phenomena at unprecedented sensitivities. It will detect and localize (<20", at 5 sigma threshold) X-ray sources quickly for immediate followup identification by two other onboard telescopes - the Soft X-ray imager (SXI) and Optical/Infrared Telescope (IRT). The large array (4.5 m^2) of imaging (0.6 mm pixel) CZT detectors in the HET, a coded-aperture telescope, will provide unprecedented high sensitivity (~0.06 mCrab Full Sky in a 2 year continuous scanning survey) in the 5 - 600 keV band. The large field of view (90 deg x 70 deg) and zenith scanning with alternating-orbital nodding motion planned for the first 2 years of the mission will enable nearly continuous monitoring of the full sky. A 3y followup pointed mission phase provides deep UV-Optical-IR-Soft X-ray and Hard X-ray imaging and spectroscopy for thousands of sources discovered in the Survey. We review the HET design concept and report the recent progress of the CZT detector development, which is underway through a series of balloon-borne wide-field hard X-ray telescope experiments, ProtoEXIST. We carried out a successful flight of the first generation of fine pixel large area CZT detectors (ProtoEXIST1) on Oct 9, 2009. We also summarize our future plan (ProtoEXIST2 & 3) for the technology development needed for the HET.Comment: 10 pages, 13 figures, 2 tables, SPIE Conference "Astronomical Telescopes and Instrumentation 2010"; to appear in Proceedings SPIE (2010