Person:

Hong, Jaesub

The OSIRIS-REx Mission was selected under the NASA New Frontiers program and is scheduled for launch in September of 2016 for a rendezvous with, and collection of a sample from the surface of asteroid Bennu in 2019. 101955 Bennu (previously 1999 RQ36) is an Apollo (near-Earth) asteroid originally discovered by the LINEAR project in 1999 which has since been classified as a potentially hazardous near-Earth object. The REgolith X-Ray Imaging Spectrometer (REXIS) was proposed jointly by MIT and Harvard and was subsequently accepted as a student led instrument for the determination of the elemental composition of the asteroid's surface as well as the surface distribution of select elements through solar induced X-ray fluorescence. REXIS consists of a detector plane that contains 4 X-ray CCDs integrated into a wide field coded aperture telescope with a focal length of 20 em for the detection of regions with enhanced abundance in key elements at 50 m scales. Elemental surface distributions of approximately 50-200 m scales can be detected using the instrument as a simple collimator. An overview of the observation strategy of the REXIS instrument and expected performance are presented here.

We have assembled a tiled array ((220 cm^2)) of fine pixel (0.6 mm) imaging CZT detectors for a balloon borne widefield hard X-ray telescope, ProtoEXIST2. ProtoEXIST2 is a prototype experiment for a next generation hard X-ray imager MlRAX-HXI on board Lattes, a spacecraft from the Agencia Espacial Brasilieira. MlRAX will survey the 5 to 200 keV sky of Galactic bulge, adjoining southern Galactic plane and the extragalactic sky with 6' angular resolution. This survey will open a vast discovery space in timing studies of accretion neutron stars and black holes. The ProtoEXIST2 CZT detector plane consists of 64 of 5 mm thick 2cm × 2cm CZT crystals tiled with a minimal gap. MIRAX will consist of 4 such detector planes, each of which will be imaged with its own coded-aperture mask. We present the packaging architecture and assembly procedure of the ProtoEXIST2 detector. On 2012, Oct 10, we conducted a successful high altitude balloon experiment of the ProtoEXISTl and 2 telescopes, which demonstrates their technology readiness for space application. Both telescopes performed flawlessly during the flight as well as on the ground. We report the results of pre-flight ground calibration and the preliminary results for the detector performance in the balloon flight.

The MIRAX X-ray observatory, the first Brazilian-led astrophysics space mission, is designed to perform an unprecedented wide-field, wide-band hard X-ray (5–200 keV) survey of Galactic X-ray transient sources. In the current configuration, MIRAX will carry a set of four coded-masks telescopes with high spatial resolution Cadmium Zinc Telluride (CZT) detector planes, each one consisting of an array of 64 closely tiled CZT pixelated detectors. Taken together, the four telescopes will have a total detection area of (959 cm^2), a large field of view (60° × 60° FWHM), high angular resolution for this energy range (6 arcmin) and very good spectral resolution ( ~ 2 keV @ 60 keV). A stratospheric balloon-borne prototype of one of the MIRAX telescopes has been developed, tested and flown by the Harvard-Smithsonian Center for Astrophysics (CfA) as part of the ProtoEXIST program. In this paper we show results of validation and calibration tests with individual CZT detectors of the ProtoEXIST second generation experiment (P2). Each one of 64 detector units of the P2 detector plane consists of an ASIC, developed by Caltech for the NuSTAR telescope, hybridized to a CZT crystal with 0.6mm pixel size. The performance of each detector was evaluated using radioactive sources in the laboratory. The calibration results show that the P2 detectors have average energy resolution of ~ 2.1 keV @ 60 keV and 2.3 @ 122 keV. P2 was also successfully tested on near-space environment on a balloon flight, demonstrating the detector unit readiness for integration on a space mission telescope, as well as satisfying all MIRAX mission requirements.

Wide-field (& 100 deg2) hard X-ray coded-aperture telescopes with high angular resolution (. 20) will enable a wide range of time domain astrophysics. For instance, transient sources such as gammaray bursts can be precisely localized without assistance of secondary focusing X-ray telescopes to enable rapid followup studies. On the other hand, high angular resolution in coded-aperture imaging introduces a new challenge in handling the systematic uncertainty: average photon count per pixel is often too small to establish a proper background pattern or model the systematic uncertainty in a time scale where the model remains invariant. We introduce two new techniques to improve detection sensitivity, which are designed for, but not limited to high resolution coded-aperture system: a self-background modeling scheme which utilizes continuous scan or dithering operations, and a Poisson-statistics based probabilistic approach to evaluate the significance of source detection without subtraction in handling the background. We illustrate these new imaging analysis techniques in high resolution coded-aperture telescope using the data acquired by the wide-field hard X-ray telescope ProtoEXIST2 during the high-altitude balloon flight in Fall, 2012. We review the imaging sensitivity of ProtoEXIST2 during the flight, and demonstrate the performance of the new techniques using our balloon flight data in comparison with simulated ideal Poisson background.