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Register a new userImaging calibration of AstroSat Cadmium Zinc Telluride Imager (CZTI)
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AstroSat is Indias first space-based astronomical observatory, launched on September 28, 2015. One of the payloads aboard AstroSat is the Cadmium Zinc Telluride Imager (CZTI), operating at hard X-rays. CZTI employs a two-dimensional coded aperture mask for the purpose of imaging. In this paper, we discuss various image reconstruction algorithms adopted for the test and calibration of the imaging capability of CZTI and present results from CZTI on-ground as well as in-orbit image calibration.
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The AstroSat satellite is designed to make multi-waveband observations of astronomical sources and the Cadmium Zinc Telluride Imager (CZTI) instrument of AstroSat covers the hard X-ray band. CZTI has a large area position sensitive hard X-ray detector equipped with a Coded Aperture Mask, thus enabling simultaneous background measurement. Ability to record simultaneous detection of ionizing interactions in multiple detector elements is a special feature of the instrument and this is exploited to provide polarization information in the 100 - 380 keV region. CZTI provides sensitive spectroscopic measurements in the 20 - 100 keV region, and acts as an all sky hard X-ray monitor and polarimeter above 100 keV. During the first year of operation, CZTI has recorded several gamma-ray bursts, measured the phase resolved hard X-ray polarization of the Crab pulsar, and the hard X-ray spectra of many bright Galactic X-ray binaries. The excellent timing capability of the instrument has been demonstrated with simultaneous observation of the Crab pulsar with radio telescopes like GMRT and Ooty radio telescope.
The High Resolution Energetic X-Ray Imager (HREXI) CZT detector development program at Harvard is aimed at developing tiled arrays of finely pixelated CZT detectors for use in wide-field coded aperture 3-200 keV X-ray telescopes. A pixel size of $simeq$ 600 $mu m$ has already been achieved in the ProtoEXIST2 (P2) detector plane with CZT read out by the NuSTAR ASIC. This paves the way for even smaller 300 $mu m$ pixels in the next generation HREXI detectors. This article describes a new HREXI calibration facility (HCF) which enables a high resolution sub-pixel level (100 $mu m$) 2D scan of a 256 $cm^2$ tiled array of 2 $times$ 2 cm CZT detectors illuminated by a bright X-ray AmpTek Mini-X tube source at timescales of around a day. HCF is a significant improvement from the previous apparatus used for scanning these detectors which took $simeq$ 3 weeks to complete a 1D scan of a similar detector plane. Moreover, HCF has the capability to scan a large tiled array of CZT detectors ($32cm times 32cm$) at 100 $mu m$ resolution in the 10 - 50 keV energy range which was not possible previously. This paper describes the design, construction, and implementation of HCF for the calibration of the P2 detector plane.
We are currently developing Cadmium Zinc Telluride (CZT) detectors for a next-generation space-borne hard X-ray telescope which can follow up on the highly successful NuSTAR (Nuclear Spectroscopic Telescope Array) mission. Since the launch of NuSTAR in 2012, there have been major advances in the area of X-ray mirrors, and state-of-the-art X-ray mirrors can improve on NuSTARs angular resolution of ~1 arcmin Half Power Diameter (HPD) to 15 or even 5 HPD. Consequently, the size of the detector pixels must be reduced to match this resolution. This paper presents detailed simulations of relatively thin (1 mm thick) CZT detectors with hexagonal pixels at a next-neighbor distance of 150 $mu$m. The simulations account for the non-negligible spatial extent of the deposition of the energy of the incident photon, and include detailed modeling of the spreading of the free charge carriers as they move toward the detector electrodes. We discuss methods to reconstruct the energies of the incident photons, and the locations where the photons hit the detector. We show that the charge recorded in the brightest pixel and six adjacent pixels suffices to obtain excellent energy and spatial resolutions. The simulation results are being used to guide the design of a hybrid application-specific integrated circuit (ASIC)-CZT detector package.
The radio as well as the high energy emission mechanism in pulsars is yet not understood properly. A multi-wavelength study is likely to help in better understanding of such processes. The first Indian space-based observatory, ASTROSAT, has five instruments aboard, which cover the electromagnetic spectrum from infra-red (1300 $AA$) to hard X-ray (380 KeV). Cadmium Zinc Telluride Imager (CZTI), one of the five instruments is a hard X-ray telescope functional over an energy range of 20-380 KeV. We aim to estimate the timing offset introduced in the data acquisition pipeline of the instrument, which will help in time alignment of high energy time series with those from two other ground-based observatories, viz. the Giant Meterwave Radio Telescope (GMRT) and the Ooty Radio Telescope (ORT). PSR B0531+21 is a well-studied pulsar with nearly aligned radio and hard X-ray pulse profiles. We use simultaneous observations of this pulsar with the ASTROSAT, the ORT and the GMRT. The pulsar was especially observed using the ORT with almost daily cadence to obtain good timing solutions. We also supplement the ORT data with archival FERMI data for estimation of timing noise. The timing offset of ASTROSAT instruments was estimated from fits to arrival time data at the ASTROSAT and the radio observatories. We estimate the offset between the GMRT and the ASTROSAT-CZTI to be -4716 $pm$ 50 $mu s$. The corresponding offset with the ORT was -29639 $pm$ 50 $mu s$. The offsets between the GMRT and Fermi-LAT -5368 $pm$ 56 $mu s$. (Abridged)
Cadmium-Zinc-Telluride Imager (CZTI) is one of the five payloads on-board recently launched Indian astronomy satellite AstroSat. CZTI is primarily designed for simultaneous hard X-ray imaging and spectroscopy of celestial X-ray sources. It employs the technique of coded mask imaging for measuring spectra in the energy range of 20 - 150 keV. It was the first scientific payload of AstroSat to be switched on after one week of the launch and was made operational during the subsequent week. Here we present preliminary results from the performance verification phase observations and discuss the in-orbit performance of CZTI.
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