arXiv subjects

H. Krawczynski

Publications and source records attributed to H. Krawczynski.

At least 127 records · Page 7Linked to original sources

arXiv2007

Detailed Studies of Pixelated CZT Detectors Grown with the Modified Horizontal Bridgman Method

The detector material Cadmium Zinc Telluride (CZT), known for its high resolution over a broad energy range, is produced mainly by two methods: the Modified High-Pressure Bridgman (MHB) and the High-Pressure Bridgman (HPB) process. This study is based on MHB CZT substrates from the company Orbotech Medical Solutions Ltd. with a detector size of 2.0x2.0x0.5 cm^3, 8x8 pixels and a pitch of 2.46 mm. Former studies have emphasized only on the cathode material showing that high-work-function improve the energy resolution at lower energies. Therfore, we studied the influence of the anode material while keeping the cathode material constant. We used four different materials: Indium, Titanium, Chromium and Gold with work-functions between 4.1 eV and 5.1 eV. The low work-function materials Indium and Titanium achieved the best performance with energy resolutions: 2.0 keV (at 59 keV) and 1.9 keV (at 122 keV) for Titanium; 2.1 keV (at 59 keV) and 2.9 keV (at 122 keV) for Indium. These detectors are very competitive compared with the more expensive ones based on HPB material if one takes the large pixel pitch of 2.46 mm into account. We present a detailed comparison of our detector response with 3-D simulations, from which we determined the mobility-lifetime-products for electrons and holes. Finally, we evaluated the temperature dependency of the detector performance and mobility-lifetime-products, which is important for many applications. With decreasing temperature down to -30C the breakdown voltage increases and the electron mobility-lifetime-product decreases by about 30% over a range from 20C to -30C. This causes the energy resolution to deteriorate, but the concomitantly increasing breakdown voltage makes it possible to increase the applied bias voltage and restore the full performance.

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arXiv2007

Blazar Observations with VERITAS

The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is an array of four 12m diameter Imaging Atmospheric Cherenkov Technique (IACT) telescopes operated at the base of Mt. Hopkins in southern Arizona. The four-telescope experiment started operation in April, 2007. GeV and TeV gamma-ray observations of blazars can be used to probe the structure and composition of their jets, and to contribute to our understanding of how supermassive black holes accrete matter. In this contribution, we present first VERITAS blazar results obtained with three and four telescopes.

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arXiv2007

Design Considerations for the Next Generation of Atmospheric Imaging Cherenkov Telescopes

We estimate the limiting angular resolution and detection area for an array of 3 large-aperture Imaging Atmospheric Cherenkov Telescopes. We consider an idealized IACT system in order to understand the limitations imposed by the intrinsic nature of the atmospheric showers and geometry of the detector configuration. The idealization includes the assumptions of a perfect optical system and the absence of the night sky background with the goal of finding the optimum camera geometry and array configuration independent of detailed assumptions about the telescope design. The showers are simulated using the ALTAI code for the altitude of 2700 m corresponding to one of possible future sites for a new northern-hemisphere array. The optimal design depends on the target energy range; for each energy we vary both the cell length (telescope spacing) and the image processing parameters in order to maximize the signal-to-noise ratio. We then present the resulting values of the detection area and the angular resolution for this energy dependent optimization. We discuss the dependence of these quantities on the field of view of the telescopes and pixel size of the camera.

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arXiv2007

Wide Field Hard X-ray Survey Telescope: ProtoEXIST1

We report our progress on the development of pixellated imaging CZT detector arrays for our first-generation balloon-borne wide-field hard X-ray (20 - 600 keV) telescope, ProtoEXIST1. Our ProtoEXIST program is a pathfinder for the High Energy Telescope (HET) on the Energetic X-ray Imaging Survey telescope (EXIST), a proposed implementation of the Black Hole Finder Probe. ProtoEXIST1 consists of four independent coded-aperture telescopes with close-tiled (~0.4 mm gaps) CZT detectors that preserve their 2.5mm pixel pitch. Multiple shielding/field-of-view configurations are planned to identify optimal geometry for the HET in EXIST. The primary technical challenge in ProtoEXIST is the development of large area, close-tiled modules of imaging CZT detectors (1000 cm2 for ProtoEXIST1), with all readout and control systems for the ASIC readout vertically stacked. We describe the overall telescope configuration of ProtoEXIST1 and review the current development status of the CZT detectors, from individual detector crystal units (DCUs) to a full detector module (DM). We have built the first units of each component for the detector plane and have completed a few Rev2 DCUs (2x2 cm2), which are under a series of tests. Bare DCUs (pre-crystal bonding) show high, uniform ASIC yield (~70%) and ~30% reduction in electronics noise compared to the Rev1 equivalent. A Rev1 DCU already achieved ~1.2% FWHM at 662 keV, and preliminary analysis of the initial radiation tests on a Rev2 DCU shows ~ 4 keV FWHM at 60 keV (vs. 4.7 keV for Rev1). We therefore expect about <~1% FWHM at 662 keV with the Rev2 detectors.

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arXiv2007

White Paper on the Status and Future of Ground-based Gamma-ray Astronomy

In recent years, ground-based gamma-ray observatories have made a number of important astrophysical discoveries which have attracted the attention of the wider scientific community. The Division of Astrophysics of the American Physical Society has requested the preparation of a white paper on the status and future of ground-based gamma-ray astronomy to define the science goals of the future observatory, to determine the performance specifications, and to identify the areas of necessary technology development. In this contribution we give a brief overview of the activities of the current white paper team and invite the international community to contribute to the white paper.

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arXiv2007

Very High Energy Observations of Gamma-Ray Burst Locations with the Whipple Telescope

Gamma-ray burst (GRB) observations at very high energies (VHE, E > 100 GeV) can impose tight constraints on some GRB emission models. Many GRB afterglow models predict a VHE component similar to that seen in blazars and plerions, in which the GRB spectral energy distribution has a double-peaked shape extending into the VHE regime. VHE emission coincident with delayed X-ray flare emission has also been predicted. GRB follow-up observations have had high priority in the observing program at the Whipple 10m Gamma-ray Telescope and GRBs will continue to be high priority targets as the next generation observatory, VERITAS, comes on-line. Upper limits on the VHE emission, at late times (>~4 hours), from seven GRBs observed with the Whipple Telescope are reported here.

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arXiv2006

Observations of the Unidentified TeV Gamma-Ray Source TeV J2032+4130 with the Whipple Observatory 10 m Telescope

We report on observations of the sky region around the unidentified TeV gamma-ray source TeV J2032+4130 carried out with the Whipple Observatory 10 m atmospheric Cherenkov telescope for a total of 65.5 hrs between 2003 and 2005. The standard two-dimensional analysis developed by the Whipple collaboration for a stand-alone telescope reveals an excess in the field of view at a pre-trials significance level of 6.1 standard deviations. The measured position of this excess is alpha(2000) =20 h 32 m 27 s, delta(2000) = 41 deg 39 min 17 s. The estimated integral flux for this gamma-ray source is about 8% of the Crab-Nebula flux. The data are consistent with a point-like source. Here we present a detailed description of the standard two-dimensional analysis technique used for the analysis of data taken with the Whipple Observatory 10 m telescope and the results for the TeV J2032+4130 campaign. We include a short discussion of the physical mechanisms that may be responsible for the observed gamma-ray emission, based on possible association with known astrophysical objects, in particular Cygnus OB2.

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arXiv2006

CZT in Space Based Hard-X-ray Astronomy

One of the key aspects of a detector material for space-borne hard X-ray and gamma-ray telescopes is the rate of prompt and delayed background events generated inside the material by charged and neutral particles striking the detector. These particles are Cosmic Rays, particles trapped in Earth's magnetic field, and secondaries from Cosmic Ray interacting with the atmosphere and the spacecraft. Here, we present a preliminary study of Cadmium Zinc Telluride (CZT) and its behaviour in space environments. We have used the simulation package MGGPOD to estimate the background of the CZT detectors in the proposed Energetic X-ray Imaging Survey Telescope (EXIST) for possible orbital parameters. The EXIST mission will make use of 6 square meters of 0.5 cm thick CZT detectors to record cosmic X-rays in the energy range from 10 keV to 600 keV. The detectors will be shielded by a fully or partly active shield. For the specific detector and shielding geometry considered here and an orbit with a low (7 deg) inclination, the background rate is dominated by the diffuse extragalactic below 200 keV. Prompt and delayed hadronic backgrounds grow increasingly important above this energy, becoming the main contributors to the total background above 1 MeV. A fully active shield performs slightly better than a half active/half passive shield.

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arXiv2006

Test of Thick Pixelated Orbotech Detectors with and without Steering Grids

We report here on the optimization of 0.5 cm thick pixelated Orbotech CZT detectors with regards to the best contacting materials and the use of steering grids. We evaluated the performance of different contacting materials. Our study differs from earlier ones in that we investigated the performance of different anode and cathode materials separately. We obtain the best performance with Au cathodes. For different anode materials Ti and In give the best energy resolutions. The detector (2.0 x 2.0 x 0.5 cm, 8 x 8 pixels) shows excellent 59 keV, 122 keV and 662 keV energy resolutions of 1.4 keV, 1.9 keV, and 7.4 keV, respectively. Furthermore, we report on using steering grids to improve on the performance of the pixelated detectors. Previously, the benefit of steering grids had been limited by additional electronic noise associated with currents between the negatively biased steering grids and the anode pixels. We are currently exploring the possibility to isolate the steering grid from the CZT substrates by a thin layer of Al$_2$O$_3$. We performed a series of measurements to determine by how much the isolation layer reduces the grid-pixel currents. Comparing the currents between two Au contacts before and after isolating one of the two contacts from the CZT with a 700 nm thick layer of Al2O3, we measure that the isolation layer reduces the currents by a factor of about 10 at 500 V. We present some results from a detector before and after deposition of an isolated steering grid. The grid indeed improves on the detectors energy resolution and detection efficiency. We show that simulations can be used to model the anode to cathode charge correlation in excellent agreement with the experimental results.

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arXiv2006

Simulation studies of CZT Detectors as Gamma-Ray Calorimeter

We describe the results of detailed 3-D Monte Carlo simulations of a "CZT calorimeter" that can be used to detect photons in the keV to several MeV range. Several astrophysics applications require the detection of photons in the energy range of keV up to several MeV with good position and energy resolution. For certain applications Cadmium Zinc Telluride (CZT) detectors might be the detector option of choice. Up to now, CZT detectors have mainly been used in the energy range between a few keV to ~1 MeV, we describe the results of detailed 3-D Monte Carlo simulations of a "CZT calorimeter" that can be used to detect photons in the keV to several MeV range. The main objective of these studies is to evaluate the feasibility of CZT calorimeters, to study their performance and detect and understand performance limiting factors. Such a calorimeter consists of many layers of closely packed pixellated CZT detector units. Our simulations of single detector units reproduce experimental results, indicating that our simulations capture the main factors that limit the performance of a detector unit. Overall the conclusion of our simulation study is that between 1 cm and 1.5 cm thick detector units can be used to build a calorimeter with good performance over the energy range from ~20 keV to ~10 MeV.

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arXiv2006

Using the Active Collimator and Shield Assembly of an EXIST-Type Mission as a Gamma-Ray Burst Spectrometer

The Energetic X-ray Imaging Survey Telescope (EXIST) is a mission design concept that uses coded masks seen by Cadmium Zinc Telluride (CZT) detectors to register hard X-rays in the energy region from 10 keV to 600 keV. A partially active or fully active anti-coincidence shield/collimator with a total area of between 15 and 35 square meters will be used to define the field of view of the CZT detectors and to suppress the background of cosmic-ray-induced events. In this paper, we describe the use of a sodium activated cesium iodide shield/collimator to detect gamma-ray bursts (GRBs) and to measure their energy spectra in the energy range from 100 keV up to 10 MeV. We use the code GEANT4 to simulate the interactions of photons and cosmic rays with the spacecraft and instrument and the code DETECT2000 to simulate the optical properties of the scintillation detectors. The shield collimator achieves a nu-F-nu sensitivity of 3 x 10^(-9) erg cm^(-2) s^(-1) and 2 x 10^(-8) erg cm^(-2) s^(-1) at 100 keV and 600 keV, respectively. The sensitivity is well matched to that of the coded mask telescope. The broad energy coverage of an EXIST-type mission with active shields will constrain the peak of the spectral energy distribution (SED) for a large number of GRBs. The measurement of the SED peak may be key for determining photometric GRB redshifts and for using GRBs as cosmological probes.

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arXiv2006

Constraints on the Nature of Jets from kpc Scale X-ray Data

Motivated by the large number of jets detected by the Chandra X-ray Observatory, and by the inverse Compton X-ray emission model (IC/CMB) for relativistic jets, we revisit two basic questions: ``If the medium that carries the jet's energy consists of hot electrons, can we use the physical length of the jet to constrain the maximum electron energy?'' and ``Why do jets have knots?'' Based on the two non-thermal emission processes for X-rays from jets, we consider constraints on the jet medium and other properties from these two simple questions. We argue that hot pairs cannot be the dominant constituent of the medium responsible for the jet's momentum flux and that some mechanisms for producing fluctuating brightness along jets (rather than a monotonically decreasing intensity) are precluded by observed jet morphologies.

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arXiv2006

Gamma-Hadron Separation Methods for the VERITAS Array of Four Imaging Atmospheric Cherenkov Telescopes

Ground-based arrays of imaging atmospheric Cherenkov telescopes have emerged as the most sensitive gamma-ray detectors in the energy range of about 100 GeV and above. The strengths of these arrays are a very large effective collection area on the order of 100,000 square meter, combined with excellent single photon angular and energy resolutions. The sensitivity of such detectors is limited by statistical fluctuations in the number of Cosmic Ray initiated air showers that resemble gamma-ray air showers in many ways. In this paper, we study the performance of simple event reconstruction methods when applied to simulated data of the Very Energetic Radiation Imaging Telescope Array System (VERITAS) experiment. We review methods for reconstructing the arrival direction and the energy of the primary photons, and examine means to improve on their performance. For a software threshold energy of 300 GeV (100 GeV), the methods achieve point source angular and energy resolutions of sigma[63%]= 0.1 degree (0.2 degree) and sigma[68%]= 15% (22%), respectively. The main emphasis of the paper is the discussion of gamma-hadron separation methods for the VERITAS experiment. We find that the information from several methods can be combined based on a likelihood ratio approach and the resulting algorithm achieves a gamma-hadron suppression with a quality factor that is substantially higher than that achieved with the standard methods used so far.

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arXiv2006

A gamma-ray burst remnant in our Galaxy: HESS J1303-631

We present the results of our investigation of the multiwavelength data on HESS J1303-631, an unidentified TeV source serendipitously discovered in the Galactic plane by the HESS collaboration. These results strongly suggest the identification of this particular source as the remnant of a Gamma-Ray Burst (GRB) that happened some few tens of thousands years ago in our Galaxy at a distance on the order of 10 kpc from us. We show, through detailed calculations of particle diffusion, interaction and radiation processes of relativistic particles in the interstellar medium, that it is possible for a GRB remnant (GRBR) to be a strong TeV emitter with no observable synchrotron emission. We predict spectral and spatial signatures that would unambiguously distinguish GRBRs from ordinary supernova remnants, including: (1) large energy budgets inferred from their TeV emission, but at the same time, (2) suppressed fluxes in the radio through GeV wavebands; (3) extended center-filled emission with an energy-dependent spatial profile; and (4) a possible elongation in the direction of the past pair of GRB jets. While GRBRs can best be detected by ground-based gamma-ray detectors, the future GLAST mission will play a crucial role in confirming the predicted low level of GeV emission.

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arXiv2006

The First VERITAS Telescope

The first atmospheric Cherenkov telescope of VERITAS (the Very Energetic Radiation Imaging Telescope Array System) has been in operation since February 2005. We present here a technical description of the instrument and a summary of its performance. The calibration methods are described, along with the results of Monte Carlo simulations of the telescope and comparisons between real and simulated data. The analysis of TeV $γ$-ray observations of the Crab Nebula, including the reconstructed energy spectrum, is shown to give results consistent with earlier measurements. The telescope is operating as expected and has met or exceeded all design specifications.

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arXiv2006

Long-Term X-Ray and TeV variability of Mrk 501

We present X-ray observations of the blazar Mrk501 obtained with RXTE during 1997, 1998, 1999, 2000, and 2004. The goal of this study is twofold: 1) characterize the long-term X-ray flux and spectral variability of the source with a model-independent analysis, and 2) investigate the X-ray and TeV correlation on long timescales. The spectral and temporal variability properties appear to be markedly different compared to those of non-jet-dominated radio-loud and radio-quiet AGN monitored with RXTE. To investigate the X-ray/TeV correlation on long timescales we compared RXTE data with HEGRA and Whipple historical light curves. We confirm the presence of a direct correlation, which appears to be stronger when the source is brighter. The analysis of individual flares shows that the X-ray\TeV correlated activity is heterogeneous. However, more sensitive TeV observations are necessary to confirm these findings, and to put tighter constraints on jet models (abridged).

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arXiv2006

Multiwavelength Observations of 1ES 1959+650, One Year After the Strong Outburst of 2002

In April-May 2003, the blazar 1ES 1959+650 showed an increased level of X-ray activity. This prompted a multiwavelength observation campaign with the Whipple 10 m gamma-ray telescope, the Rossi X-ray Timing Explorer, the Bordeaux Optical Observatory, and the University of Michigan Radio Astrophysical Observatory. We present the multiwavelength data taken from May 2, 2003 to June 7, 2003 and compare the source characteristics with those measured during observations taken during the years 2000 and 2002. The X-ray observations gave a data set with high signal-to-noise light curves and energy spectra; however, the gamma-ray observations did not reveal a major TeV gamma-ray flare. Furthermore, we find that the radio and optical fluxes do not show statistically significant deviations from those measured during the 2002 flaring periods. While the X-ray flux and X-ray photon index appear correlated during subsequent observations, the apparent correlation evolved significantly between the years 2000, 2002, and 2003. We discuss the implications of this finding for the mechanism that causes the flaring activity.

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arXiv2006

TeV Gamma-Ray Observations of the Perseus and Abell 2029 Galaxy Clusters

Galaxy clusters might be sources of TeV gamma rays emitted by high-energy protons and electrons accelerated by large scale structure formation shocks, galactic winds, or active galactic nuclei. Furthermore, gamma rays may be produced in dark matter particle annihilation processes at the cluster cores. We report on observations of the galaxy clusters Perseus and Abell 2029 using the 10 m Whipple Cherenkov telescope during the 2003-2004 and 2004-2005 observing seasons. We apply a two-dimensional analysis technique to scrutinize the clusters for TeV emission. In this paper we first determine flux upper limits on TeV gamma-ray emission from point sources within the clusters. Second, we derive upper limits on the extended cluster emission. We subsequently compare the flux upper limits with EGRET upper limits at 100 MeV and theoretical models. Assuming that the gamma-ray surface brightness profile mimics that of the thermal X-ray emission and that the spectrum of cluster cosmic rays extends all the way from thermal energies to multi-TeV energies with a differential spectral index of -2.1, our results imply that the cosmic ray proton energy density is less than 7.9% of the thermal energy density for the Perseus cluster.

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