William S. Paciesas

The νFν peak energy distributions of gamma-ray bursts observed by BATSE

The majority of gamma-ray bursts exhibit a peak in their νFν photon energy spectra at an energy Ep that is in the energy range ∼20–2000 keV of the Large Area Detectors of the Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory. If gamma-ray burst sources are at cosmological distances, then the spectra of dim bursts should be redshifted to lower energies relative to those of bright bursts. The magnitude of the shift is a function of the cosmological redshifts z of both the dim and bright burst sources and hence yields the range of redshift available to the bursts; this range is further constrained by considering cosmological model fits to the burst number-intensity distribution. We produced photon energy spectra for ∼400 bursts using data from BATSE to investigate if this expected shift in the νFν peak is observed. We find that the mean peak energies of the burst spectra are correlated with intensity: lower intensity groups of burst spectra exhibit a lower average peak energy...

CASTER: a concept for a Black Hole Finder Probe based on the use of new scintillator technologies

The primary scientific mission of the Black Hole Finder Probe (BHFP), part of the NASA Beyond Einstein program, is to survey the local Universe for black holes over a wide range of mass and accretion rate. One approach to such a survey is a hard X-ray coded-aperture imaging mission operating in the 10-600 keV energy band, a spectral range that is considered to be especially useful in the detection of black hole sources. The development of new inorganic scintillator materials provides improved performance (for example, with regards to energy resolution and timing) that is well suited to the BHFP science requirements. Detection planes formed with these materials coupled with a new generation of readout devices represent a major advancement in the performance capabilities of scintillator-based gamma cameras. Here, we discuss the Coded Aperture Survey Telescope for Energetic Radiation (CASTER), a concept that represents a BHFP based on the use of the latest scintillator technology.

Simulation study of COSMIC-a Compton telescope all-sky monitor concept for low-energy gamma-ray astronomy

The highly transient nature of cosmic low-energy gamma-ray sources offers unique insight into the astrophysics of extreme processes. Sensitive, long-term, all-sky monitoring is required to make sense of the variable emission. Instrumentation for such observations must have the combination of large effective collection area, wide field-of-view and good angular resolution. Recent advances in position-sensitive detectors and associated electronics make it possible to consider a multi-scatter Compton telescope, combined with a coded mask, as an all-sky monitor in the energy range /spl sim/10 keV to a few MeV. We present simulations of various configurations of such an instrument and show that it can provide a substantial improvement over other sky monitor strategies in this energy regime.

A maskless gamma-ray all-sky imager: BATSE/CGRO

The non-collimated detectors of BATSE (Burst And Transient Source Experiment) on the Compton Gamma Ray Observatory (CGRO) can be used as a high sensitivity hard X-ray and low energy gamma-ray all-sky imager in the energy range between 20 keV and 2 MeV. The fluxes from sources in the sky are modulated as the spacecraft orbits the Earth. The CGRO orbital precession further allows sampling of the sky in strips corresponding to the limb of the Earth at any given time. The modulation data are transformed into images by various reconstruction methods. High sensitivity images of location accuracy of about 0.1/spl deg/ and source separation of about 1/spl deg/ are obtained. >

Spectral Properties of Short Gamma‐Ray Bursts

It is well known that short GRBs have harder spectra than long GRBs, but the spectral differences between these two modes of the duration distribution have not been examined in detail. Using a database of standard model fits to GRBs in the BATSE 4B catalog, we compare the distributions of spectral parameters of short and long bursts. We also investigate the duration dependence of the same parameters within each mode. We find a consistent pattern of duration dependence: the mean values of all parameters differ significantly between the two duration classes, with short bursts being consistently harder. However, the same parameters show only weak dependence on duration within each class. We discuss the implications of these results for our understanding of GRBs.

MEGA: a medium-energy gamma-ray astronomy mission concept

The Medium Energy Gamma-ray Astronomy (MEGA) telescope concept will soon be proposed as a MIDEX mission. This mission would enable a sensitive all-sky survey of the medium-energy gamma-ray sky (0.4 - 50 MeV) and bridge the huge sensitivity gap between the COMPTEL and OSSE experiments on the Compton Gamma Ray Observatory, the SPI and IBIS instruments on INTEGRAL, and the visionary Advanced Compton Telescope (ACT) mission. The scientific goals include, among other things, compiling a much larger catalog of sources in this energy range, performing far deeper searches for supernovae, better measuring the galactic continuum and line emissions, and identifying the components of the cosmic diffuse gamma-ray emission. MEGA will accomplish these goals using a tracker made of Si strip detector (SSD) planes surrounded by a dense high-Z calorimeter. At lower photon energies (below ~ 30 MeV), the design is sensitive to Compton interactions, with the SSD system serving as a scattering medium that also detects and measures the Compton recoil energy deposit. If the energy of the recoil electron is sufficiently high (> 2 MeV) its momentum vector can also be measured. At higher photon energies (above ~ 10 MeV), the design is sensitive to pair production events, with the SSD system measuring the tracks of the electron and positron. A prototype instrument has been developed and calibrated in the laboratory and at a gamma-ray beam facility. We present calibration results from the prototype and describe the proposed satellite mission.

Fermi GBM: Main detector-level calibration results

One of the scientific objectives of NASA’s Fermi Gamma‐ray Space Telescope is the study of Gamma‐Ray Bursts (GRBs). The Fermi Gamma‐Ray Burst Monitor (GBM) was designed to detect and localize bursts for the Fermi mission. By means of an array of 12 NaI(Tl) (8 keV to 1 MeV) and two BGO (0.2 to 40 MeV) scintillation detectors, GBM extends the energy range (20 MeV to >300 GeV) of Fermi’s main instrument, the Large Area Telescope (LAT), into the traditional range of current GRB databases. The physical detector response of the GBM instrument to GRBs is determined with the help of Monte Carlo simulations, which are supported and verified by on‐ground individual detector calibration measurements. We present the principal instrument properties, which have been determined as a function of energy and angle, including the channel‐energy relation, the energy resolution and the effective area.

Testing the Compton attenuation theory of gamma-ray burst spectra

The narrow range of energies that characterize gamma-ray burst spectra motivated Brainerd to develop the Compton attenuation theory of burst spectra. We fit model spectra to 11 bright bursts to test and confirm the theory’s prediction of an x-ray excess below 10 keV.

The statistics of the search for absorption lines in burst spectra

Because spectral features have not been detected in the BATSE burst data, we study the statistical implications of what could have been detected in the spectra accumulated by BATSE. Specifically, we analyze the detectability of spectral lines in burst spectra to: 1) identify the bursts in which lines would be most detectable; 2) evaluate the consistency between the absence of a line detection by BATSE and the detections reported by previous missions; and 3) constrain the rate at which lines occur based on the observations.

Calibration of the GLAST burst monitor detectors

The GLAST Burst Monitor (GBM) will augment the capabilities of GLAST for the detection of cosmic gamma-ray bursts by extending the energy range (20 MeV to > 300 GeV) of the Large Area Telescope (LAT) towards lower energies by 2 BGO-detectors (150 keV to 30 MeV) and 12 NaI(Tl) detectors (10 keV to 1 MeV). The physical detector response of the GBM instrument for GRBs is determined with the help of Monte Carlo simulations, which are supported and verified by on-ground calibration measurements, performed extensively with the individual detectors at the MPE in 2005. All flight and spare detectors were irradiated with calibrated radioactive sources in the laboratory (from 14 keV to 4.43 MeV). The energy/channel-relations, the dependences of energy resolution and effective areas on the energy and the angular responses were measured. Due to the low number of emission lines of radioactive sources below 100 keV, calibration measurements in the energy range from 10 keV to 60 keV were performed with the X-ray radiometry working group of the Physikalisch-Technische Bundesanstalt (PTB) at the BESSY synchrotron radiation facility, Berlin.