Peter F. Bloser

Energy Spectra and High Frequency Oscillations in 4U 0614+091

We investigate the behavior of the high-frequency quasi-periodic oscillations (QPOs) in 4U 0614+091, combining timing and spectral analyses of RXTE observations. The energy spectra of the source can be described by a power law (α ~ 2.8) and a blackbody (kT ~ 1.5 keV), with the blackbody accounting for 10%-20% of the total energy flux. We find a robust correlation of the frequency, ν, of the higher frequency QPO near 1 kHz with the flux of the blackbody, FBB. The slope of this correlation, d log ν/d log FBB, is 0.27-0.37. The source follows the same relation even in observations separated by several months. The QPO frequency does not have a similarly unique correlation with the total flux or the flux of the power-law component. The rms fraction of the higher frequency QPO rises with energy from 6.8% ± 1.5% (3-5 keV) to 21.3% ± 4.0% (10-12 keV). For the lower frequency QPO, however, it is consistent with a constant value of 5.4% ± 0.9%. The results may be interpreted in terms of a beat-frequency model for the production of the high-frequency QPOs.

The MEGA advanced Compton telescope project

Abstract The goal of the Medium Energy Gamma-ray Astronomy (MEGA) telescope is to improve sensitivity at medium gamma-ray energies (0.4–50 MeV) by at least an order of magnitude over that of COMPTEL. This will be achieved with a new compact design that allows for a very wide field of view, permitting a sensitive all-sky survey and the monitoring of transient and variable sources. The key science objectives for MEGA include the investigation of cosmic high-energy particle accelerators, studies of nucleosynthesis sites using gamma-ray lines, and determination of the large-scale structure of galactic and cosmic diffuse background emission. MEGA records and images gamma-ray events by completely tracking both Compton and pair creation interactions in a tracker of double-sided silicon strip detectors and a calorimeter of CsI crystals able to resolve in three dimensions. We present initial laboratory calibration results from a small prototype MEGA telescope.

KiloHertz Quasi-Periodic oscillations in Island State of 4U 1608-52 as observed with RXTE/PCA

We report RXTE/PCA observations of 4U 1608-52 on March 15, 18, and 22 immediately after the outburst in early 1996. The persistent count rates ranged from 190 to 450 counts s-1 (1-60 keV). During this period of time, 4U 1608-52 was in the island state. We detected quasi-periodic oscillation (QPO) features in the power density spectra (PDS) at 567-800 Hz on March 15 and 22, with source fractional root mean square (rms) amplitude of 13%-17% and widths of 78-180 Hz. The average rms amplitude of these QPO features is positively correlated with the energy. Our results imply that the neutron star spin frequency is possibly between 300 and 365 Hz.

A Pair Production Telescope for Medium-Energy Gamma-Ray Polarimetry

abstract We describe the science motivation and development of a pair production telescope for medium-energy( 5–200 MeV) gamma-ray polarimetry. Our instrument concept, the Advanced Energetic Pair Telescope(AdEPT), takes advantage of the Three-Dimensional Track Imager, a low-density gaseous timeprojection chamber, to achieve angular resolution within a factor of two of the pair productionkinematics limit ( 0.6 at 70 MeV), continuum sensitivity comparable with the Fermi-LAT front detector( 10 GeV. However, neither instrument isoptimized for observations below 200 MeV or for polarizationsensitivity. Ground-based air Cherenkov telescopes have been usedto observe both galactic sources such as supernova remnants andextragalactic sources of very high energy (TeV) gamma-rays suchas active galactic nuclei (AGN) [3]. They have provided importantastrophysical information, but they also lack the capability todetect polarization. The Fermi and AGILE space-based telescopes,operating in the GeV energy range, are expected to continue tomake significant progress for the next several years. However,there remains a significant gap in our knowledge of astronomy inthe medium-energy ( 0.1–200 MeV) regime between the X-rayand high-energy gamma-ray energy ranges.The next major step in gamma-ray astrophysics, recognized asearly as the SAS-2 era [4], should be a medium-energy gamma-ray pair production telescope to fill this gap and provide answersto many important astrophysical questions. In the following, wedescribe the science motivation for this mission and the designof the Advanced Energetic Pair Telescope (AdEPT) a pair productiontelescope for medium-energy, 5to 200 MeV, gamma-raypolarimetry.2. Science motivationThe AdEPT pair production telescope for the detection of med-ium energy ( 5–200 MeV) gamma-rays with high angular resolu-tion and polarimetry capabilities will open a new window inobservational astronomy and astrophysics. Such an instrumentcan help provide answers to important questions in both astron-omy and physics. For example, it can shed light on the origin andacceleration of cosmic rays, the nature of the cosmic-ray accelera-tion of electrons in the Crab nebula to energies in excess of 10

Concept study for the next generation medium energy gamma-ray astronomy mission - MEGA

A new telescope for Medium Energy Gamma-Ray Astronomy, MEGA, is being developed for the energy band 0.4 - 50 MeV as a successor to COMPTEL on CGRO. MEGA aims to improve the sensitivity for astronomical sources by at least an order of magnitude with respect to past instruments and will fill a severe sensitivity gap between already scheduled hard-X-ray and high-energy gamma-ray missions. MEGA records and images gamma rays by completely tracking Compton and pair creation events in a stack of double sided Si-strip track detectors surrounded by a pixelated CsI calorimeter. MEGA will have an effective area of ~100 square cm, a large field of view of ~130 degrees, angular resolution of ~2 degrees, and energy resolution of ~8% (all FWHM at ~2 MeV). Key science objectives for MEGA are the investigation of cosmic high-energy accelerators, nucleosynthesis sites with gamma-ray lines, and the mapping of large-scale structures in the Galaxy and beyond. If operated on a zenith pointing satellite MEGA will be an ideal continuous all-sky monitor for transient sources. This paper describes the development of a small scale prototype and the concept of a space mission for MEGA.

POET: POlarimeters for Energetic Transients

POET (Polarimeters for Energetic Transients) is a Small Explorer mission concept proposed to NASA in January 2008. The principal scientific goal of POET is to measure GRB polarization between 2 and 500 keV. The payload consists of two wide FoV instruments: a Low Energy Polarimeter (LEP) capable of polarization measurements in the energy range from 2-15 keV and a high energy polarimeter (Gamma-Ray Polarimeter Experiment - GRAPE) that will measure polarization in the 60-500 keV energy range. Spectra will be measured from 2 keV up to 1 MeV. The POET spacecraft provides a zenith-pointed platform for maximizing the exposure to deep space. Spacecraft rotation will provide a means of effectively dealing with systematics in the polarization response. POET will provide sufficient sensitivity and sky coverage to measure statistically significant polarization for up to 100 GRBs in a two-year mission. Polarization data will also be obtained for solar flares, pulsars and other sources of astronomical interest.

RXTE Studies of X-Ray Spectral Variations with Accretion Rate in 4U 1915–05

We present the results of detailed spectral studies of the ultracompact low-mass X-ray binary (LMXB) 4U 1915-05 carried out with the Rossi X-Ray Timing Explorer (RXTE) during 1996. 4U 1915-05 is an X-ray burster (XRB) known to exhibit a ~199 day modulation in its 2-12 keV flux. Observations were performed with the PCA and HEXTE instruments on RXTE at roughly one-month intervals to sample this long-term period and study accretion rate-related spectral changes. We obtain good fits with a model consisting of a blackbody and an exponentially cut-off power law. The spectral parameters are strongly correlated with both the broadband (2-50 keV) luminosity and the position in the color-color diagram, with the source moving from a low, hard state to a high, soft state as the accretion rate increases. The blackbody component appears to drive the spectral evolution. Our results are consistent with a geometry in which the soft component arises from an optically thick boundary layer and the hard component from an extended Comptonizing corona. Comparing our results with those of a similar study of the brighter source 4U 1820-30, we find that the two ultracompact LMXBs occupy similar spectral states even though the transitions occur at very different total luminosities.

A Burst Chasing X-ray Polarimeter

Gamma-ray bursts are one of the most powerful explosions in the universe and have been detected out to distances of almost 13 billion light years. The exact origin of these energetic explosions is still unknown but the resulting huge release of energy is thought to create a highly relativistic jet of material and a power-law distribution of electrons. There are several theories describing the origin of the prompt GRB emission that currently cannot be distinguished. Measurements of the linear polarization would provide unique and important constraints on the mechanisms thought to drive these powerful explosions. We present the design of a sensitive, and extremely versatile gamma-ray burst polarimeter. The instrument is a photoelectric polarimeter based on a time-projection chamber. The photoelectric time-projection technique combines high sensitivity with broad band-pass and is potentially the most powerful method between 2 and 100 keV where the photoelectric effect is the dominant interaction process. We present measurements of polarized and unpolarized X-rays obtained with a prototype detector and describe the two mission concepts; the Gamma-Ray Burst Polarimeter (GRBP) for the U.S. Naval Academy satellite MidSTAR-2, and the Low Energy Polarimeter (LEP) onboard POET, a broadband polarimetry concept for a small explorer mission.

Prototype imaging Cd-Zn-Te array detector

The authors describe initial results of their program to develop and test Cd-Zn-Te (CZT) detectors with a pixellated array readout. Their primary interest is in the development of relatively thick CZT detectors for use in astrophysical coded aperture telescopes with response extending over the energy range {approximately}10--600 keV. The coded aperture imaging configuration requires only relatively large area pixels (1--3 mm), whereas the desired high energy response requires detector thicknesses of at least 3--5 mm. They have developed a prototype detector employing a 10 x 10 x 5 mm CZT substrate and 4 x 4 pixel (1.5 mm each) readout with gold metal contacts for the pixels and continuous gold contact for the bias on the opposite detector face. This MSM contact configuration was fabricated by RMD and tested at Harvard for uniformity, efficiency and spatial as well as spectral resolution. The authors have developed an ASIC readout (IDE-VA-1) and analysis system and report results, including {approximately}4% (FWHM) energy resolution at 60 keV. A prototype design for a full imaging detector array is discussed.

A concept for a high-energy gamma-ray polarimeter

We present a concept for an imaging gamma-ray polarimeter operating from ~50 MeV to ~1 GeV. Such an instrument would be valuable for the study of high-energy pulsars, active galactic nuclei, supernova remnants, and gamma-ray bursts. The concept makes use of pixelized gas micro-well detectors, under development at Goddard Space Flight Center, to record the electron-positron tracks from pair-production events in a large gas volume. Pixelized micro-well detectors have the potential to form large-volume 3-D track imagers with ~100 μm (rms) position resolution at moderate cost. The combination of high spatial resolution and a continuous low-density gas medium permits many thousands of measurements per radiation length, allowing the particle tracks to be imaged accurately before multiple scattering masks their original directions. The polarization of the incoming radiation may then be determined from the azimuthal distribution of the electron-positron pairs. We have performed Geant4 simulations of these processes to estimate the polarization sensitivity of a simple telescope geometry at 100 MeV.