As part of an on-going program of X-ray polarimeter development at NASA Goddard Space Flight Center, we have built a Hard X-ray Photoelectric Polarimeter (HXPP) designed for astrophysical observations in 10-60 keV band. HXPP’s detection method is based on photoelectric effect and a gas micro-pattern time projection chamber technique. This allows us to tune polarimeter sensitivity to the bandpass of interest by selection of gas type and gas pressure. Here we report on the first experimental results for measuring X-ray signal above 20 keV with HXPP filled with a mixture of 80% argon and 20% dimethyl ether at the total pressure of 2.5 atm.
We present the first application of a time projection chamber polarimeter to measure high energy X-ray polarization above 10 keV. The polarimeter is designed based on the PRAXyS soft X-ray polarimeter. The sealed gas is changed to a gas mixture of 60% argon and 40% dimethyl ether at 1 atm to be sensitive to high energy X-rays. The polarimeter performance is verified with linearly polarized, monochromatic X-rays at a synchrotron radiation facility, KEK Photon Factory BL-14A. The measured modulation factors are 42.4 ± 0.6%, 50.4 ± 0.6%, and 55.0 ± 0.6% at 12, 14, and 16 keV, respectively, and the measured polarization angles are consistent with the expected values at all energies.
The Polarimeter for Relativistic Astrophysical X-ray Sources (PRAXyS) is one of three Small Explorer (SMEX)
missions selected by NASA for Phase A study, with a launch date in 2020. The PRAXyS Observatory exploits grazing
incidence X-ray mirrors and Time Projection Chamber Polarimeters capable of measuring the linear polarization of
cosmic X-ray sources in the 2-10 keV band. PRAXyS combines well-characterized instruments with spacecraft rotation
to ensure low systematic errors. The PRAXyS payload is developed at the Goddard Space Flight Center with the Johns
Hopkins University Applied Physics Laboratory, University of Iowa, and RIKEN (JAXA) collaborating on the
Polarimeter Assembly. The LEOStar-2 spacecraft bus is developed by Orbital ATK, which also supplies the extendable
optical bench that enables the Observatory to be compatible with a Pegasus class launch vehicle.
A nine month primary mission will provide sensitive observations of multiple black hole and neutron star sources, where
theory predicts polarization is a strong diagnostic, as well as exploratory observations of other high energy sources.
The primary mission data will be released to the community rapidly and a Guest Observer extended mission will be
vigorously proposed.
The Polarimeter for Relativistic Astrophysical X-ray Sources (PRAXyS) is one of three Small Explorer (SMEX)
missions selected by NASA for Phase A study. The PRAXyS observatory carries an X-ray Polarimeter Instrument (XPI)
capable of measuring the linear polarization from a variety of high energy sources, including black holes, neutron stars,
and supernova remnants. The XPI is comprised of two identical mirror-Time Projection Chamber (TPC) polarimeter
telescopes with a system effective area of 124 cm2 at 3 keV, capable of photon limited observations for sources as faint
as 1 mCrab. The XPI is built with well-established technologies. This paper will describe the performance of the XPI
flight mirror with the engineering test unit polarimeter.
A fundamental challenge in a spaceborne application of a gas-based Time Projection Chamber (TPC) for observation of X-ray polarization is handling the large amount of data collected. The TPC polarimeter described uses the APV-25 Application Specific Integrated Circuit (ASIC) to readout a strip detector. Two dimensional photo- electron track images are created with a time projection technique and used to determine the polarization of the incident X-rays. The detector produces a 128x30 pixel image per photon interaction with each pixel registering 12 bits of collected charge. This creates challenging requirements for data storage and downlink bandwidth with only a modest incidence of photons and can have a significant impact on the overall mission cost. An approach is described for locating and isolating the photoelectron track within the detector image, yielding a much smaller data product, typically between 8x8 pixels and 20x20 pixels. This approach is implemented using a Microsemi RT-ProASIC3-3000 Field-Programmable Gate Array (FPGA), clocked at 20 MHz and utilizing 10.7k logic gates (14% of FPGA), 20 Block RAMs (17% of FPGA), and no external RAM. Results will be presented, demonstrating successful photoelectron track cluster detection with minimal impact to detector dead-time.
Polarimeters for Energetic Transients (POET) is a mission concept designed to t within the envelope of a NASA Small Explorer (SMEX) mission. POET will use X-ray and gamma-ray polarimetry to uncover the energy release mechanism associated with the formation of stellar-mass black holes and investigate the physics of extreme magnetic ields in the vicinity of compact objects. Two wide-FoV, non-imaging polarimeters will provide polarization measurements over the broad energy range from about 2 keV up to about 500 keV. A Compton scatter polarimeter, using an array of independent scintillation detector elements, will be used to collect data from 50 keV up to 500 keV. At low energies (2{15 keV), data will be provided by a photoelectric polarimeter based on the use of a Time Projection Chamber for photoelectron tracking. During a two-year baseline mission, POET will be able to collect data that will allow us to distinguish between three basic models for the inner jet of gamma-ray bursts.
X-ray polarization measurements hold great promise for studying the geometry and emission mechanisms in the strong gravitational and magnetic fields that surround black holes and neutron stars. In spite of this, the observational situation remains very limited; the last instrument dedicated to X-ray polarimetry flew decades ago on OSO-8, and the few recent measurements have been made by instruments optimized for other purposes. However, the technical capabilities to greatly advance the observational situation are in hand. Recent developments in micro-pattern gas detectors allow use of the polarization sensitivity of the photo-electric effect, which is the dominant interaction in the band above 2 keV. We present the scientific and technical requirements for an X-ray polarization observatory consistent with the scope of a NASA Small Explorer (SMEX) mission, along with a representative catalog of what the observational capabilities and expected sensitivities for the first year of operation could be. The mission is based on the technically robust design of the Gravity and Extreme Magnetism SMEX (GEMS) which completed a Phase B study and Preliminary Design Review in 2012. The GEMS mission is enabled by time projection detectors sensitive to the photo-electric effect. Prototype detectors have been designed, and provide engineering and performance data which support the mission design. The detectors are further characterized by low background, modest spectral resolution, and sub-millisecond timing resolution. The mission also incorporates high efficiency grazing incidence X-ray mirrors, design features that reduce systematic errors (identical telescopes at different azimuthal angles with respect to the look axis, and mounted on a rotating spacecraft platform), and a moderate capability to perform Target of Opportunity observations. The mission operates autonomously in a low earth, low inclination orbit with one to ten downlinks per day and one or more uplinks per week. Data and calibration products will be made available through the High Energy Astrophysics Science and Archival Research Center (HEASARC).
The design of the Time-Projection Chamber (TPC) Polarimeter for the Gravity and Extreme Magnetism Small Explorer (GEMS) was demonstrated to Technology Readiness Level 6 (TRL-6)3 and the flight detectors fabricated, assembled and performance tested. A single flight detector was characterized at the Brookhaven National Laboratory Synchrotron Light Source with polarized X-rays at 10 energies from 2.3–8.0 keV at five detector positions. The detector met all of the GEMS performance requirements. Lifetime measurements have shown that the existing flight design has 23 years of lifetime4, opening up the possibility of relaxing material requirements, in particular the consideration of the use of epoxy, to reduce risk elsewhere. We report on design improvements to the GEMS detector to enable a narrower transfer gap that, when operated with a lower transfer field, reduces asymmetries in the detector response. In addition, the new design reduces cost and risk by simplifying the assembly and reducing production time. Finally, we report on the performance of the narrow-gap detector in response to polarized and unpolarized X-rays.
We report a Monte-Carlo estimation of the in-orbit performance of a cosmic X-ray polarimeter designed to be installed on the focal plane of a small satellite. The simulation uses GEANT for the transport of photons and energetic particles and results from Magboltz for the transport of secondary electrons in the detector gas. We validated the simulation by comparing spectra and modulation curves with actual data taken with radioactive sources and an X-ray generator. We also estimated the in-orbit background induced by cosmic radiation in low Earth orbit.
We present the gain properties of the gas electron multiplier (GEM) foil in pure dimethyl ether (DME) at 190 Torr. The GEM is one of the micro pattern gas detectors and it is adopted as a key part of the X-ray polarimeter for the GEMS mission. The X-ray polarimeter is a time projection chamber operating in pure DME gas at 190 Torr. We describe experimental results of (1) the maximum gain the GEM can achieve without any discharges, (2) the linearity of the energy scale for the GEM operation, and (3) the two-dimensional gain variation of the active area. First, our experiment with 6.4 keV X-ray irradiation of the whole GEM area demonstrates that the maximum effective gain is 2 x 104 with the applied voltage of 580 V. Second, the measured energy scale is linear among three energies of 4.5, 6.4, and 8.0 keV. Third, the two-dimensional gain mapping test derives the standard deviation of the gain variability of 7% across the active area.
The primary science goal of the Polarimeters for Energetic Transients (POET) mission is to measure the polarization of gamma-ray bursts over a wide energy range, from X rays to soft gamma rays. The higher-energy portion of this band (50 - 500 keV) will be covered by the High Energy Polarimeter (HEP) instrument, a non-imaging, wide field of view Compton polarimeter. Incident high-energy photons will Compton scatter in low-Z, plastic scintillator detector elements and be subsequently absorbed in high-Z, CsI(Tl) scintillator elements; polarization is detected by measuring an asymmetry in the azimuthal scatter angle distribution. The HEP design is based on our considerable experience with the development and flight of the Gamma-Ray Polarimeter Experiment (GRAPE) balloon payload. We present the design of the POET HEP instrument, which incorporates lessons learned from the GRAPE balloon design and previous work on Explorer proposal efforts, and its expected performance on a two-year SMEX mission.
Polarimetry is a powerful tool for astrophysical observations that has yet to be exploited in the X-ray band. For satellite-borne and sounding rocket experiments, we have developed a photoelectric gas polarimeter to measure X-ray polarization in the 2–10 keV range utilizing a time projection chamber (TPC) and advanced micro-pattern gas electron multiplier (GEM) techniques. We carried out performance verification of a flight equivalent unit (1/4 model) which was planned to be launched on the NASA Gravity and Extreme Magnetism Small Explorer (GEMS) satellite. The test was performed at Brookhaven National Laboratory, National Synchrotron Light Source (NSLS) facility in April 2013. The polarimeter was irradiated with linearly-polarized monochromatic X-rays between 2.3 and 10.0 keV and scanned with a collimated beam at 5 different detector positions. After a systematic investigation of the detector response, a modulation factor ≥35% above 4 keV was obtained with the expected polarization angle. At energies below 4 keV where the photoelectron track becomes short, diffusion in the region between the GEM and readout strips leaves an asymmetric photoelectron image. A correction method retrieves an expected modulation angle, and the expected modulation factor, ~20% at 2.7 keV. Folding the measured values of modulation through an instrument model gives sensitivity, parameterized by minimum detectable polarization (MDP), nearly identical to that assumed at the preliminary design review (PDR).
The Gravity and Extreme Magnetism Small Explorer (GEMS) X-ray polarimeter Instrument (XPI) was designed to
measure the polarization of 23 sources over the course of its 9 month mission. The XPI design consists of two telescopes
each with a polarimeter assembly at the focus of a grazing incidence mirror. To make sensitive polarization
measurements the GEMS Polarimeter Assembly (PA) employed a gas detection system based on a Time Projection
Chamber (TPC) technique. Gas detectors are inherently at risk of degraded performance arising from contamination
from outgassing of internal detector components or due to loss of gas.
This paper describes the design and the materials used to build a prototype of the flight polarimeter with the required
GEMS lifetime. We report the results from outgassing measurements of the polarimeter subassemblies and assemblies,
enclosure seal tests, life tests, and performance tests that demonstrate that the GEMS lifetime is achievable. Finally we
report performance measurements and the lifetime enhancement from the use of a getter.
The Gravity and Extreme Magnetism Small explorer (GEMS) is an X-ray polarization telescope selected as a
NASA small explorer satellite mission. The X-ray Polarimeter on GEMS uses a Time Projection Chamber gas
proportional counter to measure the polarization of astrophysical X-rays in the 2-10 keV band by sensing the
direction of the track of the primary photoelectron excited by the incident X-ray.
We have simulated the expected sensitivity of the polarimeter to polarized X-rays. We use the simulation
package Penelope to model the physics of the interaction of the initial photoelectron with the detector gas and
to determine the distribution of charge deposited in the detector volume. We then model the charge diffusion in
the detector, and produce simulated track images. Within the track reconstruction algorithm we apply cuts on
the track shape and focus on the initial photoelectron direction in order to maximize the overall sensitivity of
the instrument. Using this technique we have predicted instrument modulation factors μ100 for 100% polarized
X-rays ranging from 10% to over 60% across the 2-10 keV X-ray band.
We also discuss the simulation program used to develop and model some of the algorithms used for triggering,
and energy measurement of events in the polarimeter.
J. Hill, R. Baker, J. Black, M. Browne, W. Baumgartner, E. Caldwell, J. Cantwell, A. Davies, A. Desai, P. Dickens, N. Dobson, R. Foxwell, A. Francomacaro, D. Gall, K. Gregory, S. Griffiths, A. Hayato, R. Hampshire, T. Hwang, M. Jhabvala , K. Jahoda, P. Kaaret, S. Lehtonen, N. Martin, J. Mohammed, K. Montt de Garcia, A. Morell, D. Nolan, R. Russell, M. Sampson, J. Sanders, K. Simms, M. Singer, J. Swank, T. Tamagawa, A. Weaver, S. Yerushalmi, J. Xu
The Gravity and Extreme Magnetism Small Explorer (GEMS) will realize its scientific objectives through high sensitivity linear X-ray polarization measurements in the 2-10 keV band. The GEMS X-ray polarimeters, based on the photoelectric effect, provide a strong polarization response with high quantum efficiency over a broad band-pass by a novel implementation of the time projection chamber (TPC). This paper will discuss the basic principles of the TPC polarimeter and describe the details of the mechanical and electrical design of the GEMS flight polarimeter. We will present performance measurements from two GEMS engineering test units in response to polarized and unpolarized X-rays and before and after thermal and vibration tests performed to demonstrate that the design is at a technology readiness level 6 (TRL-6).
The scientific objective of the X-ray Advanced Concepts Testbed (XACT) is to measure the X-ray polarization
properties of the Crab Nebula, the Crab pulsar, and the accreting binary Her X-1. Polarimetry is a powerful tool for
astrophysical investigation that has yet to be exploited in the X-ray band, where it promises unique insights into neutron
stars, black holes, and other extreme-physics environments. With powerful new enabling technologies, XACT will
demonstrate X-ray polarimetry as a practical and flight-ready astronomical technique. Additional technologies that
XACT will bring to flight readiness will also provide new X-ray optics and calibration capabilities for NASA missions
that pursue space-based X-ray spectroscopy, timing, and photometry.
The Gravity and Extreme Magnetism Small Explorer (GEMS) is an astrophysical observatory dedicated to X-ray
polarimetry (2-10 keV) and is being developed for launch in 2014. To maximize the polarization sensitivity of the
observatory, GEMS uses polarimeters based on the photoelectric effect with a gas micropattern time projection chamber
(TPC). We describe the TPC polarimeter concept and the details of the GEMS implementation, including factors that
affect the ultimate polarization sensitivity, including quantum efficiency, modulation factor, systematic errors, and
background.
A gamma-ray burst polarimeter (GRBP) is being developed at NASA Goddard Space Flight Center for measuring the Xray
polarization of energetic transients in the 2 - 10 keV energy range. The primary goal is to measure the polarization
of the prompt X-ray emission from gamma-ray bursts (GRBs) in order to distinguish between the possible emission
mechanisms. The instrument could also be capable of measuring polarization from other X-ray transients, such as soft
gamma repeaters (SGRs) or black hole transients. An instrument with a wide field of view is required to detect transient
events and a large collecting area is required to have sufficient sensitivity. The GRBP is a time projection chamber
(TPC) that uses negative ions as a charge carrier enabling a large volume, high spatial resolution detector. We describe a
GRBP prototype that is suitable for a sounding rocket measurement of the Crab Nebula or for measurements of bright
transient sources from a small satellite.
KEYWORDS: Point spread functions, Calibration, X-ray telescopes, Charge-coupled devices, Monte Carlo methods, Galaxy groups and clusters, Wavelet transforms, Telescopes, Error analysis, X-rays
We are exploiting the Swift X-ray Telescope (XRT) deepest GRB follow-up observations to study the cosmic
X-Ray Background (XRB) population in the 0.2-10 keV energy band. We present some preliminary results of a
serendipitous survey performed on 221 fields observed with exposure longer than 10 ks. We show that the XRT is
a profitable instrument for surveys and that it is particularly suitable for the search and observation of extended
objects like clusters of galaxies. We used the brightest serendipitous sources and the longest observations to test
the XRT optics performance and the background characteristics all over the field of view, in different energy
bands during the first 2.5 years of fully operational mission.
Joanne Hill, Scott Barthelmy, J. Kevin Black, Philip Deines-Jones, Keith Jahoda, Takanori Sakamoto, Philip Kaaret, Mark McConnell, Peter Bloser, John Macri, Jason Legere, James Ryan, Billy Smith, Bing Zhang
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.
The Swift X-ray Telescope (XRT) is a CCD based X-ray telescope designed for localization, spectroscopy and long term
light curve monitoring of Gamma-Ray Bursts and their X-ray afterglows. Since the launch of Swift in November 2004,
the XRT has undergone significant evolution in the way it is operated. Shortly after launch there was a failure of the
CCD thermo-electric cooling system, which led to the XRT team being required to devise a method of keeping the CCD
temperature below −50C utilizing only passive cooling by minimizing the exposure of the XRT radiator to the Earth. We
present in this paper an update on how the modeling of this passive cooling method has improved in first ~1000 days
since the method was devised, and the success rate of this method in day-to-day planning. We also discuss the changes
to the operational modes and onboard software of the XRT. These changes include improved rapid data product
generation in order to improve speed of rapid Gamma-Ray Burst response and localization to the community; changes to
the way XRT observation modes are chosen in order to better fine tune data acquisition to a particular science goal;
reduction of "mode switching" caused by the contamination of the CCD by Earth light or high temperature effects.
The Swift X-ray Telescope (XRT) focal plane camera is a front-illuminated MOS CCD, providing a spectral response kernel of 144 eV FWHM at 6.5 keV. We describe the CCD calibration program based on celestial and on-board calibration sources, relevant in-flight experiences, and developments in the CCD response model. We illustrate how the revised response model describes the calibration sources well. Loss of temperature control motivated a laboratory program to re-optimize the CCD substrate voltage, we describe the small changes in the CCD response that would result from use of a substrate voltage of 6V.
The Swift X-ray Telescope (XRT) is a CCD based X-ray telescope designed for localization, spectroscopy and long term light curve monitoring of Gamma-Ray Bursts and their X-ray afterglows. Shortly after launch there was a failure of the thermo-electric cooler on the XRT CCD. Due to this the Swift XRT Team had the unexpected challenge of ensuring that the CCD temperature stayed below -50C utilizing only passive cooling through a radiator mounted on the side of the Swift. Here we show that the temperature of the XRT CCD is correlated with the average elevation of the Earth above the XRT radiator, which is in turn related to the targets that Swift observes in an orbit. In order to maximize passive cooling of the XRT CCD, the XRT team devised several novel methods for ensuring that the XRT radiator's exposure to the Earth was minimized to ensure efficient cooling. These methods include: picking targets on the sky for Swift to point at which are known to put the spacecraft into a good orientation for maximizing XRT cooling; biasing the spacecraft roll angle to point the XRT radiator away from the Earth as much as possible; utilizing time in the SAA, in which all of the instruments on-board Swift are non-operational, to point at "cold targets"; and restricting observing time on "warm" targets to only the periods at which the spacecraft is in a favorable orientation for cooling. By doing this at the observation planning stage we have been able to minimize the heating of the CCD and maintain the XRT as a fully operational scientific instrument, without compromising the science goals of the Swift mission.
The X-Ray Telescope (XRT) on board the Swift satellite is a sensitive imaging spectrometer utilizing a MAT-22 CCD at the Focal plane. The system was designed to operate the CCD at -100 °C +/- 1 °C for the duration of the mission. Due to a failure of the temperature control sub-system, the CCD operates under variable thermal conditions dictated by the view factor of the radiator- heatpipe sub-system to the Earth and sun. A temperature variation of up to 5° C is seen during a single orbit due to the satellite transition from sun light into eclipse and the full operational regime of the instrument ranges from temperatures of -75°C to -45°C due to the persistent heating/cooling effects of satellite orientation to the sun and earth. To maintain the highest quality data products possible from the XRT data stream, a recalibration of the XRT is required to account for this variable thermal environment. We present the methodology for and results from a temperature dependent analysis of on-orbit XRT data, collected during the Swift commissioning phase, used to produce gain, bias and warm pixel calibration products. We also discuss the quality of XRT science products capable with these temperature dependent calibration files and future plans for updates to these calibration products.
The XRT is a sensitive, autonomous X-ray imaging spectrometer onboard the Swift Gamma-Ray Burst Observatory. The unique observing capabilities of the XRT allow it to autonomously refine the Swift BAT positions (~1-4' uncertainty) to better than 2.5 arcsec in XRT detector coordinates, within 5 seconds of target acquisition by the Swift Observatory for typical bursts, and to measure the flux, spectrum, and light curve of GRBs and afterglows over a wide dynamic range covering more than seven orders of magnitude in flux (62 Crab to < 1 mCrab). The results of the rapid positioning capability of the XRT are presented here for both known sources and newly discovered GRBs, demonstrating the ability to automatically utilise one of two integration times according to the burst brightness, and to correct the position for alignment offsets caused by the fast pointing performance and variable thermal environment of the satellite as measured by the Telescope Alignment Monitor. The onboard results are compared to the positions obtained by groundbased follow-up. After obtaining the position, the XRT switches between four CCD readout modes, automatically optimising the scientific return from the source depending on the flux of the GRB. Typical data products are presented here.
The Swift X-ray Telescope (XRT) is designed to make astrometric, spectroscopic, and photometric observations of X-ray emission from Gamma-ray Bursts and their afterglows in the energy band 0.2-10 keV. In order to provide rapid-response, automated observations of these randomly occurring objects without ground intervention, the XRT must be able to observe objects covering some seven orders of magnitude in flux, extracting the maximum possible science from each one. This requires a variety of readout modes designed to optimise the information collected in response to shifting scientific priorities as the flux from the burst diminishes.
The XRT will support four major readout modes: imaging, two timing modes and photon-counting, with several sub-modes. We describe in detail the readout modes of the XRT. We describe the flux ranges over which each mode will operate, the automated mode switching that will occur and the methods used for collection of bias information for this instrument. We also discuss the data products produced from each mode.
The Swift X-ray Telescope (XRT) is designed to make astrometric, spectroscopic and photometric observations of the X-ray emission from Gamma-ray bursts and their afterglows, in the energy band 0.2 - 10 keV. Here we report first results of the analysis of Swift XRT effective area at five different energies as measured during the end-to-end calibration campaign at the Panter X-ray beam line facility. The analysis comprises the study of the effective area both on-axis and off-axis for different event grade selection. We compare the laboratory results with the expectations and show that the measured effective area meets the mission scientific requirements.
The Swift Gamma-Ray Explorer is designed to make prompt multiwavelength observations of Gamma-Ray Bursts (GRBs) and GRB Afterglows. The X-ray Telescope (XRT) provides key capabilities that permit Swift to determine GRB positions with a few arcseconds accuracy within 100 seconds of the burst onset. The XRT utilizes a superb mirror set built for JET-X and a state-of-the-art XMM/EPIC MOS CCD detector to provide a sensitive broad-band (0.2-10 keV) X-ray imager with effective area of 135 cm2 at 1.5 keV, field of view of 23.6 x 23.6 arcminutes, and angular resolution of 18 arcseconds (HEW). The detection sensitivity is 2x10-14 erg/cm2/s in 104 seconds. The instrument is designed to provide automated source detection and position reporting within 5 seconds of target acquisition. It can also measure redshifts of GRBs for bursts with Fe line emission or other spectral features. The XRT will operate in an auto-exposure mode, adjusting the CCD readout mode automatically to optimize the science return for each frame as the source fades. The XRT will measure spectra and lightcurves of the GRB afterglow beginning about a minute after the burst and will follow each burst for days as it fades from view.
The SWIFT X-ray Telescope (XRT) is designed to make astrometric, spectroscopic and photometric observations of the X-ray emission from Gamma-ray bursts and their afterglows, in the energy band 0.2 - 10 keV. Here we report the results of the analysis of SWIFT XRT Point Spread Function (PSF) as measured during the end-to-end calibration campaign at the Panter X-Ray beam line facility. The analysis comprises the study of the PSF both on-axis and off-axis. We compare the laboratory results with the expectations from the ray-tracing software and from the mirror module tested as a single unit. We show that the measured HEW meets the mission scientific requirements. On the basis of the calibration data we build an analytical model which is able to reproduce the PSF as a function of the energy and the position within the detector.
The Swift Gamma-Ray Burst Explorer will be launched late in 2003 to make prompt multiwavelength observations of Gamma-Ray Bursts and Afterglows. The X-ray Telescope (XRT) provides key capabilities that permit Swift to determine GRB positions with several arcsecond accuracy within 100 seconds of the burst onset. The XRT is designed to observe GRB afterglows covering over seven orders of magnitude in flux in the 0.2-10 keV band, with completely autonomous operation. GRB positions are determined within seconds of target acquisition, and accurate positions are sent to the ground for distribution over the GCN. The XRT can also measure redshifts of GRBs for bursts with Fe line emission or other spectral features.
The Swift X-ray Telescope (XRT)[1] is designed to make astrometric, spectroscopic, and photometric observations of X-ray emission from Gamma-ray Bursts and their afterglows in the energy band 0.2-10 keV. The XRT has a variety of readout modes which it automatically selects in order to observe objects covering 7 orders of magnitude in flux and to extract the maximum possible science from each one, in response to the flux from the burst diminishing. The primary goal of the XRT is to locate the position of the Gamma-Ray Burst to 1 arcsec and to transmit this position to the UVOT and the ground within 100 seconds of the initial observation of the burst. We describe in detail the use of imaging mode and a centroid algorithm to determine the position of the Gamma-Ray Burst with sub-pixel accuracy.
The Penn State University Department of Astronomy and Astrophysics has been active in the design of X-ray CCD cameras for astronomy for over two decades, including sounding rocket systems, the CUBIC instrument on the SAC-B satellite and the ACIS camera on the Chandra satellite. Currently the group is designing and building an X-ray telescope (XRT), which will comprise part of the Swift Gamma-Ray Burst Explorer satellite. The Swift satellite, selected in October 1999 as one of two winners of NASA Explorer contracts, will -- within one minute -- detect, locate, and observe gamma-ray bursts simultaneously in the optical, ultraviolet, X-ray, and gamma- ray wavelengths using three co-aligned telescopes. The XRT electronics is required to read out the telescope's CCD sensor in a number of different ways depending on the observing mode selected. Immediately after the satellite re-orients to observe a newly detected burst, the XRT will enter an imaging mode to determine the exact position of the burst. The location will then be transmitted to the ground, and the XRT will autonomously enter other modes as the X-ray intensity of the burst waxes and wanes. This paper will discuss the electronics for a laboratory X-ray CCD camera, which serves as a test bed for development of the Swift XRT camera. It will also touch upon the preliminary design of the flight camera, which is closely related. A major challenge is achieving performance and reliability goals within the cost constraints of an Explorer mission.
The Swift Gamma Ray Burst Explorer will be launched in 2003 to observe hundreds of gamma-ray bursts per year and study their X-ray and optical afterglows, using a multiwavelength complement of three instruments: a wide-field Burst Alert Telescope (BAT), an X-Ray Telescope (XRT), and a UV/Optical Telescope (UVOT). The XRT is designed to study X-ray counterparts of the gamma-ray bursts and their afterglows, beginning 20 - 70 s from the time of the burst, and continuing for days or weeks. The XRT utilizes a superb mirror set built for JET-X and a state-of-the-art XMM/EPIC CCD detector to provide a sensitive broad-band (0.2 - 10 keV) X-ray imager with effective area of 110 cm2 at 1.5 keV, field of view of 23.6 X 23.6 arcminutes, and angular resolution of 15 arcseconds (HEW). The sensitivity is 2 X 10-14 erg/cm2s in 104 seconds. The telescope electronics are designed to provide automated source detection and position reporting, with a position good to 2.5 arcseconds transmitted to the ground within 100 seconds of the burst detection. The XRT will operate in an auto-exposure mode, adjusting the CCD readout mode automatically to optimize the science return for each frame as the source fades. The XRT will measure spectra and lightcurves of the GRB afterglow beginning within about a minute after the burst and will follow each burst until it fades from view, typically monitoring 2 - 3 'old' bursts at a time while waiting for a new burst to be detected.
The energy resolution degradation of the ACIS CCDs on board the Chandra X-ray Observatory has been under investigation since the effect was first recognized two months after launch. A series of laboratory CCD irradiations with electrons and protons have taken place, leading to the belief that low energy protons are responsible for the damage. In order to confirm this, an experiment has been devised to represent the flight experience of the ACIS CCDs, and the results to date are shown here.
The optical chain of the spectroscopic x-ray telescopes aboard the Constellation-X spacecraft employs a reflective grating spectrometer to provide high resolution spectra for multiple spectra as a slitless spectrometer in the spectral feature rich, soft x-ray band. As a part of the spectroscopic readout array, we provide a zero-order camera that images the sky in the soft band inaccessible to the microcalorimeters. Technological enhancements required for producing the RGS instruments are described, along with prototype development progress, fabrication and testing results.
Measuring the polarization of x-rays emitted from cosmological objects yields explanations of the structure which characterize these sources. Polarization detection efficiencies of up to 18% have been measured for two, small pixel, charge coupled devices (CCDs) using an 80% polarized monochromatic synchrotron beam between energies of 7.5 keV and 35 keV. The device efficiencies at less than 15 keV are of particular interest for astronomical purposes where imaging, spectroscopy and polarization measurements can be carried out simultaneously. Polarization measurements using a CCD rely on the preferential direction of the ejected photoelectron along the E-field of the incident x-ray beam. The resultant charge cloud is sampled by the pixellated array of the CCD. It will be shown that the CCD polarization detection efficiency (modulation factor) is a function of the pixel size and the energy of the incident photons. The effect of depletion depth and impact of a field-free layer in the detector are reviewed. The two devices used were a commercial optical CCD, Kodak KAF1400, with 6.8 by 6.8 micrometer squared pixels and a specialized CCD, designed by EEV Ltd., deeply depleted with 4 by 9 micrometer squared pixels.
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