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.
Keith Gendreau, Zaven Arzoumanian, Phillip Adkins, Cheryl Albert, John Anders, Andrew Aylward, Charles Baker, Erin Balsamo, William Bamford, Suyog Benegalrao, Daniel Berry, Shiraz Bhalwani, J. Kevin Black, Carl Blaurock, Ginger Bronke, Gary Brown, Jason Budinoff, Jeffrey Cantwell, Thoniel Cazeau, Philip Chen, Thomas Clement, Andrew Colangelo, Jerry Coleman, Jonathan Coopersmith, William Dehaven, John Doty, Mark Egan, Teruaki Enoto, Terry Fan, Deneen Ferro, Richard Foster, Nicholas Galassi, Luis Gallo, Chris Green, Dave Grosh, Kong Ha, Monther Hasouneh, Kristofer Heefner, Phyllis Hestnes, Lisa Hoge, Tawanda Jacobs, John Jørgensen, Michael Kaiser, James Kellogg, Steven Kenyon, Richard Koenecke, Robert Kozon, Beverly LaMarr, Mike Lambertson, Anne Larson, Steven Lentine, Jesse Lewis, Michael Lilly, Kuochia Alice Liu, Andrew Malonis, Sridhar Manthripragada, Craig Markwardt, Bryan Matonak, Isaac Mcginnis, Roger Miller, Alissa Mitchell, Jason Mitchell, Jelila Mohammed, Charles Monroe, Kristina Montt de Garcia, Peter Mulé, Louis Nagao, Son Ngo, Eric Norris, Dwight Norwood, Joseph Novotka, Takashi Okajima, Lawrence Olsen, Chimaobi Onyeachu, Henry Orosco, Jacqualine Peterson, Kristina Pevear, Karen Pham, Sue Pollard, John Pope, Daniel Powers, Charles Powers, Samuel Price, Gregory Prigozhin, Julian Ramirez, Winston Reid, Ronald Remillard, Eric Rogstad, Glenn Rosecrans, John Rowe, Jennifer Sager, Claude Sanders, Bruce Savadkin, Maxine Saylor, Alexander Schaeffer, Nancy Schweiss, Sean Semper, Peter Serlemitsos, Larry Shackelford, Yang Soong, Jonathan Struebel, Michael Vezie, Joel Villasenor, Luke Winternitz, George Wofford, Michael Wright, Mike Yang, Wayne Yu
During 2014 and 2015, NASA's Neutron star Interior Composition Explorer (NICER) mission proceeded success- fully through Phase C, Design and Development. An X-ray (0.2-12 keV) astrophysics payload destined for the International Space Station, NICER is manifested for launch in early 2017 on the Commercial Resupply Services SpaceX-11 flight. Its scientific objectives are to investigate the internal structure, dynamics, and energetics of neutron stars, the densest objects in the universe. During Phase C, flight components including optics, detectors, the optical bench, pointing actuators, electronics, and others were subjected to environmental testing and integrated to form the flight payload. A custom-built facility was used to co-align and integrate the X-ray "con- centrator" optics and silicon-drift detectors. Ground calibration provided robust performance measures of the optical (at NASA's Goddard Space Flight Center) and detector (at the Massachusetts Institute of Technology) subsystems, while comprehensive functional tests prior to payload-level environmental testing met all instrument performance requirements. We describe here the implementation of NICER's major subsystems, summarize their performance and calibration, and outline the component-level testing that was successfully applied.
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.
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 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.
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.
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 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.
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 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.
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.
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.
Photoelectric X-ray polarimeters based on pixel micropattern gas detectors (MPGDs) offer order-of-magnitude improvement in sensitivity over more traditional techniques based on X-ray scattering. This new technique places some of the most interesting astronomical observations within reach of even a small, dedicated mission. The most sensitive instrument would be a photoelectric polarimeter at the focus of a very large mirror, such as the planned XEUS. Our efforts are focused on a smaller pathfinder mission, which would achieve its greatest sensitivity with large-area, low-background, collimated polarimeters. We have recently demonstrated a MPGD polarimeter using amorphous silicon thin-film transistor (TFT) readout suitable for the focal plane of an X-ray telescope. All the technologies used in the demonstration polarimeter are scalable to the areas required for a high-sensitivity collimated polarimeter.
We describe the design of Lobster-ISS, an X-ray imaging all-sky monitor (ASM) to be flown as an attached payload on the International Space Station. Lobster-ISS is the subject of an ESA Phase-A study which will begin in December 2001. With an instantaneous field of view 162 x 22.5 degrees, Lobster-ISS will map almost the complete sky every 90 minute ISS orbit, generating a confusion-limited catalogue of ~250,000 sources every 2 months. Lobster-ISS will use focusing microchannel plate optics and imaging gas proportional micro-well detectors; work is currently underway to improve the MCP optics and to develop proportional counter windows with enhanced transmission and negligible rates of gas leakage, thus improving instrument throughput and reducing mass. Lobster-ISS provides an order of magnitude improvement in the sensitivity of X-ray ASMs, and will, for the first time, provide continuous monitoring of the sky in the soft X-ray region (0.1-3.5 keV). Lobster-ISS provides long term monitoring of all classes of variable X-ray source, and an essential alert facility, with rapid detection of transient X-ray sources such as Gamma-Ray Burst afterglows being relayed to contemporary pointed X-ray observatories. The mission, with a nominal lifetime of 3 years, is scheduled for launch on the Shuttle c.2009.
Gas proportional counter arrays based on the micro-well are an example of a new generation of detectors that exploit narrow anode-cathode gaps, rather than fine anodes, to create gas gain. These are inherently imaging pixel detectors that can be made very large for reasonable costs. Because of their intrinsic gain and room-temperature operation, they can be instrumented at very low power per unit area, making them valuable for a variety of space-flight applications where large-area X-ray imaging or particle tracking is required. We discuss micro-well detectors as focal plane imager for Lobster-ISS, a proposed soft X-ray all-sky monitor, and as electron trackers for the Next Generation High-Energy Gamma Ray mission. We have developed a fabrication technique using a masked UV laser that allows us both to machine micro-wells in polymer substrates and to pattern metal electrodes. We have used this technique to fabricate detectors which image X-rays by simultaneously reading out orthogonal anode and cathode strips. We present imaging results from these detectors, as well as gain and energy resolution measurements that agree well with results from other groups.
The MOnitoring X-ray Experiment (MOXE) is an X-ray all-sky monitor to be launched on the Russian Spectrum-X-Gamma satellite. It will monitor several hundred X-ray sources on a daily basis, and will be the first instrument to monitor most of the X-ray sky most of the time. MOXE will alert users of more sensitive instruments on Russia's giant high energy astrophysics observatory and of other instruments to transient activity. MOXE consists of an array of 6 X-ray pinhole cameras, sensitive from 2 to 25 keV, which views 4(pi) steradians (except for a 20 degree(s) X 80 degree(s) patch which includes the Sun). The pinhole apertures of 0.625 X 2.556 cm2 imply an angular resolution of 2 degree(s).4 X 9 degree(s).7 (FWHM on-axis). The flight instrument will mass approximately 118 kg and draw 38 Watts. For a non-focussing all-sky instrument that is limited by sky background, the limiting sensitivity is a function only of detector area. MOXE will, for a 24 hrs exposure, have a sensitivity of approximately 2 mCrab. MOXE distinguishes itself with respect to other all-sky monitors in its high duty cycle, thus having unprecedented sensitivity to transient phenomena with time scales between minutes and hours.
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