Eric H. Silver

A precision measurement of the X-ray polarization of the Crab Nebula without pulsar contamination

The linear X-ray polarization of the Crab Nebula has been precisely measured at 2.6 keV and 5.2 keV with the OSO 8 graphite crystal polarimeters. The 1.4 ms time reolution of these instruments permitted the removal of any contribution to the polarization from the pulsar. The nebular polarization is 19.2% +- 1.0% at a position angle of 156/sup 0/.4 +- 1/sup 0/.4 at 2.6 keV. At 5.2 keV the corresponding results are 19.5% +- 2.8% at 152/sup 0/.6 +- 4/sup 0/.0.

XIPE: the X-ray imaging polarimetry explorer

Abstract X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and temporal variability measurements and to imaging, allows a wealth of physical phenomena in astrophysics to be studied. X-ray polarimetry investigates the acceleration process, for example, including those typical of magnetic reconnection in solar flares, but also emission in the strong magnetic fields of neutron stars and white dwarfs. It detects scattering in asymmetric structures such as accretion disks and columns, and in the so-called molecular torus and ionization cones. In addition, it allows fundamental physics in regimes of gravity and of magnetic field intensity not accessible to experiments on the Earth to be probed. Finally, models that describe fundamental interactions (e.g. quantum gravity and the extension of the Standard Model) can be tested. We describe in this paper the X-ray Imaging Polarimetry Explorer (XIPE), proposed in June 2012 to the first ESA call for a small mission with a launch in 2017. The proposal was, unfortunately, not selected. To be compliant with this schedule, we designed the payload mostly with existing items. The XIPE proposal takes advantage of the completed phase A of POLARIX for an ASI small mission program that was cancelled, but is different in many aspects: the detectors, the presence of a solar flare polarimeter and photometer and the use of a light platform derived by a mass production for a cluster of satellites. XIPE is composed of two out of the three existing JET-X telescopes with two Gas Pixel Detectors (GPD) filled with a He-DME mixture at their focus. Two additional GPDs filled with a 3-bar Ar-DME mixture always face the Sun to detect polarization from solar flares. The Minimum Detectable Polarization of a 1 mCrab source reaches 14 % in the 2–10 keV band in 105 s for pointed observations, and 0.6 % for an X10 class solar flare in the 15–35 keV energy band. The imaging capability is 24 arcsec Half Energy Width (HEW) in a Field of View of 14.7 arcmin × 14.7 arcmin. The spectral resolution is 20 % at 6 keV and the time resolution is 8 μs. The imaging capabilities of the JET-X optics and of the GPD have been demonstrated by a recent calibration campaign at PANTER X-ray test facility of the Max-Planck-Institut für extraterrestrische Physik (MPE, Germany). XIPE takes advantage of a low-earth equatorial orbit with Malindi as down-link station and of a Mission Operation Center (MOC) at INPE (Brazil). The data policy is organized with a Core Program that comprises three months of Science Verification Phase and 25 % of net observing time in the following 2 years. A competitive Guest Observer program covers the remaining 75 % of the net observing time.

Emission-Line Intensity Ratios in Fe XVII Observed with a Microcalorimeter on an Electron Beam Ion Trap

We report new observations of emission line intensity ratios of Fe XVII under controlled experimental conditions, using the National Institute of Standards and Technology electron beam ion trap (EBIT) with a microcalorimeter detector. We compare our observations with collisional-radiative models using atomic data computed in distorted wave and R-matrix approximations, which follow the transfer of the polarization of level populations through radiative cascades. Our results for the intensity ratio of the 2p6 1S0-2p53d 1P1 15.014 A line to the 2p6 1S0-2p53d 3D1 15.265 A line are 2.94 ± 0.18 and 2.50 ± 0.13 at beam energies of 900 and 1250 eV, respectively. These results are not consistent with collisional-radiative models and support conclusions from earlier EBIT work at the Lawrence Livermore National Laboratory that the degree of resonance scattering in the solar 15.014 A line has been overestimated in previous analyses. Further observations assess the intensity ratio of the three lines between the 2p6-2p53s configurations to the three lines between the 2p6-2p53d configurations. Both R-matrix and distorted wave approximations agree with each other and our experimental results much better than most solar and stellar observations, suggesting that other processes not present in our experiment must play a role in forming the Fe XVII spectrum in solar and astrophysical plasmas.

Laboratory Astrophysics Survey of Key X-Ray Diagnostic Lines Using A Microcalorimeter on an Electron Beam Ion Trap

Cosmic plasma conditions created in an electron beam ion trap (EBIT) make it possible to simulate the dependencies of key diagnostic X-ray lines on density, temperature, and excitation conditions that exist in astrophysical sources. We used a microcalorimeter for such laboratory astrophysics studies because it has a resolving power ≈1000, quantum efficiency approaching 100%, and a bandwidth that spans the X-ray energies from 0.2 keV to 10 keV. Our microcalorimeter, coupled with an X-ray optic to increase the effective solid angle, provides a significant new capability for laboratory astrophysics measurements. Broadband spectra obtained from the National Institute of Standards and Technology EBIT with an energy resolution approaching that of a Bragg crystal spectrometer are presented for nitrogen, oxygen, neon, argon, and krypton in various stages of ionization. We have compared the measured line intensities to theoretical predictions for an EBIT plasma.

Calorimetric ionization detector

A new mode of operation for ionization detectors is described. The amount of ionization produced in a detector is determined by measuring the amount of heat generated during the carrier collection process. Very high detection sensitivities, including single carrier detection, may be achieved at cryogenic temperatures. Results from an experimental device operated at T = 0.3K is presented.

Status of the stellar x-ray polarimeter for the Spectrum-X-Gamma mission

The Stellar X-Ray Polarimeter (SXRP) uses the polarization sensitivity of a graphite Bragg crystal and a lithium Thomsom scattering target to measure the polarization of X-rays from astrophysical sources. The SXRP is a focal plane detector for the Soviet-Danish SODART telescopes which will be launched on the Soviet Spectrum-X-Gamma mission. The SXRP will be the third orbiting stellar X-ray polarimeter, and should provide an order of magnitude increase in polarization sensitivity over its predecessors.

The first search for X-ray polarization in the Centaurus X-3 and Hercules X-1 pulsars

The first search for X-ray polarization in the Cen X-3 and Her X-1 pulsars was performed by the OSO 8 polarimeters in 1975 July and 1975 August, respectively. Three-sigma upper limits to the polarization in Cen X-3 of 13.5% and 19% at 2.6 keV and 5.2 keV, respectively, were obtained when the data were averaged over the pulse and binary periods. The upper limit for Her X-1 at 2.6 keV is 60%. A search for pulse-phase dependent X-ray polarization from both objects was also performed. At the 91% confidence level, emission from Cen X-3 exhibits evidence for X-ray polarization at 2.6 keV that varies with pulse phase. Upper limits to polarization are presented for the leading and trailing edges and peak of the Her X-1 pulse at 2.6 keV.

High-Resolution, Broad-Band Microcalorimeters for X-Ray Microanalysis

Scanning electron microscope (SEM)-based x-ray analyzers have employed wavelength-dispersive x-ray (WDX) diffractometers or energy-dispersive x-ray (EDX) detectors as the spectrally resolving elements. In spite of their relatively poor energy resolution (ΔE = 130 eV at 6 keV), the solid-state EDX instruments have enjoyed much wider use than the WDX systems (ΔE = 1-10 eV) because of their convenience and efficiency. A microcalorimeter detector with 95% quantum efficiency at 6 keV has been developed that can produce spectra with an energy resolution of 7 eV over the broad energy band of 0.2-20 keV. This performance will advance the state-of-the-art for elemental analysis by virtue of its 20-fold increase in x-ray energy resolution. When coupled to an SEM it will permit the evolution of a new generation of microanalysis tools with greatly improved spatial resolution and increased sensitivity for minor elemental constituents. Since it will allow the SEM to operate at low electron energies, it will provide the ability to identify unambiguously x-ray signatures from a mixture of light and heavy elements. The latest performance of these detectors is presented along with a discussion of how they will eventually improve SEM-based microanalysis for small particle defect review and sub-micron depth studies.

Realistic inexpensive soft x-ray polarimeter and the potential scientific return

Using multilayer coated mirrors to provide high reflectivity at large graze angles, we have proposed to launch a small telescope that is capable of measuring the linear polarization of the soft x-ray fluxes from many astronomical sources. Three identical mirror-detectoer assemblies are designed for maximum efficiency at 0.25 keV, where the photon spectra of many celestial targets peak. In observations lasting 1-3 days using this low risk instrument with proven heritage, we can detect polarizations of 5-10% at 5σ due to Compton scattering or synchrotron processes in the relativistic jets of BL Lac objects, accretion disks or jets in active galactic nuclei and atmospheres of isolated pulsars. Pulsar data can be binned by pulse phase to measure the orientation of the neutron star rotation and magnetic field axes and constrain the mass to radius ratio. This project has been selected for technology development funding by the NASA Explorer Program.

Stellar X-Ray Polarimeter: a focal plane polarimeter for the Spectrum X-Gamma mission

n this paper we describe an x-ray polarimeter that will be flown on the Spectrum X-Gamma mission. The instrument exploits three distinct physical processes to measure polarization: Bragg reflection from a graphite crystal, Thomson scattering from a metallic lithium target, and pho-toemission from a cesium iodide photocathode. These three methods allow polarization measurements over an energy band from 0.3 to 12 keV. The polarimeter will make possible sensitive measurements of several hundred known x-ray sources, an increase of two orders of magnitude over the x-ray polarimeters flown to date. X-ray polarization measurements will allow us to constrain the geometry of gas flow in x-ray binaries, identify nonthermal emission in supernova remnants, test current models for x-ray emission in radio pulsars, determine the radiation mechanisms in active galactic nuclei, and search for inertial frame dragging (Lense-Thirring effect) around the putative black hole in Cygnus X-1.