D. A. Swartz

When A Standard Candle Flickers

The Crab Nebula is the only hard X-ray source in the sky that is both bright enough and steady enough to be easily used as a standard candle. As a result, it has been used as a normalization standard by most X-ray/gamma-ray telescopes. Although small-scale variations in the nebula are well known, since the start of science operations of the Fermi Gamma-ray Burst Monitor (GBM) in 2008 August, a ~7% (70 mCrab) decline has been observed in the overall Crab Nebula flux in the 15-50 keV band, measured with the Earth occultation technique. This decline is independently confirmed in the ~15-50 keV band with three other instruments: the Swift Burst Alert Telescope (Swift/BAT), the Rossi X-ray Timing Explorer Proportional Counter Array (RXTE/PCA), and the Imager on-Board the INTEGRAL Satellite (IBIS). A similar decline is also observed in the ~3-15 keV data from the RXTE/PCA and in the 50-100 keV band with GBM, Swift/BAT, and INTEGRAL/IBIS. The pulsed flux measured with RXTE/PCA since 1999 is consistent with the pulsar spin-down, indicating that the observed changes are nebular. Correlated variations in the Crab Nebula flux on a ~3 year timescale are also seen independently with the PCA, BAT, and IBIS from 2005 to 2008, with a flux minimum in 2007 April. As of 2010 August, the current flux has declined below the 2007 minimum.

X-ray optic developments at NASA's MSFC

NASAs Marshall Space Flight Center (MSFC) has a successful history of fabricating optics for astronomical x-ray telescopes. In recent years optics have been created using electroforming replication for missions such as the balloon payload HERO (High energy replicated optics) and the rocket payload FOXSI (Focusing Optics x-ray Solar Imager). The same replication process is currently being used in the creation seven x-ray mirror modules (one module comprising of 28 nested shells) for the Russian ART-XC (Astronomical Rontgen Telescope) instrument aboard the Spectrum-Roentgen-Gamma mission and for large-diameter mirror shells for the Micro-X rocket payload. In addition to MSFCs optics fabrication, there are also several areas of research and development to create the high resolution light weight optics which are required by future x-ray telescopes. Differential deposition is one technique which aims to improve the angular resolution of lightweight optics through depositing a filler material to smooth out fabrication imperfections. Following on from proof of concept studies, two new purpose built coating chambers are being assembled to apply this deposition technique to astronomical x-ray optics. Furthermore, MSFC aims to broaden its optics fabrication through the recent acquisition of a Zeeko IRP 600 robotic polishing machine. This paper will provide a summary of the current missions and research and development being undertaken at NASAs MSFC.

ART-XC/SRG: status of the x-ray optics development

The Astronomical Roentgen Telescope (ART) instrument is a hard-x-ray instrument with energy response up to 30 keV that is to be launched on board of the Spectrum Roentgen Gamma (SRG) Mission. The instrument consists of seven identical mirror modules coupled with seven CdTe strip focal-plane detectors. The mirror modules are being developed at the Marshall Space Flight Center (MSFC.) Each module has ~65 sq. cm effective area and an on-axis angular resolution of 30 arcseconds half power diameter (HPD) at 8 keV. The current status of the mirror module development and testing will be presented.

The calibration of flight mirror modules for the ART-XC instrument on board the SRG mission

MSFC is fabricating x-ray optics for the Astronomical Roentgen Telescope – X-Ray Concentrator (ART-XC or ART for short) instrument under agreements with the Russian Space Research Institute (IKI). ART-XC is one of two instruments that will be launched on the Russian-German Spectrum-Roentgen-Gamma (SRG) Mission to be launched in 20161. Delivery of the flight optics for ART-XC (7 mirror modules) is currently scheduled for summer/fall of 20142. MSFC has to date completed assembly of four modules and has performed extensive calibration on two of these. These calibrations show that the modules meet effective area requirements and greatly exceed the angular resolution requirements. Details of the calibration procedure and an overview of the results obtained to date are presented here.

X-ray testing Constellation-X optics at MSFC's 100-m facility

As NASA’s next facility-class x-ray mission, Constellation X will provide high-throughput, high-resolution spectroscopy for addressing fundamental astrophysical and cosmological questions. Key to the Constellation-X mission is the development of lightweight grazing-incidence optics for its Spectroscopy X-ray Telescopes (SXT) and for its Hard X-ray Telescopes (HXT). In preparation for x-ray testing Constellation-X SXT and HXT development and demonstration optics, Marshall Space Flight Center (MSFC) is upgrading its 100-m x-ray test facility, including development of a five degree-of-freedom (5-DoF) mount for translating and tilting test articles within the facility’s large vacuum chamber. To support development of alignment and assembly procedures for lightweight x-ray optics, Goddard Space Flight Center (GSFC) has prepared the Optical Alignment Pathfinder Two (OAP2), which will serve as a surrogate optic for developing and rehearsing x-ray test procedures. In order to minimize thermal distortion of the mirrors during x-ray testing, the Harvard-Smithsonian Center for Astrophysics (CfA) has designed and implemented a thermal control and monitoring system for the OAP2. CfA has also built an aperture wheel for masking and sub-aperture sampling of the OAP2 to aid in characterizing x-ray performance of test optics.

Calibration of the High Energy Replicated Optics to Explore the Sun (HEROES) Hard X-ray Telescope

On 2013 September 21–22, the High Energy Replicated Optics to Explore the Sun (HEROES) hard X-ray telescope flew as a balloon payload from Ft. Sumner, NM. HEROES observed the Sun, the black hole binary GRS 1915+105, and the Crab Nebula during its 27 h flight. In this paper, we describe laboratory calibration measurements of the HEROES detectors using line and continuum sources and applications of these measurements to define channel to energy (gain) corrections for observed events and to define detector response matrices. We characterize the HEROES X-ray grazing incidence optics using measurements taken in the Stray Light Facility (SLF) in Huntsville, AL, and using ray traces. We describe the application of our calibration measurements to in-flight observations of the Crab Nebula.

X-ray probes of Jupiter's auroral zones, Galilean moons, and the Io plasma torus

Remote observations from the Earth orbiting Chandra X-ray Observatory and the XMM-Newton Observatory have shown the the Jovian system is a rich and complex source of x-ray emission. The planets auroral zones and its disk are powerful sources of x-ray emission, though with different origins. Chandra observations discovered x-ray emission from the Io plasma torus and from the Galilean moons Io, Europa, and possibly Ganymede. The emission from the moons is due to bombardment of their surfaces by highly energetic magnetospheric protons, and oxygen and sulfur ions, producing fluorescent x-ray emission lines from the elements in their surfaces against an intense background continuum. Although very faint when observed from Earth orbit, an imaging x-ray spectrometer in orbit around the icy Galilean moons would provide a detail mapping of the elemental composition in their surfaces. Here we examine the necessary characteristics of such an instrument and the challenges it would face in the extreme radiation environment in which it would have to survive and operate. Such an instrument would have the ultimate goal of providing detailed high-resolution maps of the elemental abundances of the surfaces of Jupiters icy moons and Io, as well as detailed study of the x-ray mission from the Io plasma torus, Jupiters auroral zones, and the planetary disk.

Effective area of the AXAF high-resolution camera (HRC)

The Advanced X-Ray Astrophysics Facility High Resolution Camera was calibrated at NASAs X-Ray Calibration Facility during March and April 1997. We have undertaken an analysis of the effective area of the combined High Resolution Mirror Assembly/High Resolution Camera using all data presently available from these tests. In this contribution we discuss our spectral fitting of the beam-normalization detectors, our method of removing higher order contamination lines present in the spectra, and the corrections for beam non- uniformities. Using an approach based upon the mass absorption cross-section of Cesium Iodide, we determine the quantum efficiency in the microchannel plates. We model the secondary electron absorption depth as a function of energy, which we expect to be relatively smooth. This is then combined with the most recent model of the telescope to determine the ensemble effective area for the HRC. The ensemble effective area is a product of the telescope effective are, the transmission of the UV-Ion shield, and the quantum efficiency of the microchannel plates. We focus our attention on the microchannel plate quantum efficiency, using previous result for the UV-Ion shield transmission and telescope effective area. We also address future goals and concerns.

Lynx optics based on full monolithic shells: design and development

Lynx is an X-ray mission concept with superb imaging capabilities (< 1arcsec Half Energy Width, HEW) and large throughput (2 m2 effective area @1keV). Several approaches are being considered to meet the challenging technological task of the mirror fabrication. Thin and light substrates are necessary to meet mass constraints. Monolithic fused silica shells are a possible solution if their thickness can be maintained to below 4 mm for mirror shells up to 3 m diameter. In this paper we present the opto-mechanical design of the mirror assembly, the technological processes, and the results achieved so far on a prototypal shells under development. In particular, emphasis is placed on the figuring process that is based on direct polishing and on ion beam figuring and on a temporary stiffening structure designed to support the shell during the figuring and polishing operations and to manage the handling of the shell through all phases up to integration into the telescope supporting structure.