Keith C. Gendreau

The Neutron star Interior Composition ExploreR (NICER): an Explorer mission of opportunity for soft x-ray timing spectroscopy

The Neutron star Interior Composition ExploreR (NICER) is a proposed NASA Explorer Mission of Opportunity dedicated to the study of the extraordinary gravitational, electromagnetic, and nuclear-physics environments embodied by neutron stars. NICER will explore the exotic states of matter within neutron stars, where density and pressure are higher than in atomic nuclei, confronting theory with unique observational constraints. NICER will enable rotation-resolved spectroscopy of the thermal and non-thermal emissions of neutron stars in the soft (0.2–12 keV) X-ray band with unprecedented sensitivity, probing interior structure, the origins of dynamic phenomena, and the mechanisms that underlie the most powerful cosmic particle accelerators known. NICER will achieve these goals by deploying, following launch in December 2016, an X-ray timing and spectroscopy instrument as an attached payload aboard the International Space Station (ISS). A robust design compatible with the ISS visibility, vibration, and contamination environments allows NICER to exploit established infrastructure with low risk. Grazing-incidence optics coupled with silicon drift detectors, actively pointed for a full hemisphere of sky coverage, will provide photon-counting spectroscopy and timing registered to GPS time and position, with high throughput and relatively low background. In addition to advancing a vital multi-wavelength approach to neutron star studies through coordination with radio and γ-ray observations, NICER will provide a rapid-response capability for targeting of transients, continuity in X-ray timing astrophysics investigations post-RXTE through a proposed Guest Observer program, and new discovery space in soft X-ray timing science.

Radiation damage to charge coupled devices in the space environment

We have investigated the characteristics of radiation damage to charge coupled devices (CCDs) in the space environment. The X-ray astronomy satellite ASCA launched on February 20, 1993 in low Earth orbit carries CCDs specially developed for soft X-ray detection. We have traced the performance of the CCDs for 3 years. We have observed both the gradual decrease of charge transfer efficiency (CTE) and the increase of dark current. These are phenomenologically explained by the increase of charge traps due to irradiation by high energy charged particles. However, some of the effects of the radiation damage in the CCD are quite non-uniform over the chip on various scales. We discuss characteristics of the charge traps and possible origins of the non-uniformity.

Temperature and iron abundance variation of the gas in the Perseus cluster

We present the first two-dimensional map of the temperature and iron abundance in the Perseus cluster. Analysis of spectra obtained using the Gas Imaging Spectrometer on ASCA shows nonaxisymmetric variations in both the temperature and iron abundance. Traveling west from the cluster center, the temperature increases to 9 keV at 20 min and then decreases rapidly to 5 keV at 40 min. There is a hot (greater than 10 keV) region to the northwest of the cluster center. The abundance is approximately constant over much of the surveyed region, but there is evidence for an increased abundance in the northwest hot area and a gradual decrease in a westerly direction.

Laboratory astrophysics using a spare XRS microcalorimeter

The XRS instrument on Astro-E is a fully self-contained microcalorimeter x-ray instrument capable of acquiring, optimally filtering, and characterizing events for 32 independent pixels. We have recently integrated a full engineering model XRS detector system into a laboratory cryostat for use on the electron beam ion trap (EBIT) at Lawrence Livermore National Laboratory. The detector system contains a microcalorimeter array with 32 instrumented pixels heat sunk to 60 mK using an adiabatic demagnetization refrigerator. The instrument has a composite resolution of 8 eV at 1 keV and 11 eV at 6 keV with a minimum of 98% quantum efficiency and a total collecting area of 13 mm2. This will allow high spectral resolution, broadband observations of plasmas with known ionization states that are produced in the EBIT experiment. Unique to our instrument are exceptionally well characterized 1000 Angstrom thick aluminum on polyimide infrared blocking filters. The detailed transmission function including the edge fine structure of these filters has been measured in our laboratory using a variable spaced grating spectrometer. This will allow the instrument to perform the first broadband absolute flux measurements with the EBIT instrument. The instrument performance as well as the results of preliminary measurements of Fe K and L shell at fixed electron energy, Fe emission with Maxwellian electron distributions, and phase resolved spectroscopy of ionizing plasmas will be discussed.

The neutron star interior composition explorer (NICER): mission definition

Over a 10-month period during 2013 and early 2014, development of the Neutron star Interior Composition Explorer (NICER) mission [1] proceeded through Phase B, Mission Definition. An external attached payload on the International Space Station (ISS), NICER is scheduled to launch in 2016 for an 18-month baseline mission. Its prime scientific focus is an in-depth investigation of neutron stars—objects that compress up to two Solar masses into a volume the size of a city—accomplished through observations in 0.2–12 keV X-rays, the electromagnetic band into which the stars radiate significant fractions of their thermal, magnetic, and rotational energy stores. Additionally, NICER enables the Station Explorer for X-ray Timing and Navigation Technology (SEXTANT) demonstration of spacecraft navigation using pulsars as beacons. During Phase B, substantive refinements were made to the mission-level requirements, concept of operations, and payload and instrument design. Fabrication and testing of engineering-model components improved the fidelity of the anticipated scientific performance of NICER’s X-ray Timing Instrument (XTI), as well as of the payload’s pointing system, which enables tracking of science targets from the ISS platform. We briefly summarize advances in the mission’s formulation that, together with strong programmatic performance in project management, culminated in NICER’s confirmation by NASA into Phase C, Design and Development, in March 2014.

Laboratory Measurements of the Relative Intensity of the 3s → 2p and 3d → 2p Transitions in Fe XVII

The intensity ratios of the 3s → 2p and 3d → 2p lines in Fe XVII were measured on the Livermore electron beam ion trap employing a complementary set of spectrometers, including a high-resolution crystal spectrometer and the Goddard 32 pixel calorimeter. The resulting laboratory data are in agreement with satellite measurements of the Sun and astrophysical sources in collisional equilibrium such as Capella, Procyon, and NGC 4636. The results disagree with earlier laboratory measurements and assertions that processes not accounted for in laboratory measurements must play a role in the formation of the Fe XVII spectra in solar and astrophysical plasmas.

Direct measurements at the sub-pixel level of the X-ray detection efficiency of the CCD on board the ASCA satellite

We present here a measurement at sub-pixel resolution of the X-ray detection efficiency of the CCD detector on board the ASCA satellite. A mesh placed in front of the CCD restricted the interaction position of the X-ray inside the CCD pixel. We clearly show that all the primary charge is collected in one pixel when the X-ray interaction is sufficiently far from the pixel boundary. Conversely, some photo-ionized charge is collected in neighboring pixels when the X-ray interacts near the pixel boundary. We also obtained the thickness of various elements gate structure, and measure channel stop dimensions. From these data, we can obtain more accurate estimates of the X-ray responsivity of the CCD.

MAXIM Pathfinder x-ray interferometry mission

The MAXIM Pathfinder (MP) mission is under study as a scientific and technical stepping stone for the full MAXIM X-ray interferometry mission. While full MAXIM will resolve the event horizons of black holes with 0.1 microarcsecond imaging, MP will address scientific and technical issues as a 100 microarcsecond imager with some capabilities to resolve microarcsecond structure. We will present the primary science goals of MP. These include resolving stellar coronae, distinguishing between jets and accretion disks in AGN. This paper will also present the baseline design of MP. We will overview the challenging technical requirements and solutions for formation flying, target acquisition, and metrology.

The Milli-Arc-Second Structure Imager, MASSIM: A New Concept for a High Angular Resolution X-ray Telescope

MASSIM, the Milli-Arc-Second Structure Imager, is a mission that has been proposed for study within the context of NASAs Astrophysics Strategic Mission Concept Studies program. It uses a set of achromatic diffractive-refractive Fresnel lenses on an optics spacecraft to focus 5-11 keV X-rays onto detectors on a second spacecraft flying in formation 1000 km away. It will have a point-source sensitivity comparable with that of the current generation of major X-ray observatories (Chandra, XMM-Newton) but an angular resolution some three orders of magnitude better. MASSIM is optimized for the study of jets and other phenomena that occur in the immediate vicinity of black holes and neutron stars. It can also be used for studying other astrophysical phenomena on the milli-arc-second scale, such as those involving proto-stars, the surfaces and surroundings of nearby active stars and interacting winds. We describe the MASSIM mission concept, scientific objectives and the trade-offs within the X-ray optics design. The anticipated performance of the mission and possible future developments using the diffractive-refractive optics approach to imaging at X-ray and gamma-ray energies are discussed.

X-ray pulsar navigation algorithms and testbed for SEXTANT

The Station Explorer for X-ray Timing and Navigation Technology (SEXTANT) is a NASA funded technology-demonstration. SEXTANT will, for the first time, demonstrate real-time, on-board X-ray Pulsar-based Navigation (XNAV), a significant milestone in the quest to establish a GPS-like navigation capability available throughout our Solar System and beyond. This paper describes the basic design of the SEXTANT system with a focus on core models and algorithms, and the design and continued development of the GSFC X-ray Navigation Laboratory Testbed (GXLT) with its dynamic pulsar emulation capability. We also present early results from GXLT modeling of the combined NICER X-ray timing instrument hardware and SEXTANT flight software algorithms.