Stephen L. O'Dell

Advanced X-Ray Astrophysics Facility (AXAF)

AXAF is an x-ray observatory designed to study x-ray emission from al categories of astronomical objects, from normal stars to quasars. AXAF has broad scientific objectives and outstanding capability to provide high resolution images, spectrometric imaging and high resolution dispersive spectroscopy over the energy bandwidth from 0.1 to 10-keV. This is a significant year in the development of AXAF, to be launched in late 1998. Major elements of the observatory, the optics and the scientific instruments, are now nearing completion in preparation for calibration later this year.

CHANDRA PHASE-RESOLVED X-RAY SPECTROSCOPY OF THE CRAB PULSAR

We present the first phase-resolved study of the X-ray spectral properties of the Crab pulsar that covers all pulse phases. The superb angular resolution of the Chandra X-Ray Observatory enables distinguishing the pulsar from the surrounding nebulosity, even at pulse minimum. Analysis of the pulse-averaged spectrum measures interstellar X-ray extinction due primarily to photoelectric absorption and secondarily to scattering by dust grains in the direction of the Crab Nebula. We confirm previous findings that the line of sight to the Crab is underabundant in oxygen, although more so than recently measured. Using recent abundances and cross sections from Wilms, Allen, and McCray, we find ½O=H �¼ð 3:33 � 0:25 Þ� 10 � 4 . Analysis of the spectrum as a function of pulse phase measures the low-energy X-ray spectral index even at pulse minimum—albeit with large statistical uncertainty—and we find marginal evidence for variations of the spectral index. The data are also used to set a new (3 � ) upper limit to the temperature of the neutron star of logT1 < 6:30. Subject headings: atomic processes — ISM: general — pulsars: individual (Crab Pulsar) — stars: neutron — techniques: spectroscopic — X-rays: stars

On understanding the figures of merit for detection and measurement of x-ray polarization

The prospects for accomplishing X-ray polarization measurements appear to have grown in recent years after a more than 35-year hiatus. Unfortunately, this long hiatus has brought with it some confusion over the statistical uncertainties associated with polarization measurements of astronomical sources. The heart of this confusion stems from a misunderstanding (or potential misunderstanding) of a standard figure of merit-the minimum detectable polarization (MDP)-that one of us introduced many years ago. We review the relevant statistics, and quantify the differences between the MDP and the uncertainty of an actual polarization measurement. We discuss the implications for future missions.

Advanced X-ray Astrophysics Facility (AXAF): An overview

The Advanced X-ray Astrophysics Facility (AXAF) is the x-ray component of NASAs Great Observatories. To be launched in late 1998, AXAF will provide unprecedented capabilities for high-resolution imaging, spectrometric imaging, and high-resolution dispersive spectroscopy, over the x-ray band from about 0.1 keV to 10 keV. With these capabilities, AXAF observations will address many of the outstanding questions in astronomy, astrophysics, and cosmology.

Mirror Technology Development for the International X-ray Observatory Mission

The International X-ray Observatory mission is a collaborative effort of NASA, ESA, and JAXA. It will have unprecedented capabilities in spectroscopy, imaging, timing and polarization measurement. A key enabling element of the mission is a flight mirror assembly providing unprecedented large effective area (3 m2) and high angular resolution of (5 arcseconds half-power diameter). In this paper we outline the conceptual design of the mirror assembly and development of technology to enable its construction.

The Imaging X-ray Polarimetry Explorer (IXPE)

The Imaging X-ray Polarimetry Explorer (IXPE) expands observation space by simultaneously adding polarization measurements to the array of source properties currently measured (energy, time, and location). IXPE will thus open new dimensions for understanding how X-ray emission is produced in astrophysical objects, especially systems under extreme physical conditions—such as neutron stars and black holes. Polarization singularly probes physical anisotropies—ordered magnetic fields, aspheric matter distributions, or general relativistic coupling to black-hole spin—that are not otherwise measurable. Hence, IXPE complements all other investigations in high-energy astrophysics by adding important and relatively unexplored information to the parameter space for studying cosmic X-ray sources and processes, as well as for using extreme astrophysical environments as laboratories for fundamental physics.

The Development of Hard-X-Ray Optics at MSFC

We have developed the electroformed-nickel replication process to enable us to fabricate light-weight, high-quality mirrors for the hard-x-ray region. Two projects currently utilizing this technology are the production of 240 mirror shells, of diameters ranging from 50 to 94 mm, for our HERO balloon payload, and 150- and 230-mm-diameter shells for a prototype Constellation-X hard-x-ray telescope module. The challenge for the former is to fabricate, mount, align and fly a large number of high-resolution mirrors within the constraints of a modest budget. For the latter, the challenge is to maintain high angular resolution despite weight-budget-driven mirror shell thicknesses (100 μm) which make the shells extremely sensitive to fabrication and handling stresses, and to ensure that the replication process does not degrade the ultra-smooth surface finish (~3Å) required for eventual multilayer coatings. We present a progress report on these two programs.

Radiation environment of the Chandra X-Ray Observatory

The Chandra X-ray Observatory, the x-ray component of NASAs Great Observatories, provides unprecedented subarcsecond imaging, imaging spectrometry, and high-resolution dispersive spectroscopy of cosmic x-ray sources. During the initial phase of operation, some of the focal-plane charge-coupled devices (CCDs) -- namely, the front-illuminated devices -- experienced an unanticipated increase in charge-transfer inefficiency (CTI). Investigation of this anomaly determined the root cause to be radiation damage by weakly penetrating protons, entering the telescopes aperture and scattered off the mirrors into the focal plane. Subsequent changes in operating procedures have slowed the rate of increase of the CTI of the front- illuminated CCDs to acceptable levels. There has been no measurable degradation of the back-illuminated CCDs.

X-Ray Polarimetry and Its Potential Use for Understanding Neutron Stars

We review the state of the art for measuring the X-ray polarization of neutron stars. We discuss how valuable precision measurements of the degree and po- sition angle of polarization as a function of energy and, where relevant, of pulse phase, would provide deeper in- sight into the details of the emission mechanisms. We then review the current state of instrumentation and its poten- tial for obtaining relevant data. Finally, we conclude our discussion with some opinions as to future directions.

Calibration of the AXAF Observatory: Overview

The Advanced X-ray Astrophysics Facility (AXAF) will soon begin its exploration of the x-ray universe, providing unprecedented angular and spectral resolution. Also unprecedented is the ambitious goal of calibrating the AXAF observatory to an accuracy of a few percent. Toward this end, AXAF science and engineering teams undertook an extensive calibration program at component, subsystem, and system levels. This paper is an overview of the system-level calibration activities, conducted over the past year at the Marshall Space Flight Center (MSFC) X-Ray Calibration Facility (XRCF).