NASA High Energy Research Articles

Limits on Radiative Capture γ-Ray Lines and Implications for Energy Content in Flare-Accelerated Protons

Measurements of radiative-capture γ-ray lines can provide information on both the energy content of nonthermal protons below 1 MeV and the temperature in the region where they interact. We have derived upper limits on the fluences in three proton-capture lines of 12C and 13C in the flare-averaged gamma-ray spectrum from 19 X-class flares observed with the Solar Maximum Mission (SMM). The most significant limit comes from the 2.37 MeV line that is excited by 0.46 MeV protons. We estimate an upper limit on the energy content in the accelerated protons by extrapolating the power-law spectrum derived at higher energies down to the resonant energy. The derived upper limit on the temperature, ∼ 5–10×107 K, is higher than measured in the flaring region with other techniques, even for this optimistic energy content. It is possible that NASA's High Energy Solar Spectroscopic Imager (HESSI) will be sensitive enough to detect the 2.37 MeV line if temperatures exceed ∼2×107 K.

The NASA high energy solar physics (HESP) mission for the next solar maximum

The NASA High Energy Solar Physics (HESP) mission offers the opportunity for major breakthroughs in our understanding of the fundamental energy release and particle acceleration processes at the core of the solar flare problem. HESP's primary strawman instrument, the High Energy Imaging Spectrometer (HEISPEC), will provide X-ray and γ-ray imaging spectroscopy, i.e., high-resolution spectroscopy at each spatial point in the image. It has the following unique capabilities; (1) high-resolution (∼keV) spectroscopy from 2 keV - 20 MeV to resolve flare gamma-ray lines and sharp features in the continuum; (2) hard X-ray imaging with 2″ angular resolution and tens of millisecond temporal resolution, commensurate with the travel and stopping distances and times for the accelerated electrons; (3) gamma-ray imaging with 4″–8″ resolution with the capability of imaging in specific lines or continuum regions; (4) moderate resolution imaging of energetic (20 MeV to ∼1 GeV) gamma-rays and neutrons. Additional strawman instruments include a Bragg crystal spectrometer for diagnostic information and a soft X-ray/XUV/UV imager to map the flare coronal magnetic field and plasma structure. The HESP mission also includes extensive ground-based observational and supporting theory programs. Presently HESP is planned for a FY 1995 new start and late 1999 launch, in time for the next solar activity maximum.

Measurements of X-ray source positions by the scanning modulation collimator on HEAO 1

This paper reports preliminary results on the positions of X-ray sources obtained with the modulation collimator on the first NASA High Energy Astronomy Observatory, HEAO 1. Calibration on Sco X-1 establishes the experiment capabilities to be within 7 sec as limited by aspect errors. For the reported X-ray sources certain ambiguities in their positions are resolved and the previous optical identifications of four sources are confirmed, two of which are X-ray bursters and one of which is the black hole candidate Cir X-1. An accurate position for a new transient source is also presented.

A Diagnostic Emulator for HEAO software development

Diagnostic Emulation is the application of microprogramming to the emulation of an operational computer to support software development and verification for that computer. A conventional technique, Interpretive Computer Simulation (ICS), has been used for many years in support of such software development and verification efforts. The ICS method is becoming less cost effective. For the development of attitude control software for NASA's High Energy Astronomical Observatory (HEAO) diagnostic emulation was considered as an alternative. This paper describes the motivation for using an ICS or diagnostic emulator to validate command and control software, the potential advantages of microprogramming, and the analysis which led to the development of the HEAO Diagnostic Emulator rather than the use of conventional techniques. Also discussed are the emulator implementation tradeoff analysis, the resulting program design, and the interface between the microprogram and the assembly language modules. The results to date are described, including the capability, speed, and reliability of the program and initial user response.

Design of a Celestial Thomson-Scattering X-Ray Polarimeter

A general discussion of the scientific importance and status of stellar x-ray polarimetry is presented. A stellar x-ray polarimeter designed to fit into the bottom half of the NASA Orbiting Solar Observatory-I (OSO-I) wheel compartment or other similar spacecraft is described. In this design, the linear polarization is obtained as a function of energy. The sensitivity of the polarimeter in the 4 - 24 keV energy range was optimized with the aid of a Monte Carlo simulation computer program and is given for several important celestial x-ray sources. Estimates of sensitivity thresholds for a much larger polarimeter, suitable for flight in the NASA High-Energy Astronomy Observatory (HEAO), are also given. The minimum detectable polarization for several x-ray sources is given. For example, the minimum detectable polarization at the 99% confidence level for the Crab nebula in a 24-h observation time is 1.4% for an OSO polarimeter in a compartment (50 × 30 × 30) cm and 0.27% for a HEAO polarimeter in a compartment (100 × 100 × 30) cm. These satellite experiments are feasible and will yield significant polarization results which will have an important bearing on our understanding of x-ray source mechanisms.