1. HIREGS Solar Observations -- The First Two Flights
High resolution spectroscopy of gamma-ray and hard x-ray emissions from solar flares can provide a qualitatively new window on flare particle acceleration processes since all the nuclear gamma-ray lines produced by the energetic ions and most of the important features of the hard x-ray continuum produced by the energetic electrons are unresolved by the scintillator detectors that have been used in spacecraft. High resolution gamma-ray spectroscopy can also yield new information on elemental abundances in the solar atmosphere.
The High Resolution Gamma-Ray and Hard X-Ray Spectrometer (HIREGS) is designed to detect photons from 20 keV to 18 MeV with an energy resolution of 1.5 to several keV, and photons up to ~160 MeV with ~0.5 MeV resolution. In this energy range, many processes associated with solar flare activity can be studied. Interactions of accelerated ions with the ambient solar atmosphere produce gamma-ray lines, the strongest of which are at 2.223 MeV from neutron capture in 1H, 0.511 MeV from positron annihilation, and 6.129, 4.438, 1.634, 1.369, 1.779, and 0.847 MeV from 16O, 12C, 20Ne, 24Mg, 28Si, and 56Fe de-excitations, respectively. All of these lines have been observed from solar flares. A feature at ~0.45 MeV due to lines at 0.429 and 0.478 MeV from 7Li and 7Be, resulting from the interaction of alpha particles with He nuclei, as well as neutrons and gamma-rays produced in pion decay have also been observed.
The 2.223 MeV deuterium line and the 511 keV electron-positron annihilation line are the most likely lines to be observed from solar flares because they are not only strong but also delayed with respect to the primary flare emissions. The time scale over which deuterium is formed and over which positrons annihilate can be several minutes or more, meaning that the flare continuum may no longer be present, and the sensitivity for detection of these lines will consequently be greater than the sensitivity for detection of prompt lines. The observed intensity and time profile of the 2.223 MeV line depend on the 3He/1H ratio in the photosphere because the radiationless 3He(n,p)3H reaction competes with the 1H(n,g)2H reaction. This line also indicates the presence of stored energetic ions -- primarily 30-100 MeV protons that produce neutrons through p-p and p-a interactions during flare times and possibly during periods of low solar activity.
Observations of the 511 keV line can provide information on the temperature, density, and location of the ambient medium with which the positrons annihilate. Positrons are produced from the decay of pions, from the decay of radioactive nuclei, and from the deexcitation of excited nuclei, which are produced through energetic ion interactions. The positrons may annihilate either directly with electrons or after forming positronium, depending on the temperature and density of the ambient medium. The positronium fraction can be observed because the triplet state of positronium decays into a three-photon continuum whereas the singlet state and direct annihilation both produce two 511 keV photons in the particles' rest frame.
HIREGS was twice flown on a 29 million cubic foot helium balloon from McMurdo Station, Antarctica. Balloon flights over Antarctica during the austral summer have several advantages. With the Sun always up, the solar panels that power the instrument can be illuminated most of the time, and the balloon altitude, which normally varies over a day-night cycle, can be maintained with a minimal use of ballast. These advantages along with the fact that the high altitude wind pattern is circular around the continent allow long duration balloon flights of one to several weeks to be achieved. In the case of HIREGS, where the primary purpose was to observe solar flares, the instrument could observe the Sun 24 hours per day thus maximizing the probability of observing a large flare; however, because the elevation of the Sun above the horizon was relatively low -- roughly 11 to 33 degrees over the course of a day -- the atmospheric absorption of hard x-rays along the instrument's line-of-sight was high (e.g., from 90% to 99.8% at 35 keV).
The HIREGS instrument consists of an array of twelve 6.7 cm dia. x 6.2 cm long n-type germanium coaxial detectors. Each set of four detectors is held by a cryostat that is attached to a 50-liter liquid nitrogen dewar, and the three cryostats are enclosed on the sides and bottom by a 5 cm thick bismuth germanate (BGO) anticoincidence shield. In front is a 10 cm thick drilled CsI collimator which provides a 24-degree FWHM field-of-view. On top of the CsI is a 2.7 mm thick sheet of lead to help prevent, by blocking hard x-rays, the active shielding from becoming saturated in the event of a large solar flare.
The data system was designed to handle the high count rates expected during large solar flares. Every two detectors are monitored by a dedicated (satellite) microprocessor with 32 Mbytes of temporary "burst" memory that act as a data buffer between the detectors and the main processor in order to prevent the loss of data during periods of high count rate. Four bytes of information are stored per detector event and are packaged by the satellites into 72-byte frames, which are passed on request to the main processor on a serial data link. Also, each of the satellites counts the events and measures dead times for their respective detectors. The main processor collects the data from each of the satellites and packages them into 160-byte data frames, which are sent to the tape system and/or telemetry. The main processor has a 44-Mbyte RAM disk for temporary data storage, and the exabyte tape drive can store up to 4.6 Gbytes, enough to hold all the data expected from the balloon flights. In addition to the satellite and main processors, there is an independent processor that performs other functions such as command, navigation, pointing, and housekeeping.
Data is stored in two formats, referred to as event data and underflight data. The event data consists mostly of detailed information on each detector event and is stored in the tape system. The underflight data contains primarily spectra that are accumulated in the main processor and are stored in the 44 Mbyte temporary memory, from which it is telemetered down to an LC-130 aircraft that is flown under the balloon every few days. It was intended that this underflight data be a backup in the event that the data tape was not recovered; and the event information was "compressed" into spectra so that several days worth of data could be stored in the temporary memory. A small amount of housekeeping information was also transmitted to the ground via the ARGOS and INMARSAT satellite networks.
HIREGS was launched from McMurdo Station, Antarctica on January 10, 1992 and again on December 31, 1992. The balloon circumnavigated the continent in 14 days on the first flight and 10 days on the second, at an altitude of roughly 130 thousand ft. during the first flight (corresponding to an atmospheric depth of ~3.5 g/cm2) and 120-127 thousand ft. during the second (5.3-3.7 g/cm2). On the first flight, the instrument consisted of only four detectors, and there was an additional lead-tin-copper passive collimator that reduced the field-of-view to 30 x 120 for hard x-rays (#300 keV). Also, during only the first flight was underflight data retrieved, and data was stored as either event data or underflight data, but never as both at the same time. During the second flight, aircraft were not available to retrieve underflight data, but all the data that was collected was recovered from the tape system in the form of event data.
2. HIREGS '94 -- The Latest Mission
HIREGS was launched again from Antarctica on January 9, 1995 to study the Galactic center region and several other interesting sources. The flight lasted 23 days, and the entire instrument and gondola were recovered with an LC130 cargo plane. The data are currently being analyzed, so results are not yet available. The following description of the scientific objectives was written prior to the flight:
Given a successful flight, HIREGS will record the best spectroscopic measurements to date, which will allow us to study the shapes of the 0.511 MeV electron-positron annihilation line, the 1.809 MeV 26Al nucleosynthesis line, and the cyclotron absorption features of 2 accreting X-ray pulsars. We will also be able to monitor transient sources such as the "Great Annihilator" -- the flaring black hole candidate 1E1740.7-2942 near the Galactic center.
2.1. The Narrow Positron Annihilation Line (0.511 MeV)
The annihilation of positrons has important consequences in high energy astrophysics. Positrons are formed in nuclear processes and in photon-photon collisions around compact objects. The annihilation of positrons with electrons produces characteristic gamma-ray photons at 0.511 MeV, as well as a unique continuum spectrum below this energy.
Previous observations of the galactic distribution of the narrow 0.511 MeV line are strongly peaked in the direction of the Galactic Center, indicating a point-like or spheroid distribution from this region. The source of these positrons could be the black hole candidate 1E1740.7-2942. Positrons formed through photon-photon collisions near the compact object could escape into the surrounding molecular cloud, and possibly even into the interstellar medium (ISM), before annihilating. Other compact objects near the Galactic Center could also be adding to the production of the positrons.
Once the positrons are formed, they can annihilate through several different processes, each of which has important implications for the shape of the 0.511 MeV line. Line shapes have been calculated for the four phases of the ISM. Line shapes from the cold neutral medium (CNM) show a narrow line due to direct annihilation with atomic electrons superimposed on a broader line due to positronium formation and annihilation in flight. For the warm neutral medium (WNM) and the warm ionized medium (WIM), the lines are very narrow, and for the hot ionized medium (HIM) the lines are very broad.
Assuming the positrons are free to propagate, and thus annihilate, in all four phases of the ISM, and assuming the model of the ISM by McKee & Ostriker (1977), then the effective full-width-at-half-maximum (FWHM) of the 0.511 MeV line would be 4.6 keV. In this model, the positrons are most likely to annihilate in the CNM, thus the line would have a broad base resulting from positronium formation and annihilation in flight.
However, the presence of interstellar magnetic fields could have a strong effect on this line shape. These fields could exclude positrons from the CNM, thus greatly narrow the effective FWHM of the 0.511 MeV line to 1.46 keV.
The exclusion of mildly relativistic positrons from the cold cloud cores would contradict low-energy cosmic-ray heating models for clouds, since the exclusion of the low energy positrons would indicate the exclusion of both high and low energy cosmic rays as well. However, since the CNM contains roughly 90% of the mass of the ISM, exclusion of cosmic rays would mean that they see only about 10% of the average gas density that models previously assumed. This implies that either the lifetime or the galactic scale height of the cosmic rays must be an order of magnitude smaller.
So far, observations of the line shape by Ge spectrometers have been ambiguous. Thus, the centerpiece of the HIREGS '94 long duration balloon flight in Antarctica will be the best measurement of the 0.511 MeV line shape to date. With a 14 day flight, HIREGS will be able to measure the Galactic Center 0.511 MeV line flux to 19s. This detection would define the line shape to a high enough accuracy to distinguish between the separate models.
2.2. The 26Al Decay Line at 1.809 MeV
The decay of 26Al to 26Mg results in the emission of a characteristic photon at 1.809 MeV. Because of large yields of 26Al in theoretical models of supernovae nucleosynthesis, and the long lifetime of the isotope, two independent predictions were made in 1977 about the utility of the 1.809 MeV line for mapping out regions of nucleosynthesis in the Galactic Disk. The line was first detected, from the direction of the Galactic Center, by the germanium (Ge) spectrometer aboard the HEAO-3 satellite.
Recent preliminary observations by CGRO/COMPTEL to map the extended 1.809 MeV emission have yielded some surprising results. The COMPTEL data shows evidence of the extended 1.809 MeV emission along the plane as was expected -- but the data also show localized enhancements, or "hot spots", along the plane. These hot spots suggest that much of the 1.809 MeV emission might be originating from 26Al formed in local supernovae, and that the measurements of the galactic 1.809 MeV intensities, and thus 26Al yields, will have to be revised to account for the local sources.
As a part of its flight, HIREGS 494 will observe the 1.809 MeV line from the Galactic Center, and also from two of the hot spots reported by COMPTEL. These observations will be at the level of 12s, providing important information on the line shapes, and potentially distinguishing the local sources by their Doppler shifts.
2.3. Cyclotron Line Sources
Cyclotron scattering resonance features (CSFRs) can occur in the X-ray spectra of accreting sources with strong magnetic fields; thus, measurements of these features in source spectra can provide unique information on the magnetic fields and accretion geometries of compact objects. To date, cyclotron absorption features have been seen in 9 accreting X-ray pulsars. HIREGS 494 will monitor two of these sources, Vela X-1 and GX301-2, which were first discovered to have CSFRs by the Ginga satellite. Based upon spectral models, we expect good resolution measurements of the energies and line shapes of the fundamental absorption resonances, and possible detection of higher harmonic features, if they are present.
3. INTEGRAL -- A Few Years Down the Road
In addition to the balloon program, our group is involved in the development of new technology for use in the INTEGRAL Satellite Mission that has been selected by the European Space Agency.