The Physics of Cluster Mergers

Clusters of galaxies generally form by the gravitational merger of smaller clusters and groups. Major cluster mergers are the most energetic events in the Universe since the Big Bang. Some of the basic physical properties of mergers will be discussed, with an emphasis on simple analytic arguments rather than numerical simulations. Semi-analytic estimates of merger rates are reviewed, and a simple treatment of the kinematics of binary mergers is given. Mergers drive shocks into the intracluster medium, and these shocks heat the gas and should also accelerate nonthermal relativistic particles. X-ray observations of shocks can be used to determine the geometry and kinematics of the merger. Many clusters contain cooling flow cores; the hydrodynamical interactions of these cores with the hotter, less dense gas during mergers are discussed. As a result of particle acceleration in shocks, clusters of galaxies should contain very large populations of relativistic electrons and ions. Electrons with Lorentz factors gamma~300 (energies E = gamma m_e c^2 ~ 150 MeV) are expected to be particularly common. Observations and models for the radio, extreme ultraviolet, hard X-ray, and gamma-ray emission from nonthermal particles accelerated in these mergers are described.Comment: 38 pages with 9 embedded Postscript figures. To appear in Merging Processes in Clusters of Galaxies, edited by L. Feretti, I. M. Gioia, and G. Giovannini (Dordrecht: Kluwer), in press (2001

Low-Cost Attitude Determination and Control for Small Satellites

This paper addresses the need to develop small satellite technology which will enable small satellites to perform large satellite missions. The Center for Aerospace Technology (CAST) at Weber State University (WSU) has an 18 year history of small satellite innovation. Previous satellites include: NUSAT, WEBERSAT, and PHASE 3d. CAST is currently fabricating two new small satellites with advanced capabilities. CATSAT, a USRA program, will determine the origin of gamma-ray bursts and is a joint project with the University of New Hampshire and the University of Leicister in England. JAWSAT, a joint project with the U.S. Air Force Academy, will be the first payload launched by a converted minuteman missile. Both missions require active attitude determination and control previously unavailable for satellites of this class. In support of these two missions CAST has undertaken the task of developing satellite instrumentation designed specifically for small satellite applications. Size, weight, power consumption and cost minimization were incorporated into the design philosophy. New enabling technology includes the use of the State-Sampled Network for sensor integration, attitude determination and attitude control. The overall development history is chronicled with emphasis relating to issues of reliability and acceptance testing

The CATSAT Student Explorer Mission

CATSAT (Cooperative Astrophysical and Technology SATellite) is one of three missions being developed under NASA/USRA\u27s Student Explorer Demonstration Initiative (STEDI) for launch in 1997-98. STEDI is a pilot program to assess the efficacy of smaller, low-cost spaceflight missions ... that is matched to the traditional process of research and development at universities . This program allows $4 million and 2-3 years for all aspects of the mission, i.e. instrument and satellite development, integration, testing, mission operations and data analysis. CATSAT is an astrophysics mission being developed in collaboration by three university teams. Its mission is to study the nature and distance scale of Cosmic Gamma Ray Bursters. Its prime instrumentation is a Soft X-Ray spectrometer (0.5-15 keV), with a total area of 190 cm2 and 5.5 sr. FOV, to measure the photoelectric absorption along the line-of-sight and thus determine a distance scale to the burst source. This sensor is supported by three context sensors to determine intensity, spectral and directional information. These include four Hard X-Ray spectrometers (15-400 keV, 4x45 cm2 ), a Directional Gamma-ray Spectrometer (0.3-6 MeV, 135 cm2) and an array of nine X-ray Albedo sensors (15-400 keV, 9x80 cm2 ) which are also sensitive to polarization in the burst\u27s x-ray emissions. The science payload will generate 24 Mbytes of data per 12 hours. CATSAT is expected to be launched in mid 1998 into a 550 Km polar sun synchronous 6am-6pm orbit. The mission form factor is a rectangular box with a base of 72 cm, a height of 102 cm and a launch mass of 135 Kg. A combination of body mounted cells and deployable solar panels produce a total of 150 watts. The mission is 3-axis stabilized utilizing reaction wheels and magnetic torquers to provide continuous solar-(near zenith) pointing to 5°, in orbits up to 30° from its initial terminator orbit. Solar and earth-horizon sensors are used for attitude determination and will provide after-the-fact knowledge to 1°. An onboard GPS receiver will provide universal time and orbital navigation data. The GPS information together with an onboard three-axis magnetometer will be used for a coarse attitude solution during the initial turn-on as well as other orbital emergencies, and also act as a backup for the primary sensors. Commanding and science data retrieval will take place every 12 hours by student operating teams at the university sites. A 10 watt low power standby mode is included to manage orbital emergencies between contacts

Measurements of electron anisotropy in solar flares using albedo with RHESSI X-ray data

The angular distribution of electrons accelerated in solar flares is a key parameter in the understanding of the acceleration and propagation mechanisms that occur there. However, the anisotropy of energetic electrons is still a poorly known quantity, with observational studies producing evidence for an isotropic distribution and theoretical models mainly considering the strongly beamed case. We use the effect of photospheric albedo to infer the pitch-angle distribution of X-ray emitting electrons using Hard X-ray data from RHESSI. A bi-directional approximation is applied and a regularised inversion is performed for eight large flare events to deduce the electron spectra in both downward (towards the photosphere) and upward (away from the photosphere) directions. The electron spectra and the electron anisotropy ratios are calculated for a broad energy range, from about ten up to ∼ 300 keV, near the peak of the flares. The variation of electron anisotropy over short periods of time lasting 4, 8 and 16 seconds near the impulsive peak has been examined. The results show little evidence for strong anisotropy and the mean electron flux spectra are consistent with the isotropic electron distribution. The 3σ level uncertainties, although energy and event dependent, are found to suggest that anisotropic distribution with anisotropy larger than ∼ three are not consistent with the hard X-ray data. At energies above 150 – 200 keV, the uncertainties are larger and thus the possible electron anisotropies could be larger