A flight experiment to determine GPS photochemical contamination accumulation rates

It was recently suggested that photochemically deposited contamination, originating from volatiles outgassed by a spacecraft, may be responsible for the anomalous degradation in power seen on the GPS Block 1 vehicles. In an attempt to confirm, or deny, the photochemical deposition rates predicted, a study was undertaken to design a flight experiment to be incorporated on the GPS vehicles currently in production. The objective was to develop an inexpensive, light weight instrument package that would give information on the contamination levels within a few months of launch. Three types of apparatus were studied, Quartz Crystal Microbalances, (QCM's), modified solar cells, and calorimeters. A calorimeter was selected due primarily to its impact on the production schedule of the GPS vehicles. An analysis of the sensitivity of the final design is compared to the predicted contamination accumulation rates in order to determine how long after launch it will take the experiment to show the effects of photochemical contamination

RXTE and ASCA Constraints on Non-thermal Emission from the A2256 Galaxy Cluster

An 8.3 hour observation of the Abell 2256 galaxy cluster using the Rossi X-ray Timing Explorer proportional counter array produced a high quality spectrum in the 2 - 30 keV range. Joint fitting with the 0.7 - 11 keV spectrum obtained with the Advanced Satellite for Astrophysics and Cosmology gas imaging spectrometer gives an upperlimit of 2.3x10^-7 photons/cm^2/sec/keV for non-thermal emission at 30 keV. This yields a lower limit to the mean magnetic field of 0.36 micro Gauss (uG) and an upperlimit of 1.8x10^-13 ergs/cm^3 for the cosmic-ray electron energy density. The resulting lower limit to the central magnetic field is ~1 - 3 uG While a magnetic field of ~0.1 - 0.2 uG can be created by galaxy wakes, a magnetic field of several uG is usually associated with a cooling flow or, as in the case of the Coma cluster, a subcluster merger. However, for A2256, the evidence for a merger is weak and the main cluster shows no evidence of a cooling flow. Thus, there is presently no satisfactory hypothesis for the origin of an average cluster magnetic field as high as >0.36 uG in the A2256 cluster.Comment: 8 pages, Astrophysical Journal (in press

Global analysis of muon decay measurements

We have performed a global analysis of muon decay measurements to establish model-independent limits on the space-time structure of the muon decay matrix element. We find limits on the scalar, vector and tensor coupling of right- and left-handed muons to right- and left-handed electrons. The limits on those terms that involve the decay of right-handed muons to left-handed electrons are more restrictive than in previous global analyses, while the limits on the other non-standard model interactions are comparable. The value of the Michel parameter eta found in the global analysis is -0.0036 \pm 0.0069, slightly more precise than the value found in a more restrictive analysis of a recent measurement. This has implications for the Fermi coupling constant G_F.Comment: 5 pages, 3 table

Magnetic Field Evolution in Merging Clusters of Galaxies

We present initial results from the first 3-dimensional numerical magnetohydrodynamical (MHD) simulations of magnetic field evolution in merging clusters of galaxies. Within the framework of idealized initial conditions similar to our previous work, we look at the gasdynamics and the magnetic field evolution during a major merger event in order to examine the suggestion that shocks and turbulence generated during a cluster/subcluster merger can produce magnetic field amplification and relativistic particle acceleration and, as such, may play a role in the formation and evolution of cluster-wide radio halos. The ICM, as represented by the equations of ideal MHD, is evolved self-consistently within a changing gravitational potential defined largely by the collisionless dark matter component represented by an N-body particle distribution. The MHD equations are solved by the Eulerian, finite-difference code, ZEUS. The particles are evolved by a standard particle-mesh (PM) code. We find significant evolution of the magnetic field structure and strength during two distinct epochs of the merger evolution.Comment: 21 pages, 7 figures, Figure 2 is color postscript. Accepted for publication in Ap

Performance evaluation of novel square-bordered position-sensitive silicon detectors with four-corner readout

We report on a recently developed novel type of large area (62 mm x 62 mm) position sensitive silicon detector with four-corner readout. It consists of a square-shaped ion-implanted resistive anode framed by additional low-resistivity strips with resistances smaller than the anode surface resistance by a factor of 2. The detector position linearity, position resolution, and energy resolution were measured with alpha-particles and heavy ions. In-beam experimental results reveal a position resolution below 1 mm (FWHM) and a very good non-linearity of less than 1% (rms). The energy resolution determined from 228Th alpha source measurements is around 2% (FWHM).Comment: 13 pages, 10 figures, submitted to Nucl. Instr. and Meth.

The Energy Spectrum of Primary Cosmic Ray Electrons in Clusters of Galaxies and Inverse Compton Emission

Models for the evolution of the integrated energy spectrum of primary cosmic ray electrons in clusters of galaxies have been calculated, including the effects of losses due to inverse Compton (IC), synchrotron, and bremsstrahlung emission, and Coulomb losses to the intracluster medium (ICM). The combined time scale for these losses reaches a maximum of ~3e9 yr for electrons with a Lorentz factor ~300. Only clusters in which there has been a substantial injection of relativistic electrons since z <~ 1 will have any significant population of primary cosmic ray electrons at present. In typical models, there is a broad peak in the electron energy distribution extending to gamma~300, and a steep drop in the electron population beyond this. In clusters with current particle injection, there is a power-law tail of higher energy electrons with an abundance determined by the current rate of injection. A significant population of electrons with gamma~300, associated with the peak in the particle loss time, is a generic feature of the models. The IC and synchrotron emission from these models was calculated. In the models, EUV and soft X-ray emission are nearly ubiquitous. This emission is produced by electrons with gamma~300. The spectra are predicted to drop rapidly in going from the EUV to the X-ray band. The IC emission also extends down the UV, optical, and IR bands with a fairly flat spectrum. Hard X-ray (HXR) and diffuse radio emission due to high energy electrons (gamma~10e4) is present only in clusters which have current particle acceleration. Assuming that the electrons are accelerated in ICM shocks, one would only expect diffuse HXR/radio emission in clusters which are currently undergoing a large merger.Comment: Accepted for publication in the Astrophysical Journal, with minor revisons to wording for clarity and one additional reference. 19 pages with 16 embedded Postscript figures in emulateapj.sty. Abbreviated abstract belo

Constraining the Accretion Rate Onto Sagittarius A* Using Linear Polarization

Two possible explanations for the low luminosity of the supermassive black hole at the center of our galaxy are (1) an accretion rate of order the canonical Bondi value (roughly 10^{-5} solar masses per year), but a very low radiative efficiency for the accreting gas or (2) an accretion rate much less than the Bondi rate. Both models can explain the broad-band spectrum of the Galactic Center. We show that they can be distinguished using the linear polarization of synchrotron radiation. Accretion at the Bondi rate predicts no linear polarization at any frequency due to Faraday depolarization. Low accretion rate models, on the other hand, have much lower gas densities and magnetic field strengths close to the black hole; polarization may therefore be observable at high frequencies. If confirmed, a recent detection of linear polarization from Sgr A$^*$ above 150 GHz argues for an accretion rate of order 10^{-8} solar masses per year, much less than the Bondi rate. This test can be applied to other low-luminosity galactic nuclei.Comment: final version accepted by ApJ; references added, somewhat shortene

On The Origin of Radio Halos in Galaxy Clusters

Previously it has been recognized that radio halos in galaxy clusters are preferentially associated with merging systems as indicated by substructure in the X-ray images and temperature maps. Since, however, many clusters without radio halos also possess substructure, the role of mergers in the formation of radio halos has remained unclear. By using power ratios to relate gravitational potential fluctuations to substructure in X-ray images, we provide the first quantitative comparison of the dynamical states of clusters possessing radio halos. A correlation between the 1.4 GHz power (P_{1.4}) of the radio halo (or relic) and the magnitude of the dipole power ratio (P_1/P_0) is discovered such that approximately P_{1.4} ~ P_1/P_0; i.e., the strongest radio halos appear only in those clusters currently experiencing the largest departures from a virialized state. From additional consideration of a small number of highly disturbed clusters without radio halos detected at 1.4 GHz, and recalling that radio halos are more common in clusters with high X-ray luminosity (Giovannini, Tordi, & Feretti), we argue that radio halos form preferentially in massive (L_x >~ 0.5 x 10^{45} erg/s) clusters experiencing violent mergers (P_1/P_0 >~ 0.5 x 10^{-4}) that have seriously disrupted the cluster core. The association of radio halos with massive, large-P_1/P_0, core-disrupted clusters is able to account for both the vital role of mergers in accelerating the relativistic particles responsible for the radio emission as well as the rare occurrence of radio halos in cluster samples.Comment: 4 pages, 1 figure, Accepted for Publication in The Astrophysical Journal Letters, updated reference

Gamma-ray probe of cosmic-ray pressure in galaxy clusters and cosmological implications

Cosmic rays produced in cluster accretion and merger shocks provide pressure to the intracluster medium (ICM) and affect the mass estimates of galaxy clusters. Although direct evidence for cosmic-ray ions in the ICM is still lacking, they produce gamma-ray emission through the decay of neutral pions produced in their collisions with ICM nucleons. We investigate the capability of the Gamma-ray Large Area Space Telescope (GLAST) and imaging atmospheric Cerenkov telescopes (IACTs) for constraining the cosmic-ray pressure contribution to the ICM. We show that GLAST can be used to place stringent upper limits, a few per cent for individual nearby rich clusters, on the ratio of pressures of the cosmic rays and thermal gas. We further show that it is possible to place tight (<~10%) constraints for distant (z <~ 0.25) clusters in the case of hard spectrum, by stacking signals from samples of known clusters. The GLAST limits could be made more precise with the constraint on the cosmic-ray spectrum potentially provided by IACTs. Future gamma-ray observations of clusters can constrain the evolution of cosmic-ray energy density, which would have important implications for cosmological tests with upcoming X-ray and Sunyaev-Zel'dovich effect cluster surveys.Comment: 12 pages, 5 figures; extended discussions; accepted by MNRA