Wojtek Hajdas

Deducing Electron Properties from Hard X-Ray Observations

X-radiation from energetic electrons is the prime diagnostic of flare-accelerated electrons. The observed X-ray flux (and polarization state) is fundamentally a convolution of the cross-section for the hard X-ray emission process(es) in question with the electron distribution function, which is in turn a function of energy, direction, spatial location and time. To address the problems of particle propagation and acceleration one needs to infer as much information as possible on this electron distribution function, through a deconvolution of this fundamental relationship. This review presents recent progress toward this goal using spectroscopic, imaging and polarization measurements, primarily from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Previous conclusions regarding the energy, angular (pitch angle) and spatial distributions of energetic electrons in solar flares are critically reviewed. We discuss the role and the observational evidence of several radiation processes: free-free electron-ion, free-free electron-electron, free-bound electron-ion, photoelectric absorption and Compton backscatter (albedo), using both spectroscopic and imaging techniques. This unprecedented quality of data allows for the first time inference of the angular distributions of the X-ray-emitting electrons and improved model-independent inference of electron energy spectra and emission measures of thermal plasma. Moreover, imaging spectroscopy has revealed hitherto unknown details of solar flare morphology and detailed spectroscopy of coronal, footpoint and extended sources in flaring regions. Additional attempts to measure hard X-ray polarization were not sufficient to put constraints on the degree of anisotropy of electrons, but point to the importance of obtaining good quality polarization data in the future.

Gamma-ray burst polarization: Limits from Rhessi measurements

Using the RHESSI satellite as a Compton polarimeter, a recent study claimed that the prompt emission of GRB 021206 was almost fully linearly polarized. This was challenged by a subsequent reanalysis. We present a novel approach, applying our method to the same data. We identify Compton scattering candidates by carefully filtering events in energy, time, and scattering geometry. Our polarization search is based on time-dependent scattering rates in perpendicular directions, thus optimally excluding systematic errors. We perform simulations to obtain the instruments polarimetric sensitivity, and these simulations include photon polarization. For GRB 021206, we formally find a linear polarization degree of ΠGRB = 41%, concluding that the data quality is insufficient to constrain the polarization degree in this case. We further applied our analysis to GRB 030519B and again found a null result.

POLAR, a compact detector for Gamma Ray Bursts photon polarization measurements

The design and the simulated performances of a compact detector dedicated to the measurement of GRB photon polarization is presented. Such a detector would permit to answer the question “are most of the GRB strongly polarized?” in a mission of one year in space.

Search for gamma-ray burst classes with the RHESSI satellite

A sample of 427 gamma-ray bursts (GRBs), measured by the RHESSI satellite, is studied statistically with respect to duration and hardness ratio. Standard statistical tests are used, such as

From the Reference SEU Monitor to the Technology Demonstration Module On-Board PROBA-II

\chi^2

The ESA Standard Radiation Environment Monitor program first results from PROBA-I and INTEGRAL

, F-test and the maximum likelihood ratio test, in order to compare the number of GRB groups in the RHESSI database with that of the BATSE database. Previous studies based on the BATSE Catalog claim the existence of an intermediate GRB group, besides the long and short groups. Using only the GRB duration

The Proton Irradiation Facility at the Paul Scherrer Institute

T_{90}

Linear Energy Transfer of Heavy Ions in Silicon

as information and

The Giant Flare of 2004 December 27 from SGR 1806–20

\chi^2

Radiation environment along the INTEGRAL ? orbit measured with the IREM monitor

or F-test, we have not found any statistically significant intermediate group in the RHESSI data. However, maximum likelihood ratio test reveals a significant intermediate group. Also using the 2-dimensional hardness /