Felix Ryde

Data acquisition system for the PoGOLite astronomical hard X-ray polarimeter

The PoGOLite is a new balloon-borne instrument to measure the polarization of hard X-rays / soft gamma-rays in the 25-80 keV energy range for the first time. In order to detect the polarization, PoGOLite measures the azimuthal angle asymmetry of Compton scattering and the subsequent photo- absorption in an array of detectors. This array consists of 217 well-type phoswich detector cells (PDCs) surrounded by a side anti-coincidence shield (SAS) composed of 54 segments of BGO crystals. At balloon altitude, the intensity of backgrounds due to cosmic-ray charged particles, atmospheric gamma-rays and neutrons is extremely high, typically a few hundred Hz per unit. Hence the data acquisition (DAQ) system of PoGOLite is required to handle more than 270 signals simultaneously, and detect weak signals from astrophysical objects (lOOmCrab, 1.5 cs-1 in 25-80 keV ) under such a severe environment. We have developed a new DAQ system consisting of front-end electronics, waveform digitizer, field programmable gate array (FPGA) and a microprocessor. In this system, all output signals of PDC / SAS are fed into individual charge-sensitive amplifier and then digitized to 12 bit accuracy at 24MSa/s by pipelined analog to digital converters. A DAQ board for the PDC records waveforms which will be examined in an off-line analysis to distinguish signals from the background events and measure the energy spectrum and polarization of targets. A board for the SAS records hit pattern to be used for background rejection. It also continuously records a pulse-height analysis (PHA) histogram to monitor incident background flux. These basic functions of the DAQ system were verified in a series of beam tests.

On the origin of the dark gamma-ray bursts

The origin of dark bursts—i.e. that have no observed afterglows in X-ray, optical/NIR and radio ranges—is unclear yet. Different possibilities—instrumental biases, very high redshifts, extinction in the host galaxies—are discussed and shown to be important. On the other hand, the dark bursts should not form a new subgroup of long ngamma-ray bursts themselves.

Model-independent methods of describing GRB spectra using BATSE MER data

The gamma-ray inverse problem is discussed. Four methods of spectral deconvolution are presented here and applied to the BATSEs MER data type. We compare these to the Band spectra.

Properties of the intermediate type of gamma-ray bursts

Gamma‐ray bursts can be divided into three groups (“short”, “intermediate”, “long”) with respect to their durations. The third type of gamma‐ray bursts — as known — has the intermediate duration. We show that the intermediate group is the softest one. An anticorrelation between the hardness and the duration is found for this subclass in contrast to the short and long groups.

Principal-component analysis of gamma-ray bursts' spectra

The principal-component analysis is a statistical method, which lowers the number of important variables in a data set. The use of this method for the bursts’ spectra and afterglows is discussed in this paper. The analysis indicates that three principal components are enough among the eight ones to describe the variablity of the data. The correlation between the spectral index α and the redshift nsuggests that the thermal emission component becomes more dominant at larger redshifts.

The conspicuous gamma-ray burst of 30 May 1996

The spectra of the majority of bursts exhibit a low-energy power law index, alpha, that is either a constant or becomes softer with time. However, in the burst of 30 May 1996 alpha becomes harder. Here we show that this behavior can be explained by a hybrid model consisting of a thermal and a non-thermal component. In this burst the power-law index of the non-thermal component changes drastically from s ~ -1.5 to s ~ -0.67 at approximately 5 seconds after the trigger, thereby revealing, at low energies, the thermal component with its hard Rayleigh-Jeans tail. This leads to the large alpha-values that are found if the Band function is fitted to the spectra. We suggest that the change in s could be due to a transition from fast to slow cooling of the electrons emitting in the BATSE range. This could be due to the fact that the magnetic field strength becomes weaker.

Shape of the decay phase of gamma-ray burst pulses

We study the decay phase of GRB pulses belonging to the large sub-sample for which two important correlations between spectral hardness and intensity, and spectral hardness and photon fluence, respectively, are valid. These pulses exhibit a common temporal behavior, i.e., the spectral hardness and intensity of the decay phase of the pulses follow a reciprocal function in time.

A new method for studying the hardness-intensity correlation in gamma-ray bursts

We introduce a new method to study the hardness-intensity correlation (HIC) within gamma-ray bursts, in which the intensity is represented by the peak value of the EFE spectrum. We compare it to the traditional method where the intensity over a finite energy range is used. This new method gives stronger correlations and is useful in studying several aspects of the HIC.

Track jumps in hardness-intensity correlations of GRB pulse decays

The hardness-intensity correlation (HIC) during the decay phase of gamma-ray burst pulses can often be described by a simple power law. The pulse decays in multi-pulse bursts present power-law indices that are equal to within the errors. We find that in some pulses the HIC changes to a parallel power law, a feature that we graphically call a track jump. The use of the index constancy is proposed as an auxiliary tool for pulse identification, and examples of its application are given.