Z. Bagoly

DETAILED CLASSIFICATION OF SWIFT 'S GAMMA-RAY BURSTS

Earlier classification analyses found three types of gamma-ray bursts (short, long, and intermediate in duration) in the BATSE sample. Recent works have shown that these three groups are also present in the RHESSI and BeppoSAX databases. The duration distribution analysis of the bursts observed by the Swift satellite also favors the three-component model. In this paper, we extend the analysis of the Swift data with spectral information. We show, using the spectral hardness and duration simultaneously, that the maximum likelihood method favors the three-component against the two-component model. The likelihood also shows that a fourth component is not needed.

A DISTINCT PEAK-FLUX DISTRIBUTION OF THE THIRD CLASS OF GAMMA-RAY BURSTS: A POSSIBLE SIGNATURE OF X-RAY FLASHES?

Gamma-ray bursts (GRBs) are the most luminous events in the universe. Going beyond the short-long classification scheme, we work in the context of three burst populations with the third group of intermediate duration and softest spectrum. We are looking for physical properties which discriminate the intermediate duration bursts from the other two classes. We use maximum likelihood fits to establish group memberships in the duration-hardness plane. To confirm these results we also use k-means and hierarchical clustering. We use Monte Carlo simulations to test the significance of the existence of the intermediate group and we find it with 99.8% probability. The intermediate duration population has a significantly lower peak flux (with 99.94% significance). Also, long bursts with measured redshift have higher peak fluxes (with 98.6% significance) than long bursts without measured redshifts. As the third group is the softest, we argue that we have related them with X-ray flashes among the GRBs. We give a new, probabilistic definition for this class of events.

Anisotropy in the sky distributions of the short and intermediate gamma-ray bursts: Breakdown of the cosmological principle?

After the discovery of the anisotropy in the sky‐distribution of intermediate gamma‐ray bursts recently also the distribution of the short gamma‐ray bursts is proven to be anisotropic. The impact of these behaviors on the validity of the cosmological principle is shortly discussed.

A possible interrelation between the estimated luminosity distances and internal extinctions of type Ia supernovae

We studied the statistical properties of the luminosity distance and internal extinction data of type Ia supernovae in the lists published by Tonry et al. (2003) and Barris et al. (2004). After selecting the luminosity distance in an empty Universe as a reference level we divided the sample into low z < 0.25 and high z ≥ 0.25 parts. We further divided these subsamples by the median of the internal extinction. Performing sign tests using the standardized residuals between the estimated logarithmic luminosity distances and those of an empty universe, on the four subsamples separately, we recognized that the residuals were distributed symmetrically in the low redshift region, independently from the internal extinction. On the contrary, the low extinction part of the data of z ≥ 0.25 clearly showed an excess of the points with respect to an empty Universe which was not the case in the high extinction region. This diversity pointed to an interrelation between the estimated luminosity distance and internal extinction. To characterize quantitatively this interrelation we introduced a hidden variable making use of the technics of factor analysis. After subtracting that part of the residual which was explained by the hidden variable we obtained luminosity distances which were already free from interrelation with internal extinction. Fitting the corrected luminosity distances with cosmological models we concluded that the SN Ia data alone did not exclude the possibility of the Λ = 0 solution. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Angular distribution of GRBs

We studied the complete randomness of the angular distribution of BATSE gamma-ray bursts (GRBs). Based on their durations and peak fluxes, we divided the BATSE sample into 5 subsamples (short1, short2, intermediate, long1, long2) and studied the angular distributions separately. We used three methods to search for non-randomness in the subsamples: Voronoi tesselation, minimal spanning tree, and multifractal spectra. To study any non-randomness in the subsamples we defined 13 test-variables (9 from Voronoi tesselation, 3 from the minimal spanning tree and one from the multifractal spectrum). We made Monte Carlo simulations taking into account the BATSE’s sky-exposure function. We tested therandomness by introducing squared Euclidean distances in the parameter space of the test-variables. We recognized that the short1, short2 groups deviate significantly (99.90%, 99.98%) from the fully random case in the distribution of the squared Euclidean distances but this is not true for the long samples. In the intermediate group, the squared Euclidean distances also give significant deviation (98.51%).

Different satellites—different GRB redshift distributions?

The measured redshifts of gamma‐ray bursts (GRBs), which were first detected by the Swift satellite, seem to be bigger on average than the redshifts of GRBs detected by other satellites. We analyzed the redshift distribution of GRBs triggered and observed by different satellites (Swift[1], HETE2[2], BeppoSax, Ulyssses). After considering the possible biases significant difference was found at the p = 95.70% level in the redshift distributions of GRBs measured by HETE and the Swift.

Analysis of the Swift Gamma-Ray Bursts duration

Two classes of gamma‐ray bursts have been identified in the BATSE catalogs characterized by durations shorter and longer than about 2 seconds. There are, however, some indications for the existence of a third type of burst. Swift satellite detectors have different spectral sensitivity than pre‐Swift ones for gamma‐ray bursts. Therefore it is worth to reanalyze the durations and their distribution and also the classification of GRBs. Using The First BAT Catalog the maximum likelihood estimation was used to analyzed the duration distribution of GRBs. The three log‐normal fit is significantly (99.54% probability) better than the two for the duration distribution. Monte‐Carlo simulations also confirm this probability (99.2%).

Observational difference between gamma and X‐ray properties of optically dark and bright GRBs

Using the discriminant analysis of the multivariate statistical analysis we compared the distribution of the physical quantities of the optically dark and bright GRBs, detected by the BAT and XRT on board of the Swift Satellite. We found that the GRBs having detected optical transients (OT) have systematically higher peak fluxes and lower HI column densities than those without OT.

Probing the Isotropy in the Sky Distribution of the Gamma-Ray Bursts

The statistical tests - done by the authors - are surveyed, which verify the null-hypothesis of the intrinsic randomness in the angular distribution of gamma-ray bursts collected at BATSE Catalog. The tests use the counts-in-cells method, an analysis of spherical harmonics, a test based on the two-point correlation function and a method based on multiscale methods. The tests suggest that the intermediate subclass of gamma-ray bursts are distributed anisotropically.

Searching for galactic sources in the Swift GRB catalog

Context. Since the early 1990s Gamma Ray Bursts have been accepted to b of extra-galactic origin due to the isotropic distributio n observed by BATSE and the redshifts observed via absorption line spectroscopy. Nevertheless, upon further examinatio at least one case turned out to be of galactic origin. This particular eve nt presented a Fast Rise, Exponential Decay (FRED) structur e which leads us to believe that other FRED sources might also be Galactic. Aims. This study was set out to estimate the most probable degree of contamination by galactic sources that certain samples of F REDs have. Methods. In order to quantify the degree of anisotropy the average dip olar and quadripolar moments of each sample of GRBs with respect to the galactic plane were calculated. This was then compared to the probability distribution of simulated samp les comprised of a combination of isotropically generated sources and gal actic sources. Results. We observe that the dipolar and quadripolar moments of the se lected subsamples of FREDs are found more than two standard deviations outside those of random isotropically generate d samples.The most probable degree of contamination by gala ctic sources for the FRED GRBs of theSwift catalog detected until February 2011 that do not have a known redshift is about 21 out of 77 sources which representes roughly equal to 27%. Furthermore we obse rve, that by removing from this sample those bursts that have any type of indirect redshift indicator and multiple peaks the most p robable contamination increases up to 34% (17 out of 49 sourc es). Conclusions. It is probable that a high degree of contamination by galacti c sources occurs among the single peak FREDs observed by Swift. So we encourage further studies on these type of events in or der to determine the nature of what could be an exotic type of galactic source.