L. G. Balázs

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.

Searching for differences in Swift's intermediate GRBs

Context. Gamma-ray bursts are usually classified in terms their high-energy emission into either short-duration or long-duration bursts, which presumably reflect two different types of progenitors. However, it has been shown on statistical grounds that a third, intermediate population is needed in this classification scheme, although an extensive study of the properties of this class has so far not been performed. The large amount of follow-up studies generated during the Swift era allows us to have a sufficient sample to attempt a study of this third population through the properties of their prompt emission and their afterglows. Aims. To understand the differences of the intermediate population, we study a sample of GRBs observed by Swift during its first four years of operation. The sample contains only bursts with measured redshifts since these data help us to derive intrinsic properties. Methods. We search for differences in the properties of the three groups of bursts, which we quantify using a Kolmogorov-Smirnov test whenever possible. Results. Intermediate bursts are found to be less energetic and have dimmer afterglows than long GRBs, especially when considering the X-ray light curves, which are on average one order of magnitude fainter than long bursts. There is a less significant trend in the redshift distribution that places intermediate bursts closer than long bursts. Except for this, intermediate bursts show similar properties to long bursts. In particular, they follow the E-peak versus E-iso correlation and have, on average, positive spectral lags with a distribution similar to that of long bursts. As for long GRBs, they normally have an associated supernova, although some intermediate bursts have been found to contain no supernova component. Conclusions. This study shows that intermediate bursts differ from short bursts, but exhibit no significant differences from long bursts apart from their lower brightness. We suggest that the physical difference between intermediate and long bursts could be explained by being produced by similar progenitors, of the former being the ejecta thin shells and the latter thick shells.

Gamma photometric redshifts for long gamma-ray bursts

Max-Planck-Institut fu¨r Astronomie, D-69117 Heidelberg, 17 K¨onigstuhl, Germanye-mail: vavrek@mpia-hd.mpg.deReceived 18 June, 2002/Accepted 18 November 2002Abstract. It is known that the soft tail of the gamma-ray bursts’ spectra show excesses from the exact power-lawdependence. In this article we show that this departure can be detected in the peak flux ratios of different BATSEDISCSC energy channels. This effect allows to estimate the redshift of the bright long gamma-ray bursts in theBATSE Catalog. A verification of these redshifts is obtained for the 8 GRB which have both BATSE DISCSCdata and measured optical spectroscopic redshifts. There is good correlation between the measured and estimatedredshifts, and the average error is ∆z ≈ 0.33. The method is similar to the photometric redshift estimation ofgalaxies in the optical range, hence it can be called as ”gamma photometric redshift estimation”. The estimatedredshifts for the long bright gamma-ray bursts are up to z ≃ 4. For the the faint long bursts - which should beup to z ≃ 20 - the redshifts cannot be determined unambiguously with this method.Key words. Cosmology: large-scale structure of Universe – gamma-rays: bursts

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.

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 pu2009=u200995.70% level in the redshift distributions of GRBs measured by HETE and the Swift.

Classifying GRB 170817A/GW170817 in a Fermi duration–hardness plane

GRBxa0170817A, associated with the LIGO-Virgo GW170817 neutron-star merger event, lacks the short duration and hard spectrum of a Short gamma-ray burst (GRB) expected from long-standing classification models. Correctly identifying the class to which this burst belongs requires comparison with other GRBs detected by the Fermi GBM. The aim of our analysis is to classify Fermi GRBs and to test whether or not GRBxa0170817A belongs—as suggested—to the Short GRB class. The Fermi GBM catalog provides a large database with many measured variables that can be used to explore gamma-ray burst classification. We use statistical techniques to look for clustering in a sample of 1298 gamma-ray bursts described by duration and spectral hardness. Classification of the detected bursts shows that GRBxa0170817A most likely belongs to the Intermediate, rather than the Short GRB class. We discuss this result in light of theoretical neutron-star merger models and existing GRB classification schemes. It appears that GRB classification schemes may not yet be linked to appropriate theoretical models, and that theoretical models may not yet adequately account for known GRB class properties. We conclude that GRBxa0170817A may not fit into a simple phenomenological classification scheme.

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.