Possible structure in the GRB sky distribution at redshift two

Context. Research over the past three decades has revolutionized cosmology while supporting the standard cosmological model. However, the cosmological principle of Universal homogeneity and isotropy has always been in question, since structures as large as the survey size have always been found each time the survey size has increased. Until 2013, the largest known structure in our Universe was the Sloan Great Wall, which is more than 400 Mpc long located approximately one billion light years away. Aims. Gamma-ray bursts are the most energetic explosions in the Universe. As they are associated with the stellar endpoints of massive stars and are found in and near distant galaxies, they are viable indicators of the dense part of the Universe containing normal matter. The spatial distribution of gamma-ray bursts can thus help expose the large scale structure of the Universe. Methods. As of July 2012, 283 GRB redshifts have been measured. Subdividing this sample into nine radial parts, each containing 31 GRBs, indicates that the GRB sample having 1.6 < z < 2.1 differs significantly from the others in that 14 of the 31 GRBs are concentrated in roughly 1/8 of the sky. A two-dimensional Kolmogorov-Smirnov test, a nearest-neighbour test, and a Bootstrap Point-Radius Method explore the significance of this clustering. Results. All tests used indicate that there is a statistically significant clustering of the GRB sample at 1.6 < z < 2.1. Furthermore, this angular excess cannot be entirely attributed to known selection biases, making its existence due to chance unlikely. Conclusions. This huge structure lies ten times farther away than the Sloan Great Wall, at a distance of approximately ten billion light years. The size of the structure defined by these GRBs is about 2000-3000 Mpc, or more than six times the size of the Sloan Great Wall.Comment: accepted for publication in Astronomy & Astrophysic

Investigation the connection between the intermediate gamma-ray bursts and X-ray flashes

Gamma-ray bursts can be divided into three groups: short-, intermediate and long-duration bursts. While the progenitors of the short and long ones are relatively known, the progenitor objects of the intermediate-duration bursts are generally unknown, however, recent statistical studies suggest, that they should be related to the long-duration bursts. In this work we investigate whether there is a differ- ence between the global parameters of the X-ray flashes and intermediate-duration group of gamma-ray bursts. The statistical tests do not show any significant discrepancy regarding most of the parameters, except the BAT photon index, which is only a consequence of the defintion of the X-ray flashes.Comment: 9 pages, 10 figures, submitted to Astronomische Nachrichte

Testing the randomness in the sky-distribution of gamma-ray bursts

We have studied the complete randomness of the angular distribution of gamma-ray bursts (GRBs) detected by the Burst and Transient Source Experiment (BATSE). Because GRBs seem to be a mixture of objects of different physical nature, we divided the BATSE sample into five subsamples (short1, short2, intermediate, long1, long2) based on their durations and peak fluxes, and we studied the angular distributions separately. We used three methods, Voronoi tesselation, minimal spanning tree and multifractal spectra, to search for non-randomness in the subsamples. To investigate the eventual non-randomness in the subsamples, we defined 13 test variables (nine from the Voronoi tesselation, three from the minimal spanning tree and one from the multifractal spectrum). Assuming that the point patterns obtained from the BATSE subsamples are fully random, we made Monte Carlo simulations taking into account the BATSE's sky-exposure function. The Monte Carlo simulations enabled us to test the null hypothesis (i.e. that the angular distributions are fully random). We tested the randomness using a binomial test and by introducing squared Euclidean distances in the parameter space of the test variables. We concluded that the short1 and short2 groups deviate significantly (99.90 and 99.98 per cent, respectively) from the full randomness in the distribution of the squared Euclidean distances; however, this is not the case for the long samples. For the intermediate group, the squared Euclidean distances also give a significant deviation (98.51 per cent)

A Remarkable Angular Distribution of the Intermediate Subclass of Gamma‐Ray Bursts

We develop a method of testing the null hypothesis of intrinsic randomness in the angular distribution of gamma-ray bursts collected in the Current BATSE Catalog. The method is a modified version of the well-known counts-in-cells test and fully eliminates the nonuniform sky-exposure function of the BATSE instrument. Applying this method to the case of all gamma-ray bursts, we found no intrinsic nonrandomness. The test also did not find intrinsic nonrandomness for the short and long gamma-ray bursts. However, using the method on the new, intermediate subclass of gamma-ray bursts, the null hypothesis of intrinsic randomness for 181 intermediate gamma-ray bursts is rejected on the 96.4% confidence level. Taking 92 dimmer bursts from this subclass, we obtain a surprising result: this "dim" subclass of the intermediate subclass has an intrinsic nonrandomness on the 99.3% confidence level. On the other hand, the 89 "bright" gamma-ray bursts show no intrinsic nonrandomness

Cosmology with Gamma-Ray Bursts Using k-correction

In the case of Gamma-Ray Bursts with measured redshift, we can calculate the k-correction to get the fluence and energy that were actually produced in the comoving system of the GRB. To achieve this we have to use well-fitted parameters of a GRB spectrum, available in the GCN database. The output of the calculations is the comoving isotropic energy E_iso, but this is not the endpoint: this data can be useful for estimating the {\Omega}M parameter of the Universe and for making a GRB Hubble diagram using Amati's relation.Comment: 4 pages, 6 figures. Presented as a talk on the conference '7th INTEGRAL/BART Workshop 14 -18 April 2010, Karlovy Vary, Czech Republic'. Published in Acta Polytechnic

Direction Dependent Background Fitting for the Fermi GBM Data

We present a method for determining the background of Fermi GBM GRBs using the satellite positional information and a physical model. Since the polynomial fitting method typically used for GRBs is generally only indicative of the background over relatively short timescales, this method is particularly useful in the cases of long GRBs or those which have Autonomous Repoint Request (ARR) and a background with much variability on short timescales. We give a Direction Dependent Background Fitting (DDBF) method for separating the motion effects from the real data and calculate the duration (T90 and T50, as well as confidence intervals) of the nine example bursts, from which two resulted an ARR. We also summarize the features of our method and compare it qualitatively with the official GBM Catalogue. Our background filtering method uses a model based on the physical information of the satellite position. Therefore, it has many advantages compared to previous methods. It can fit long background intervals, remove all the features caused by the rocking behaviour of the satellite, and search for long emissions or not-triggered events. Furthermore, many part of the fitting have now been automatised, and the method have been shown to work for both Sky Survey mode and ARR mode data. Future work will provide a burst catalogue with DDBF.Comment: 16 pages, 28 figure

An Observational Evidence for the Difference Between the Short and Long Gamma-Ray Bursts

The intrinsic fluence and duration distributions of gamma-ray bursts are well represented by log-normal distributions. This allows a bivariate log-normal distribution fit to be made to the BATSE short and long bursts separately. A statistically significant difference between the long and short groups is found. We argue that the effect is probably real. Applying the Cramér’s theorem these results lead to some predictions for models of long and short bursts

Factor analysis of the spectral and time behavior of long GRBs

A sample of 197 long BATSE GRBs is studied statistically. In the sample 11 variables, describing for any burst the time behavior of the spectra and other quantities, are collected. The application of the factor analysis on this sample shows that five factors describe the sample satisfactorily. Both the pseudo-redshifts coming from the variability and the Amati-relation in its original form are disfavored.Comment: In GAMMA-RAY BURSTS 2007: Proceedings of the Santa Fe Conferenc

Physical Difference Between the Short and Long Gamma-Ray Bursts

We provided separate bivariate log-normal distribution fits to the BATSE short and long burst samples using the durations and fluences. We show that these fits present evidence for a power-law dependence between the fluence and the duration, with statistically significant different indexes for the long and short subgroups. We argue that the effect is probably real, and the two subgroups are different physical phenomena. This may provide a potentially useful constraint for models of long and short bursts