A giant ring-like structure at 0.78<z<0.86 displayed by GRBs

According to the cosmological principle, Universal large-scale structure is homogeneous and isotropic. The observable Universe, however, shows complex structures even on very large scales. The recent discoveries of structures significantly exceeding the transition scale of 370 Mpc pose a challenge to the cosmological principle. We report here the discovery of the largest regular formation in the observable Universe; a ring with a diameter of 1720 Mpc, displayed by 9 gamma ray bursts (GRBs), exceeding by a factor of five the transition scale to the homogeneous and isotropic distribution. The ring has a major diameter of $43^o$ and a minor diameter of $30^o$ at a distance of 2770 Mpc in the 0.78<z<0.86 redshift range, with a probability of $2times 10^{-6}$ of being the result of a random fluctuation in the GRB count rate. Evidence suggests that this feature is the projection of a shell onto the plane of the sky. Voids and string-like formations are common outcomes of large-scale structure. However, these structures have maximum sizes of 150 Mpc, which are an order of magnitude smaller than the observed GRB ring diameter. Evidence in support of the shell interpretation requires that temporal information of the transient GRBs be included in the analysis. This ring-shaped feature is large enough to contradict the cosmological principle. The physical mechanism responsible for causing it is unknown.

Classification of Fermi Gamma-RAY Bursts

The Fermi GBM Catalog has been recently published. Previous classification analyses of the BATSE, RHESSI, BeppoSAX, and Swift databases found three types of gamma-ray bursts. Now we analyzed the GBM catalog to classify the GRBs. PCA and Multiclustering analysis revealed three groups. Validation of these groups, in terms of the observed variables, shows that one of the groups coincides with the short GRBs. The other two groups split the long class into a bright and dim part, as defined by the peak flux. Additional analysis is needed to determine whether this splitting is only a mathematical byproduct of the analysis or has some real physical meaning.

Dependence of the optical brightness on the gamma and X-ray properties of GRBs

The Swift satellite made a real break through with measuring simultaneously the gamma X-ray and optical data of GRBs, effectively. Although, the satellite measures the gamma, X-ray and optical properties almost in the same time a significant fractions of GRBs remain undetected in the optical domain. In a large number of cases only an upper bound is obtained. Survival analysis is a tool for studying samples where a part of the cases has only an upper (lower) limit. The obtained survival function may depend on some other variables. The Cox regression is a way to study these dependencies. We studied the dependence of the optical brightness (obtained by the UVOT) on the gamma and X-ray properties, measured by the BAT and XRT on board of the Swift satellite. We showed that the gamma peak flux has the greatest impact on the afterglows optical brightness while the gamma photon index and the X-ray flux do not. This effect probably originates in the energetics of the jet launched from the central engine of the GRB which triggers the afterglow.

Classification of Swifts gamma-ray bursts

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 class. 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. We analyze, the maximum likelihood estimation, the bursts duration distribution, published in The First BAT Catalog, whether it contains two, three or more groups. 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%). Similarly, in previous results we found that the fourth component is not needed. The relative frequencies of the distribution of the groups are 7% short 35% intermediate and 58% long. Although the relative frequencies of the groups are different than in the BATSE GRB sample, the difference in the instrument spectral sensitivities can explain this bias on a natural way. This means theoretical models may be needed to explain three different type of gamma-ray bursts.

Testing the Randomness in the Sky-Distribution of Gamma-Ray Bursts

We studied the complete randomness of the angular distribution of gamma-ray bursts (GRBs) detected by BATSE. Since GRBs seem to be a mixture of objects of different physical nature we divided the BATSE sample into 5 subsamples (short1, short2, intermediate, long1, long2) based on their durations and peak fluxes and 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 (9 from the Voronoi tesselation, 3 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 BATSEs sky-exposure function. The MC simulations enabled us to test the null hypothesis i.e. that the angular distributions are fully random. We tested the randomness by binomial test and introducing squared Euclidean distances in the parameter space of the test-variables. We concluded that the short1, short2 groups deviate significantly (99.90%, 99.98%) from the fully randomness in the distribution of the squared Euclidean distances but it is not the case at the long samples. At the intermediate group the squared Euclidean distances also give significant deviation (98.51%).