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 2 × 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.
Context. We present a method for determining the background of the gamma-ray bursts (GRBs) of the Fermi Gamma-ray Burst Monitor (GBM) 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 that have autonomous repoint request (ARR) and a background with much variability on short timescales. Aims. Modern space instruments, like Fermi, have some specific motion to survey the sky and catch gamma-ray bursts in the most effective way. However, GBM bursts sometimes have highly varying backgrounds (with or without ARR), and modelling them with a polynomial function of time is not efficient -one needs more complex, Fermi-specific methods. This article presents a new direction dependent background fitting method and shows how it can be used for filtering the lightcurves. Methods. First, we investigate how the celestial position of the satellite may have influence on the background and define three underlying variables with physical meaning: celestial distance of the burst and the detector's orientation, the contribution of the Sun and the contribution of the Earth. Then, we use multi-dimensional general least square fitting and Akaike model selection criterion for the background fitting of the GBM lightcurves. Eight bursts are presented as examples, of which we computed the duration using background fitted cumulative lightcurves. Results. We give a direction dependent background fitting (DDBF) method for separating the motion effects from the real data and calculate the duration (T 90 , T 50 , and 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. Conclusions. 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 parts of the fitting have now been automatised, and the method has been shown to work for both sky survey mode and ARR mode data. Future work will provide a burst catalogue with DDBF.
Detecting the thermal emission from double neutron star merger events is a challenging task because of the quick fading of the observed flux. In order to create an efficient observing strategy for their observing method, it is crucial to know their intrinsic rate. Unfortunately, the numerous models existing today predict this rate on a very wide range. Hence, our goal in this paper is to investigate the effect of different levels of approximations on the relative rate predictions. Also, we study the effect of distinct ejecta mass layouts on the light curve. We find that the ratio of the expected kilonova detections of the spherical to axisymmetrical models is 6:1 (or 2:1, depending on the input parameter set applied in our work). Nevertheless, the light-curve shape is only slightly affected by the various ejecta alignments. This means that different ejecta layouts can produce light curves with similar shapes making it a challenging task to infer the structure of the matter outflow. Thus, we conclude that the uncertainty in the rate predictions arising from the various ejecta mass distribution models is negligible compared to the errors present in other input parameters (e.g. binary neutron star merger rate). In addition, we show that up to moderate redshifts (z ≲ 0.2) the redshift distribution type (observed or uniform in volume) does not affect the expected relative rate estimations.
Gamma-ray bursts (GRBs) 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 (IBs) are generally unknown, however, recent statistical studies suggest, that they can be related to the long-duration bursts. Another types of GRBs are the so-called X-ray flashes (XRFs) and X-ray rich GRBs (XRRs). The former ones radiate more intensively in the X-ray bands than common GRBs, but in the cases of XRFs the main component of the emission is produced entirely at X-ray wavelengths. Also, the XRFs and IBs show some similarities regarding their prompt-and afterglow properties. In this work we investigate whether there is a difference 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 XRFs.
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