Testing Isotropic Universe Using the Gamma-Ray Burst Data of Fermi / GBM

Abstract

The sky distribution of Gamma-Ray Bursts (GRBs) has been intensively studied by various groups for more than two decades. Most of these studies test the isotropy of GRBs based on their sky number density distribution. In this work we propose an approach to test the isotropy of the Universe through inspecting the isotropy of the properties of GRBs such as their duration, fluences and peak fluxes at various energy bands and different time scales. We apply this method on the {\em Fermi} / Gamma-ray Burst Monitor (GBM) data sample containing 1591 GRBs. The most noticeable feature we found is near the Galactic coordinates $l\approx 30^{\circ }$, $b\approx 15^{\circ }$ and radius $r\approx 20^{\circ }-40^{\circ }$. The inferred probability for the occurrence of such an anisotropic signal (in a random isotropic sample) is derived to be less than a percent in some of the tests while the other tests give results consistent with isotropy. These are based on the comparison of the results from the real data with the randomly shuffled data samples. Considering large number of statistics we used in this work (which some of them are correlated to each other) we can anticipate that the detected feature could be result of statistical fluctuations. Moreover, we noticed a considerably low number of GRBs in this particular patch which might be due to some instrumentation or observational effects that can consequently affect our statistics through some systematics. Further investigation is highly desirable in order clarify about this result, e.g. utilizing a larger future {\em Fermi} / GBM data sample as well as data samples of other GRB missions and also looking for possible systematics.

17 pages, 10 figures, 4 tables, accepted for publication in The Astrophysical Journal

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• Cosmology Intertwined: A Review of the Particle Physics, Astrophysics, and Cosmology Associated with the Cosmological Tensions and Anomalies
• Large angular scale multipoles at redshift ~0.8
• Role of Non-conserved Gravity Theory and Electric Charge in Constructing Complexity-free Stellar Models: A Novel Approach under Non-minimal Coupling
• An Oppositeness in the Cosmology: Distribution of the Gamma-Ray Bursts and the Cosmological Principle
• Extending Finch-Skea Isotropic Model to Anisotropic Domain in Modified $f(\mathcal{R},\mathcal{T})$ Gravity
• Does the GRB Duration Depend on Redshift?
• Implications of Rastall Theory on Stellar Solutions admitting Vanishing Complexity: A New Perspective
• Physical Features of Geometrically Deformed Anisotropic Charged Three-dimensional BTZ Black Holes
• Scanning the Universe for Large-Scale Structures using Gamma-Ray Bursts
• Reanalysing large-scale structure using an updated gamma-ray burst spatial density approach
• Measuring cosmic dipole with the GRB luminosity-time relation
• Probing dipole and quadrupole anisotropy in Gamma-ray bursts from Swift dataset