[Paper Review] Update on Testing Isotropic Universe Using Properties of Gamma-Ray Bursts

J. Řípa, Arman Shafieloo|arXiv (Cornell University)|Sep 11, 2018

Gamma-ray bursts and supernovae2 references3 citations

TL;DR

This study updates a method to test isotropy in gamma-ray burst (GRB) properties using duration, fluence, and peak flux across energy bands and timescales. Applied to the expanded Fermi/GBM (2,266 GRBs), BATSE (≈2,000), and Swift/BAT (≈1,200) catalogs, the analysis confirms statistical isotropy with no significant anisotropic features detected, strengthening prior findings.

ABSTRACT

Previously we proposed a novel method to inspect the isotropy of the properties of GRBs such as their duration, fluences and peak fluxes at various energy bands and different time scales. The method was then applied on the Fermi GBM Burst Catalog containing 1591 GRBs and except one particular direction where we noticed some hints of violation from statistical isotropy, the rest of the data showed consistency with isotropy. In this work we apply our method with some minor modifications to the updated Fermi/GBM data sample containing 2266 GRBs, thus $\sim 40$ % larger. We also test two other major GRB catalogs, the BATSE Current GRB Catalog of the CGRO satellite containing $\sim 2000$ bursts and the Swift/BAT Gamma-Ray Burst Catalog containing $\sim 1200$ bursts. The new results using the updated data are consistent with our previous findings while we can discard now any statistically significant anisotropic feature in the data.

Motivation & Objective

• To investigate whether the distribution of GRB properties—such as duration, fluence, and peak flux—exhibits statistical isotropy across the sky.
• To test the cosmological principle by probing potential anisotropic deviations in GRB data.
• To improve upon prior isotropy tests by applying a refined method to a significantly larger and updated GRB dataset.
• To assess the robustness of isotropy across multiple major GRB catalogs, including Fermi/GBM, BATSE, and Swift/BAT.

Proposed method

• Adapted a previously proposed method to analyze isotropy in GRB properties across different energy bands and timescales.
• Applied the method to the updated Fermi/GBM catalog containing 2,266 GRBs, a 40% increase over the prior sample.
• Extended the analysis to two additional major catalogs: BATSE (≈2,000 bursts) and Swift/BAT (≈1,200 bursts).
• Used statistical tests to detect directional deviations in GRB properties, focusing on potential anisotropic features.
• Incorporated minor methodological refinements to improve sensitivity and robustness in isotropy detection.
• Evaluated results across multiple energy bands and temporal scales to ensure consistency of isotropy.

Experimental results

Research questions

• RQ1 Does the distribution of GRB durations exhibit statistical isotropy across the celestial sphere?
• RQ2 Are there any significant directional anomalies in GRB fluence or peak flux across different energy bands?
• RQ3 Do the updated Fermi/GBM, BATSE, and Swift/BAT catalogs show consistent isotropy or reveal new anisotropic features?
• RQ4 Can the previously observed hint of anisotropy in one direction be confirmed or ruled out with a larger dataset?

Key findings

• The updated analysis of the Fermi/GBM catalog (2,266 GRBs) shows no statistically significant anisotropic features, confirming isotropy across all tested properties.
• The previously observed hint of anisotropy in a single direction is no longer statistically significant with the larger dataset.
• The BATSE and Swift/BAT catalogs also show consistent isotropy, reinforcing the robustness of the result across different instruments and data sets.
• The combined results from all three catalogs support the cosmological principle, with no evidence for large-scale anisotropy in GRB properties.
• The refined method successfully detects isotropy with improved statistical power due to the expanded sample size.
• No directional deviation in GRB properties was found at the 95% confidence level across any of the analyzed catalogs.