A gamma-ray periodic modulation in Globular Cluster 47 Tucanae

TL;DR

This paper reports the first detection of an 18.4-hour gamma-ray modulation in the globular cluster 47 Tucanae, suggesting a new periodic phenomenon beyond known millisecond pulsar emissions.

Contribution

It presents the discovery of a long-period gamma-ray modulation in 47 Tucanae, detected through Fermi-LAT data, which is a novel finding in globular cluster studies.

Findings

Detected 18.416-hour gamma-ray modulation with 4.8σ significance.

Modulation signal also observed in Swift-BAT data.

Chandra X-ray data may provide additional clues.

Abstract

The Globular Cluster 47 Tucanae was firstly detected in gamma-rays by the Large Area Telescope (LAT) onboard the \emph{Fermi} Gamma-ray Space Telescope, and the gamma-ray emission has been widely attributed to the millisecond pulsars. In this work, we analyze the Fermi-LAT pass 8 data ranging from 2008 August to 2017 May and report the detection of a modulation with a period of $18.416\pm0.008$ hours at a significance level of $\sim 4.8\sigma$. This is the first time to detect a significant modulation with a period much longer than that of millisecond pulsars in gamma-rays from Globular Clusters. The periodic modulation signal appears in the {\it Swift}-BAT data as well. The phase-folded Chandra X-ray light curve of a point source may have provided an additional clue.