A Gamma-Ray Periodic Modulation in Globular Cluster 47 Tucanae

The globular cluster 47 Tucanae was first detected in gamma rays by the Large Area Telescope (LAT) on board the 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 ± 0.008 hr at a significance level of ∼4.8σ. This is the first time detecting 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 Swift Burst Alert Telescope (BAT) data as well. The phase-folded Chandra X-ray light curve of a point source may have provided an additional clue.