An update on Fermi-LAT transients in the Galactic plane, including strong activity of Cygnus X-3 in mid-2020

TL;DR

This study analyzes 13 years of Fermi-LAT data to detect and characterize transient gamma-ray sources in the Galactic plane, revealing new sources and detailed behavior of Cygnus X-3 in 2020.

Contribution

It introduces an improved variable-size sliding-time-window analysis and extends data coverage, leading to new detections of gamma-ray transients and detailed insights into Cygnus X-3 activity.

Findings

Detected three novae and one gamma-ray binary with the new method.

Identified a gamma-ray flare from Cygnus X-3 in 2020.

Observed gamma-ray emission modulation with orbital period.

Abstract

We present a search for Galactic transient gamma-ray sources using 13 years of the Fermi Large Area Telescope data. The search is based on a recently developed variable-size sliding-time-window (VSSTW) analysis and aimed at studying variable gamma-ray emission from binary systems, including novae, gamma-ray binaries, and microquasars. Compared to the previous search for transient sources at random positions in the sky with 11.5 years of data, we included gamma rays with energies down to 500 MeV, increased a number of test positions, and extended the data set by adding data collected between February 2020 and July 2021. These refinements allowed us to detect an additional three novae, V1324 Sco, V5855 Sgr, V357 Mus, and one gamma-ray binary, PSR B1259-63, with the VSSTW method. Our search revealed a gamma-ray flare from the microquasar, Cygnus X-3, occurred in 2020. When applied to equal quarters of the data, the analysis provided us with detections of repeating signals from PSR B1259-63, LS I +61 303, PSR J2021+4026, and Cygnus X-3. While the Cygnus X-3 was bright in gamma rays in mid-2020, it was in a soft X-ray state and we found that its gamma-ray emission was modulated with the orbital period.