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

Abstract

ABSTRACT We present a search for Galactic transient γ-ray sources using 13 yr 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 γ-ray emission from binary systems, including novae, γ-ray binaries, and microquasars. Compared to the previous search for transient sources at random positions in the sky with 11.5 yr of data, we included γ-rays with energies down to 500 MeV, increased a number of test positions, and extended the data set by adding data collected between 2020 February and 2021 July. These refinements allowed us to detect additional three novae, V1324 Sco, V5855 Sgr, V357 Mus, and one γ-ray binary, PSR B1259-63, with the VSSTW method. Our search revealed a γ-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 γ-rays in mid-2020, it was in a soft X-ray state and we found that its γ-ray emission was modulated with the orbital period.