Detection of Increasing γ-Ray Emissions from 4FGL J1718.5+4237 with Fermi-LAT

Abstract

We report a gradual brightening γ-ray source, 4FGL J1718.5+4237, in 0.1–500.0 GeV, which may be associated with a blazar NVSS J171822+423948 with a redshift of ∼2.7. We analyzed 15.25 yr of γ-ray data recorded by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope and detected significant γ-ray emissions in the direction of the blazar with a test statistic (TS) of ∼135. Based on timing analysis using a 1 yr time bin, we have observed a gradual brightening in γ-ray emissions from the target. In our analysis, we categorize them into two states: Quiet (TS ∼ 0) and Loud (TS ∼ 226) states, with the distinction occurring in 2016 August (MJD 57602.69). From the Quiet state to the brightest period (the last data point), the γ-ray flux in 0.1–500.0 GeV increased by more than 12-fold from <0.2 × 10−8 photons cm−1 s−1 to 2.6 × 10−8 photons cm−1 s−1. Additionally, we studied the spectral properties in detail for the Loud state and the overall data. While no significant variation was detected, both exhibited a spectral index Γ of ∼2.6. The origin of the brightening γ-ray emissions from the target is not yet clear. Future long-term multi-wavelength observations and studies will provide insight into the astrophysical mechanisms of the target.