GCN Circular 35746

S. Buson (DESY; Univ. of Wurzburg) on behalf of the Fermi Large Area Telescope Collaboration

The LHAASO collaboration has reported the detection of TeV activity from the blazar 1ES 1959+650 (ATel #16437) between  MJD 60347.02 / 60348.33 (2024-02-08 00:28:48  / 07:55:12 UTC). Furthermore, enhanced emission has been observed also at X-rays (ATel #16449). The blazar is associated with the >100 MeV gamma-ray source  4FGL J2000.0+6508 (The Fourth Fermi-LAT catalog, Data Release 4; The Fermi-LAT collaboration 2022, ApJS, 260, 53). In the past it has been suggested as a candidate neutrino emitter,  after two high-energy neutrinos were detected coincidentally with TeV flares observed by Whipple from the direction of 1ES 1959+650, as reported by the AMANDA collaboration (Halzen and Hooper, 2005).

Archival observations at MeV-GeV energies are available from the Fermi-Large Area Telescope. The source was modelled adopting the LAT catalog shape, i.e. a logparabola model, allowing the flux and spectral parameter alpha to vary in the fit. Integrating the LAT data over the period from 04 August 2008 to 18 February 2024, the observed flux is (5.00 +/- 0.05) x 10^{-8} ph/cm^2/s, and alpha = 1.780 +/- 0.008. A preliminary inspection of the LAT data revels that the blazar displays strong variability at gamma rays, from short to long timescales (days to years timescales). The most prominent flare, occurred as a prolonged, intense activity commencing around 2015. A one-year binned light curve highlights that this enhanced state persisted from approximately MJD 57237 to 59062 (August 2015 to August 2020). The flux peak was observed between August 2015 and August 2016, with an average flux of (1.12 +/- 0.03) x 10^{-7} ph/cm^2/s (>100 MeV, statistical uncertainty only), which is approximately twice the full-mission average flux, and alpha = 1.74 +/- 0.02.

Preliminary analysis integrating recent observations spanning from MJD 60338 to 60357 (January 29 – February 17, 2024), indicates that over the last month, the blazar has been experiencing a high state, with a monthly averaged flux (E>100 MeV) of flux= (1.8 +/- 0.2) x 10^{-7} ph/cm^2/s, and alpha = 1.67 +/- 0.05. This represents an increase of approximately four times the average flux reported in the fourth Fermi LAT source catalog and is comparable in brightness to the monthly-average flaring activity observed in 2015/2016. Upon integrating the LAT data over the time interval (~one day) corresponding to the detection of gamma rays by LHAASO, the LAT observed flux is (3.8 +/- 0.8)  x 10^{-7} ph/cm^2/s and alpha = 1.72 +/- 0.12. No significant changes are observed in the spectral shape of the source at gamma rays.

We strongly encourage multiwavelength observations of this source. Since Fermi normally operates in an all-sky scanning mode, regular monitoring of this source will continue. For this source the Fermi-LAT contact person is Sara Buson (sara.buson at desy.de).

The Fermi LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.