Fermi-LAT detection of recent gamma-ray activity from Cygnus X-3

ATel #17843; P. van Zyl (SARAO, South African Radio Astronomy Observatory), P. Monti-Guarnieri (University of Trieste and INFN Trieste), G. La Mura (INAF-OAC), J. Valverde (Marquette University), and M. Giroletti (INAF-IRA Bologna), on behalf of the Fermi Large Area Telescope Collaboration
on 14 Jun 2026; 15:18 UT
Credential Certification: Pfesesani van Zyl (pfesi24@gmail.com)

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The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed renewed gamma-ray activity from the high mass X-ray binary Cygnus X-3, also known as 4FGL J2032.6+4053 (The Fermi-LAT collaboration 2020, ApJS, 247, 1), with coordinates R.A. = 308.10737 deg, Decl. = 40.95775 deg (J2000; Petrov 2012, MNRAS, 419, 1097).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on June 13, 2026, with a daily averaged gamma-ray flux (E>100MeV) of (2.6+/- 0.4) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of approximately 20 relative to the average flux reported in the fourth data release of the fourth Fermi-LAT source catalog (4FGL-DR4, Ballet et al. 2023, arXiv:2307.12546). The corresponding photon index is 2.6 +/- 0.1, and it is consistent with the 4FGL-DR4 value of 2.68 +/- 0.04 within uncertainties. On a weekly time scale the source is significantly detected by the "Fermi All-sky Variability Analysis" (FAVA) between June 1 and June 8 in the low energy band (100 MeV - 800 MeV), but not in the high energy one (800 MeV - 300 GeV). It is however significantly detected in the energy range 100 MeV - 300 GeV between June 4 and June 11, being also associated with a E=25 GeV photon, detected on June 11 at 11:23:02 UT with a source association probability p > 0.975. The Fermi-LAT Collaboration has previously reported flaring activity from this source in ATel #10243. Radio and gamma-ray flaring activity has also recently been reported in ATel #17836.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. A preliminary light curve for Cygnus X-3 can be accessed via the Fermi-LAT Light Curve Repository at 4FGL J2032.6+4053. Since the source is part of the list of LAT daily monitored sources, its daily light curve can also be accessed at Cygnus X-3. We encourage multifrequency observations of this source. For this source the Fermi-LAT contact person is Stephane Corbel (stephane.corbel@cea.fr).

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.