Fermi-LAT detection of enhanced gamma-ray activity from the BL Lac 4C +56.27
ATel #16913; M. Giroletti (INAF/IRA Bologna), D. Bernard (LLR, Ecole Polytechnique & CNRS / IN2P3), on behalf of the Fermi Large Area Telescope Collaboration
on 18 Nov 2024; 16:48 UT
Credential Certification: Marcello Giroletti (giroletti@ira.inaf.it)
Subjects: Gamma Ray, >GeV, AGN, Blazar
The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed enhanced gamma-ray activity from a source positionally consistent with the BL Lac object 4C +56.27, also known as 4FGL J1824.1+5651 (The Fermi-LAT collaboration 2020, ApJS, 247, 33), with coordinates R.A. = 276.02945 deg, Decl. = +56.85041 deg (J2000; Le Bali et al. 2016, AJ, 151, 79L), and redshift z=0.664 (Lawrence et al. 1986, AJ 91, 494).
Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on November 16, 2024, with a daily averaged gamma-ray flux (E>100MeV) of (0.54+/-0.12) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of 14 relative to the average flux reported in the fourth Fermi-LAT catalog (4FGL). This is the highest LAT daily flux ever observed for this source. The corresponding photon index is 1.86+/-0.14, indicating a significantly harder spectrum than the 4FGL value of 2.39+/-0.02.
Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. A preliminary light curve for 4FGL J1824.1+5651 can be accessed via the Fermi-LAT Light-Curve Repository at https://fermi.gsfc.nasa.gov/ssc/data/access/lat/LightCurveRepository/source.html?source_name=4FGL_J1824.1+5651. We encourage multifrequency observations of this source. For this source, the Fermi-LAT contact person is M. Giroletti (marcello.giroletti@inaf.it).
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.
