ATel draft: Fermi-LAT detection of renewed gamma-ray activity from the BL Lac 4C +56.27

ATel #16926; Pfesesani V. van Zyl (SARAO/HartRAO), Pietro Monti-Guarnieri (University of Trieste & INFN Trieste)
on 29 Nov 2024; 18:07 UT
Distributed as an Instant Email Notice Transients
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 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 Bail et al. 2016, AJ, 151, 79), 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 28, 2024 , with a daily averaged gamma-ray flux (E>100MeV) of (0.8+/-0.1) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of about 20 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. A corresponding photon index of 1.6+/-0.1 indicates a significantly harder spectrum than the 4FGL value of 2.39+/-0.02. The source also experienced a notable enhancement in HE emission, leading to the detection of eight photons having E > 10 GeV, associated with the source at a confidence level larger than 0.999. The highest energy event was a 44 GeV photon detected at 11:18:59 UT. Further likelihood analysis dedicated to the HE domain over the period of MET = 753842586.0 s (2024-11-21 00:43:01.000 UTC) to MET = 754551251.0 s (2024-11-29 05:34:06.000 UTC) revealed a flux above 10 GeV of 1.33+/-0.27 X 10^-8 photons cm^-2 s^-1 , a 260-fold increase over the 3FHL catalog value of 5.20+/-1.29 X 10^-11 photons cm^-2 s^-1. It is worth noting that this source was recently reported to be in an excited state on November 18, 2024, in ATel #16913.

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.