Fermi-LAT detection of enhanced gamma-ray activity from the FSRQs S3 1444+17 and 4C +27.50

ATel #17055; P. Benke (GFZ, MPIfR), G. La Mura (INAF - Astronomical Observatory of Cagliari), on behalf of the Fermi Large Area Telescope Collaboration
on 27 Feb 2025; 21:32 UT
Credential Certification: Petra Benke (pbenke@mpifr-bonn.mpg.de)

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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 flat-spectrum radio quasar S3 1444+17, also known as 4FGL J1446.7+1719 (4FGL; The Fermi-LAT collaboration 2020, ApJS, 247, 33), with coordinates R.A. = 221.64728 deg, Dec. = +17.35211 deg (J2000; Xu et al. 2019, ApJS, 242, 5), and redshift z=1.026 (Healey et al. 2008, ApJS, 175, 97).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on February 26, 2025, with a daily averaged gamma-ray flux (E>100MeV) of (0.5+/-0.1) x 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of 60 relative to the average flux reported in the fourth Fermi-LAT catalog (4FGL-DR4, Ballet et al. 2024, arXiv:2307.12546). The corresponding photon index is 1.7+/-0.2, indicating a significantly harder spectrum than the 4FGL-DR4 value of 2.6+/-0.1. The Fermi LAT Collaboration has previously reported flaring activity with a similar gamma-ray flux from this source in ATel #16948. In addition, two high-energy photons of 14.7 GeV and 27.9 GeV are spatially consistent with this blazar at a confidence level p > 0.996.

The Fermi LAT has also observed enhanced gamma-ray activity from a source positionally consistent with the flat-spectrum radio quasar 4C +27.50, also known as 4FGL J2321.9+2734 (4FGL-DR4), with coordinates R.A. = 350.49943 deg, Dec. = 27.54623 deg (J2000; Johnston et al. 1995, AJ, 110, 880) and redshift z=1.255 (Sexton et al. 2022, ApJS, 260, 33).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on February 26, 2025, with a daily averaged gamma-ray flux (E>100MeV) of (2.5+/-0.3) x 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of 130 relative to the average flux reported in 4FGL-DR4. The corresponding photon index is 2.0+/-0.1, indicating a harder spectrum than the 4FGL-DR4 value of 2.24+/-0.04. On the same day, two high-energy photons of 11.1 GeV and 16.6 GeV were associated with the source at a confidence level p > 0.999. The Fermi LAT Collaboration has previously reported flaring activity from this source in the ATels #15549, #15746, #16718 and #17035.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. Preliminary Fermi-LAT light curves can be accessed via the Fermi-LAT Light-Curve Repository for PKS 1127-14 . 4C +27.50 belongs to the "LAT Monitored Sources" and consequently, a preliminary estimation of the daily gamma-ray flux observed by Fermi-LAT will be publicly available at the LAT daily monitored source list. We encourage multifrequency observations of these sources. For S3 1444+17, the Fermi-LAT contact person is Ömer Faruk Çoban (coban@ice.csic.es), and for 4C +27.50 is Giovanni La Mura (giovanni.lamura@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.