Fermi-LAT detection of renewed gamma-ray activity from the FSRQs 4C +27.50 and PKS 1440-389

ATel #17378; P. van Zyl (South African Radio Astronomy Observatory) and F. Longo (University of Trieste and INFN Trieste) on behalf of the Fermi Large Area Telescope Collaboration
on 8 Sep 2025; 18:32 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 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 September 7, 2025, with a daily averaged gamma-ray flux (E>100MeV) of (1.2+/-0.2) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase by a factor of about 60 times, 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 2.3+/-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 from this source in ATels #17055, #17035, #16718, #15746 and #15549.

The Fermi LAT has also observed renewed gamma-ray activity from a source positionally consistent with the flat-spectrum radio quasar PKS 1440-389, also known as 4FGL J1443.9-3908 (Abdollahi, et al. 2020, ApJS, 247, 33), with radio coordinates R.A. = 220.98833 deg, Decl. = -39.14436 deg (J2000; Healey et al. 2007, ApJS, 171, 61), and redshift z=0.1385 (Goldoni et al. 2021, A&A 650, A106).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on September 7, 2025, with a daily averaged gamma-ray flux (E>100MeV) of (0.5+/-0.2) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase by a factor of about 20 times, relative to the average flux reported in 4FGL-DR4. The corresponding photon index is 2.2+/-0.3, indicating a similar spectrum to the 4FGL-DR4 value of 2.24+/-0.04. The Fermi LAT Collaboration has previously reported flaring activity from this source in the ATel #15635.

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 1440-389. 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 PKS 1440-389, the Fermi-LAT contact persons are S. Ciprini (stefano.ciprini[at]ssdc.asi.it) and C.C. Cheung (teddy.cheung[at]nrl.navy.mil), 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.