Fermi-LAT detection of enhanced gamma-ray activity from 4C +33.06 and VHE emission from PKS 1440-389

ATel #17769; F. Casaburo (INAF - OAR, INFN - Rome Tor Vergata, Sapienza University Rome), G. La Mura (INAF-OAC), and S. Ciprini (INFN - Rome Tor Vergata, ASI- SSDC), on behalf of the Fermi Large Area Telescope Collaboration
on 27 Apr 2026; 22:02 UT
Credential Certification: Giovanni La Mura (giovanni.lamura@inaf.it)

Tweet

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 4C +33.06, also known as 4FGL J0304.5+3349 (The Fermi-LAT collaboration 2020, ApJS, 247, 1), with coordinates R.A. = 46.17234 deg, Dec. = 33.81209 deg (J2000; Xu et al., 2019, ApJS, 242, 5), and redshift z=0.68 (Alvarez Crespo et al., 2025, A&A, 694, 46).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on April 26, 2026, with a daily averaged gamma-ray flux (E>100MeV) of (0.6+/-0.1) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of more than 70 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 2.0+/-0.2, indicating a significantly harder spectrum than the 4FGL-DR4 value of 2.58 +/- 0.08. This is the first time that the Fermi-LAT Collaboration reports enhanced gamma-ray activity from 4C +33.06.

The Fermi-LAT has also observed renewed emission from a source positionally consistent with the BL Lac PKS 1440-389, also known as 4FGL J1443.9-3908 (The Fermi-LAT collaboration 2020, ApJS, 247, 1), with coordinates R.A. = 220.98835 deg, Dec. = -39.14445 deg (J2000; Cutri et al., 2013, AllWISE), and redshift z= 0.138 (Goldoni et al., 2021, A&A, 650, 106).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state, with a daily averaged gamma-ray flux (E>100MeV) of (0.3+/-0.1) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of more than 10 relative to the average flux reported in 4FGL-DR4. The corresponding photon index is 1.7+/-0.2, and it is consistent with the 4FGL-DR4 value of 1.83+/-0.02 within the uncertainties. The flux enhancement was accompanied by the emission of a VHE photon of energy E = 110 GeV with association probability of p > 0.999. The Fermi-LAT Collaboration has previously reported flaring activity from this source in ATels #17378 and #15635.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of these sources will continue. A preliminary light curve for 4C +33.06 can be accessed via the Fermi-LAT Light Curve Repository at 4FGL J0304.5+3349. A preliminary light curve for PKS 1440-389 can also be accessed via the Fermi-LAT Light Curve Repository at 4FGL J1443.9-3908. We encourage multifrequency observations of these sources. For 4C +33.06, the Fermi-LAT contact person is Fausto Casaburo (fausto[dot]casaburo[at]inaf[dot]it). For PKS 1440-389, the Fermi-LAT contact people are S. Ciprini (stefano.ciprini[at]ssdc.asi.it) and C.C. Cheung (teddy.cheung[at]nrl.navy.mil).

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.