Einstein Probe X-ray detection of the FSRQ 4C +10.45 following renewed gamma-ray activity

ATel #17935; J. W. Hu, C. C. Jin (NAO, CAS), H. Feng (IHEP, CAS), R. D. Liang, W. Chen, H. W. Pan (NAO, CAS), Y. J. Zhang (PKU) on behalf of the Einstein Probe Collaboration, Adithiya Dinesh (Universidad Complutense de Madrid), Janeth Valverde (Marquette University), Chiara Bartolini (INFN Bari), G. La Mura (INAF - Astronomical Observatory of Cagliari), on behalf of the Fermi Large Area Telescope Collaboration.
on 1 Aug 2026; 21:00 UT
Credential Certification: Giovanni La Mura (giovanni.lamura@inaf.it)

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On July 23, 2026, the Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, observed gamma-ray flaring activity from a source positionally consistent with the flat-spectrum radio quasar 4C +10.45, also known as 4FGL J1608.7+1029 (Abdollahi et al. 2022, ApJS, 260, 53), with coordinates R.A. = 242.19251 deg, Decl. = +10.48549 deg (J2000; Le Bail et al. 2016, AJ, 151, 79L), and redshift z = 1.233 (SDSS-DR13, Albareti et al. 2017, ApJS, 233, 25). The gamma-ray activity was reported in ATel #17924. Enhanced optical activity from the source was also reported in ATel #17916 on July 21, 2026 from Hans-Haffner-Sternwarte observatory.

Following these reports, a one-orbit target-of-opportunity observation of 4C +10.45 was performed with the Follow-up X-ray Telescope (FXT) on board Einstein Probe, starting at 2026-07-25 22:43:07 (UTC), with an exposure time of approximately 1.6 ks. An X-ray source positionally consistent with 4C +10.45 was significantly detected. The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power-law model with a fixed Galactic hydrogen column density of 5 X 10^20 cm^-2 and a photon index of 1.38 (+0.20/-0.19). The derived average unabsorbed 0.5-10 keV flux is 4.16 (+1.07/-0.82) X 10^-12 erg cm^-2 s^-1. The uncertainties quoted above are at the 90% confidence level.

The source has previously been detected with the Neil Gehrels Swift Observatory and is listed in the Second Swift-XRT Point Source Catalogue as 2SXPS J160846.0+102907. The catalogue reports an unabsorbed 0.3-10 keV flux of (2.34+/-0.05) X 10^-12 erg cm^-2 s^-1.

During July 25, 2026, which covered the Einstein Probe observation, 4C +10.45 was not significantly detected by the Fermi-LAT. Assuming a power-law spectrum with photon index 2.0, the corresponding 95% confidence upper limit on the photon flux (E>100MeV) is 0.8 X 10^-6 photons cm^-2 s^-1. However, preliminary analysis of the three-day interval from July 23 to July 25, 2026, indicates significant gamma-ray emission from 4C +10.45, with an average gamma-ray flux (E > 100 MeV) of (0.5 +/- 0.1) x 10^-6 photons cm^-2 s^-1 and TS around 70. This corresponds to a flux increase by a factor of approximately 20 relative to the 4FGL-DR4 average, while the corresponding photon index of 2.55 +/- 0.20 is consistent with the catalog value of 2.53 +/- 0.03.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. A preliminary light curve for 4C +10.45 can be accessed via the Fermi-LAT Light-Curve Repository and via the Monitored Source List. For this source, the Fermi-LAT contact person is Adithiya Dinesh (adinesh@ucm.es).

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. We further acknowledge the commitment of the Einstein Probe Team, Observatory Duty Scientists, and PI, who made these observations possible.

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.