First detection of flaring very-high-energy gamma-ray emission from PKS 1725+123 with the MAGIC and LST-1 telescopes
ATel #17344; David Paneque (Max Planck Institute for Physics), Masahiro Teshima (Max Planck Institute for Physics), Ryuji Takeishi (Institute for Cosmic Ray Research, University of Tokyo), Seiya Nozaki (Institute for Cosmic Ray Research, University of Tokyo), Mathilde Croissonnier (IFAE Barcelona), Yusuke Suda (Hiroshima University), Axel Arbet-Engels (Max Planck Institute for Physics) and Jorge Otero Santos (INFN Padova) on behalf of the MAGIC and LST CTAO collaborations
on 19 Aug 2025; 21:31 UT
Credential Certification: Axel Arbet-Engels (aarbet@mpp.mpg.de)
Subjects: Gamma Ray, TeV, VHE, AGN, Blazar
The MAGIC and CTAO LST Collaborations report the first detection of very-high-energy (VHE; E>100 GeV) gamma-ray emission from PKS 1725+123 (R.A. 262.02938 deg, Dec. 12.26097 deg, J2000.0). The preliminary analysis of the MAGIC data taken on the night of 2025/08/18 to 2025/08/19 resulted in the detection of PKS 1725+123 with a statistical significance of more than 5 standard deviations after about 2 hours of observations. The LST-1 telescope observed simultaneously, and a preliminary analysis of the LST-1 data further reveals a detection at the level of 5 standard deviations.
PKS 1725+123 is a flat-spectrum radio quasar located at redshift z=0.586. This source has been in an active state in 2025 at different energy bands, as reported in ATel #17127 and ATel #17316. Our monitoring pipeline revealed that Fermi-LAT detected several high-energy (E>10 GeV) photons, including >100 GeV photons, on August 17 and the gamma-ray spectral index was harder than 2. The MAGIC and LST-1 observations were triggered by this flaring activity in gamma-rays.
MAGIC and LST-1 observations on PKS 1725+123 will continue during the next days. Follow-up observations (gamma-rays, X-rays, optical, NIR, radio) are strongly encouraged. The contact persons for these observations are David Paneque (dpaneque@mppmu.mpg.de), Masahiro Teshima (mteshima@mpp.mpg.de) and Seiya Nozaki (nozaki@icrr.u-tokyo.ac.jp)
The preliminary analysis has been performed by Ryuji Takeishi (take@icrr.u-tokyo.ac.jp), Mathilde Croissonnier (croisonnier.mathilde@gmail.com) and Yusuke Suda (ysuda@hiroshima-u.ac.jp).
MAGIC is a system of two 17m-diameter Imaging Atmospheric Cherenkov Telescopes located at the Canary island of La Palma, Spain, and designed to perform gamma-ray astronomy in the energy range from 50 GeV to greater than 50 TeV.
LST-1 is a prototype of the Large-Sized Telescope for the Cherenkov Telescope Array Observatory, and is located on the Canary island of La Palma, Spain. The telescope design is optimized for observation of gamma rays in the range from 20 GeV to 3 TeV.
