Increased high energy γ-ray activity of Mrk 421

[ SS1-1M Collaboration, 14. 1. 2026 ]

The SST-1M Collaboration issued its second Astronomer’s Telegram (ATel #17597) on 14 January, reporting the detection of enhanced very-high-energy γ-ray activity from the nearby blazar Markarian 421 on 12 January 2026.

Markarian 421 (Mrk 421 or Mkn 421) is one of the closest and brightest high-frequency peaked blazars at a redshift of z = 0.031, observed at very-high-energy (VHE; E ≳ 100 GeV) γ rays.

Blazars are a class of Active Galactic Nuclei (AGN) with relativistic jets ejected from the central supermassive black hole (SMBH) that are closely aligned with the observer’s line of sight, making them a prime laboratory for studying particle acceleration and emission processes in
active galactic nuclei.

Mrk 421 is a highly variable source across the entire electromagnetic spectrum,  exhibiting flux changes on timescales ranging from minutes to
months. In the VHE band, it often undergoes intense flaring episodes, during which its γ-ray flux can increase by more than an order of magnitude, often accompanied by spectral hardening and complex temporal evolution.

The observations were performed in stereoscopic imaging mode between 01:27 and 04:37 UTC using the two SST-1M Cherenkov telescopes currently in operation at the Ondřejov Observatory, Czech Republic. This detection is part of the ongoing monitoring program of Mrk 421 and
is consistent with the flare reported independently by the VERITAS Collaboration (ATel #17594).

Following the initial detection, Mrk 421 remained in an active state for an extended period. The stereoscopic configuration of the SST-1M system
allowed the temporal evolution of the flare, including its rise and decay phases, to be clearly resolved.

[ View full content at ATel site ]