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Blazar TXS 2013-370 gamma ray flare

Astronomers Detect Extraordinary Gamma-Ray Flare From Blazar TXS 2013+370

Images showing the total intensity of TXS 2013+370 on 11 February 2021. Displayed left is the 22 GHz view, with the 43 GHz and 86 GHz observations positioned at the top right and bottom right respectively. Credit: arXiv (2025). DOI: 10.48550/arxiv.2511.15601.

Jets Aimed at Earth: What Makes Blazars Unique

Structure and Classification of Blazars

Astronomers have used very long baseline interferometry (VLBI) to study the gamma-ray loud blazar known as TXS 2013+370. The findings, shared on the arXiv preprint server on 19 November, revealed the presence of an extraordinary gamma-ray flare from the source.

Blazars are compact, quasar-like celestial objects powered by supermassive black holes located at the centers of giant active elliptical galaxies. They form part of a broader category of active galaxies that host active galactic nuclei (AGN) and are the most frequent sources of gamma-ray emissions beyond the Milky Way. A defining characteristic of blazars is the presence of highly energetic, relativistic jets that are aligned almost directly towards Earth.

Blazars are generally grouped into two types, based on their optical signatures. Flat-spectrum radio quasars (FSRQs) display strong and broad optical emission lines, whereas BL Lacertae objects (BL Lacs) lack such prominent spectral features.

Catching the Gamma-Ray Flare

Observing the 2020-2021 Outburst

TXS 2013-370 is a highly energetic, gamma-ray-bright blazar situated near the Galactic plane, with a redshift of roughly 0-86. At its core lies a supermassive black hole weighing an estimated 400 million times the mass of the Sun.

On 6 December 2020, the blazar began exhibiting heightened gamma-ray activity, eventually erupting into a full-scale flare. In response, a team led by Giorgos Michailidis of the Aristotle University of Thessaloniki launched VLBI observations using the Very Long Baseline Array, capturing the event with remarkable clarity.

"We performed polarimetric VLBI observations of TXS 2013-370 at 22, 43 and 86 GHz during an extraordinary GeV outburst on 11 February 2021, achieving angular resolutions of around ∼0.1 mas. This marks the first multi-frequency polarimetric VLBI investigation of this source," the team reported.

Uncovering the Jet Structure

Discovery of the Newly Formed Jet Feature N2

Initial observations of the flare showed that TXS 2013-370 is a compact, core-dominated blazar with a gently curving jet stretching to the south-west of its bright central region. The images reveal a prominent core accompanied by several separate jet features, with the structure becoming clearer and more detailed at higher observing frequencies.

The team also detected a newly formed feature within the curved jet, now labelled N2. Positioned around 60 microarcseconds from the core, this component appears to be linked to increased activity across multiple wavelengths.

Locating the Gamma-Ray Production Site

Connection Between Gamma-Ray and Radio Variations

The study determined that the gamma-ray production site in TXS 2013-370 lies either at or beyond the edge of the BLR. Consequently, the dusty torus acts as the primary photon source, supplying infrared radiation that interacts with the jet and is up-scattered to gamma-ray levels through external Compton emission. A clear association was observed between gamma-ray variations and 15 GHz radio changes, with the gamma-ray activity leading by about 102 days.

A comparison between the 2021 flare and an earlier episode in 2009 indicates that both flares originate from the same compact subparsec/parsec region. This implies that shifts in lag times reflect evolving opacity conditions, not a change in where the energy is released.

What the Findings Mean for Blazar Science

Insights Into Jet Evolution and Emission Mechanisms

  • Strengthens evidence that blazar gamma-ray flares originate outside the broad-line region.
  • Confirms repeated flaring from the same compact regions, suggesting stable jet-launching conditions.
  • Reveals a newly formed jet feature (N2), contributing to long-term jet evolution studies.
  • Provides rare multi-frequency polarimetric data across 22-86 GHz.

Study Contributors

  • Giorgos Michailidis (Aristotle University of Thessaloniki)
  • VLBA observatory team
  • Additional collaborators from gamma-ray and radio astronomy institutes

Source

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