EI2GYB > ASTRO 27.11.25 13:00l 54 Lines 5116 Bytes #200 (0) @ WW
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Subj: Astronomers capture an exceptional gamma-ray flare from a b
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Astronomers capture an exceptional gamma-ray flare from a blazar
Astronomers have performed very long baseline interferometry (VLBI) observations of a gamma-ray loud blazar known as TXS 2013+370. The observations, posted November 19 on the arXiv preprint server, resulted in the detection of an exceptional gamma-ray flare from this object.
Jets aimed at our planet
In general, blazars are very compact quasi-stellar objects (quasars) associated with supermassive black holes (SMBHs) at the centers of active, giant elliptical galaxies. They belong to a larger group of active galaxies that host active galactic nuclei (AGN), and are the most common sources of gamma-rays outside our galaxy. The characteristic features of blazars are relativistic jets pointed almost exactly toward Earth.
Blazars can be divided into two classes, based on their optical emission properties: flat-spectrum radio quasars (FSRQs) that feature prominent and broad optical emission lines, and BL Lacertae objects (BL Lacs), which do not.
Catching a gamma-ray flare
TXS 2013+370 is a powerful gamma-ray loud blazar at a redshift of approximately 0.86, located close to the Galactic plane. It hosts a supermassive black hole with an estimated mass of about 400 million solar masses.
Since December 6, 2020, TXS 2013+370 has started to experience an elevated gamma-ray emission, which evolved into a flaring activity. A team of astronomers led by Giorgos Michailidis of the Aristotle University of Thessaloniki in Greece, decided to initiate VLBI observations of this blazar with the Very Long Baseline Array (VLBA), which allowed them to catch the gamma-ray flare in unprecedented detail.
"In this work, we conducted polarimetric VLBI observations of TXS 2013+370 at 22, 43, and 86 GHz during an exceptional GeV outburst on February 11, 2021, achieving angular resolutions down to ?0.1 mas. This represents the first multi-frequency polarimetric VLBI study of this source," the researchers write.
Uncovering the jet structure
First of all, the flare observations revealed that TXS 2013+370 is a compact, core-dominated source with a curved jet structure extending southwestward from the bright core region. The images show that the blazar consists of a dominant core and several distinct jet components, with the overall jet structure becoming increasingly well-resolved at higher frequencies.
The observations found that the curved jet of TXS 2013+370 contains a newly emerged component, which received the designation N2. This component is located some 60 microarcseconds from the blazar's core and is associated with enhanced multi-wavelength activity.
Location of the gamma-ray flare
It turned out that the gamma-ray emission site of TXS 2013+370 lies beyond or at the broad-line region (BLR) edge. This makes the blazar's dusty torus the primary photon reservoir, with infrared photons intercepted by the jet and up-scattered to gamma-rays also via external Compton (EC) emission. A strong correlation and time lag was found between the gamma-ray and 15 GHz variability, indicating that the high-energy activity leads the radio by about 102 days.
By comparing the 2021 gamma-ray flare to the previous one, which occurred in 2009, the authors of the paper concluded that both locate the gamma-ray emission to the same subparsec/parsec region. This suggests that lag variations reflect changing opacity conditions rather than a moving dissipation site.
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