HomeAstronomy & SpaceAstronomyMid-infrared flare observed in the galaxy NGC 3786

Mid-infrared flare observed in the galaxy NGC 3786

Astronomers from South Korea and China used NASA’s Wide-field Infrared Survey Explorer (WISE) to detect a mid-infrared flare in NGC 3786, an active spiral galaxy. The researchers believe this unusual flare caused a tidal disruption event (TDE). The discovery was detailed in a paper published on the arXiv pre-print server on August 17.

NGC 3786 (also known as PGC 36158 and UGC 6621) is a type 1.8 Seyfert spiral galaxy in the constellation Ursa Major with a redshift of about 0.009. It has an apparent size of 2.1 by 1.1 arcminutes and forms an interacting pair of galaxies known as Arp 294.

A group of astronomers led by Suyeon Son of Daegu’s Kyungpook National University reports a strange infrared-only flaring event in NGC 3786. The discovery is based on WISE’s multi-epoch photometric data.

“By comparing this dataset to light curves in the optical band, we stumbled upon an infrared-only flare in NGC 3786, which appeared to have occurred around mid-2020,” the researchers wrote in their paper.

This bright mid-infrared flare had an amplitude of about 0.5 and 0.8 mag in the WISE W1 and W2 bands, respectively. These values are 68 times higher than they were before the flare. However, no significant increase in brightness was observed in optical monitoring data obtained with the Zwicky Transient Facility from 2019 to 2021. (ZTF).

Son’s team obtained optical and near-infrared (NIR) spectroscopic data using Gemini Observatory’s GMOS and GNIRS spectrographs to shed more light on the nature of this flare. They discovered that broad emission lines in Paschen-alpha (Pa) and Paschen-beta (Pa) appear for the first time, while broad hydrogen (H) emission is only detected marginally in the post-flare spectrum.

Furthermore, the newly appearing broad emission lines were redshifted up to 900 km/s. It was discovered that such a high-velocity offset is frequently observed in changing-look (CL) active galactic nuclei (AGN), possibly as a result of the eccentric tidal debris produced by the so-called tidal disruption event (TDE) phenomenon.

According to research, TDEs can emit infrared radiation from circumnuclear dust heated by the X-ray/ultraviolet continuum from the accretion disc. TDEs are occasionally reported to be accompanied by CL behaviour in AGNs, where the appearance or disappearance of broad emission lines and thermal continuum from the accretion disc occurs, according to the researchers.

The findings suggest that in the case of NGC 3786, the observed mid-infrared flare is highly obscured by circumnuclear dust and is associated with the CL AGN phenomenon, according to the paper’s authors. They concluded that this flare was caused by a TDE because it is the most likely explanation for the velocity shifts in the broad emission lines.


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