First mid-infrared detection and modeling of a flare from Sgr A*

TL;DR

This paper reports the first mid-infrared detection of a flare from Sgr A* using JWST, revealing synchrotron cooling and magnetic field insights through multi-wavelength observations and modeling.

Contribution

It presents the first MIR detection of Sgr A* flare and models the emission as synchrotron radiation from cooling high-energy electrons.

Findings

MIR flare lasted about 40 minutes, similar to NIR and X-ray flares.

Spectral index steepened as the flare ended, indicating synchrotron cooling.

Magnetic field strengths estimated at 40-70 Gauss in the emission zone.

Abstract

The time-variable emission from the accretion flow of Sgr A*, the supermassive black hole at the Galactic Center, has long been examined in the radio-to-mm, near-infrared (NIR), and X-ray regimes of the electromagnetic spectrum. However, until now, sensitivity and angular resolution have been insufficient in the crucial mid-infrared (MIR) regime. The MIRI instrument on JWST has changed that, and we report the first MIR detection of Sgr A*. The detection was during a flare that lasted about 40 minutes, a duration similar to NIR and X-ray flares, and the source's spectral index steepened as the flare ended. The steepening suggests synchrotron cooling is an important process for Sgr A*'s variability and implies magnetic field strengths $\sim$40--70 Gauss in the emission zone. Observations at $1.3~\mathrm{mm}$ with the Submillimeter Array revealed a counterpart flare lagging the MIR flare by $\approx$10 minutes. The observations can be self-consistently explained as synchrotron radiation from a single population of gradually cooling high-energy electrons accelerated through (a combination of) magnetic reconnection and/or magnetized turbulence.