First Mid-infrared Detection and Modeling of a Flare from Sgr A*. II. Mid-IR Spectral Energy Distribution and Millimeter Polarimetry

TL;DR

This study combines new Galactic Center extinction measurements with mid-infrared and millimeter observations of Sgr A*'s flare, revealing spectral index evolution and polarization features that suggest a single hotspot origin for multi-wavelength variability.

Contribution

It provides the first detailed mid-infrared spectral energy distribution and millimeter polarimetry analysis of a Sgr A* flare, incorporating new extinction data and proposing a quantitative flare onset definition.

Findings

Spectral index peaks at 0.45 during the flare

Instantaneous change in spectral index at flare start

Millimeter polarization shows a clockwise Stokes Q-U loop

Abstract

S. D. von Fellenberg et al. (2025a, Paper I) reported the first mid-infrared detection of a flare from Sgr A*. The JWST/MIRI/MRS observations were consistent with an orbiting hotspot undergoing electron injection with a spectrum that subsequently breaks from synchrotron cooling. However, mid-infrared extinction measurements appropriate for these data were not yet determined, and therefore the temporal evolution of the absolute spectral index remained unknown. This work applies new Galactic Center extinction measurements to the flare observations. The evolution of the spectral index after the peak is fully consistent with that reported in Paper I with a maximum absolute mid-infrared spectral index $\alpha_{\rm{MIR}}=0.45\pm0.01_{\rm{stat}}\pm0.08_{\rm{sys}}$ during the second mid-infrared flare peak, matching the known near-infrared spectral index during bright states ($\alpha_{\rm{NIR}}\approx0.5$). There was a near-instantaneous change in the mid-infrared spectral index of $\Delta\alpha_{\rm{MIR}}=0.33\pm0.06_{\rm{stat}}\pm0.11_{\rm{sys}}$ at the flare onset. We propose this as a quantitative definition for this infrared flare's beginning, physically interpreted as the underlying electron distribution's transition into a hard power-law distribution. This paper also reports the SMA millimeter polarization during the flare, which shows a small, distorted, but overall clockwise-oriented Stokes Q--U loop during the third mid-infrared peak. Extrapolating the mid-infrared flux power law to the millimeter yields a variable flux consistent with the observed 220 GHz emission. These results, together with the Paper I modeling, plausibly suggest a single hotspot produced both the mid-infrared and millimeter variability during this event. However, additional flares are required to make a general statement about the millimeter and mid-infrared connection.