Multi-wavelength Variability of Sagittarius A* in July 2019

TL;DR

This study presents a multi-wavelength timing analysis of Sagittarius A* during a three-day campaign, revealing correlated variability and potential causal links between NIR, X-ray, and submm emissions, with implications for accretion activity.

Contribution

It provides the first coordinated multi-wavelength timing analysis of Sgr A* with detailed lag measurements and modeling of flare mechanisms, highlighting possible accretion rate changes.

Findings

NIR and X-ray flares are temporally correlated with submm flux increases.

A potential lag of about 34 minutes from X-ray/NIR to submm emission was observed.

High electron densities consistent with increased accretion rates are suggested during the flare event.

Abstract

We report timing analysis of near-infrared (NIR), X-ray, and sub-millimeter (submm) data during a three-day coordinated campaign observing Sagittarius A*. Data were collected at 4.5 micron with the Spitzer Space Telescope, 2-8 keV with the Chandra X-ray Observatory, 3-70 keV with NuSTAR, 340 GHz with ALMA, and at 2.2 micron with the GRAVITY instrument on the Very Large Telescope Interferometer. Two dates show moderate variability with no significant lags between the submm and the infrared at 99% confidence. July 18 captured a moderately bright NIR flare (F_K ~ 15 mJy) simultaneous with an X-ray flare (F ~ 0.1 cts/s) that most likely preceded bright submm flux (F ~ 5.5 Jy) by about +34 (+14 -33) minutes at 99% confidence. The uncertainty in this lag is dominated by the fact that we did not observe the peak of the submm emission. A synchrotron source cooled through adiabatic expansion can describe a rise in the submm once the synchrotron-self-Compton NIR and X-ray peaks have faded. This model predicts high GHz and THz fluxes at the time of the NIR/X-ray peak and electron densities well above those implied from average accretion rates for Sgr A*. However, the higher electron density postulated in this scenario would be in agreement with the idea that 2019 was an extraordinary epoch with a heightened accretion rate. Since the NIR and X-ray peaks can also be fit by a non-thermal synchrotron source with lower electron densities, we cannot rule out an unrelated chance coincidence of this bright submm flare with the NIR/X-ray emission.