The two states of Sgr A* in the near-infrared: bright episodic flares on top of low-level continuous variability

TL;DR

This study analyzes near-infrared variability of Sgr A* over five years, revealing continuous emission with a lognormal flux distribution and identifying bright flares, including the brightest ever observed.

Contribution

It provides a detailed characterization of Sgr A*'s NIR variability, including flux distribution and the identification of bright flares, expanding understanding of its emission behavior.

Findings

Sgr A* exhibits continuous near-infrared emission with variability on weekly timescales.

Flux distribution follows a lognormal pattern at low fluxes, with deviations at higher fluxes.

The brightest flare observed reached 27.5 mJy, the highest recorded for Sgr A*.

Abstract

In this paper we examine properties of the variable source Sgr A* in the near-infrared (NIR) using a very extensive Ks-band data set from NACO/VLT observations taken 2004 to 2009. We investigate the variability of Sgr A* with two different photometric methods and analyze its flux distribution. We find Sgr A* is continuously emitting and continuously variable in the near-infrared, with some variability occurring on timescales as long as weeks. The flux distribution can be described by a lognormal distribution at low intrinsic fluxes (<~5 mJy, dereddened with A_{Ks}=2.5). The lognormal distribution has a median flux of approximately 1.1 mJy, but above 5 mJy the flux distribution is significantly flatter (high flux events are more common) than expected for the extrapolation of the lognormal distribution to high fluxes. We make a general identification of the low level emission above 5 mJy as flaring emission and of the low level emission as the quiescent state. We also report here the brightest Ks-band flare ever observed (from August 5th, 2008) which reached an intrinsic Ks-band flux of 27.5 mJy (m_{Ks}=13.5). This flare was a factor 27 increase over the median flux of Sgr A*, close to double the brightness of the star S2, and 40% brighter than the next brightest flare ever observed from Sgr~A*.