A Swift study of long-term changes in the X-ray flaring properties of Sagittarius A*

TL;DR

This study analyzes over a decade of Swift X-ray data to investigate long-term variability in Sagittarius A*'s flaring activity, revealing non-stationary flare rates and periods of increased activity.

Contribution

It provides the first detailed long-term analysis of Sgr A*'s X-ray flaring rate variability using extensive Swift data, highlighting non-stationary behavior.

Findings

Flares show non-stationary flux distributions over years.

In some years, flare activity is consistent with a Poisson process.

Increased flaring activity observed between 2006-2007 and 2017-2019.

Abstract

The radiative counterpart of the supermassive black hole at the Galactic Centre, Sagittarius A*, displays flaring emission in the X-ray band atop a steady, quiescent level. Flares are also observed in the near-infrared band. The physical process producing the flares is not fully understood and it is unclear if the flaring rate varies, although some recent works suggest it has reached unprecedented variability in recent years. Using over a decade of regular X-ray monitoring of Neil Gehrels Swift Observatory, we studied the variations in count rate of Sgr A* on time scales of years. We decomposed the X-ray emission into quiescent and flaring emission, modelled as a constant and power law process, respectively. We found that the complete, multi-year dataset cannot be described by a stationary distribution of flare fluxes, while individual years follow this model better. In three of the ten studied years, the data is consistent with a purely Poissonian quiescent distribution, while for five years only an upper limit of the flare flux distribution parameter could be determined. We find that these possible changes cannot be explained fully by the different number of observations per year. Combined, these results are instead consistent with a changing flaring rate of Sgr A*, appearing more active between 2006-2007 and 2017-2019, than between 2008-2012. Finally, we discuss this result in the context of flare models and the passing of gaseous objects, and discuss the extra statistical steps taken, for instance to deal with the background in the Swift observations.