Abstract
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, multiyear data set 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 5 yr, 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.
Highlights
Sagittarius A* (Sgr A∗) is the electromagnetic counterpart of the supermassive black hole at the centre of the Milky Way galaxy
We study the Cumulative Distribution Function (CDF) of count rates of Sgr A∗ using data accumulated between 2006 and 2019 with the Neil Gehrels Swift observatory (Swift; Gehrels et al 2004)
We found that the quiescent component of Sgr A∗ emission is best represented by a pure Poisson distribution with mean count rate of Q = 22.3+−11..64 cts/ks, and a power law process with ζ = 0.57+−00..1182; for both parameters, throughout this paper, we will report the mode, with uncertainties corresponding to the 1σ level
Summary
Sagittarius A* (Sgr A∗) is the electromagnetic counterpart of the supermassive black hole at the centre of the Milky Way galaxy It has an estimated mass of ∼ 4 × 106 M , but its bolometric lue-mail: jakob.vandeneijnden@physics.ox.ac.uk minosity is ∼ 9 orders of magnitude fainter than the Eddington luminosity for an object of this mass (Genzel et al 2010; Morris et al 2012). The X-ray emission from Sgr A∗ is observed to be composed of a quiescent component, corresponding to a luminosity of LX 3 × 1033erg s−1 in the 2 − 10 keV energy range, which is interrupted ∼daily by flares (e.g. Baganoff et al 2001; Goldwurm et al 2003; Genzel et al 2010; Markoff 2010; Neilsen et al 2013; Degenaar et al 2013) These flares are ∼ 1 − 2 orders of magnitude more luminous than its quiescent emission, with the brightest ones reaching values of LX (1 − 5) × 1035erg s−1 (e.g. Nowak et al 2012; Haggard et al 2019). The emission mechanism and the physical process producing Sgr A∗’s flares are not completely understood yet (Markoff et al 2001; Liu & Melia 2002; Yuan et al 2003; Liu et al 2004; Cadezet al. 2008), Ponti et al (2017) developed the first simultaneous multiwaveband campaign measuring the spectral index in nIR and X-ray bands, showing that synchrotron emission with a cooling break is a viable process for Sgr A∗’s flaring emission
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