Fifteen years of XMM-Newton and Chandra monitoring of Sgr A★: evidence for a recent increase in the bright flaring rate

We present a study of the X-ray flaring activity of Sgr A^⋆ during all the 150 XMM-Newton and Chandra observations pointed at the Milky Way centre over the last 15 years. This includes the latest XMM-Newton and Chandra campaigns devoted to monitoring the closest approach of the very red Brγ emitting object called G2. The entire data set analysed extends from 1999 September through 2014 November. We employed a Bayesian block analysis to investigate any possible variations in the characteristics (frequency, energetics, peak intensity, duration) of the flaring events that Sgr A^⋆ has exhibited since their discovery in 2001. We observe that the total bright or very bright flare luminosity of Sgr A^⋆ increased between 2013 and 2014 by a factor of 2-3 (∼3.5σ significance). We also observe an increase (∼99.9 per cent significance) from 0.27 ± 0.04 to 2.5 ± 1.0 d^-1 of the bright or very bright flaring rate of Sgr A^⋆, starting in late summer 2014, which happens to be about six months after G2's pericentre passage. This might indicate that clustering is a general property of bright flares and that it is associated with a stationary noise process producing flares not uniformly distributed in time (similar to what is observed in other quiescent black holes). If so, the variation in flaring properties would be revealed only now because of the increased monitoring frequency. Alternatively, this may be the first sign of an excess accretion activity induced by the close passage of G2. More observations are necessary to distinguish between these two hypotheses.