Echoes of multiple outbursts of Sagittarius A^⋆revealed byChandra

Abstract

Context. The relatively rapid spatial and temporal variability of the X-ray radiation from some molecular clouds near the Galactic center shows that this emission component is due to the reflection of X-rays generated by a source that was luminous in the past, most likely the central supermassive black hole, Sagittarius A . Aims. Studying the evolution of the molecular cloud reflection features is therefore a key element to reconstruct Sgr A 's past activity. The aim of the present work is to study this emission on small angular scales in order to characterize the source outburst on short time scales. Methods. We use Chandra high-resolution data collected from 1999 to 2011 to study the most rapid variations detected so far, those of clouds between 5 and 20 from Sgr A towards positive longitudes. Our systematic spectral-imaging analysis of the reflection emission, notably of the Fe Kα line at 6.4 keV and its associated 4-8 keV continuum, allows us to characterize the variations down to 15 angular scale and 1-year time scale. Results. We reveal for the first time abrupt variations of few years only and in particular a short peaked emission, with a factor of 10 increase followed by a comparable decrease, that propagates along the dense filaments of the "Bridge" cloud. This 2-year peaked feature contrasts with the slower 10-year linear variations we reveal in all the other molecular structures of the region. Based on column density constraints, we argue that these two different behaviors are unlikely to be due to the same illuminating event.Conclusions. The variations are likely due to a highly variable active phase of Sgr A sometime within the past few hundred years, characterized by at least two luminous outbursts of a few-year time scale and during which the Sgr A luminosity went up to at least 10 39 erg s −1 .

On the origin of the 6.4keV line in the Galactic Center region

“…• In our opinion the photon model of Ponti et al (2010); Ryu et al (2013); Clavel et al (2013) reproduces naturally these spatial and temporal characteristics of the 6.4 keV emission in the GC. The photon model has two important advantages: the front of primary photons occupies a very extended region 100 yr after the explosion, and its spatial distribution may have very sharp leading and back fronts that reproduces easily the 10 yr temporal variations of the 6.4 keV emission from GC molecular clouds.…”

“…However, if interpretation of the 6.4 keV time variability of e.g Ponti et al (2010); Ryu et al (2013); Clavel et al (2013) is correct, then the electron model is unable to reproduce the simultaneous short time variability of the iron line emission from clouds which are distant from each other by hundred pc. Alternatively we can speculate that a random distribution of local electron sources could also provide the necessary effect of the line variations in different clouds that are seen by chance.…”

“…In this respect the electron model can be considered as quite reasonable for local production of a flux of the 6.4 keV line from some of molecular clouds (see Yusef-Zadeh et al, 2013b) and even for interpretation of time variability of this flux as assumed by Yusef-Zadeh et al (2013a). The question is whether this model is able to explain the observed simultaneous variability of the 6.4 keV emission with the characteristic time about ≥ 10 yr for the clouds which are at distances about 100 pc from each other as follows from the analyses of Ponti et al (2010); Ryu et al (2013) and Clavel et al (2013). It seems incredible that this simultaneous variability is due to independent local sources of electrons nearby the clouds.…”

“…Inui et al, 2009;Terrier et al, 2010). Analysis of the Chandra data provided by Clavel et al (2013) showed that this activity of SgrA* can be presented as short X-ray flares whose duration is about 10 yr. In principle, past flares of Sgr A* can be found on time scales of ten thousand years (see Cramphorn & Sunyaev, 2002) For the last ten years the 6.4 keV intensity of Sgr B2 has dropped down in about three times in comparison with its maximum value in 2000 (see Nobukawa et al, 2011).…”

The Herschel view of the Galactic center

“…The CMZ exhibits diffuse and variable X-ray emission in the 6.4 keV fluorescent K-α line produced by neutral iron atoms (Clavel et al 2013). The accepted interpretation is that Sgr A* has experienced major outbursts within the last few hundred years.…”