Polarized NIR and X-ray flares from Sagittarius A*

Abstract

Context. Stellar dynamics indicate the presence of a supermassive 3−4 × 10 6 M black hole at the Galactic Center. It is associated with the variable radio, near-infrared, and X-ray source Sagittarius A* (SgrA*). Aims. The goal is the investigation and understanding of the physical processes responsible for the variable emission from SgrA*. Methods. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope (July 2005, May 2007) and the ACIS-I instrument aboard the Chandra X-ray Observatory (July 2005). Results. We find that for the July 2005 flare the variable and polarized NIR emission of SgrA* occurred synchronous with a moderately bright flare event in the X-ray domain with an excess 2−8 keV luminosity of about 8 × 10 33 erg/s. We find no time lag between the flare events in the two wavelength bands with a lower limit of ≤10 min. The May 2007 flare shows the highest sub-flare to flare contrast observed until now. It provides evidence for a variation in the profile of consecutive sub-flares. Conclusions. We confirm that highly variable and NIR polarized flare emission is non-thermal and that there exists a class of synchronous NIR/X-ray flares. We find that the flaring state can be explained via the synchrotron self-Compton (SSC) process involving up-scattered X-rays from the compact source component. The observations can be interpreted in a model involving a temporary disk with a short jet. In the disk component the flux density variations can be explained by spots on relativistic orbits around the central supermassive black hole (SMBH). The profile variations for the May 2007 flare can be interpreted as a variation of the spot structure due to differential rotation within the disk.

Millimeter to X-ray flares from Sagittarius A*

“…In addition multi-wavelength observations of flares suggest an adiabatically expanding blob model (Eckart et al 2006a(Eckart et al , 2008Yusef-Zadeh et al 2006a, which can explain the observed time lags between the infrared/X-ray and millimeter flares. However, a general model that provides a quantitative explanation of the frequency-dependent variability of SgrA* in the millimeter and sub-millimeter wavelengths, including the contribution of thermal electrons and relativistic electrons that result in NIR/X-ray flares, is not yet available (see comments in Dexter et al 2010).…”

“…With a total bandwidth of about 60 GHz, the system is optimized for the 345 GHz atmospheric window. A detailed description of the LABOCA data recording and analysis in the context of SgrA* is given by e.g., Eckart et al (2008) and Garcia-Marin et al (2011a,b). For completeness, we summarize some of the essentials that are relevant to this paper.…”

“…A more detailed description of the data reduction and analysis of the images was given by e.g. Eckart et al (2008) and Garcia-Marin et al (2011a,b).…”

“…Source structure on event-horizon scales has been detected by Doeleman et al (2008) and Fish et al (2011) by means of very long baseline interferometry (VLBI) at 1.3 mm wavelength. In the radio and millimeter wavelength regime, SgrA* is found to be variable (Zhao et al 2003;Mauerhan et al 2005;Eckart et al 2008;Marrone et al 2008;Li et al 2009;Yusef-Zadeh et al 2008. Its spectral energy distribution (SED) shows an inverted spectrum from the radio to the (sub-)millimeter domain (Falcke et al 2000).…”

“…In addition to either spiral arm models (Karas et al 2007;Falanga et al 2007) or jet/jet-base models (Falcke & Markoff 2000Markoff 2005;Markoff et al 2007), recent attempts to explain the emission from the compact region around SgrA* have been based on a hot-spot/disk model (Eckart et al 2006b;Broderick & Loeb 2006), multi-spot models, or even evolving hot-spot models (Eckart et al 2008;Yuan et al 2009; see also Pecháček et al 2008;Zamaninasab et al 2011).…”