The flare activity of Sagittarius A*

Abstract

Context. We report new simultaneous near-infrared/sub-millimeter/X-ray observations of the Sgr A* counterpart associated with the massive 3−4 × 10 6 M black hole at the Galactic Center. Aims. We investigate the physical processes responsible for the variable emission from Sgr A*. Methods. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope and the ACIS-I instrument aboard the Chandra X-ray Observatory as well as the Submillimeter Array SMA on Mauna Kea, Hawaii, and the Very Large Array in New Mexico. Results. We detected one moderately bright flare event in the X-ray domain and 5 events at infrared wavelengths. The X-ray flare had an excess 2−8 keV luminosity of about 33 × 10 33 erg/s. The duration of this flare was completely covered in the infrared and it was detected as a simultaneous NIR event with a time lag of ≤10 min. Simultaneous infrared/X-ray observations are available for 4 flares. All simultaneously covered flares, combined with the flare covered in 2003, indicate that the time-lag between the NIR and X-ray flare emission is very small and in agreement with a synchronous evolution. There are no simultaneous flare detections between the NIR/X-ray data and the VLA and SMA data. The excess flux densities detected in the radio and sub-millimeter domain may be linked with the flare activity observed at shorter wavelengths. Conclusions. We find that the flaring state can be explained with a synchrotron self-Compton (SSC) model involving up-scattered submillimeter photons from a compact source component. This model allows for NIR flux density contributions from both the synchrotron and SSC mechanisms. Indications for an exponential cutoff of the NIR/MIR synchrotron spectrum allow for a straightforward explanation of the variable and red spectral indices of NIR flares.

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).…”

“…The sky background was extracted from the median of stacks of dithered exposures of a dark cloud -a region practically empty of stars -about 400 north and 713 west of the target. Standard data reduction was applied (see, e.g., Eckart et al 2006a). The images were stacked and mosaicked, the point spread functions (PSF) were extracted with StarFinder (Diolaiti et al 2000) for deconvolution with the Lucy-Richardson (LR;Lucy 1974) algorithm.…”

“…The model is represented by an expanding uniform blob of relativistic electrons with an energy-dependent particle number density ρ(E) ∝ E −p threaded by a magnetic field. In the context of SgrA*, this model was explained in detail by, e.g., Eckart et al (2006aEckart et al ( , 2008 and Yusef-Zadeh et al (2008). For completeness, we summarize the definition of essential quantities and features of the model.…”

“…Its spectral energy distribution (SED) shows an inverted spectrum from the radio to the (sub-)millimeter domain (Falcke et al 2000). It displays order-of-magnitude flares in the infrared and X-ray domain (Baganoff et al 2001;Genzel et al 2003;Eckart et al 2004Eckart et al , 2006aGhez et al 2004). Baganoff et al (2003) detected a very low quiescent X-ray luminosity.…”

The infrared emission of the dust clouds close to Sgr A*

“…-dust cloud on the line of sight, heated by a nearby/embedded star: this mechanism has been invoked by [3,5] and is of course a possible explanation;…”