SimultaneousChandra, CSO, and VLA Observations of Sgr A*: The Nature of Flaring Activity

Abstract

Sgr AÃ , the massive black hole at the center of the Galaxy, varies in radio through X-ray emission on hourly timescales. The flare activity is thought to arise from the innermost region of an accretion flow onto Sgr A Ã . We present simultaneous light curves of Sgr A Ã in radio, submillimeter and X-rays that show a possible time delay of 110 AE 17 minutes between X-ray and 850 m suggesting that the submillimeter flare emission is optically thick. At radio wavelengths, we detect time lags of 20:4 AE 6:8, 30 AE 12, and 20 AE 6 minutes between the flare peaks observed at 13 and 7 mm (22 and 43 GHz) in three different epochs using the VLA. Linear polarization of 1% AE 0:2% and 0:7 AE 0:1% is detected at 7 and 13 mm, respectively, when averaged over the entire observation on 2006 July 17. A simple model of a bubble of synchrotron-emitting electrons cooling via adiabatic expansion can explain the time delay between various wavelengths, the asymmetric shape of the light curves, and the observed polarization of the flare emission at 43 and 22 GHz. The derived physical quantities that characterize the emission give an expansion speed of v exp $ 0:003Y0:1 c, magnetic field of B $ 10Y70 G, and particle spectral index p $ 1Y2. These parameters suggest that the associated plasma cannot escape from Sgr A Ã unless it has a large bulk motion.

Millimeter to X-ray flares from Sagittarius A*

“…Hence, these models are consistent with the observed millimeter variability of SgrA*. However, since Dexter et al (2009Dexter et al ( , 2010) concentrate only on modeling the (sub-)millimeter emission, their modeling could not reproduce the observed NIR/X-ray flares and corresponding (sub-)millimeter flares (Eckart et al 2008;Marrone et al 2008;Dodds-Eden et al 2009;Yusef-Zadeh et al 2008, which are most likely physically associated. The authors also clearly state that -in the framework of their modeling -it is unclear how their description of variability is linked to the NIR/X-ray flare activity.…”

“…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.…”

“…In general, the flare profiles are more complex. They are not necessarily symmetric with respect to the peak (see the first X-ray flare detection by Baganoff et al 2001) and significant differences between the NIR and X-ray flare profiles have been reported (e.g., Eckart et al 2006a;Marrone et al 2008;Yusef-Zadeh et al 2008;Sabha et al 2010;Dodds-Eden et al 2009.…”

“…3.2). Until now, several events could successfully be described in the framework of adiabatic expansion (e.g., Eckart et al 2006aEckart et al , 2008Yusef-Zadeh et al 2008Trap et al 2011). This indicates that the relevance of this process to SgrA* has as high probability of being confirmed in the very near future, e.g., through quasi-simultaneous multi-frequency measurements of SgrA* flares, for instance, with advanced multi-channel observing systems like ALMA.…”

“…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).…”