Millimeter to X-ray flares from Sagittarius A*

Abstract

Context. We report on new simultaneous observations and modeling of the millimeter, near-infrared, and X-ray flare emission of the source Sagittarius A* (SgrA*) associated with the super-massive (4 × 10 6 M ) black hole at the Galactic center. Aims. We study the applicability of the adiabatic synchrotron source expansion model and study physical processes giving rise to the variable emission of SgrA* from the radio to the X-ray domain. Methods. Our observations were carried out on 18 May 2009 using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope, the ACIS-I instrument aboard the Chandra X-ray Observatory, the LABOCA bolometer at the Atacama Pathfinder EXperiment (APEX), and the CARMA mm telescope array at Cedar Flat, California. Results. The X-ray flare had an excess 2−8 keV luminosity between 6 and 12×10 33 erg s −1 . The observations reveal flaring activity in all wavelength bands that can be modeled as the signal from an adiabatically expanding synchrotron self-Compton (SSC) component.Modeling of the light curves shows that the sub-mm follows the NIR emission with a delay of about three-quarters of an hour with an expansion velocity of about v exp ∼ 0.009c. We find source component sizes of around one Schwarzschild radius, flux densities of a few Janskys, and spectral indices α of about +1 (S (ν) ∝ ν −α ). At the start of the flare, the spectra of the two main components peak just short of 1 THz. To statistically explain the observed variability of the (sub-)mm spectrum of SgrA*, we use a sample of simultaneous NIR/X-ray flare peaks and model the flares using a synchrotron and SSC mechanism. Conclusions. These parameters suggest that either the adiabatically expanding source components have a bulk motion larger than v exp or the expanding material contributes to a corona or disk, confined to the immediate surroundings of SgrA*. For the bulk of the synchrotron and SSC models, we find synchrotron turnover frequencies in the range of 300−400 GHz. For the pure synchrotron models, this results in densities of relativistic particles of the order of 10 6.5 cm −3 and for the SSC models, the median densities are about one order of magnitude higher. However, to obtain a realistic description of the frequency-dependent variability amplitude of SgrA*, models with higher turnover frequencies and even higher densities are required.

The Milky Way’s Supermassive Black Hole: How Good a Case Is It?

“…Baganoff et al (2001) report the first discovery of rapid X-ray flaring from SgrA* using the Chandra observatory in the (0.5-7 keV)-band implying that this part of the X-ray emission is due to accretion of gas onto a SMBH. Eckart et al (2012b) report on new simultaneous observations and modeling of the millimeter, near-infrared, and X-ray flare emission of the source SgrA*. The authors study how and if the concept of the adiabatic synchrotron source expansion can be applied to the variable emission of SgrA*.…”

“…Observations of X-ray and NIR flares from Sgr A* (Mossoux et al, 2015;Neilsen et al, 2015b;Barrière et al, 2014;Eckart et al, 2012c;Porquet et al, 2008;Eckart et al, 2006a;Ghez et al, 2004;Eckart et al, 2003;Genzel et al, 2003;Baganoff et al, 2003Baganoff et al, , 2001 suggest that the emission comes from structures smaller than about ten Schwarzschild radii of a ∼ 4 × 10 6 M ⊙ million solar mass black hole. This is more than two orders of magnitude more compact than the radius of a neutrino ball with a neutrino mass discussed in this context.…”

“…As for possibilities of definitely ruling out the boson star scenario, simultaneous multiwavelength measurements of the emission from Sgr A* (e.g. Eckart et al, 2012c) will allow us to constrain the emission mechanism and therefore the compactness of the emitting region around Sgr A* even further. Probably within the next decade it will be possible to image the "shadow" cast by the putative black hole through deflection of light rays using global radio interferometry at sub-millimeter wavelengths.…”

Wisps in the Galactic center: Near-infrared triggered observations of the radio source Sgr A* at 43 GHz

“…The intensity of Sgr A* shows spontaneous flux density outbursts at radio to X-ray frequencies, commonly referred to as flares (Mauerhan et al 2005;Marrone et al 2006;Yusef-Zadeh et al 2008;Eckart et al 2008aEckart et al ,b,c, 2009Eckart et al , 2012Lu et al 2011;Miyazaki et al 2012). These intensity irregularities appear on timescales ranging from 1-2 h (main flares) down to 7-10 min (sub-flares) (Eckart et al 2006a) with stronger activity at shorter wavelengths (Baganoff et al 2001;Genzel et al 2003;Ghez 2004;Eckart et al 2006b,c).…”

“…While the flux variability of Sgr A* has been studied by many authors (e.g., Zhao et al 2003;Mauerhan et al 2005;Marrone et al 2006Marrone et al , 2008Doeleman et al 2008;Eckart et al 2008aEckart et al ,b,c, 2009Eckart et al , 2012Yusef-Zadeh et al 2008Li et al 2009;Kunneriath et al 2010;Sabha et al 2010;Zamaninasab et al 2010;Fish et al 2011;Miyazaki et al 2012), size measurements during flares have not been performed as frequently (e.g., Rogers et al 1994;Krichbaum et al 1998;Lo et al 1998;Doeleman et al 2001Doeleman et al , 2008Shen et al 2005Shen et al , 2006Huang et al 2007;Lu et al 2011;Bower et al 2014). Recently, Bower et al (2014) performed triggered multi-frequency VLBI observations of Sgr A* at NIR, X-ray, and 43 GHz that revealed an elliptical intrinsic source size of 35.4 × 12.6 R s with an rms variation of ∼5% and maximum peak-to-peak change of 15%.…”