Flares of Sagittarius A^*at Short Millimeter Wavelengths

Abstract

We have performed monitoring observations of the flux density toward the Galactic center compact radio source, Sagittarius A* (Sgr A*), which is a supermassive black hole, from 1996 to 2005 using the Nobeyama Millimeter Array of the Nobeyama Radio Observatory, Japan. These monitoring observations of Sgr A* were carried out in the 3-and 2-mm (100 and 140 GHz) bands, and we have detected several flares of Sgr A*. We found intraday variation of Sgr A* in the 2000 March flare. The twofold increase timescale is estimated to be about 1.5 hr at 140 GHz. This intraday variability suggests that the physical size of the flare-emitting region is compact on a scale at or below about 12 AU (≈150 RS; Schwarzschild radius). On the other hand, clear evidence of long-term periodic variability was not found from a periodicity analysis of our current millimeter data set. this period, we detected no significant mm-flare in unfavorable weather conditions.Reference [20] reported the presence of a 106 day periodicity in the centimeter flux variability of Sgr A* from the analysis of data observed with the VLA over 20 years. We showed that the millimeter light curve exhibits distinct high and low activity states, by folding our NMA data with this period of 106 days [14]. These results may indicate a quasi-periodic flux variability of Sgr A* at mm-wavelengths. Then we carried out a periodicity analysis using the Lomb-Scargle periodogram method for our NMA data at 3-mm [18]. However, no significant peak in the normalized power spectra is found for the entire 3-mm data set. For the subset of these data after 2000, the power spectra has a peak at 0.0101d −1 corresponding to a ~99 day period, with a significance level of 0.37 (false alarm probability). Thus, from the analysis, this periodicity is not significant for our data [18]. Our current sampling is probably insufficient to detect a periodicity, if we consider possible fluctuations in the period, as indicated in recent analysis of VLA cm-data [21].

The Variability of Polarized Radiation from Sagittarius A*

“…To provide additional insight on the nature of flaring activity such as whether the variable emission is driven by an outflow or by infall, we have begun a program to study the light curves as well as polarization measurements of Sgr A*'s flare emission at radio wavelengths. The flare emissions in radio, sub-millimeter and millimeter wavelengths are known to vary on hourly time scales (Marrone et al 2007;Miyazaki et al 2006;Maurehan et al 2005;Yusef-Zadeh et al 2006a). The near-IR measurements show a high degree of linearly polarized emission during the 10-20 minute time scale of typical flare emission.…”

Near-infrared flares from accreting gas around the supermassive black hole at the Galactic Centre

“…The higher fractional flare amplitudes in X-rays are probably just a reflection of the much fainter quiescent emission. Although the millimetre/submillimetre emission of SgrA* is also variable, almost all of these variations are on timescales of several days to a few hundred days 21,22 . One exception is a one-hour-duration, 30% amplitude event seen in March 2000 at a wavelength of 2mm (ref.…”

“…One exception is a one-hour-duration, 30% amplitude event seen in March 2000 at a wavelength of 2mm (ref. 22). Although we do not have a simultaneous SED of the infrared flares, our data suggest that they may be bluer than the quiescent emission with a flux density that is approximately constant as a function of frequency ( Fig.…”