Simultaneous Multi-Wavelength Observations of Sgr A* During 2007 April 1-11

The Astrophysical Journal (Impact Factor: 5.99). 10/2009; 706(1):348. DOI: 10.1088/0004-637X/706/1/348
Source: arXiv


We report the detection of variable emission from Sgr A* in almost all
wavelength bands (i.e. centimeter, millimeter, submillimeter, near-IR and
X-rays) during a multi-wavelength observing campaign. Three new moderate flares
are detected simultaneously in both near-IR and X-ray bands. The ratio of X-ray
to near-IR flux in the flares is consistent with inverse Compton scattering of
near-IR photons by submillimeter emitting relativistic particles which follow
scaling relations obtained from size measurements of Sgr A*. We also find that
the flare statistics in near-IR wavelengths is consistent with the probability
of flare emission being inversely proportional to the flux. At millimeter
wavelengths, the presence of flare emission at 43 GHz (7mm) using VLBA with
milli-arcsecond spatial resolution indicates the first direct evidence that
hourly time scale flares are localized within the inner 30$\times$70
Schwarzschild radii of Sgr A*. We also show several cross correlation plots
between near-IR, millimeter and submillimeter light curves that collectively
demonstrate the presence of time delays between the peaks of emission up to
three hours. The evidence for time delays at millimeter and submillimeter
wavelengths are consistent with the source of emission being optically thick
initially followed by a transition to an optically thin regime. In particular,
there is an intriguing correlation between the optically thin near-IR and X-ray
flare and optically thick radio flare at 43 GHz that occurred on 2007 April 4.
This would be the first evidence of a radio flare emission at 43 GHz delayed
with respect to the near-IR and X-ray flare emission.

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    ABSTRACT: The source of emission from Sgr A*, the supermassive black hole at the Galactic Center, is still unknown. Flares and data from multiwavelength campaigns provide important clues about the nature of Sgr A* itself. Here we attempt to constrain the physical origin of the broadband emission and the radio flares from Sgr A*. We developed a time-dependent jet model, which for the first time allows one to compare the model predictions with flare data from Sgr A*. Taking into account relevant cooling mechanisms, we calculate the frequency-dependent time lags and photosphere size expected in the jet model. The predicted lags and sizes are then compared with recent observations. Both the observed time lags and size-frequency relationships are reproduced well by the model. The combined timing and structural information strongly constrain the speed of the outflow to be mildly relativistic, and the radio flares are likely to be caused by a transient increase in the matter channelled into the jets. The model also predicts light curves and structural information at other wavelengths which could be tested by observations in the near future. We show that a time-dependent relativistic jet model can successfully reproduce: (1) the quiescent broadband spectral energy distribution of Sgr A*, (2) the observed 22 and 43 GHz light curve morphologies and time lags, and (3) the frequency-size relationship. The results suggest that the observed emission at radio frequencies from Sgr A* is most easily explained by a stratified, optically thick, mildly relativistic jet outflow. Frequency-dependent measurements of time-lags and intrinsic source size provide strong constraints on the bulk motion of the jet plasma. Comment: Accepted for publication in Astronomy & Astrophysics Letters. Four pages, 3 figures
    Preview · Article · Nov 2009 · Astronomy and Astrophysics
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    ABSTRACT: We review our current understanding to the accretion and ejection processes in Sgr A*. Roughly speaking, they correspond to the quiescent and flare states of the source respectively. The high-resolution {\it Chandra} observations to the gas at the Bondi radius combined with the Bondi accretion theory, the spectral energy distribution from radio to X-ray, and the radio polarization provide us strict constraints and abundant information to the theory of accretion. We review these observational results and describe how the advection-dominated accretion flow model explains these observations. Recently more attentions have been paid to flares in Sgr A*. Many simultaneous multi-wavelength campaigns have been conducted, aiming at uncovering the nature of flares. The main observational properties of flares are briefly reviewed. Especially, the time lag between the peaks of flare at two radio frequencies strongly indicates that the flare is associated with ejection of radio-emitting blobs from the underlying accretion flow. Such kind of episodic jets is distinctive from the continuous jets and are quite common in black hole systems. We introduce the magnetohydrodynamical model for the formation of episodic jets recently proposed based on the analogy with the theory of coronal mass ejection in the Sun. We point out that the various observational appearances of flares should be explained in the framework of this model, since ejection and flare originate from the same physical process. Comment: 12 pagaes, 4 figures; Proceedings overview article to be published in "The Galactic Center: A Window on the Nuclear Environment of Disk Galaxies", ed. Mark Morris, Daniel Q. Wang and Feng Yuan
    Preview · Article · Feb 2010
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    ABSTRACT: We discuss mm-wavelength radio, 2.2-11.8um NIR and 2-10 keV X-ray light curves of the super massive black hole (SMBH) counterpart of Sagittarius A* (SgrA*) near its lowest and highest observed luminosity states. The luminosity during the low state can be interpreted as synchrotron emission from a continuous or even spotted accretion disk. For the high luminosity state SSC emission from THz peaked source components can fully account for the flux density variations observed in the NIR and X-ray domain. We conclude that at near-infrared wavelengths the SSC mechanism is responsible for all emission from the lowest to the brightest flare from SgrA*. For the bright flare event of 4 April 2007 that was covered from the radio to the X-ray domain, the SSC model combined with adiabatic expansion can explain the related peak luminosities and different widths of the flare profiles obtained in the NIR and X-ray regime as well as the non detection in the radio domain.
    Full-text · Article · Mar 2010 · Astronomy and Astrophysics
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