Recent radio observations by the Very Long Baseline Array at 7 and 3.5 mm produced the high-resolution images of the compact radio source located at the center of our Galaxy (Sgr A*) and detected its wavelength-dependent intrinsic sizes at the two wavelengths. This provides us with a good chance of testing previously proposed theoretical models for Sgr A*. In this Letter, we calculate the size based on the radiatively inefficient accretion flow (RIAF) model proposed by Yuan, Quataert, & Narayan. We find that after taking into account the scattering of the interstellar electrons, the predicted sizes are consistent with the observations. We further predict an image of Sgr A* at 1.3 mm that can be tested by future observations.
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"All such theoretical interpretations have observational support as it was recently reported that both X-ray flares and the outflows are originated from the vicinity of the black hole (Junor et al. 1999;Biretta et al. 2002;Falcke 1999). The typical size of the post-shock flow which possibly responsible for various diverse phenomenon is reasonably coherent with the observational results (Yuan et al. 2006). In this work, we use pseudo-Kerr potential introduced by (Mondal and Chakrabarti 2006) that successfully describe the similar space time geometry around a Kerr black hole as far as the transonic properties of the flow is concerned. "
[Show abstract][Hide abstract]ABSTRACT: We examine the behaviour of accretion flow around a rotating black hole in presence of cooling. We obtain global flow solutions
for various accretion parameters that govern the accreting flow. We show that standing isothermal shock wave may develop in
such an advective accretion flow in presence of cooling. This shocked solution has observational consequences as it successfully
provides the possible explanations of energy spectra as well as generation of outflows/jets of various galactic and extra-galactic
black hole candidates. We study the properties of isothermal shock wave and find that it strongly depends on the cooling efficiency.
We identify the region in the parameter space spanned by the specific energy and specific angular momentum of the flow for
standing isothermal shock as a function of cooling efficiencies and find that parameter space gradually shrinks with the increase
of cooling rates. Our results imply that accretion flow ceases to contain isothermal shocks when cooling is beyond its critical
Full-text · Article · May 2009 · Astrophysics and Space Science
[Show abstract][Hide abstract]ABSTRACT: This review summarizes our current understanding of low-luminosity AGNs in the context of the advection-dominated accretion flow (ADAF). Special attention is paid to the best investigated source, the supermassive black hole in our galactic center, Sgr A*. ADAF can naturally explain the most important features of these sources such as the low luminosity and the lack of the big blue bump in their spectral energy distribution. The emission from the jet usually dominates over the ADAF in the radio waveband and sometimes in other wavebands as well especially the X-ray
[Show abstract][Hide abstract]ABSTRACT: The nonthermal radio source Sgr A∗ at the Galactic center has been known to be extremely compact since its discovery. Following a brief introduction to its size measurements (including its discovery, interstellar scattering effect and its elongated structure), we describe the detection of an intrinsic size of only 1 AU for Sgr A∗. When combined with the mass estimate of Sgr A∗, these size measurements provide strong evidence that Sgr A∗ is a super‐massive black hole. Results from the simulation of the black hole shadow image and the corresponding visibility analysis indicate that future sub‐millimeter VLBI experiments are critical and promising to probe the Galactic center black hole. High quality multi‐epoch radio interferometric images of Sgr A∗ with a wavelength coverage from long centimeter to short millimeter are needed in order to pursue the two‐dimensional structure for the scattering angle and the intrinsic source structure, and the corresponding size variation.