Abstract
Millimeter very-long baseline interferometry (mm-VLBI) provides the novel capacity to probe the emission region of a handful of supermassive black holes on sub-horizon scales. For Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, this provides access to the region in the immediate vicinity of the horizon. Broderick et al. (2009) have already shown that by leveraging spectral and polarization information as well as accretion theory, it is possible to extract accretion-model parameters (including black hole spin) from mm-VLBI experiments containing only a handful of telescopes. Here we repeat this analysis with the most recent mm-VLBI data, considering a class of aligned, radiatively inefficient accretion flow (RIAF) models. We find that the combined data set rules out symmetric models for Sgr A*'s flux distribution at the 3.9-sigma level, strongly favoring length-to-width ratios of roughly 2.4:1. More importantly, we find that physically motivated accretion flow models provide a significantly better fit to the mm-VLBI observations than phenomenological models, at the 2.9-sigma level. This implies that not only is mm-VLBI presently capable of distinguishing between potential physical models for Sgr A*'s emission, but further that it is sensitive to the strong gravitational lensing associated with the propagation of photons near the black hole. Based upon this analysis we find that the most probable magnitude, viewing angle, and position angle for the black hole spin are a=0.0(+0.64+0.86), theta=68(+5+9)(-20-28) degrees, and xi=-52(+17+33)(-15-24) east of north, where the errors quoted are the 1-sigma and 2-sigma uncertainties.
Highlights
Despite being invoked to power a variety of energetic astrophysical phenomena, the detailed structure and dynamics of black hole accretion flows remain a central problem in astrophysics
The first successful mm-VLBI observation of Sgr A∗ with Earth-scale baselines was performed in 2007 April, during which visibilities were measured on the 4.6 × 103 km baseline between Mauna Kea, Hawaii and Mount Graham, Arizona (Doeleman et al 2008)
We describe three classes of model images: those associated with radiatively inefficient accretion flows (RIAFs) of the form discussed in Broderick & Loeb (2006a), symmetric, and asymmetric Gaussians
Summary
Despite being invoked to power a variety of energetic astrophysical phenomena, the detailed structure and dynamics of black hole accretion flows remain a central problem in astrophysics. The first successful mm-VLBI observation of Sgr A∗ with Earth-scale baselines was performed in 2007 April, during which visibilities were measured on the 4.6 × 103 km baseline between Mauna Kea, Hawaii and Mount Graham, Arizona (Doeleman et al 2008) By fitting these with a Gaussian model, Doeleman et al (2008) found a typical intrinsic source size of 37+−53 μas (after correcting for the subdominant broadening due to interstellar electron scattering), smaller than the black hole silhouette. Since that time a number of groups have analyzed the 2007 mm-VLBI data using various physically motivated accretion models for the emission region (Broderick et al 2009; Huang et al 2009; Moscibrodzka et al 2009; Dexter et al 2010), inferring from these efforts the black hole spin vector.
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
