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PERSISTENT ASYMMETRIC STRUCTURE OF SAGITTARIUS A* ON EVENT HORIZON SCALES
Fish, Vincent L.,Johnson, Michael D.,Doeleman, Sheperd S.,Broderick, Avery E.,Psaltis, Dimitrios,Lu, Ru-Sen,Akiyama, Kazunori,Alef, Walter,Algaba, Juan Carlos,Asada, Keiichi,Beaudoin, Christopher,Bert American Astronomical Society 2016 The Astrophysical journal Vol.820 No.2
<P>The Galactic Center black hole Sagittarius. A* (Sgr A*) is a prime observing target for the Event Horizon Telescope (EHT), which can resolve the 1.3 mm emission from this source on angular scales comparable to that of the general relativistic shadow. Previous EHT observations have used visibility amplitudes to infer the morphology of the millimeter-wavelength emission. Potentially much richer source information is contained in the phases. We report on 1.3 mm phase information on Sgr. A* obtained with the EHT on a total of 13 observing nights over four years. Closure phases, which are the sum of visibility phases along a closed triangle of interferometer baselines, are used because they are robust against phase corruptions introduced by instrumentation and the rapidly variable atmosphere. The median closure phase on a triangle including telescopes in California, Hawaii, and Arizona is nonzero. This result conclusively demonstrates that the millimeter emission is asymmetric on scales of a few Schwarzschild radii and can be used to break 180 degrees rotational ambiguities inherent from amplitude data alone. The stability of the sign of the closure phase over most observing nights indicates persistent asymmetry in the image of Sgr. A* that is not obscured by refraction due to interstellar electrons along the line of sight.</P>
Akiyama, Kazunori,Lu, Ru-Sen,Fish, Vincent L.,Doeleman, Sheperd S.,Broderick, Avery E.,Dexter, Jason,Hada, Kazuhiro,Kino, Motoki,Nagai, Hiroshi,Honma, Mareki,Johnson, Michael D.,Algaba, Juan C.,Asada, IOP Publishing 2015 The Astrophysical journal Vol.807 No.2
<P>We report on 230 GHz (1.3 mm) very long baseline interferometry (VLBI) observations of M87 with the Event Horizon Telescope using antennas on Mauna Kea in Hawaii, Mt. Graham in Arizona, and Cedar Flat in California. For the first time, we have acquired 230 GHz VLBI interferometric phase information on M87 through measurement of the closure phase on the triangle of long baselines. Most of the measured closure phases are consistent with 0 degrees as expected by physically motivated models for 230 GHz structure such as jet models and accretion disk models. The brightness temperature of the event-horizon-scale structure is similar to 1 x 10(10) K derived from the compact flux density of similar to 1 Jy and the angular size of similar to 40 mu as similar to 5.5 R-s, which is broadly consistent with the peak brightness of the radio cores at 1-86 GHz located within similar to 10(2) R-s. Our observations occurred in the middle of an enhancement in very-high-energy (VHE) gamma-ray flux, presumably originating in the vicinity of the central black hole. Our measurements, combined with results of multi-wavelength observations, favor a scenario in which the VHE region has an extended size of similar to 20-60 R-s.</P>