Resolving the Inner Jet Structure of 1924-292 with the Event Horizon Telescope

We present the first 1.3 mm (230 GHz) very long baseline interferometrymodel image of an active galactic nucleus (AGN) jet using closure phasetechniques with a four-element array. The model image of the quasar1924-292 was obtained with four telescopes at three observatories: theJames Clerk Maxwell Telescope on Mauna Kea in Hawaii, the Arizona RadioObservatory's Submillimeter Telescope in Arizona, and two telescopes ofthe Combined Array for Research in Millimeter-wave Astronomy inCalifornia in 2009 April. With the greatly improved resolution comparedwith previous observations and robust closure phase measurement, theinner jet structure of 1924-292 was spatially resolved. The inner jetextends to the northwest along a position angle of -53° at adistance of 0.38 mas from the tentatively identified core, in agreementwith the inner jet structure inferred from lower frequencies, and makinga position angle difference of ~80° with respect to the centimeterjet. The size of the compact core is 0.15 pc with a brightnesstemperature of 1.2 × 10¹¹ K. Compared with thosemeasured at lower frequencies, the low brightness temperature may arguein favor of the decelerating jet model or particle-cascade models. Thesuccessful measurement of closure phase paves the way for imaging andtime resolving Sgr A* and nearby AGNs with the Event Horizon Telescope.