Person:

Gurwell, Mark

Sagittarius A*, the ~4 × 106 M ☉ black hole candidate at the Galactic center, can be studied on Schwarzschild radius scales with (sub)millimeter wavelength very long baseline interferometry (VLBI). We report on 1.3 mm wavelength observations of Sgr A* using a VLBI array consisting of the JCMT on Mauna Kea, the Arizona Radio Observatory's Submillimeter Telescope on Mt. Graham in Arizona, and two telescopes of the CARMA array at Cedar Flat in California. Both Sgr A* and the quasar calibrator 1924–292 were observed over three consecutive nights, and both sources were clearly detected on all baselines. For the first time, we are able to extract 1.3 mm VLBI interferometer phase information on Sgr A* through measurement of closure phase on the triangle of baselines. On the third night of observing, the correlated flux density of Sgr A* on all VLBI baselines increased relative to the first two nights, providing strong evidence for time-variable change on scales of a few Schwarzschild radii. These results suggest that future VLBI observations with greater sensitivity and additional baselines will play a valuable role in determining the structure of emission near the event horizon of Sgr A*.

Submillimeter Array observations of Orion-KL at ∼ 1′′ resolution in the 440 µm/690 GHz band reveal new insights about the continuum and line emission of the region. The 440 µm continuum flux density measurement from source I allows us to differentiate among the various proposed physical models: Source I can be well modeled by a “normal” protostellar SED consisting of a proton-electron free-free emission component at low frequencies and a strong dust component in the submillimeter bands. Furthermore, we find that the protostellar object SMA1 is clearly distinct from the hot core. The non-detection of SMA1 at cm and infrared wavelengths suggests that it may be one of the youngest sources in the entire Orion-KL region. The molecular line maps show emission mainly from the sources I, SMA1 and the hot core peak position. An analysis of the CH 3CN(37 K − 36 K) K-ladder ( K = 0...3) indicates a warm gas component of the order 600 ± 200K. In addition, we detect a large fraction ( ∼ 58%) of unidentified lines and discuss the difficulties of line identifications at these frequencies.

We report on 230 GHz (1.3 mm) 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 closure phase on the triangle of long baselines. Most of the measured closure phases are consistent with 0◦ 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 ∼ 1 × 1010 K derived from the compact flux density of ∼ 1 Jy and the angular size of ∼ 40 µas ∼ 5.5 Rs, which is broadly consistent with the peak brightness of the radio cores at 1-86 GHz located within ∼ 102 Rs. Our observations occurred in the middle of an enhancement in very-high-energy (VHE) γ-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 ∼20-60 Rs.