The Event Horizon Telescope: Current Observations of SgrA*

1. Abstract The Event Horizon Telescope (EHT) is a (sub)mm wavelength VLBI network that will achieve angular resolutions sufficient to resolve and image the Event Horizons of the nearest super massive black holes. SgrA*, the 4 million solar mass super massive black hole candidate at the Galactic Center, has the largest known apparent Event Horizon and is the primary target of the EHT. Previously reported VLBI observations at 1.3 mm taken in 2007 with three elements of the EHT have confirmed that SgrA* has structure on scales of just a few Schwarzschild radii. In this presentation we report on 2009 observations of Sgr A* with a similar array. We confirm the previous long-baseline detections at higher signal to noise and report on new findings. These results provide a compelling scientific case for (a) technological improvements, currently in development, to increase the sensitivity of EHT telescopes, and (b) the inclusion of additional telescopes in the EHT array in the near future in order to increase angular resolution. Following Hot-Spots in the Orbiting Accretion Flow SgrA* exhibits variability in total flux density, which can be modeled as a localized region of hot plasma orbiting the black hole. Such hot-spots will be strongly lensed as they orbit, and (sub)mm-VLBI can reveal these structural changes by computing the closure phase – the sum of interferometric phases around a triangle of VLBI baselines – as a function of time. An example is shown in the figure below in which a hot spot at the Innermost Stable Circular Orbit with 27 minute period exhibits clear and periodic closure phase signatures on the ARO/SMT – Hawaii – CARMA triangle of baselines. Left Below: upper panel is simulated accretion flow with orbiting hot spot, and lower panel is same with ISM scattering included. Right Below: closure phase signatures showing periodic structures due to orbiting material. Timing of this period can be used to estimate black hole spin (Doeleman et al, ApJ, v695, 59, 2009; Fish et al, ApJ). Previous Observations of SgrA* 1.3mm VLBI observations in April 2007 detected structure on scales of ~4 Schwarzschild radii in SgrA*. Due to the strong gravitational lensing near the black hole, an emission region that is centered on the black hole will present a larger apparent size to distant observers. The size measured in 2007 is smaller than the theoretical minimum, which suggests that SgrA* itself may be offset from the black hole, possibly due to doppler enhancement of an approaching side of an accretion disk (or jet). The 2007 data also could be fit with a ‘ring’ like model that echos GRMHD simulations in which a ‘shadow’ or ‘silhouette’ lies in the center of the emission region due to the strong field gravity. Thes data were obtained on a 3-station array including CARMA, JCMT and ARO/SMT. The 2007 results could not distinguish between the Gaussian and ring-like models, primarily due to a lack of detections on the CARMA-JCMT baseline (Doeleman et al, Nature, v455, 78, 2008). The Event Horizon Telescope: Current Observations of SgrA* New VLBI Observations In April 2009, the SgrA* experiment was repeated but using two CARMA dishes instead of one, and with an effective doubling of recorded bandwidth to increase sensitivity. Robust detections were found on the CARMA-JCMT baseline, strengthening the case for a Gaussian model for the most compact structure. The preliminary results shown in the graph of correlated flux density vs baseline length to the right also show that there appears to be some larger emission component (>200mas) as suggested by the reduction in correlated flux on the ~600Ml baselines compared to the total flux density of SgrA* (dashed horizontal line). Imaging with a Larger (sub)mm-VLBI Array By adding more antennas to the current (sub)mm-VLBI array, an Event-Horizon Telescope can be assembled, which will enable imaging of the SgrA* and M87 black holes with ~Schwarzschild radius scale angular resolution. We have simulated imaging fidelity assuming 7-station and 13-station arrays using telescopes on the globes at lower left. Between 7 and 13 telescopes, conventional imaging techniques can be used to discern signatures of strong gravity, and specifically to measure the size and shape of the predicted black hole ‘shadow’. Below we show two models of emission surrounding the SgrA* black hole. Left panels are models (~40 Rsch across); middle panels show image reconstructions using a 7-station VLBI array; right panels show image reconstructions using 13 stations. Models courtesy of Broderick&Loeb (MNRAS, 367, 905, 2006) and Noble & Gammie (CQGra, 24, 259, 2007). See also: Broderick et al, ApJ, v697,45, 2009) and Fish et al, ApJL, v692, 14, 2009. Sheperd Doeleman1, Vincent Fish1 & the EHT Collaboration 1MIT Haystack Observatory Plot of correlated flux density vs Baseline length for the 1.3mm VLBI SgrA* detections from April 5,6 2009. Cyan and Blue points show data on baselines from the two CARMA dishes to the ARO/SMT. Green and Red points show data on baselines from the CARMA dishes to the JCMT. Magenta points show detections on the baseline between ARO/SMT and the JCMT. Open and filled points are used to distinguish between the two 480MHz passbands that were observed. The solid black curve shows a 43mas Gaussian model of the compact emission, while the dotted line shows a ring model with inner diameter of 35mas and outer diameter of 80mas. The horizontal dashed line shows the total flux density (3.3Jy) of SgrA*. • An Event Horizon Telescope • New 1.3mm VLBI Observations have confirmed Schwarzschild radius scale structures in SgrA*, the supermassive black hole candidate at the Galactic Center. • New preliminary results do not yet suggest evidence for the expected ‘shadow’ effect caused by strong gravity of the black hole. • Progress towards increasing the number of telescopes in the array and the bandwidth of next generation VLBI backends will allow us to assemble an Event Horizon Telescope, capable of resolving and eventually imaging a black hole. • Additional developments include: combining the ALMA array dishes into a single VLBI element, upgrading the bandwith and polarimetry capabilities of current receivers, and observing at 0.8mml where ISM scattering is reduced. This Work Supported by the National Science Foundation The yellow triangle on the globe connects the three VLBI sites used in these 1.3mm observations of SgrA*. Phase I of the full Event Horizon Telescope will include the seven sites marked with red squares. Phases II and III will focus on incorporating the sites marked in yellow to allow full imaging of SgrA* and M87.