Talks

It has now become clear that the radio jet in the giant elliptical galaxy M87 must turn on very close to the black hole. This implies the efficient acceleration of leptons within the jet at scales much smaller than feasible by the typical dissipative events usually invoked to explain jet synchrotron emission. Here we show that the stagnation surface, the separatrix between material that falls back into the black hole and material that is accelerated outward forming the jet, is a natural site of pair formation and particle acceleration. This occurs via an inverse-Compton pair catastrophe driven by unscreened electric fields within the charge-starved region about the stagnation surface and substantially amplified by a post-gap cascade. For typical estimates of the jet properties in M87, we find excellent quantitive agreement between the predicted relativistic lepton densities and those required by recent high-frequency radio observations of M87.

The radio galaxy M87 is an excellent laboratory for investigating the formation process of the relativistic jet and the production mechanism of high energy particles and photons in the vicinity of super-massive black holes. VLBI observations at 1.3 mm can address at least two issues concerning the fundamental nature of M87. The first is the event-horizon-scale structure of the jet launching region, and another one is the production mechanism of very high energy (VHE; > 100 GeV) photons at there. In this talk, I report on new 1.3 mm VLBI observations of M87 with the EHT during the VHE enhancement in 2012. These observations provide the first measurements of closure phase, imposing new constraints on accretion-disk/jet models for M87, and also the first constraints on the innermost structure of the relativistic jet on scales of a few Rs during VHE variability. I discuss results and their implications for jet/disk models and also VHE models for M87, as well as future prospects of EHT observations of M87.

Because of the existence of a prominent jet, M87 provides an ideal source with which the EHT can critically test current jet launching paradigms. The EHT has already placed weak limits on the black hole spin based on generic arguments regarding limits on the image size. However, careful modeling of the jet structure can produce much more stringent constraints on both the black hole properties and the mechanisms responsible for the generation of the observed radio emission. I will describe how this modeling is performed, what the remaining chief uncertainties are and why we believe they can be controlled, and how existing EHT observations of M87 support models of electromagnetically launched jets powered by black hole spin.

VLBI observations at the highest possible frequency penetrate the opacity barrier in the nuclear regions of radio-galaxies and blazars, which are synchrotron self-absorbed at longer wavelength. This facilitates a direct and sharper than ever view into the 'heart' of Active Galactic Nuclei (AGN), into region in which BH physics and general relativity effects become important and where radio jets are launched. Here we report on new results from global 3mm and 1.3mm VLBI observations adding the APEX and IRAM to the Event Horizon Telescope. New images and core size estimates for a number of AGN jets and for Sgr A* are presented and discussed.

Mass accretion rate on the SMBHs is one of the fundamental parameters used to investigate AGNs. Faraday Rotation Measure (RM) observations at mm/sub-mm wavelengths is one of the powerful methods to derive the mass accretion rate of hot accretion flows towards our galactic center, Sgr A* (e.g., Marrone et al. 2006). Based on this scheme, we conducted an SMA observation to apply this method to M 87, which is one of the primary target for our submm VLBI observations, in February 2013. We succeeded to derive an RM of (2.1 ± 1.8) × 10^5 rad m^-2, it gives the range of the mass accretion rate (M_dot) between 0 and 9.2 × 10^-4 M_sun yr^-1 at the distance of 21 rs from the SMBH. Our estimated M.is already two orders of magnitude smaller than the M_dot at the outer part of the accretion flow (~10^5 r_s) of 0.1 M_sun yr^-1 determined by X-ray observations (Di Matteo et al. 2003). This significant suppression of the M_dot at the inner region is expected with the radiatively inefficient accretion flow (RIAF) model. With future submm VLBI polarimetry towards jetted sources including M 87, we will derive the profile of accretion flow along the jet. It is very important itself for the study of the accretion process onto the SMBH, but also provide fundamental properties to derive BH parameters from the BH shadow imaging.

The super-massive black hole in the center of the Milky Way, Sgr~A*, displays a nearly flat radio spectrum which is typical for jets in Active Galactic Nuclei. Indeed, time-dependent, magnetized models of radiatively inefficient accretion flows, which are commonly used to explain emission of Sgr A* also often produce jet-like outflows. However, the emission from these models so far has failed to reproduce the flat radio spectrum. We show that current GRMHD simulations can naturally reproduce the flat spectrum, when using a two-temperature plasma in the disk and a constant electron temperature plasma in the jet. This assumption is consistent with current state-of-the art simulations, in which the electron temperature evolution is not explicitly modeled. The model images and spectra are consistent with the radio sizes and spectrum of Sgr~A*. The model can also reproduce the radio images of the jet base in M87.

Stellar dynamical measurements of black hole masses have become the de facto standard method. I will give a brief review of how this measurement method works, along with arguments for its overall reliability and caveats. Then I will turn my attention to the case of the black hole in M87. The black hole is undeniably large ? billions of solar masses ? but has a stellar dynamical mass measurement in disagreement with gas dynamical mass measurements at about the 2 sigma level. I will discuss potential systematic uncertainties in both measurements and avenues to reconciling the discrepancy.