The Event Horizon Telescope: 1.3mm VLBI of M87

1. Abstract The radio galaxy M87 (Virgo A) offers a unique opportunity to study large-scale jet formation and very-high-energy (VHE) γ-ray emission from an active galactic nucleus.  New estimates of the mass of the black hole at the center of M87 (6.4 billion solar masses) yield an apparent Schwarzschild radius of ~8 microarcseconds.  This size scale can be probed only by very long baseline interferometry (VLBI) at high frequencies.  VLBI observations of M87 taken in April, 2009, used four antennas at three locations (Hawaii, California, Arizona) and observed at 230 GHz (1.3mm) to achieve a fringe spacing of 60 microarcseconds. The resulting size estimates for the mm wavelength core of M87 set stringent constraints on the processes giving rise to the well-studied radio jet in this source.  The impact on models of gamma-ray emission in the core of M87 is discussed. Observations and Model Fitting Observations were made from April 3, 2009 to April 8, 2009 using a VLBI array including the JCMT in Hawaii, the SMTO in Arizona, and two antennas of the CARMA array in California. Data were processed on the Mark4 VLBI correlator at MIT Haystack Observatory, corrected for atmospheric coherence effects, and calibrated to produce a correlated flux density (Jy) for each baseline as a function of time. The figure below shows the correlated flux density as a function of baseline length, and a fit to a source model consisting of two circular Gaussian components. The size of the larger component is not well determined, but must be greater than ~200as (FWHM), while the size of the smaller component is tightly constrained to be ~38as (4.5 RS) based on the long-baseline detections. The large component is consistent with the size scales of the inner jet seen with 7mm VLBI. The compact component is smaller than the expected apparent size of the Innermost Stable Circular Orbit (ISCO) for a 6.4 billion solar mass black hole. M87 At a distance of 16 Mpc (z=0.00436), M87 is one of the closest supermassive black hole candidates in the universe. The possible 4 million solar mass black hole at the center of the Galaxy, Sagittarius A*, is far closer but not nearly as massive as M87. The Schwarzschild radii of these two objects each have an angular size of about 9 as, larger than any other potential black holes. At these angular sizes, the only technique capable of resolving the sources is VLBI because the long baselines offer the greatest resolution. The active galactic nucleus of M87 was the first extragalactic source other than a blazar to be detected at TeV (1012eV) energies (Lenain et al. 2008). This very-high-energy (VHE) emission is variable on a timescale of just two days (Neronov et al. 2007). This variability has been used to derive an upper limit on the size of the VHE emitting region based on a light crossing time argument. M87 also has a relativistic jet of material that extends for kiloparsecs. The origin of the TeV emission and jet are not currently known. Physical processes such as synchrotron radiation and inverse Compton scattering are invoked in several different models of TeV emission (Lenain et al. 2008, Neronov et al. 2007, Reimer et al. 2004, Rieger et al. 2008, Tavecchio et al. 2008). Jet formation is often associated with a magnetic field getting frozen into the accretion disc, causing it to get twisted into a helix (Marscher 2006, Meier 2008). The Event Horizon Telescope: 1.3mm VLBI of M87 CARMA1-CARMA2 SMTO-CARMA1 SMTO-CARMA2 Image of the jet of M87 taken by VLBA at 43 GHz (Walker et al. 2008). JCMT-CARMA1 JCMT-CARMA2 Past VLBI Observations Previous VLBI of M87, conducted at a wavelength of 7mm, have imaged the jet with angular resolution corresponding to 30x60 Schwarzschild radii (Acciari et al 2009). These observations associated the TeV emission from M87 with the inner radio core, but did not have sufficient resolution to probe the region just outside the Event Horizon of the central black hole. By decreasing the observing wavelength to 1.3mm, the VLBI resolution can be matched to the inner few Schwarzschild radii. The apparent size of the emitting region is estimated to be ~5δRS due to TeV variability, or about 10RS, where the Doppler Factor (δ) is estimated to be ~2 (Acciari et al. 2009). By obtaining a robust size estimate for the radio emitting region at 1.3mm, the size of the VHE emitting region can be constrained. It is assumed the VHE and radio emission regions have comparable size because 43 GHz VLBA data shows that a VHE gamma-ray flares were accompanied by a strong increase in radio flux (Acciari et al. 2009). Plot of the correlated flux density vs Baseline length for the 1.3mm VLBI M87 detections. The stations associated with each grouping are labeled. The two black curves show two circular Gaussian sources used to model M87, and the red curve is the sum of the two components. JCMT-SMTO David Schenck123, Shep Doeleman2, Vincent Fish2 & the EHT Collaboration 1University of Arizona, 2MIT Haystack Observatory, 3MIT Haystack Observatory REU 2009 • Conclusions • Compact structure on the order of 4.5 RS was observed in M87. • This size is smaller than the ISCO of the black hole. • The measurement of the size of the radio emitting region along with previous TeV/43 GHz VLBI observations limits the size of the TeV emitting region. • The observed size is broadly consistent with radiative process that boost synchrotron emission through inverse Compton scattering near the black hole. • The size determined by 1.3mm VLBI suggests that the M87 jet originates through magnetic fields co-rotating with a spinning black hole, though some jet models from non-spinning black holes also produce emission regions consistent with the size we observe (Broderick & Loeb 2009). The yellow triangle on the globe connects the three VLBI sites used in these 1.3mm observations of M87. 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