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The Greenland Telescope (GLT) Project*
"Direct Confirmation of Black Holes with Submillimeter VLBI"

* collaborating with Smithsonian Astrophysical Observatory, MIT Haystack Observatory, & National Radio Astronomy Observatory

Bringing Black Holes into Reality

Simulated submm VLBI images of the SMBH
shadow of M87 including the GLT. Using
ray-tracing we model the case
of a non-rotating, six billion Solar mass SMBH
enclosed by optically thin, free-falling material.
Image credit: Hung-Yi Pu, Chih-Yin Tseng (ASIAA)
A direct confirmation of black holes (BH) in the Universe is one of the ultimate goals in modern physics and astronomy. With such an observation we can access matter and electromagnetic fields under extremely strong gravity for the first time. A BH shadow is expected against the bright enhanced annulus of emission around a BH. The size of this annulus, “Event Horizon” as defined by the Schwarzschild radius (rs), is lensed and self-magnified by its strong gravity. Therefore, a detection of a BH shadow is a direct confirmation of the existence of a BH, and provides a test for general relativity in the strong gravity regime. In addition to the detection of strong lensing, a BH spin can also be probed by a precise image of the shape and the axis of the shadow as it is expected that the BH shadow would be compressed perpendicularly to the spin axis of the BH. Furthermore, since we observe BHs as the silhouette against accretion disks and/or relativistic jets, BH shadow imaging will simultaneously provide images of the innermost regions of accretion disks and the formation regions of relativistic jets. However, the apparent size (depending on the distance to the source and the intrinsic size of the BH) of the shadow is very tiny. Thus, a special telescope system is required for such observations: that is the Very Long Baseline Interferometry (VLBI) at submillimeter (submm) wavelengths.

How to Observe Black Holes?

VLBI is the pursuit of highest angular resolution in observational astronomy. The angular resolution of an interferometer is proportional to λ/D, where λ is the observing wavelength and D is the length of the baseline between two telescopes. Consequently, observations at shorter wavelengths and/or longer baselines are essential for a higher angular resolution. The GLT project is deploying a new submm VLBI station to Greenland. Combining submm wavelengths and intercontinental baselines, submm VLBI will achieve resolutions of several tens of micro-arcseconds (μas). This is the equivalent to the apparent size of a US one cent coin on the moon seen from the Earth. Our primary scientific goal is to image the shadow of the supermassive black hole (SMBH) of about six billion Solar mass in the active galactic nucleus (AGN) M87 at the center of the Virgo cluster of galaxies at the distance of 17 Mpc. The expected SMBH shadow size of 40-50 μas requires superbly high angular resolution, suggesting that submm VLBI is the only way to obtain a shadow image. The Summit station on the Greenland ice plateau enables us to establish baselines longer than 9,000 km with the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile and the Submillimeter Array (SMA) in Hawaii, while at the same time providing a unique uv-coverage for imaging M87. Our VLBI network will achieve a superior angular resolution of about 20 μas at 350 GHz, corresponding to ~ 2.5 times the Schwarzschild radius of the SMBH in M87.

Expected baselines for submm VLBI observations at 350 GHz. Earth-size, very long baselines over 9,000 km will provide a resolution ~ 20 μas to resolve a size of few times the SMBH radius in M87.

Required angular resolutions to image the SMBH shadow in the nearby AGN M87. Dashed line: estimated resolution of a radio interferometer with a baseline of 9,000km. Dotted line: angular diameter of the SMBH shadow θ=3√3rs ~ 42 μas (rs ∼8 μas), corresponding to a 6.6 billion Solar mass BH. Imaging the SMBH shadow of M87 will be possible with submm VLBI with λ < 1mm, as indicated with the vertical colored lines of 230 GHz (purple), 350 GHz (magenta), and 690 GHz (orange), respectively. Image credit: M Nakamura (ASIAA)

Submm VLBI Operation with New Antenna

ALMA-NA prototype antenna in Socorro, New Mexico.
Image credit: ASIAA.
Submm VLBI observations are conducted under international collaborations. We are attempting to play a leading role in these observations by proposing a new submm VLBI array consisting of the SMA in Hawaii, the ALMA in Chile and a new single-dish telescope at an excellent site. ASIAA is actively involved in all these telescope projects. Our new telescope is the North-American (NA) ALMA prototype antenna, a 12-meter diameter antenna designed for mm and submm wavelengths (0.3 to 10 mm, or 30 to 950 GHz). In July 2010, the US National Science Foundation (NSF) announced a call for expression of interests for this telescope. The Smithsonian Astrophysical Observatory (SAO) was awarded the telescope in April 2011, under close collaboration with ASIAA as well as the MIT Haystack Observatory and the National Radio Astronomy Observatory (NRAO).

Site Selection of New Submm VLBI Station in Greenland

ASIAA Radiometer in Greenland.
Image credit: ASIAA.
We examined possible sites for a new submm telescope location. Our main requirements are: (1) excellent observing conditions to perform high-quality observations at submm and even shorter wavelengths; (2) location providing long baselines with other key stations (e.g., SMA, ALMA). Based upon the precipitable water vapor (PWV) data measured by the NASA satellites Aqua and Terra/MODIS, we selected the Summit station in Greenland as the best candidate site. For further evaluation, we have started a site-testing campaign at the Summit station in 2011 with a radiometer to measure the atmospheric transparency at 225 GHz. Current data indicate a median opacity of 0.06 for the winter seasons between August 2011 and June 2013. These excellent weather conditions will also enable us to do single-dish THz work.

Current Antenna Status

Disassembly of the ALMA NA Prototype in Nov. 2012.
Image credit: ASIAA.
The telescope was tested for functionality at the VLA site in Socorro, NM, in 2011/2012. We also conducted photogrammetry surface measurements, installed a new antenna control software and an optical pointing telescope. In November 2012, the antenna was disassembled and parts were shipped to various places for retrofitting for low-temperature operation down to -55°C. First re-assembly, cold environment tests and first commissioning observations are expected for 2015 in Thule. Around 2016, the telescope will be moved to its final destination, the Summit station, as the Greenland Telescope (GLT). The project is currently working together with the NSF Office of Polar Research Programs for logistics in Greenland and CH2M HILL in close collaboration with staff at the Harvard Smithsonian Center for Astrophysics (CfA).

ALMA Phase-up Project

The introduction of ALMA into the submm VLBI network will be crucial, as this will increase the sensitivity by a factor of ten due to the large collecting area of ALMA. ASIAA has a very active participation in an international collaboration led by the MIT Haystack Observatory aiming towards the ALMA phase-up project (APP). An international consortium is presently constructing the APP system which will be available as a facility instrument. This system will have impact on a variety of scientific topics, including accretion and outflow processes around SMBHs in AGNs, tests of general relativity near SMBHs, jet launching and collimation from AGNs and microquasars, pulsar and magnetar emission processes, the chemical history of the Universe and the evolution of fundamental constants across cosmic time, maser science and astrometry (arXiv:1309.3519)

DiFX Correlator

ASIAA Cluster for VLBI DiFX correlator.
Image credit: ASIAA.
ASIAA has acquired a CPU cluster with broadband interconnect network connection (InfiniBand) capabilities. We started building and testing the DiFX software correlator which is easy to upgrade, and we are planning to handle the massive data rates from submm VLBI observations. We performed a test correlation and succeeded to get fringes using 1.3 mm VLBI data from the Event Horizon Telescope (EHT) observation in 2012. We are now aiming at solving discrepant sampling schemes between ALMA and other submm VLBI stations (including the GLT/EHT) by DiFX enhancements. We plan to test these enhancements using data generated from a data simulator which is currently under development at ASIAA. In October 2013, ASIAA acquired a Mark 5c playback system and has a plan to purchase a Mark 6 as well.

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