Schematic diagram of the 1.3 mm VLBI observations of Sagittarius A* (Sgr A*) in the Galactic centre, which were performed in 2013. The insets show possible shapes of the source emission that are consistent with the measurements. For better visualization of the angular dimensions, a white circle of 50 micro-arcseconds in diameter is superimposed on the models. The location of the APEX telescope on the southern hemisphere in Chile now provides longer interferometric baselines, leading to a doubling of the angular resolution in comparison to earlier observations. This setup enables a spatial resolution of only 3 Schwarzschild radii in Sgr A*. © Eduardo Ros/Thomas Krichbaum (MPIfR)
A global array of telescopes, including APEX, reveals the finest details so far on event horizon scales in the centre of our Galaxy
The 12 m radio telescope APEX in Chile has been outfitted with special equipment including broad bandwidth recorders and a stable hydrogen maser clock for performing joint interferometric observations with other telescopes at wavelengths as short as 1.3 mm and the goal to obtain the ultimate picture of the black hole shadow. The addition of APEX to the so-called Event Horizon Telescope (EHT), which until recently consisted of antennas only in the northern hemisphere, reveals new and unprecedented details in the structure of Sgr A* at the centre of the Milky Way. The increased angular resolution provided by the APEX telescope now reveals details in the asymmetric and not point-like source structure, which are as small as 36 million km. This corresponds to dimensions that are only 3 times larger than the hypothetical size of the black hole (3 Schwarzschild This corresponds to dimensions that are only 3 times larger than the hypothetical size of the black hole (3 Schwarzschild radii).
Astronomers are hunting for the ultimate proof of Einstein’s theory
of general relativity, which is to obtain a direct image of the shadow
of a black hole. This is possible by combining radio telescopes spread
over the globe using a technique which is called Very Long Baseline
Interferometry (VLBI). The participating telescopes are located at high
altitudes to minimize the disturbance from the atmosphere and on remote
sites with clear skies, allowing to observe the compact radio source
Sagittarius A* (Sgr A*) at the centre of the Milky Way.
The research team observed Sgr A* in 2013 using VLBI telescopes at
four sites. The telescopes include the APEX telescope in Chile, the
CARMA array in California, the JCMT and the phased SMA in Hawaii, and
the SMT telescope in Arizona. Sgr A* was detected with all stations and
the longest baseline length reached up to almost 10,000 kilometers,
indicating an ultra-compact and asymmetric (not point-like) source
structure.
“The participation of the APEX telescope almost doubles the length of
the longest baselines in comparison to earlier observations and leads
to a spectacular resolution of 3 Schwarzschild radii only”, says Ru-Sen
Lu from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn,
Germany, the lead author of the publication. “It reveals details in the
central radio source which are smaller than the expected size of the
accretion disk”, adds Thomas Krichbaum, initiator of the mm-VLBI
observations with APEX.
The location of APEX in the southern hemisphere considerably improves
the image quality for a source as far south in the sky as Sagittarius
A* (−29 degrees declination). APEX has paved the way towards the
inclusion of the large and extremely sensitive ALMA telescope into the
EHT observations, which are now being performed once a year.
“We have worked hard at an altitude of more than 5000 meters to
install the equipment to make the APEX telescope ready for VLBI
observations at 1.3 mm wavelength”, says Alan Roy, also from MPIfR who
leads the VLBI team at APEX. “We are proud of the good performance of
APEX in this experiment.”
The team employed a model-fitting procedure to investigate the
event-horizon-scale-structure of Sgr A*. “We started to figure out what
the horizon-scale structure may look like, rather than just draw generic
conclusions from the visibilities that we sampled. It is very
encouraging to see that the fitting of a ring-like structure agrees very
well with the data, though we cannot exclude other models, e.g., a
composition of bright spots.”, adds Ru-Sen Lu. Future observations with
more telescopes added to the EHT will sort out residual ambiguities in
the imaging.
The black hole at the center of the our galaxy is embedded in a dense
interstellar medium, which may affect the propagation of
electromagnetic waves along the line of sight. “However, the
interstellar scintillation, which in principle may lead to image
distortions, is not a strongly dominating effect at 1.3 mm wavelength ”,
says Dimitrios Psaltis from the University of Arizona, who is the EHT
project scientist.
“The results are an important step to ongoing development of the
Event Horizon Telescope”, says Sheperd Doeleman from the
Harvard-Smithsonian Center for Astrophysics and director of the EHT
project. “The analysis of new observations, which since 2017 also
include ALMA, will bring us another step closer to imaging the black
hole in the centre of our Galaxy.”
The Atacama Pathfinder Experiment (APEX) is a collaboration
between the Max Planck Institute for Radio Astronomy (MPIfR), the Onsala
Space Observatory (OSO), and the European Southern Observatory (ESO) to
construct and operate a modified prototype antenna of ALMA (Atacama
Large Millimetre Array) as a single dish on the Chajnantor plateau at an
altitude of 5,100 metres above sea level (Atacama Desert, Chile). The
telescope was manufactured by VERTEX Antennentechnik in Duisburg,
Germany. The operation of the telescope is entrusted to ESO.
The research team consists of Ru-Sen Lu, Thomas P. Krichbaum, Alan L.
Roy, Vincent L. Fish, Sheperd S. Doeleman, Michael D. Johnson, Kazunori
Akiyama, Dimitrios Psaltis, Walter Alef, Keiichi Asada, Christopher
Beaudoin, Alessandra Bertarini, Lindy Blackburn, Ray Blundell, Geoffrey
C. Bower, Christiaan Brinkerink, Avery E. Broderick, Roger Cappallo,
Geoffrey B. Crew, Jason Dexter, Matt Dexter, Heino Falcke, Robert
Freund, Per Friberg, Christopher H. Greer, Mark A. Gurwell, Paul T. P.
Ho, Mareki Honma, Makoto Inoue, Junhan Kim, James Lamb, Michael
Lindqvist, David MacMahon, Daniel P. Marrone, Ivan Martí-Vidal, Karl M.
Menten, James M. Moran, Neil M. Nagar, Richard L. Plambeck, Rurik A.
Primiani, Alan E. E. Rogers, Eduardo Ros, Helge Rottmann, Jason SooHoo,
Justin Spilker, Jordan Stone, Peter Strittmatter, Remo P. J. Tilanus,
Michael Titus, Laura Vertatschitsch, Jan Wagner, Jonathan Weintroub,
Melvyn Wright, Ken H. Young, J. Anton Zensus and Lucy M. Ziurys.
Authors with MPIfR affiliation include Ru-Sen Lu, the first author,
Thomas Krichbaum, Alan Roy, Walter Alef, Alessandra Bertarini, Karl
Menten, Eduardo Ros, Helge Rottmann, Anton Zensus, and Heino Falcke.
Local Contact:
Dr. Ru-Sen Lu
Phone:+49 228 525-292
Email: rslu@mpifr-bonn.mpg.de
Max-Planck-Institut für Radioastronomie, Bonn
Dr. Thomas Krichbaum
Phone:+49 228 525-295
Email: tkrichbaum@mpifr-bonn.mpg.de
Max-Planck-Institut für Radioastronomie, Bonn
Dr. Norbert Junkes
Press and Public Outreach
Phone:+49 228 525-399
Email: njunkes@mpifr-bonn.mpg.de
Max-Planck-Institut für Radioastronomie, Bonn
Original Paper:
Ru-Sen Lu et al., 2018, The Astrophysical Journal, Vol. 859, No. 1 (DOI: 10.3847/1538-4357)
Links :
Radio Astronomy / VLBI
Research Department "Radio Astronomy. VLBI" at MPIfR Bonn
APEX
The Atacama Pathfinder Experiment (APEX)
CARMA
Combined Array for Research in Millimeter-wave Astronomy (CARMA), Bishop, California
JCMT
James Clerk Maxwell Telescope, Mauna Kea, Hawaii (JCMT)
SMA
Submillimeter Array (SMA), Mauna Kea, Hawaii
SMT (Heinrich Hertz Telescope)
Submillimeter Telescope (SMT), Mt. Graham, Arizona
EHT
Event Horizon Telescope (EHT)
Event Horizon Telescope
MPIfR web page for the EHT observations in April 2017
