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APEX takes a glimpse into the heart of darkness


The APEX telescope in Chile was 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 reveals new and unprecedented details in the structure of Sgr A* at the centre of the Milky Way. The increased angular resolution provided by APEX now reveals details in the asymmetric and not point-like source structure, which are as small as 36 million km. This corresponds to dimensions of only 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.

Schematic diagram of the VLBI observations of Sgr A* from 2013. The insets show possible shapes of the source emission that are consistent with the measurements.

Eduardo Ros, Thomas Krichbaum (MPIfR)

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.

Original Paper:

Ru-Sen Lu et al.: Detection of Intrinsic Source Structure at ∼ 3 Schwarzschild Radii with Millimeter-VLBI Observations of Sagittarius A*, 2018, The Astrophysical Journal, Vol. 859, No. 1 (DOI: 10.3847/1538-4357):

Local Contact:

Dr. Ru-Sen Lu
Max-Planck-Institut für Radioastronomie, Bonn.
Fon: +49 228 525-292

Dr. Thomas Krichbaum
Max-Planck-Institut für Radioastronomie, Bonn.
Fon: +49 228 525-295

Dr. Norbert Junkes,
Press and Public Outreach
Max-Planck-Institut für Radioastronomie, Bonn.
Fon: +49 228 525-399

Weitere Informationen:

Norbert Junkes | Max-Planck-Institut für Radioastronomie
Further information:

Further reports about: ALMA APEX Atacama MPIfR Max-Planck-Institut Radioastronomie Sagittarius A* black hole darkness

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