The APEX 12-meter Radio Telescope in Chile has been equipped with specialized equipment, including a broadband recorder and a stable hydrogen-maser clock to perform common interferometric observations with other telescopes at wavelengths as small as 1.3mm and aiming for the ultimate image to get the black hole shadow. The addition of APEX to the Event Horizon Telescope (EHT), which until recently consisted only of antennas in the Northern Hemisphere, shows new and unprecedented details in the structure of Sgr A * in the center of the Milky Way. The increased angular resolution of the APEX telescope now reveals details in the asymmetric and non-point source structure, which is only 36 million km in size. 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).
The results are published in the Astrophysical Journal
Astronomers are seeking the ultimate proof of Einstein's general theory of relativity, which is intended to provide a direct image of the shadow of a black hole. This is possible through the combination of radio telescopes spread across the globe using a technique called Very Long Baseline Interferometry (VLBI). The participating telescopes are at high altitudes to minimize interference from the atmosphere and remote locations with clear skies so that the compact Sagittarius A * (Sgr A *) radio source can be observed in the center of the Milky Way.
The research team observed Sgr A * in 2013 with VLBI telescopes at four locations. Telescopes include the APEX telescope in Chile, the CARMA array in California, the JCMT and the phased array SMA in Hawaii, and the SMT telescope in Arizona. Sgr A * was detected with all stations and the longest baseline length reached nearly 10,000 kilometers, indicating an ultra-compact and asymmetric (non-point-like) source structure.
"The APEX telescope's participation almost doubles the length of the longest baseline compared to previous observations, resulting in a spectacular resolution of just 3 Schwarzschild radii," says Ru-Sen Lu of the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, the main author of the publication. "It reveals details in the central radio source that are smaller than the expected size of the accretion disk," adds Thomas Krichbaum, initiator of mm-VLBI observations with APEX.
The location of APEX in the Southern Hemisphere improves image quality for a source as far south as Sagittarius A * (-29 degrees declination). APEX has paved the way for the inclusion of the large and extremely sensitive ALMA telescope in the EHT observations, which are conducted once a year.
"We worked hard at over 5000 meters to install the equipment to prepare the APEX telescope for VLBI observations at 1.3mm wavelength," says Alan Roy, also of the MPIfR, who controls the VLBI team at APEX. "We are proud of the good performance of APEX in this experiment."
The team used a model fitting procedure to examine the event horizon scale structure of Sgr A *. "We started to figure out what the horizon-scale structure might look like rather than just drawing general conclusions from our sampled visibility, and it's very heartening to see that fitting a ring-like structure is very consistent with the data, although we do not rule out other models can, for example, be a composition of bright spots, "adds Ru-Sen Lu. Future observations with more telescopes being added to the EHT will sort out remaining ambiguities in the imaging.
The black hole in the center of our galaxy is embedded in a dense interstellar medium that can affect the propagation of electromagnetic waves along the line from view. "However, interstellar scintillation, which in principle can lead to image distortions, is not a strongly dominant effect at 1.3 mm wavelength," says Dimitrios Psaltis of the University of Arizona, the EHT project scientist.
"The results are an important step in the evolution of the Event Horizon Telescope," said Sheperd Doeleman of the Harvard-Smithsonian Center for Astrophysics and director of the EHT project. "The analysis of new observations, including ALMA since 2017, will take us one step closer to imaging the black hole in the center of our galaxy."
The APEX telescope is involved in the keenest observation of all time
Ru-Sen Lu et al. Evidence of Intrinsic Source Structure at ~ 3 Schwarzschild Radii with Millimeter VLBI Observations by SAGITTARIUS A *, The Astrophysical Journal (2018). DOI: 10.3847 / 1538-4357 / aabe2e