In astronomy, some of the most profound discoveries happen by accident. As the saying goes, “The most exciting phrase in science is not ‘eureka!’ but ‘that’s funny.’” This was certainly the case with Matus Rybak – a postdoctoral researcher at Leiden University – and his colleagues were observing RXJ1131-1231, a quasar located 6 billion light-years away in the constellation Crater. This active galactic nucleus (AGN) is a favorite among astronomers because of the supermassive black hole (SMBH) at its center and the fact that there is an intervening galaxy between it and Earth.
This galaxy magnifies the light from RXJ1131-1231 by a factor of three, a phenomenon known as macrolensing, related to “gravitational lensing.” When observing this quasar for variations in brightness, Rybak and an international team of astronomers witnessed a microlensing event caused by an intervening star. This “double zoom,” which the team stumbled upon by accident, allowed them to study the quasar more closely and resolve millimeter radiation emanating from an SMBH for the first time.
The research team was composed of astronomers from the Netherlands Institute for Space Research (SRON), the STAR Institute, the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), the Institute for Particle Physics and Astrophysics, the Institut de Ciències del Cosmos (ICC), the Institución Catalana de Investigación y Estudios Avanzados (ICREA), the Kapteyn Astronomical Institute, the South African Radio Astronomy Observatory (SARAO), the European Southern Observatory (ESO), and multiple universities.
Images of RX J1131-1231 taken by NASA’s Chandra X-ray Observatory and the Hubble Space Telescope. Credit: NASA/CXC/Univ of Michigan/R.C.Reis et al (X-ray)/NASA/STScI (optical)
In 2008, Rybak and colleagues – including co-authors Dominique Sluse from the STAR Institute and Frédéric Courbin from ICREA and the University of Barcelona – conducted pioneering research in microlensing in visible light. Between 2015 and 2020, they observed RXJ1131-1231 to look for signs of cold gas and monitor variations in its brightness. When they observed the quasar again recently using the Atacama Large Millimeter-submillimeter Array (ALMA), they saw that three images of the quasar varied in brightness independently of each other. As Rybak explained in a Netherlands Research School for Astronomy (NOVA) press release:
That’s a smoking gun for microlensing, a phenomenon that occurs when a star is located between the foreground galaxy and the observer. We immediately knew we had to pursue this further. The combination of microlensing and macrolensing reveals things that remain invisible even with the best telescopes in the world and with macrolensing alone.
The team’s observations revealed that the quasar flickers in millimeter radiation on timescales of years, which they think is coming from the hot, magnetic torus (the “corona”) surrounding the SMBH. Millimeter radiation has been observed near black holes before, but it was unclear if this was caused by gas and dust or if some other mechanism was at work. Based on their analysis, the team concluded that this radiation was mainly produced by cold gas and dust, which is a remarkable find. They will be conducting follow-up observations shortly using the Chandra X-ray Telescope.
Their ultimate goal is to study the temperature and magnetic environment close to the black hole, which is significant because those conditions are known to influence the surrounding galaxy. The team’s findings were presented in a paper recently accepted for publication in the journal Astronomy & Astrophysics.
Further Reading: NOVA, Astronomy & Astrophysics