Event horizon telescope: after two black holes, he immortalized a very distant quasar

Event horizon telescope

Eht , acronym of Event horizon telescope , is the international scientific collaboration which, for the first time in the world, has given us the first true image of a black hole . Indeed, two: the first time in April 2019, when the telescope (or, better, the network of telescopes), immortalized M87 , the supermassive black hole located in the center of the galaxy Messier 87 , about 55 million light years away from the Earth weighs just over 7 billion solar masses; the second time last June, when Sagittarius A, the supermassive black hole at the center of our galaxy, was instead "photographed". Both works had (and have) a scientific scope that goes well beyond the (suggestive) images produced: the one on Sagittarius, for example, unequivocally confirmed that the one at the center of our galaxy is a black hole (a hypothesis that actually was not the only one in the field) and has allowed us to study many characteristics, improving our understanding of the underlying dynamics. Today Eht is making headlines again for a new study, published in The Astrophysical Journal , relating to an object that is probably even more mysterious than black holes: the so-called quasars , a curious acronym of QUASi-stellaR radio source , among the light sources brightest lights in the Universe , whose origin and behavior is still largely mysterious . The authors of the work, in particular, were able to "photograph" NRAO 530, a quasar about seven and a half billion light years away from the Earth, with an incredible spatial resolution (of the order of a light year): the study, hopefully , will provide us with valuable information on how these exotic cosmic objects work.

What are quasars

The first quasars - a term coined by astrophysicist Hong-Yee Chiu in 1964 - were observed, again using radio telescopes, in the early 1960s. In the beginning they were thought to be objects very similar to stars (hence the name); only in the eighties were other models proposed according to which they were rather active galaxies, whose enormous brightness is due to the friction caused by gas and dust falling into a supermassive black hole forming the so-called accretion disk. To date this is still the most accredited theory: "Quasars - explains Rocco Lico , postdoc researcher at the Instituto de Astrofisica de Andalucía , in Spain, associated with the National Institute of Astrophysics (Inaf) and co-author of the work (awarded from the EHT collaboration with one of the Early-career Awards 2022) - are supermassive elliptical galaxies that have a supermassive black hole at their center (i.e. whose mass is in the order of millions or billions of solar masses). The black hole rotates and attracts gas and dust, forming around itself an accretion disk. The action of strong magnetic fields causes part of these gases to be 'thrown out' forming huge relativistic jets ". The quasar just observed, in particular, belongs to the category of blazars, i.e. quasars whose jets are oriented along the line of observation, emit in gamma rays and was discovered in 1966 by the Green Bank Telescope.

The quasar NRAO 530 “photographed” by Eht with different signal processing methods

S. Jorstad, M. Wielgus, et al., Apj, 2023

The Event horizon telescope

The Event horizon telescope, the instrument that allowed the discovery, is a network of radio telescopes around the world whose signals are then processed and combined in order to have as a result an "image" coming from a single "virtual" telescope. EHT has eight radio telescopes: the Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Pathfinder Experiment (Apex), in Chile; the Large Millimeter Telescope Alfonso Serrano (Lmt), in Mexico; Iram, in Spain; the Submillimeter Telescope (Smt), in Arizona; the James Clerk Maxwell Telescope (JCMT) and Submillimeter Array (SMA), in Hawaii; the South Pole Telescope (Spt), in Antarctica.

A "collateral" discovery

"Actually - says Lico - the observation of NRAO 530 is not the main target of the Event horizon telescope. We studied it because we needed it to 'calibrate' the instrument and then use it to observe Sagittarius A*. In fact, this quasar has characteristics that make it perfect for calibration: it is a compact source and does not have much temporal variability " . Before the observations, in fact, the telescopes that are part of the EHT network must be calibrated to understand what the "noise" of the instruments is and subtract it from the observed signal: to do this it is necessary to point them towards a source that is as "uniform" as possible. Just like NRAO 530. In addition to providing useful data for the calibration process, however, the quasar also proved to be very interesting due to its nature: "We observed - continues Lico - some very interesting substructures. In particular, the images we took reveal the presence of a bright and compact region, the so-called 'radio core' of the source, from which a relativistic jet extends for about 60 micro-arcseconds. Core and jet show substructures on the light-year scale. Furthermore, we were able to obtain more information on the magnetic field, which is 'twisted' along the jet, just as we expect from a rotating black hole and as already observed in other quasars.This result will allow us to better understand what the 'engine' of quasars is and how it works , and in particular what is the role of the magnetic field in the process of formation of relativistic jets and the connection with the central black hole" .