How the Event Horizon Telescope is shining light on black holes

Early this July, a team of radio astronomers located a supermassive black hole, 55 million times as massive as the sun, in a neighbouring galaxy Centaurus A, 165000 light years away. (One light year is the distance covered travelling at a speed of light, that is, 300000 km per sec.) The findings published in the journal Nature Astronomy show the exact location of the black hole in the heart of the galaxy.

The findings reveal the birth of a gigantic radiation jet emanating from the black hole’s core — a signature phenomenon occurring around them. However, the laws of physics defining these events are still shrouded in mystery, and astronomers are actively researching to understand them. The present study has captured images of the galaxy’s core in unprecedented detail with the help of the Event Horizon Telescope (EHT).

When stars die, their matter shrinks, packing into a tiny space compared to the original size. As mass shrinks, the gravity becomes dense, to a point where the mass collapses within itself, forming a black hole. Neighbouring stars, gases, dust and space objects spiral around the gravity-dense hole at the speed of light; those closer to the centre get sucked into the abyss.

Nothing entering a black hole can escape it – not even light; however, the process releases an enormous amount of energy. Surprisingly, some matter at the edge of the black hole gains the velocity to ‘escape’ the gravitational pull and bursts out as intense radiation, spewing charged matter far out into the galaxy.

The threshold around the edge of a black hole where the escape velocity exceeds the speed of light is called the event horizon. As there is no light, black holes are not visible; the radiations are detectable only at the outer edges of the jet, which appear as a halo around the dark core. Currently, the event horizon is the only way astronomers can detect black holes that indicate an active galaxy.

The earth-sized telescope

The radiations emanating from the event horizon are radiofrequency wavelength, and owing to the vast distances, the signals are faint. Moreover, the signals are almost vertical by the time they reach earth. Therefore, to focus on the enormous events occurring millions of light-years away, we need telescopes with angular resolutions comparable to the size of the event horizon.

In other words, earth-sized (or more), powerful radio telescopes alone can capture the happenings at the event horizons in their entirety. However, realising such a large practical telescope is challenging. So, in 2009, astronomers devised a clever technique and synchronised several radio antennae and formed the Event Horizon Telescope project. Together they function as one virtual earth-sized telescope.

The EHT is a sum of parts, integrating data from eight extensive telescope facilities, scattered across the globe, all pointing towards the target galaxy.

Each of the telescope facilities has arrays of radio antennae that pick up the faraway signals. The observations are performed by a technique called Very Long Baseline Interferometry (VLBI). Algorithms and statistical methods unify the data from the individual facilities, providing high image resolutions to see the faraway objects. A global team of more than 300 scientists keep a constant vigil for the signals and analyse them.

Just the beginning

From 2009 to 2017, the EHT team has been collecting data on neighbouring galaxies and archiving it. In 2017, the team conducted extensive week-long observations on a galaxy named M87; they analysed the collected data, which revealed the first-ever images of a black hole to the world in 2019. The historic moment was just the beginning for many more iconic observations, which helps us understand the enigma surrounding black holes.

With ongoing analyses of archival data, experts are unravelling several unknown facets of black holes. For example, analysing the behaviour of the black hole images over the years, they found that the black hole of the M87 galaxy has a persistent, crescent-like shadow around it.

The finding was consistent with the prediction of Einstein’s general theory of relativity. However, the data has also shown wobbly rings around the black hole, which give a glimpse of the dynamic structure of the event horizon. In another analysis, they imaged the magnetic fields close to the edge of the black hole – another first-ever feat.

In the current study, the team has revealed stunning images of the galaxy Centaurus A, with ten-fold higher frequency and sixteen times sharper image resolutions (reportedly, a magnification factor of a billion). The pictures show how massive jets of radiation form at the edge of black holes. This study will help solve the mystery of how jets are launched at the black hole’s core.

The next-gen telescopes

The EHT consortium will expand further — by synchronising with more ground-based radio telescopes and arrays of satellite observatories. Currently, three new telescope arrays have been integrated into the system. With this strong contingent, astronomers envisage conducting further challenging investigations like rigorously testing Einstein’s theory of relativity and the origin of cosmic rays.

Scientists believe that the jets emanating from black holes could be the source of cosmic rays that constantly bombard the earth. Analysing black holes in greater detail and investigating the acceleration of the particles from the jets will enable us to understand the effect of black holes on the environment.

In April, the team targeted M87 again, one Sagittarius A* in our galaxy, and many more distant ones. The results of these observations will unfold in the coming years opening new vistas about the universe’s machinations.