Flashes Of Light From The Hidden Side Of The Black Hole (Cosmology)

Observing the supermassive black hole at the heart of I Zwicky 1 – a galaxy with an active core 876 million light-years from us – a team of astronomers led by Dan Wilkins of Stanford University has noticed X-photon reverberations from all regions. of the accretion disk, including those “behind” the black hole. We talk about it with Elisa Costantini, astrophysicist at the Dutch space research institute Sron and co-author of the article published today in Nature

Looking the other side of the coin? Now it also applies to black holes, at least if we want to explain some of the behaviors of light in their accretion disk . The analysis of the X-ray emission coming from the supermassive black hole in the galaxy I Zwicky 1 highlighted the reflection of photons – visible as flashes of light – from the “hidden” side of the black hole. The variation in energy of these photons makes their origin evident in relation to space. Photons reflected from the opposite side of the accretion disk are “bent” around the black hole and magnified by the intense gravitational field. The study , published today in Nature, was led by a team of scientists from the universities of Stanford and Penn State in the US, Saint Mary in Canada and Sron, the Netherlands’ space research institute. Elisa Costantini , born in Rimini, graduated in Bologna and with a PhD in extraterrestrial physics, obtained in 2004 at the Max Planck Institute, in Germany , talks about it . After a post doc at the University of Utrecht, in the Netherlands, since 2008 she has been an astronomer associated with Sron.

First of all, let us guide us in the geography of a black hole: event horizon, accretion disk, crown.

“A black hole is an object about a hundred million times the mass of the Sun, compressed into a relatively small space. Suffice it to say that a black hole that has the same mass as the Sun would have a diameter of six kilometers. The event horizon is what defines the virtual surface of the black hole (the black hole does not have a solid surface) and is in fact the horizon beyond which not even light can escape its gravity – hence the black hole name. If the black hole is active, then it is constantly absorbing gas from its surroundings. This gas spirals around the black hole forming a disk. This phase of growth lasts a long time, about ten million years. In all this time the matter continues to grow, compact, heat up enormously, especially near the black hole. Such temperatures are reached – we are talking about millions of Kelvin degrees – that they ionize the gas, therefore they separate the electrons from the atom. These free electrons form a structure, which we call the corona, above the disk. We can sum it up by saying that there is an accretion disk around the black hole and a corona of electrons hovering above ».

Now that we know how to proceed, can you explain to us what your study revealed and what is the importance of the results achieved?

“In this study we have interpreted the change in brightness, as a function of time, of an active galactic nucleus, thus a galaxy hosting a supermassive black hole. The fact that an accretion disk varies in brightness is not an unusual fact, indeed, and in most cases it is not possible to give a certain explanation for these phenomena. So let’s say they are random, unpredictable. But this time we saw flashes of light in the X-band followed by others, weaker, detected at different energies. And this thing repeated itself twice: we had a flash and immediately after another with the same substructures, and it cannot be accidental. So, using a theoretical model that explains how photons behave in the presence of a black hole, we came to the conclusion that the secondary flashes were nothing more than photons reflected from behind the black hole. The light curves, therefore the variations in brightness as a function of time,

X-rays are emitted by a corona of energetic particles around the black hole. Some of these rays reach the observer directly, but others (red lines) illuminate the internal regions of the accretion disk (in orange) and their reflection from the disk (blue lines) is observed. The strong bending of light in the gravitational field around the black hole focuses the rays towards the black hole and on the inner regions of the disk. The rays reflected from the back of the disk can be bent around the black hole (in rotation), allowing X-rays to re-emerge from regions of the disk that would otherwise be hidden behind the black hole. Credits: DR Wilkins et al., Nature, 2021

How do you distinguish the photons arriving from one side of the disk and the other?

“The model tells us. The photons come from an emission line – the so-called iron line – which is produced at a certain energy and is tracer of the disc. Applying classical mechanics, the Doppler effect and general relativity to the model, we see that as a function of time the emission line can have a certain profile that can be found in the light curve ».

And the observation of these photons slightly “bent” around the black hole is in agreement with general relativity?

«Yes, because we started from the prescriptions given by general relativity. So we assumed that the environment around the black hole behaved this way. The data are not inconsistent with the theory. However, this is only a first step. As mentioned, this new methodology can be applied to many cases, extending and confirming the results ».

Where is the galaxy you studied, I Zwicky 1, located? And why did you choose to study his de black hole?

“It’s in the constellation of Pisces, and it’s 876 million light-years away from us. This galaxy is interesting from various points of view. Over the past fifteen years my collaborators and I have observed it three times with dedicated pointings of the ESA’s X-telescope, Xmm-Newton, and have discovered that its accretion disk emits a wind of gas traveling at more than 1000 km / if it behaves as a function of time in a peculiar way, unlike other objects we know. It is a galaxy that we were exploring even before the study started ».

Did the phenomenon you observed surprise you or was it your purpose from the start?

“The first author, Dan Wilkins of Stanford University, had started with a classical approach to studying the temporal characteristics of this black hole. As mentioned before, the fact of being able to interpret a phenomenon that in general we consider to be random or in any case an overlap of so many phenomena that it cannot be fully understood, was a real surprise ».

Featured image: Elisa Costantini, astronomer at Sron, the Netherlands space research institute, co-author of the study published today in Nature © INAF

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