In April 2019, scientists released the first image of a black hole in the galaxy M87 using the Event Horizon Telescope (EHT). However, that remarkable achievement was just the beginning of the science story to be told. McGill University astronomers were part of this global effort.
Data from 19 observatories are now being released that promise to give unparalleled insight into this black hole and the system it powers, and to improve tests of Einstein’s General Theory of Relativity.
“We knew that the first direct image of a black hole would be groundbreaking,” said Kazuhiro Hada of the National Astronomical Observatory of Japan, a co-author of a new study published recently in The Astrophysical Journal Letters describing the large set of data. “But to get the most out of this remarkable image, we need to know everything we can about the black hole’s behavior at that time by observing over the entire electromagnetic spectrum.”
Light from darkness
The immense gravitational pull of a supermassive black hole can power jets of particles that travel at almost the speed of light across vast distances. M87’s jets produce light spanning the entire electromagnetic spectrum, from radio waves to visible light to gamma rays. This pattern is different for each black hole. Identifying this pattern gives crucial insight into a black hole’s properties (for example, its spin and energy output), but this is a challenge because the pattern changes with time.
Scientists compensated for this variability by coordinating observations with many of the world’s most powerful telescopes on the ground and in space, collecting light from across the spectrum. This is the largest simultaneous observing campaign ever undertaken on a supermassive black hole with jets.
McGill Professor Daryl Haggard, alongside graduate student Hope Boyce, helped lead the acquisition and analysis of the Chandra X-ray data, and coordination of the multi-wavelength campaign. The X-ray data, in particular, are key for understanding the total energy output of the core (near the black hole) and the jet, and to figuring out whether the black hole is in the midst of an outburst or in a quiet state.
“Luckily, during these observations, the black hole relatively quiet, allowing us a look at the typical hot material in the vicinity of the black hole,” said Haggard, who is an Associate Professor in the Department of Physics and the McGill Space Institute and holds the Canada Research Chair in Multi-Messenger Astrophysics. “These observations give us a first detailed view of the regions where the jet and the hot material near the event horizon link together, offering insight into how such a huge jet (spanning 1,000,000 light years, yes, that’s a million light years!) can be created by black hole whose size matches that of our Solar System (a mere 0.01 light years).”
A global effort
The data were collected by a team of 760 scientists and engineers from nearly 200 institutions, spanning 32 countries or regions, and using observatories funded by agencies and institutions around the globe. The observations were concentrated from the end of March to the middle of April 2017.
Each telescope delivers different information about the behaviour and impact of the 6.5-billion-solar-mass black hole at the centre of M87, which is located about 55 million light-years from Earth.
This simultaneous, multi-wavelength data set gives researchers a ‘snapshot’ of the black hole and its enormous jet at the same moment in time, so scientists can see how the energy and material from black hole and jet are linked, without confusion introduced by variability.
“This incredible set of observations includes many of the world’s best telescopes,” said co-author Juan Carlos Algaba of the University of Malaya in Kuala Lumpur, Malaysia. “This is a wonderful example of astronomers around the world working together in the pursuit of science.”
Improving tests of Einstein’s theory of relativity
The first results show that the intensity of the light produced by material around M87’s supermassive black hole was the lowest that had ever been seen. This produced ideal conditions for viewing the ‘shadow’ of the black hole, as well as being able to isolate the light from regions close to the event horizon from those tens of thousands of light-years away from the black hole.
The combination of data from these telescopes, and current (and future) EHT observations, will allow scientists to conduct important lines of investigation into some of astrophysics’ most significant and challenging fields of study. For example, scientists plan to use these data to improve tests of Einstein’s Theory of General Relativity. Currently, uncertainties about the material rotating around the black hole and being blasted away in jets, in particular the properties that determine the emitted light, represent a major hurdle for these GR tests.
So far, all observations of black holes have confirmed scientific suspicion that the theory of General Relativity (GR) works in this “strong gravity’ regime. To make this an even stronger statement and really test GR, we need to understand the hot material near the black hole and being driven out in the jet, and to do this, scientists need multi-wavelength observations like the ones presented here.
Understanding the origins of cosmic rays
A related question that is addressed by today’s study concerns the origin of energetic particles called “cosmic rays,” which continually bombard the Earth from outer space. Their energies can be a million times higher than what can be produced in the most powerful accelerator on Earth, the Large Hadron Collider.
The huge jets launched from black holes, like the ones shown in today’s images, are thought to be the most likely source of the highest energy cosmic rays, but there are many questions about the details, including the precise locations where the particles get accelerated. Because cosmic rays produce light via their collisions, the highest-energy gamma rays can pinpoint this location, and the new study indicates that these gamma-rays are likely not produced near the event horizon – at least not in 2017. A key to settling this debate will be comparison to the observations from 2018, and the new data being collected this week.
“Understanding the particle acceleration is really central to our understanding of both the EHT image as well as the jets, in all their ‘colours’,” said co-author Sera Markoff, from the University of Amsterdam. “These jets manage to transport energy released by the black hole out to scales larger than the host galaxy, like a huge power cord. Our results will help us calculate the amount of power carried, and the effect the black hole’s jets have on its environment.”
New observations taking place now
The release of this new treasure trove of data coincides with the EHT’s 2021 observing run, which leverages a worldwide array of radio dishes, the first since 2018. Last year’s campaign was canceled because of the COVID-19 pandemic, and the previous year was suspended because of unforeseen technical problems. Recently, EHT astronomers targeted the supermassive black hole in M87 again, the one in our Galaxy (called Sagittarius A*), together with several more distant black holes for six nights. Compared to 2017 the array has been improved by adding three more radio telescopes: the Greenland Telescope, the Kitt Peak 12-meter Telescope in Arizona, and the NOrthern Extended Millimeter Array (NOEMA) in France.
“With the release of these data, combined with the resumption of observing and an improved EHT, we know many exciting new results are on the horizon,” said co-author Mislav Baloković of Yale University.