In 2018, it was revealed that a pioneering telescope the size of Earth had taken, for the first time, an image of a black hole. That same instrument, the Event Horizon Telescope (EHT), has now witnessed the same black hole erupt with a powerful and unexpected explosion. Scientists hope that by studying this emission, they can better model the structure surrounding supermassive black holes.
The flare, which lasted for about three days in April and May 2018, erupted from the supermassive black hole designated M87*, which lies at the heart of the galaxy M87, located around 55 million light-years away. The 25 ground-based and orbiting telescopes that comprise the EHT saw the outburst as high-energy light called gamma rays.
Not only was this the first time M87* had flared since 2010, but the eruption was also more energetic than typical flares from this black hole.
Supermassive black holes are thought to exist at the center of all large galaxies, including our own, the Milky Way.
M87* stands out from the Milky Way’s central supermassive black hole, Sagittarius A* (Sgr A*). Our home supermassive black hole has a mass equal to that of around 4.3 million suns, while M87* has a mass equal to about 5.4 billion suns!
But M87* also differs from Sgr A* as this more distant black hole is voraciously feeding. That feeding is responsible for the jets connected to high-energy flares, such as the gamma-ray eruption the EHT spotted in 2018.
“Together with the EHT’s submillimeter observations, the new data collected in multiple bands of radiation offer a unique opportunity to understand the properties of the gamma-ray emission region, link it to potential changes in the M87 jet, and enable more sensitive tests of general relativity,” project leader and University of Trieste researcher Giacomo Principe said in a statement. “These observations can shed light on some of the main questions of astrophysics that are still unsolved.
“How do the powerful relativistic jets that are observed in some galaxies originate? Where are the particles responsible for the emission of gamma rays accelerated? What phenomenon accelerates them to energies of the trillions of electron volts? What is the origin of cosmic rays?”
Black holes are messy eaters
What really sets black holes apart is the preponderance of matter that surrounds them. While some like Sgr A* exist in relatively empty larders (if our black hole was a human, it would exist on a diet of one grain of rice every million years), others like M87* have an abundance of matter to feed upon.
Yet, though our image of a black hole is one of an all-consuming, all-devouring cosmic titan from which nothing escapes, supermassive black holes like M87* are actually quite wasteful eaters. Like grumpy toddlers, most of the food intended for these black holes ends up violently flung away.
The matter that surrounds supermassive black holes exists in a flattened cloud called an accretion disk and in the form of superheated gas called “plasma” because it still has angular momentum, or spin. This angular momentum also means that this plasma can’t fall directly to the black hole; instead, it swirls around the central supermassive black hole and is gradually fed to it.
However, supermassive black holes are also surrounded by powerful magnetic fields. These channel material from the accretion disk to the poles of the black hole. At some point, these particles are accelerated to near-light speeds and are blasted out as high-energy jets.
These jets are accompanied by bursts of electromagnetic radiation like the gamma-ray flare that the EHT witnessed.
The energetic outburst from M87* seen by the EHT showed that the near light-speed jet erupting from around this black hole is extended to a surprising distance.
The jet is tens of millions of times wider than the black hole itself. The size difference is so vast it is akin to a blue whale erupting from a single bacteria.
Quite how black holes launch these jets is still something of a mystery, one which EHT scientists hope these observations could help to get to the bottom of.
“In particular, these results offer the first-ever chance to identify the point at which the particles that cause the flare are accelerated, which could potentially resolve a long-standing debate about the origin of cosmic rays (very high-energy particles) from space detected on Earth,” Principe continued.
The EHT is all about collaboration between instruments, and these results are a striking example of that.
In addition to the telescopes that already combine to turn the EHT into an Earth-sized telescope, this campaign turned to space-based instruments like Fermi, NuSTAR, Chandra, and Swift.
“Fermi-LAT has revealed a notable increase in flux in the same period as the other observatories, helping to identify the region of gamma-ray emission during these increases in brightness,” Fermi-head Elisabetta Cavazzuti said. “M87 It is a laboratory that demonstrates once again the importance of having coordinated observations at multiple wavelengths and also well sampled to fully characterize the spectral variability of the source, variability that probably extends over different time scales, with the widest possible vision, complete across the entire electromagnetic spectrum.”
Thanks to the collaboration between these and other telescopes, scientists were able to distinguish a clear change in the angle of the jet from the nucleus of M87. This seems to occur on an annual basis.
The team also noticed correlated changes in the event horizon, the light-trapping outer boundary of every black hole. This suggests a connection between event horizons and the powerful jets launched by black holes.
“In the first image during the 2018 observational campaign, it was seen that this ring was not homogeneous, and therefore had asymmetries (i.e., brighter areas),” Principe concluded. “The subsequent observations conducted in 2018 and linked to this publication scientific studies confirmed the data, but highlighted that the position angle of the asymmetry had changed.”
The team’s research was published on Friday (Dec. 13) in the journal Astronomy & Astrophysics.
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