Billions of years ago, two black holes much more massive than the Sun - 31 and 19 solar masses each - merged together in a distant galaxy. On January 4, 2017, these gravitational waves, traveling through the Universe at the speed of light, finally reached Earth, where they squeezed and stretched our planet into several atoms. This was enough for the two LIGO detectors in Washington DC and Louisiana to pick up the signal and reconstruct exactly what happened. For the third time in history, we have directly observed gravitational waves. Meanwhile, telescopes and observatories around the world, including those in Earth's orbit, were looking for a completely different signal: something like light or electromagnetic radiation that these merging black holes could emit.
An illustration of two merging black holes of comparable mass to those seen at LIGO. It is expected that such a fusion should produce very little electromagnetic signals, but the presence of a highly heated substance near such objects can change this.
According to our best physics models, merging black holes should not emit any light at all. A massive singularity surrounded by an event horizon can emit gravitational waves due to the changing curvature of space-time, as it revolves around another giant mass, and general relativity implies this. Since gravitational energy in the form of radiation must come from somewhere, the final black hole after the merger will be several solar masses lighter than the sum of the sources that generated it. This is completely in line with two other mergers that LIGO observed: about 5% of the original masses were converted into pure energy in the form of gravitational radiation.
The masses of known binary black hole systems, including three confirmed LIGO mergers and one merger candidate
But if there is something outside of these black holes, such as an accretion disk, a firewall, a hard shell, a diffuse cloud, or anything else, the acceleration and heating of this material can create electromagnetic radiation that travels along with our gravitational waves. After the first LIGO detection, the Fermi Gamma-ray Burst Monitor stated that it had detected a burst of high energy coinciding with the time of the gravitational wave signal. Unfortunately, the ESA satellite not only failed to confirm Fermi's results, but the scientists working there discovered a flaw in Fermi's analysis of their data, completely discrediting their results.
Merging of two black holes through the eyes of an artist, with an accretion disk. The density and energy of matter should not be enough here to create gamma rays or X-ray bursts, but who knows what nature is capable of.
The second merger did not show such hints of electromagnetic signals, but this is not surprising: black holes were significantly lower in mass, so any signal they generated would be correspondingly lower in magnitude. But the third merger was also large in mass, more comparable to the first than to the second. Although Fermi said nothing, and ESA's Integral satellite also remained silent, there were two hints that electromagnetic radiation might have occurred. The AGILE satellite of the Italian Space Agency recorded a faint, short-lived flare that occurred half a second before the merger at LIGO, and X-ray, radio and optical observations combined were identified strangely.
If any of this could be attributed to the merging of black holes, it would be completely incredible. We know so little about black holes in general, what can we say about merging ones. We've never seen them with our own eyes, although the Event Horizon Telescope will kind of take a picture before the end of this year. We found out just this year that black holes do not have hard shells surrounding the event horizon, but this fact was also statistical. So when it comes to the possibility that black holes might have electromagnetic leaks, it's worth keeping an open mind.
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Distant, massive quasars exhibit supermassive black holes in their cores, and their electromagnetic leaks are easy to detect. But we have not yet seen merging black holes (especially those with low masses, less than 100 suns) emit anything that can be detected.
Unfortunately, none of these observations provide the necessary data to lead us to conclude that merging black holes can emit anything in the electromagnetic spectrum. In general, it is quite difficult to obtain convincing evidence, since even the twin LIGO detectors, working with incredible accuracy, cannot pinpoint the location of the gravitational wave signal with more accuracy than up to a constellation or three. Since gravitational waves and electromagnetic waves travel at the speed of light, it is extremely unlikely that there will be a nearly 24-hour delay between the two. In addition, the transient event occurs at a distance that does not allow it to be associated with a gravitational wave.
Observatory area of the AGILE observatory at the time of LIGO observations with the possible location of the gravitational wave source shown in purple outlines
AGILE's observations could potentially hint that something interesting is happening. The moment the gravitational wave event was detected, AGILE was aimed at an area of space that contains 36% of the LIGO study area. According to scientists, "the excess of detected X-ray photons" appeared somewhere above the usual average background. But looking at the data, the first question scientists are asking is: How convincing are they?
Seconds before the LIGO merger, they pulled out an interesting event labeled E2 in the three charts above. After a thorough analysis in which they correlated what they see and what kind of random fluctuations can occur naturally, they concluded that something interesting had happened with a 99.9% probability. In other words, they saw a real signal, not a random fluctuation. There are many objects in the Universe that emit gamma and X-rays that make up the background. But can the incident be linked to the gravitational merger of two black holes?
Computer simulations of two merging black holes with the production of gravitational waves. The question is, does this signal accompany any electromagnetic burst?
If so, why didn't the other satellites see it? At the moment, we can conclude that if black holes had an electromagnetic part, it:
- extremely weak
- is born only at low energies
- has no bright optical, radio or gamma ray component
- does not occur simultaneously with the release of gravitational waves.
Binary black holes of 30 solar masses, first detected by LIGO, are difficult to form without direct collapse. Now, when they have already been observed twice, it became clear that such pairs of black holes are quite common. Do they have electromagnetic radiation?
In addition, everything we see fits perfectly with the fact that merging black holes do not have an electromagnetic part. But could this be because we do not have enough data? If we build more gravitational wave detectors, see more mergers of high mass black holes, better locate them, see more transient events - we can find out the answer to that question. If missions and observatories that should collect such data are built, commissioned and put into orbit, if necessary, then in 15 years we will receive scientific confirmation.
ILYA KHEL