A Cosmic “laser” Detected 8 Billion Light Years Away: What Is It?

On February 13, the arXiv database hosted one of the most exciting papers of the year for spatiophiles. Aimed towards the ends of the Universe, the MeerKAT radio telescope intercepted an extraordinary radio signal, deciphered by a team of researchers from the University of Pretoria. This emission, whose radiation traveled approximately 8 billion light years from the HATLAS J142935.3–002836 system, was so intense that it is among the most powerful ever observed at such a distance.

It was not a laser as the title suggests, but a maser (Microwave Amplification by Stimulated Emission of Radiation). What’s the difference? A laser is a beam of photons of light; A maser is a beam of microwave photons. For the most powerful of them, we use the term gigamaser to designate sources of exceptional brightness in a very narrow frequency band. This one clearly boxes in this category, establishing itself as one of the most extreme signals captured by our radio telescopes. Where could it have come from? About the death of a star? From an extraterrestrial civilization that tried to contact us?

The burning of HATLAS

In reality, MeerKAT captured signs of a collision between two massive galaxies in the HATLAS systemsaturated with gas. During the shock, which spanned millions of years, immense clouds of hydroxyl (OH) molecules were strongly compressed and agitated by gravitational interactions and turbulence in the environment. Hydroxyl is a molecule composed of an oxygen atom and a hydrogen atom, which, when placed in certain extreme physical conditions, passes into a particular energy state called ” of population inversion “. During the collision, that’s exactly what happened.

In this state, each molecule accumulates a surplus of energy which it is capable of releasing in the form of radiation. When a first molecule emits a radio photon, it can trigger stimulated emission in other molecules in the cloud. Each excited molecule can then release an identical photon in turn, set on the same frequency and in the same direction, gradually amplifying the signal.

This process transforms the gas cloud into an amplifying medium : the electromagnetic waves emitted by each molecule no longer add up randomly (which would cancel part of the signal), but are superimposed in phase. This synchronization creates constructive interference: the amplitude of the electromagnetic field can then increase very quickly as the signal passes through the cloud. The resulting radiation is a beam whose power is concentrated in a single direction (collimation phenomenon): this is why this megamaser was so powerful.

However, in the case of the HATLAS J142935.3–002836 system, the signal power benefited from a second level of amplification: the gravitational lensing effect. By crossing a massive galaxy in its trajectory, the beam crossed a gravitational field intense enough to locally bend space-time.

This deformation had the effect of a converging lens, amplifying and directing the signal observable from Earth. What MeerKAT captured is therefore the end product of double amplification: a quantum amplification within the gas cloud (the maser) and a geometric amplification on the galactic scale (the lens).

The echo of a titanic crash

According to the team’s calculations, the gigamaser emitted by HATLAS released a luminosity equivalent to 300,000 Sunsbut focused only on a single frequency band corresponding to the hydroxyl spectral lines. This is the integrated luminosity before lensing correction: OH molecules only emit at certain precise frequencies, which concentrates the signal energy on these narrow bands of the spectrum.

These lines are the characteristic signatures of the energy transitions of OH molecules, which only emit or absorb photons at certain frequencies. Not all of the signal’s power was distributed across the entire electromagnetic spectrum, which multiplied its perceived intensity by MeerKAT.

An insane power, therefore, but which has greatly diminished over the course of its journey. As the signal was sent when the Universe was in its adolescent phase (it was 5.8 billion years old), he crossed it surviving countless gravitational turbulences and chaotic magnetic fields, fspanning distances that our minds struggle to conceive. The MeerKAT antennas did not receive only a little tickle of the original signala masterful demonstration of gravitational optics and the power of our instruments, without which this gigamaser would have remained forever undetectable.