Beyond our galaxy, a repeating gamma-ray burst was recently detected—a phenomenon that has never been observed before. Since, so far, no known theory can fully explain it, its true nature remains unknown.
Gamma-ray bursts (GRBs) are perhaps the most energetic explosions that can occur in the Universe. These unimaginably violent cosmic explosions were first detected by chance in the late 1960s, by U.S. military satellites designed to detect violations of the agreement between the U.S. and the USSR on the partial ban on nuclear testing. Since then, these mysterious flashes have been detected on numerous occasions, and according to all previous observations, they fall into two broad categories: in long-duration GRBs (longer than 2 sec) and in short-duration GRBs (shorter than 2 sec).
For several years now, astronomers have believed that short-duration GRBs are caused by the collision and merger of two neutron stars. This was confirmed as recently as 2017, when gravitational waves and the electromagnetic radiation released by the collision of two such stellar remnants were detected for the first time (for more information, see the article Collision of Neutron Stars). Long-duration GRBs, which are much more powerful, on the other hand, are associated with the explosive death of the Universe's giant stars, whose cores collapse into black holes. Finally, another phenomenon that can cause gamma-ray bursts is the tearing apart of a star by a black hole, in a phenomenon called Tidal Disruption Event.
This particular GRB was detected on July 2 by NASA's Fermi gamma-ray space telescope, which recorded not just one, but three bursts from the same source over the course of several hours. Subsequently, in fact, the researchers found that, according to data from the Einstein Probe X-ray space observatory, the source had already been active nearly a day earlier. This type of long-lasting and repeated gamma-ray burst had never been observed before. In fact, most GRBs last from a few milliseconds to several minutes at most, and precisely because the phenomenon that causes them is literally catastrophic for their source, gamma-ray bursts never recur.
As Antonio Martin-Carrillo, an astronomer at University College Dublin in Ireland and co-author of the related study recently published in the scientific journal *The Astrophysical Journal Letters*, further explains, “This phenomenon surprised us not only because it exhibited repeated intense activity, but also because it appeared to be periodic, something that has never been observed before.”
At first, astronomers thought the flare must have originated in our galaxy. Subsequent observations, however, using the VLT telescope and the Hubble Space Telescope, led them to conclude that the flare originated from another galaxy. The size and brightness of the host galaxy suggest that it may be billions of light-years away, but more data will be needed to determine this distance with greater precision.
As we mentioned earlier, one of the possible scenarios that astronomers are investigating in order to decipher the physical mechanism that caused this long-lasting and recurring gamma-ray burst is the collapse of a giant star. If, however, this is indeed the case, it would be a collapse entirely unlike any other we know of, because in that case its duration would not exceed a few seconds.
Alternatively, Martin-Carrillo explains, “the periodicity of the flares could be caused by a star being torn apart by the tidal forces of a black hole, that is, by a Tidal Disruption Event (TDE). However, unlike more typical TDEs, explaining the properties of this outburst would require an unusual star to be destroyed by an even more unusual black hole, likely an intermediate-mass black hole. “Both options would be unprecedented, making this phenomenon extremely unique.”
Given these facts, the origin of the GRB in the study in question remains unknown. The investigation, however, continues…
Photo: Artist's rendering of a gamma-ray burst (Credit: (ESO)
Source: Astronomers Spot Mysterious Gamma-Ray Burst, Unlike Any Detected Before | ESO