An article published in the journal “Nature Communications” describes an analysis of the gamma-ray burst cataloged as GRB150101B. A team of researchers led by Eleonora Troja of NASA pointed out the similarity between this gamma-ray burst and the electromagnetic signals emitted by the event cataloged as GW170817, the neutron star merger detected at both electromagnetic waves and gravitational waves. A year after the sensational announcement of that event, the authors of this new research believe they have discovered another one of that type.

On New Year’s Day 2015, NASA’s Fermi gamma-ray telescope detected a gamma-ray burst that was cataloged as GRB150101B which was weak by the standards of that type of event and of short duration. This discovery was followed by the location of the event thanks to the Swift space observatory and this allowed a very quick start of follow-up observations with other instruments including the Chandra and Hubble space telescopes and the Discovery Channel Telescope (DCT) in Arizona.

The observations made at different electromagnetic frequencies made it possible to obtain a lot of information on the gamma-ray burst GRB150101B which were later compared with those collected during the extraordinary observation of a neutron star merger at electromagnetic and gravitational waves at the same time. The event GW170817 marked the beginning of a new era for astronomy, the one defined as multimessenger, integrating the growing branch based on the detection of gravitational waves with the classical one based on electromagnetic waves in a collaboration among several international scientific entities.

Eleonora Troja, a researcher at the University of Maryland and NASA, and her collaborators studied the event GRB150101B noting similarities with the gamma-ray burst occurred at the event GW170817. Both gamma-ray bursts were unusually weak and short-lived and were sources of blue light and X-rays that were long lasting. In both cases, most likely from Earth we saw an explosion of the type called kilonova that was off-axis, which means that their jet didn’t point exactly towards the Earth. These are the only two short and off-axis gamma-ray bursts discovered so far.

The image (X-ray: NASA/CXC/GSFC/UMC/E. Troja et al., Optical and infrared: NASA/STScI) shows three different perspectives on the event GRB150101B. At the center there’s the Hubble Space Telescope image showing the galaxy in which the event took place, at the top there are two Chandra space observatory X-ray images that shows that event on January 9, 2015 on the left, with a visible jet under it, and February 10, 2015 on the right, after the jet faded.

The event GRB150101B was detected only at electromagnetic waves and for this reason only today has come to the spotlight while the event GW170817 has been at the center of astronomers’ attention for one year. GRB150101B originated about 1.7 billion light years away while GW170817 was much closer, about 130 million light years away. This means that even if the LIGO and VIRGO experiments had been active, they would almost certainly not have detected the gravitational waves generated by GRB150101B.

The consequence is that there are less data on the event GRB150101B in quantity and quality. For example, it’s not possible to estimate the masses of the two objects that probably merged. There’s the possibility that it was the merger between a black hole and a neutron star that still caused what looks like a kilonova.

The study of the event GRB150101B remains important because it adds data about a type of phenomenon that until a year ago was only theoretical known as a kilonova. Luigi Piro of INAF, Rome, another author of this research, compared it to a DNA test in which some characteristics of the two events are compared as if they were genetic markers to recognize members of the same family. The LIGO experiment can’t currently detect events so far away and in any case it wasn’t active at the beginning of 2015 but the information gathered thanks to the event GW170817 allow to recognize other similar ones.

Gravitational wave astronomy has a very short life so there will be many developments. ESA tested its LISA Pathfinder space probe to verify the possibility of having a space observatory for gravitational waves and will develop the Laser Interferometer Space Antenna (LISA) project, however the mission should start no earlier than the 2030s. In the meantime, we can hope that other events of this type will be detected and it’s possible that more of them from the past weren’t recognized and will be reconsidered after an examination of archive data.