GW170817, detected on August 17, 2017, was the first multi-messenger astronomical source, seen in gravitational waves and across the whole electromagnetic spectrum. Much of the physics of this source has been understood thanks to the high quality data collected for months after the initial detection. We now know that it was due to the collision between two neutron stars, a class of very massive and compact stars that were in orbit around each other and eventually merged forming a black hole. During the collision material was flung out in all directions. Most of the material was sent in the equatorial direction, where new atoms – such as gold and platinum – were formed through rapid neutron capture. Some material was sent in the polar direction, but exactly how much and with what energy is not known, since our observing geometry is far from the polar axis. For that reason, it had been impossible to ascertain whether a short gamma-ray burst also took place with the star collision.

Short gamma-ray bursts are some of the brightest explosions recorded in present day universe. They are produced when extremely fast outflows are sent in our direction by leftover material that accretes onto a newly formed black hole. Scientists believe they should be caused by a neutron star collision, but direct evidence is not yet available. When we detect  the burst directly, it is so bright that outshines all the signs of the neutron star collision. Groundbreaking research performed by the astrophysics group led by Dr. Lazzati and accepted for publication in Physical Review Letters, however, has shown that the unusual increase of the luminosity of GW170817 over time is a sign that a short GRB did happen right after the merger, albeit along a different direction. The figure displays the model predictions (solid lines) along with the observations (symbols), showing the excellent agreement of the model with the data.