Sometimes in astronomy, you get lucky. The sky is big, and many astronomically interesting events are extremely rare and brief. This means you have to be looking at exactly the right place at the right time and with the right instrument to detect key events.
We can increase our chances of catching rare events by keeping survey scopes crawling the sky for flickers of light, and today we have observatories like the Neil Gehrels Swift Observatory keeping watch in gamma-ray light, and the Zwicky Transient Facility is out there watching in visible light. At the same time, many of the world’s telescopes have agreements in place that say “in case of something awesome, drop what you’re doing and go look”.
Back in May, this combination of watching with Swift and dropping everything to go look with just about everything else may have allowed the astronomical community to catch the birth of a magnetar, a special kind of neutron star that has a massive magnetic field capable of releasing tremendous bursts of energy.
It all started with a short flash of gamma-rays. Short gamma-ray bursts (GRBs), events that release massive amounts of energy for less than two seconds, are caused by two neutron stars merging into a new, single object. These events are called kilonovae, and back in August 2017, one such event was observed in light, gravitational waves, and neutrinos. That historic event produced a black hole but it now appears that in some instances, the merging neutron stars take a different, more luminous path.
When the Swift telescope spotted that new short gamma-ray burst in May, it triggered a myriad of telescopes to redirect their efforts to its location in the sky. From Hubble to the Very Large Array (VLA) to Keck, these scopes and many more all measured everything they could, and because Hubble was able to be involved, the observing team quickly realized that the afterglow of light was ten times more luminous than expected. If these merging objects had formed a black hole, the only source of light would have been the warm material shocked during the kilonova. If the merging neutron stars were small enough, however, they could instead form a new, larger neutron star, which would not only be glowing hot but would be transferring energy to the surrounding material via its strong magnetic field.
According to study co-author Tanmoy Laskar: You basically have these magnetic field lines that are anchored to the star [and] that are whipping around at about a thousand times a second, and this produces a magnetized wind. These spinning field lines extract the rotational energy of the neutron star formed in the merger, and deposit that energy into the ejecta from the blast, causing the material to glow even brighter.
Magnetars are fairly rare objects that are thought to most often form via the death of a single massive star. This event – the merger of two smaller neutron stars – shows that magnetars can form in more than one way. It’s thought that this is a rarer way for magnetars to form, but we will need to keep looking and keep getting lucky in order to get a better understanding of the real statistics. For now, this research is published in the Astronomical Journal with lead author Wen-fai Fong.
“The Broad-Band Counterpart of the Short GRB 200522A at z = 0.5536: A Luminous Kilonova or a Collimated Outflow with a Reverse Shock?” W. Fong et al., 2020 Nov. 12, Astrophysical Journal (preprint on arxiv.org)