NEUTRON STARS, KILONOVAE, AND MAGNETARS, OH MY!
WHEN STARS COLLIDE, the result is often explosive. That’s especially true when they are a pair of superdense stellar remnants like neutron stars. The fireworks show, called a kilonova, unleashes more energy than the Sun will produce during its 10-billion-year lifetime. Kilonovae shine as a result of the radioactive decay of heavy elements — like gold and platinum — that are produced during the merger and blasted outward. These events last less than two seconds and produce short gamma-ray bursts.
And wouldn’t you know it, the light from one such collision reached Earth on May 22, 2020. After traveling nearly 5.5 billion light-years, the brilliant flash was first detected by NASA’s Neil Gehrels Swift Observatory. Then, telescopes across the world quickly turned their eyes to the aftermath of the explosion.
As the most luminous kilonova event on record, the find was already groundbreaking, resulting in some of the most detailed observations to date. But data from the Hubble Space Telescope took that even further. When light from this event, GRB 200522A, reached Earth,
Hubble observed the event across the across a broad swath of the electromagnetic spectrum, finding the event’s infrared emissions 10 times greater than predicted. The most likely explanation, researchers say, is that the remnant of the collision was feeding energy into the emission.
“We don’t know the upper mass limit of neutron stars and the lower mass limit of black holes,” says
Wen-Fei Fong of Northwestern University, lead author of the study. “But if you take two neutron stars and smash them together — and you assume most of that mass ends up in the new object — then more than likely you’re going to be in the regime of black holes.” But not all of that mass is going into that new object, making it possible to instead form a so-called heavy neutron star. These heavy neutron stars are thought to be unstable, collapsing into a black hole in a handful of milliseconds.
Researchers suspect that the collision formed a heavy magnetar, creating the perfect storm to power the resulting kilonova. A magnetar’s magnetic field lines whipping around can deposit some of the rotational energy from the newly formed object into the ejecta, causing that material to glow brighter than expected.
If true, this will be the first time researchers have seen evidence of merging neutron stars giving birth to a magnetic monster. In order to know for certain, scientists will have to keep their eyes trained on this area of the sky. If a magnetar really is lighting it up, then within a few years, the ejected material from the burst will begin appearing in radio wavelengths.
“If we hadn’t looked with the right telescopes, we would have never known this was a weird feature,” says Fong. “I’ve been studying short gamma-ray bursts for over 10 years now, and I’m just amazed that [the universe] never ceases to throw surprises our way. So, I’m excited for when we start detecting more of these [gamma-ray] sources.”