Neutron star collision sheds new light on short gamma ray bursts
Scientists have detected a short burst of gamma rays from the collision of two neutron stars that put out more energy in a half-second than the Sun could in its 10-billion-year lifespan. Designated 200522A, the flash originated 5.47 billion light-years from Earth and glowed with a brightness 10 times greater than a nova after it resulted in the formation of a new, highly magnetic, neutron star.
Gamma ray bursts are extremely high energy phenomena that occur during events like the implosion of a supernova. These bursts can last for up to several hours, but when they go for less than two seconds, they're called short gamma ray bursts. Until recently, these short bursts were thought to be due to either the collision of two neutron stars generating an enormous, brief burst of radiation before collapsing into a black hole, or the collision of a neutron star and a black hole, which absorbs the star.
Either way, you end up with a black hole – at least, that's what the conventional theory suggested. Then, on May 22, 2020, NASA’s Neil Gehrels Swift Observatory detected 200522A. Other observatories were brought on, including the Hubble Space Telescope, the Very Large Array (VLA), the Las Cumbres Observatory Global Telescope (LCOGT), and the W.M. Keck Observatory.
These observed the short gamma ray burst across the entire electromagnetic spectrum, from radio to X-rays. What the Hubble found was that the burst hadn't disappeared in the expected fashion. Instead, it was glowing in the infrared band of the spectrum 10 times brighter than anticipated. This means that instead of forming a black hole, the collision of the neutron stars produced a new star called a magnetar.
Neutron stars are stars that have about the same mass as the Sun, but have a diameter of only about 12 miles (20 km). The expectation was that when two of these stars spiral into one another, the collision should produce a burst of gamma rays, create heavy elements like uranium and then an afterglow called a kilonova, which is 1,000 times brighter than a nova, though only a tenth the brightness of a supernova.
The problem is that 200522A is 10 times too bright or at the low end of the supernova scale. Instead of a black hole, the neutron stars merged, as did their magnetic fields, to form a magnetar – one massive star with one massive magnetic field. The field lines spin at a rate of thousands of times per second, extracting energy from the rotation of the star and pumping it into the matter ejected by the collision, causing it to glow brightly.
"Hubble really sealed the deal in the sense that it was the only one to detect infrared light," says Wen-fai Fong, an astronomer at Northwestern University and leader of the study. "Amazingly, Hubble was able to take an image only three days after the burst. You need another observation to prove that there is a fading counterpart associated with the merger, as opposed to a static source. When Hubble looked again at 16 days and 55 days, we knew we had not only nabbed the fading source, but that we had also discovered something very unusual. Hubble’s spectacular resolution was also key in disentangling the host galaxy from the position of the burst and to quantify the amount of light coming from the merger."
According to the study team, the James Webb Space Telescope and other instruments under development will be able to detect such events at even greater distances while providing more detailed spectrographic analysis.
The research will appear in The Astrophysical Journal. and the event is illustrated in the animation below.