Space and Physics
PUBLISHED
Neutron star collisions are pretty important in the universe, and, perhaps surprisingly, also in our everyday lives. A substantial fraction of heavy elements that regulate functions in our bodies, as well as precious and rare metals, are formed when two of these extraordinarily dense objects merge into one.
According to a new paper currently awaiting peer review, we might have found the aftermath of one such collision right here in our galaxy – if confirmed, it would be the first to be discovered in the Milky Way. The remnant's name is HESS J1507−622, and since it was spotted in 2009, it has been an odd duck.
Neutron stars are the product of certain supernovae, the explosions that happen when massive stars bigger than the Sun come to the ends of their lives. The ultra-dense core such cosmic booms leave behind packs more than the mass of the Sun into a city-sized sphere, causing protons and electrons to smush together under their own gravity, their positive and negative charges cancelling out and leaving only neutrons.
Two become one
When two neutron stars orbit each other, they may eventually collide and merge. The resulting interaction is yet another big boom, something called a kilonova. But while that name feels like it should belong to a burst of light 1000 times brighter than a supernova, kilonovae are actually the dimmer of the two phenomena.
The kilo part of the name is because their peak luminosity is about 1,000 times brighter than an entirely separate class of explosion, a regular nova. These occur when small, dense stars called white dwarf steal material from another star, causing that material to ignite and glow more brightly.
Still, that's not to say that kilonovae are small potatoes; each one releases an amount of energy in just a few instants that it would take the Sun hundreds of thousands of years to produce.
Part of that is released in a bright flash of gamma rays, an event known as a gamma-ray burst. These are usually very short-lived when produced by neutron star mergers, lasting less than two seconds. This was confirmed directly in observations of GW170817, a collision between neutron stars in the shell elliptical galaxy NGC 4993, about 140 million light years away.
So that makes it a little odd that the gamma-ray emission from HESS J1507−622 continues to this day, though at a much lower intensity than what we would expect from the burst itself. Yet HESS J1507−622 also doesn’t look like any other kind of gamma-ray source – of which there are several – lacking an associated X-ray or radio-wave counterpart, which should usually come along as a kind of package deal.
To explain the emission, researchers have previously proposed that the source was indeed a kilonova, just one that is producing gamma rays by a totally different route. They suggested that when the two neutron stars collided, they would have ejected a lot of highly accelerated protons, launching them into the interstellar medium where they would collide with other material and produce gamma rays.
Unfortunately, the modelled kilonova would have needed a lot more energy if it were to accelerate protons sufficiently to match the gamma rays we are seeing.
The new work suggests an alternative. Since electrons are over 1,800 times lighter than protons, a kilonova of the size we expect would have been able to accelerate them to energies high enough to produce our anomalous gamma rays.
If this scenario is confirmed by other supporting evidence, it would make HESS J1507−622 the first known kilonova remnant in the Milky Way. It is between 12,000 and 47,000 light-years from Earth, and it might have exploded between 200 and 3,000 years ago (from our point of view).
A paper describing this proposed scenario has been submitted to The Astrophysical Journal Letters and is available on the ArXiv.
