Paul M. Sutter is an astrophysicist at SONK Stony Brook and the Flatiron Institute, host of Ask an astronaut and Space Radio, and author of How to die in space. He contributed this article to Space.com’s Expert voices: opinions and insights.
Short gamma ray bars, which, as the name suggests, are short explosions of high-energy gamma rays, tend to appear far away from their host systems.
Astronomers have been thinking for years that this means they get a ‘kick’ when they are born. But new observations prove otherwise: we just missed all the stars in their environment.
Related: 8 Astonishing Astronomy Mysteries
Short and fast
It took astronomers a long time to find out what causes short gamma rays in the universe. These bursts, which lasted less than about two seconds, were first noticed by the U.S. military when they developed gamma-ray detectors to sniff out sneaky Soviet nuclear tests. When the whistleblowers went wild (and the panic disappeared), US officials realized that the cosmos in the distance was much more active than they were. Opponents of the Cold War.
The reason astronomers were so difficult was that short gamma-ray bursts are a) short and b) rare. It’s a nasty combination for a group of people who depend on seeing the same thing over and over again to deal with it.
Whatever caused these short bursts, it was cruelly energetic and relatively small, even smaller than a star. Astronomers can estimate the last bit based on the duration of the event. The average short gamma-ray burst event is about 0.2 seconds long, and if you want an object in space to do something (such as, for example, inflate), then its action is always limited by the speed of light. . If the event lasts 0.2 seconds, it means that one point of the object cannot be further than 0.2 light seconds from the other side. This is about four times the diameter of Earth.
When it comes to the secret, astronomers finally exploded an afterglow in 2005 due to a short gamma ray. Prior to the discovery of X-ray flashes that lasted for hours after the main event, all of the eruptions were isolated cases.
And it was only in 2017 when astronomers got the final, decisive idea, when a short gamma-ray event coincided with the detection of a gravity wave. The specific gravitational wave bore the signature of two neutron stars fusing together – a so-called kilonova.
Neutron stars: definition and facts
Far from home
Although astronomers eventually discovered what caused short gamma-ray bursts, one major mystery remained: their location. Unlike their prolonged cousins (long gamma-ray bursts), many of the short-lived ones tended to come from regions of the universe relatively far away from galaxies. They are not part of the normal star population.
Linking a powerful, rare event like this to its environment is a useful astronomical trick. Before we, for example, thoroughly understood what causes the different species supernovae, astronomers have noted that the type II class tends to come from elliptical and spiral galaxies, while type I basically comes from everywhere. It helped us understand their identity: Type II is derived from the death of massive stars, which are produced in abundance in star-shaped elliptical and spiral shapes, while Type I is derived from the destruction of white dwarfs, a much more common and long-lived object that can live anywhere.
Astronomers are therefore amazed at the location of very short gamma rays. They certainly come from stars (the neutron stars behind the kilonova events are the remaining core of large stars), but the short gamma-ray bursts were not embedded in a population of older stars … or any stars, for that matter. .
This has led astronomers to suspect that before neutron stars collide in a kilonova flash, complex dynamics ‘kick’ them out of their home and away from their host. galaxies. Then the neutron stars wander together in the lonely intergalactic depths, leading to a short gamma ray, that one explosion of light is the only sign of their existence.
Stacks and stacks of stars
Another possibility, as suggested by a paper recently appeared in the preprint journal arXiv, is that we misunderstand everything.
By far most stars in a galaxy are concentrated in the center or within a thin disk. Less than 2% of all stars are usually found in the region where the “halo, “which can range from 10.00 to 100,000 parsecs away from the galaxy. (One parsec is about 3.26 light-years.) Hence the simple reasoning that when we see that a large part of the short gamma ray bursts from within the halo and stars there are relatively rare, then the neutron stars that led to the eruption must have come somewhere else.
However, because galaxies are so bright and there are relatively few stars in the halo, it is difficult to measure the number of halo stars for a particular galaxy. Thus, when astronomers say something like ‘short gamma-ray bursts come from a very empty halo’, it is only true in an average, statistical sense.
The astronomers behind the new study looked very deeply and deeply into the deep galaxies that offered short gamma-ray bursts – and found that the events were not as isolated as they seemed. In all cases, they found populations of ancient stars extending from the galactic disk to the bursts of the bars. Those old stars probably have stellar remnants, like neutron stars, that would eventually collide in a kilonova explosion – and a concomitant short gamma ray.
The conclusion is that no kick is needed. There is not even a mystery. Short gamma-ray bursts that occur far beyond their host galaxies are there because there are actually more stars in their vicinity than we thought. Neutron stars are not randomly ejected from their host systems as often as we thought. Which, given how violent the universe can be, is perhaps the more surprising answer.
Learn more: “No velocity kicks are needed to explain large-distance offsets of Ca-rich supernovae and short GRBs“
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