Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask an astronaut and Space radio, and author of How to die in space. He contributed to this article at Space.com Expert voices: opinions and perspectives.
Short gamma-ray bursts, which, as the name suggests, are short bursts of high-energy gamma rays, tending to occur far from their host galaxies.
For years, astronomers have believed that this means they receive a “blow” when they are born. But the new observations prove otherwise: we were just missing all the stars in their vicinity.
Related: 8 mysterious astronomy mysteries
Short and fast
It took astronomers a long time to figure out what in the universe is causing short gamma-ray bursts. These explosions, which lasted less than about two seconds, were first observed by the United States military when it developed in-orbit gamma-ray detectors to sniff out Soviet nuclear tests. When the detectors went off (and the panic disappeared), American officials realized that the distant cosmos was much more active than Opponents of the Cold War.
The reason astronomers had such a difficult time was that short gamma-ray bursts are a) short and b) rare. This is an ugly combo for a group of people who depend on being able to see the same thing countless times to gain control.
Whatever caused these brief explosions was brutally energetic and relatively small, even smaller than a star. Astronomers could estimate the last bit based on the duration of the event itself. 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, say, explode), its actions are always limited by the speed of light. If the event lasts 0.2 seconds, then this means that one end of the object cannot be more than 0.2 light seconds away from the other end. This reaches about four times the diameter Earth.
Adding to the mystery, it wasn’t until 2005 that astronomers finally caught light after a brief gamma-ray burst. Before that discovery of X-ray flashes that lasted for hours after the main event, all the explosions were isolated businesses.
And it wasn’t until 2017 that astronomers got the last decisive clue, when a short gamma-ray event coincided with the detection of a gravitational waves. This special gravitational wave bore the signature of two merging neutron stars – the so-called kilonova.
Neutron stars: definition and facts
Far away from home
Even though astronomers have finally realized what caused short-range gamma-ray bursts, a great mystery remains: their location. Unlike their longer summers (long gamma-ray bursts), many of the shorter ones tended to come from regions of The universe relatively far from galaxies. They are not part of the normal stellar population.
Linking a powerful and rare event like this to its surroundings is a useful astronomical trick. For example, before you fully understand what causes the different types of Supernova, astronomers have observed that class II tends to come from elliptical and spiral galaxies, while type I comes from virtually anywhere. This helped us understand their identity: type II comes from the death of massive stars, which are made in abundance in elliptical and star-forming spirals, while type I comes from the destruction white dwarfs, a much more common and long-lasting object that can live anywhere.
And so astronomers were puzzled by the location of many short-range gamma-ray bursts. They certainly come from stars (the neutron stars behind the kilonova events are the remaining cores of large stars), but the short bursts of gamma rays have not been incorporated into an older star population … or any stars at all. .
This led astronomers to suspect that before neutron stars struck in a kilonova flash, complicated dynamics “kicked them out” of the house and away from their host. galaxies. Then, wandering the solitary intergalactic depths, the neutron stars unite, leading to a short explosion of gamma rays, that explosion of light the only sign of their existence.
Stacks and stacks of stars
Another possibility, as a paper suggests which appears recently in the arXiv prepress log, is that we are doing everything wrong.
By far the vast majority of stars in a galaxy are concentrated right in the center or in a thin disk. Usually less than 2% of all stars are in the region called “Hate, “which can extend from 10.00 to 100,000 parsecs away from the galaxy itself. (A parsec is about 3.26 light-years.) Hence the simple reasoning that if we see a large proportion of short explosions of gamma rays coming from inside the halo and the stars there are relatively rare, then the neutron stars that led to the explosion must have come from elsewhere.
However, because galaxies are so bright and there are relatively few stars in the halo, it is actually notoriously difficult to measure the number of halo stars for a particular galaxy. Thus, when astronomers said something like “short gamma-ray bursts come from a very empty halo,” this is true only in the statistical sense.
So the astronomers behind the new study took a very deep, long look at the galaxies that hosted short-range gamma-ray bursts – and found that those events were not as isolated as they seemed. In all cases, they found populations of old stars moving away from the galactic disk and in the regions of the explosions. These old stars are more likely to have stellar remnants, such as neutron stars, that would eventually collide in a kilonova explosion – and an associated short gamma-ray burst.
The bottom line is that no blow is needed. There is no real mystery. The short bursts of gamma rays that occur far outside the host galaxies are there, because there are actually more stars in their vicinity than I thought. Neutron stars are not randomly evacuated from host galaxies as often as we would have thought. Which, given how violent the universe can be, is probably the most surprising answer.
Find out more: “No speed bumps are needed to explain the long-range offsets of Ca-rich supernovae and short GRBs“
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