Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host Ask a Spaceman and Space Radio, and author of How to die in spaceHe contributed this article to Space.com’s Expert voices: opinions and insights
Short gamma-ray bursts, which, as the name implies, are short bursts of high-energy gamma rays, appear to appear far away from their host galaxies.
For years, astronomers have thought that this means they get a ‘kick’ when they are born. But new observations prove otherwise: we just missed all the stars in their vicinity.
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Short and fast
It took astronomers a long time to figure out what caused short gamma bursts in the universe. These eruptions, which lasted less than about two seconds, were first noticed by the US military when it developed gamma-ray detectors orbiting them to detect sneaky Soviet nuclear tests. When the detectors went wild (and the panic dissipated), U.S. officials realized that the distant cosmos was much more active than their Cold War opponents
The reason astronomers had such a hard time was that short gamma-ray bursts are a) short and b) rare. That’s an annoying combination for a group of people who depend on being able to see the same thing over and over to get to grips with it.
Whatever triggered these brief bursts, it was brutally energetic and relatively small, even smaller than a star. Astronomers were able to estimate that last bit based on the duration of the event itself. The average short gamma-ray burst lasts about 0.2 seconds, and if you want an object in space to do something (like blow it up), its actions are always limited by the speed of light. If the event lasts 0.2 seconds, it means that one end of the object should be no further than 0.2 light seconds from the other end. That works out to about four times the diameter of Soil
To add to the mystery, it wasn’t until 2005 that astronomers finally got an afterglow from a brief gamma-ray burst. Prior to the discovery of X-ray flashes that lasted hours after the main event, all eruptions were isolated cases.
And it wasn’t until 2017 that astronomers got the last, decisive clue, when a brief gamma ray coincided with the detection of a gravitational waveThat particular gravitational wave bore the signature of two neutron stars merging – called a kilonova.
Neutron Stars: Definition and Facts
Far from home
Although astronomers had finally discovered the cause of short gamma-ray bursts, one big mystery remained: their location. Unlike their long-lasting cousins (long gamma-ray bursts), many of the short bursts came 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. For example, before we fully understood what causes the different types supernovaeastronomers noted that the Type II class mostly comes from elliptical and spiral galaxies, while Type I comes from pretty much everywhere. That helped us understand their identity: Type II comes from the death of massive stars, which are manufactured in abundance into star-forming elliptical stars and spirals, while Type I comes from the destruction of white dwarfs, a much more common and long-lived object that can live anywhere.
And so astronomers have been amazed at the location of many short gamma-ray bursts. They certainly come from stars (the neutron stars behind the kilonova events are the remnants of cores of large stars), but the short gamma bursts weren’t embedded in a population of older stars … or no stars at all, for that matter. .
This led astronomers to suspect that before neutron stars hit each other in a kilonova flash, complicated dynamics “ kick ” them out of their home and away from their host. galaxiesThen they wander through the lonely intergalactic depths and the neutron stars merge, leading to a brief gamma-ray burst, that one ray of light 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 are all wrong.
The vast majority of stars in a galaxy are concentrated in the center or in a thin disk. Typically less than 2% of all stars are found in the region containing the ‘halo, “which can range from 10.00 to 100,000 parsec of the actual galaxy. (One parsec is about 3.26 light years.) Hence the simple reasoning that if we see a large proportion of short gamma-ray bursts from within the halo, stars there are relatively rare, so the neutron stars that led to the eruption must come from somewhere else.
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 any given galaxy. So if astronomers have said something like “short gamma-ray bursts come out of a very empty halo,” that is only true in an average statistical sense.
So the astronomers behind the new study looked very deeply and long at the galaxies that had short gamma-ray bursts – and found that those events weren’t as isolated as they seemed. In all cases, they found populations of ancient stars reaching far from the galactic disk into the regions of the eruptions. Those old stars are more likely to have stellar remnants, like neutron stars, that would eventually collide in a kilonova explosion – and an accompanying short gamma-ray burst.
The conclusion is that a kick is not necessary. There is not even much of a mystery. Short gamma-ray bursts that take place far beyond their host galaxies exist because there are actually more stars in their vicinity than we thought. Neutron stars are not randomly ejected from their host galaxies as often as we thought. Which, given how violent the universe can be, is perhaps the most surprising answer.
Find out more: “No speed kicks are needed to explain long-range compensations from Ca-rich supernovae and short GRBs
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