Paul M. Sutter is an astrophysicist in SUNY Stony Brook and the Flatiron Foundation, host Just ask Spaceman a Space Radio, and author How to die in space. This article contributed to Space.com’s Expert Voices: Opinions and Insights.
Short gamma ray pulses, which, as the name implies, are short bursts of high-energy gamma rays, tending 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 have just been losing all the stars in their neighborhood.
Related: 8 mysteries of baffling astronomy
Short and fast
It took a long time for astronomers to discover what in the universe was causing short gamma ray pulses. These bursts, lasting less than two seconds, were first spotted by the United States army when they developed orbital gamma ray detectors to smell the Soviet nuclear tests. When the sensors went wild (and the panic went away), US officials realized that the distant cosmos was far more active than their Cold War opponents.
The reason astronomers were having such a difficult time was that short gamma ray pulses were a) short and b) rare. That’s a nasty combo for a group of people who rely on being able to see the same thing over and over to get it.
Whatever was causing these short bursts, it was energetic and relatively small, even smaller than a star. Astronomers could estimate that last piece 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 (like, say, blow up), then its actions are always constrained by speed the light. If the event lasts for 0.2 seconds, then that means one end of the object cannot be more than 0.2 light-seconds away from the other end. That comes out to about four times diameter Ground.
Adding to the mystery, it was only until 2005 that astronomers caught a back-ear of a short gamma ray burst. Before those X-ray flashes were discovered for hours after the main event, all the ramifications were isolated issues.
And it wasn’t until 2017 that astronomers got the final, definite clue, when a short gamma ray event coincided with the detection of gravitational wave. The signature of two neutron stars merged in that particular gravity wave – a so-called kilonova.
Neutron stars: Definition and facts
Far away from home
Although astronomers have finally figured out what caused short gamma ray eruptions, one great mystery remained: their location. Unlike their longer cousins (long gamma ray pulses), many of the short ones tended to come from regions of the universe relatively far away from galaxies. They are not part of the normal stellar population.
Linking a powerful, rare event like this to its surroundings is a useful astronomical feat. For example, before we fully understand what causes the different types of supernovas, astronomers noticed that the Type II class tends to come from elliptical and spiral galaxies, whereas Type I is essentially from wherever. That helped us understand their identities: Type II comes from massive star deaths, extensively manufactured within ellipticals and star-forming spirals, while Type I comes from destruction white dwarf, a much more common and long-lasting object that can live anywhere.
And so astronomers have been amazed by the location of many short gamma-ray bursts. They definitely came from stars (the neutron stars behind the kilonova events are the cores of giant stars), but the short gamma ray bursts were not embedded in a population of older stars … nor any stars of absolutely, for that matter. .
This led astronomers to suspect that before neutron stars slammed together in a kilonova flash, that complex dynamics “kicked” them out of their home and away from their host galaxies. Then, wandering the lonely intergalactic depths, the neutron stars combine, resulting in a short gamma ray burst, that one blast of light is the only indication of their existence.
Stacks and stacks of stars
Another possibility, as suggested by a paper appeared recently in the print edition arXiv, is that we get it all wrong.
The vast majority of stars in a galaxy are concentrated in the very center or within a thin disk. Typically, less than 2% of all stars are found in the region called “halo, “which can extend anywhere from 10,00 to 100,000 parsons away from the galaxy properly. (One parsec is about 3.26 light years.) So the simple logic is that if we see a large proportion of short gamma ray bursts coming from inside the halo, with the stars there being relatively rare, then the neutron stars that led to the burst must have come from somewhere else.
However, due to the brightness of galaxies, and relatively few stars in the halo, it is extremely difficult to measure the number of halo stars for any given galaxy. So when astronomers have said something like “short gamma ray pulses come from a very empty halo,” that is only true in a statistical sense on average.
So the astronomers behind the new study took very long, deep look at the galaxies that have sustained short gamma ray pulses – and found that those events were not as isolated as they appeared. In each case, they found populations of old stars extending away from the galactic disk and into the bursting regions. Those old stars are more likely to have stellar remains, such as neutron stars, which would eventually collide in a kilonova blast – and a related short gamma ray burst.
The conclusion is that no kick is needed. There is not even a mystery really. Short gamma ray pulses that occur far outside their host galaxies are there because there are more stars in their vicinity than we thought. Neutron stars are not randomly ejected from their host galaxies as often as we might have thought. Which, given how violent the universe can be, is perhaps the most surprising answer.
Learn more: “Speed kicking is not required to explain large distance offset of Ca-rich superpowers and short GRBs“
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