Introduction
Gamma ray bursts (GRBs) are some of the most energetic phenomena in the universe since the Big Bang itself. They emit enough energy to sterilize entire regions of their home galaxies, and occur in a galaxy once every few million years or so. GRB 22 1009a was so powerful that it temporarily blinded the Fermi Gamma Ray Telescope. Follow up observations have been made at all wavelengths to figure out what caused it, and these findings have called into question our understanding of GRBs.
GRB 22 1009a
When the Geral Swift Observatory and XMM Newton telescopes imaged the bright X-ray Afterglow of GRB 22 1009a, they found more than 20 rings of X-ray light, scattering off dust clouds in our galaxy. It is estimated that a burst this powerful might occur only once every 10,000 years – making it the most energetic GRB to reach Earth since the dawn of human civilization. This is why astronomers dubbed GRB 22 1009a the brightest of all time, or the BOAT.
Follow Up Observations
A series of papers about the BOAT were published in a special issue of the Astrophysical Journal. GRBs are thought to mark the beginning of a supernova, but neither Hubble nor the James Webb Space Telescopes found any evidence of one. It was also surprisingly faint in the radio spectrum, despite the fact that GRBs should emit very strongly at all wavelengths.
The brightest ever seen
What Are Gamma Ray Bursts?
Gamma ray bursts (GRBs) are generally classified by their duration. Short GRBs last under two seconds and are thought to be caused when a pair of neutron stars collide. This collision sets off what is called a kilonova. By contrast, long GRBs can last anywhere from two seconds to several hours. It is thought that these longer bursts occur when the core of a very massive star collapses to form a black hole.
The Afterglow
Both scenarios produce an initial burst of gamma ray emission, followed by a longer lasting afterglow. The afterglow is created when the leading edge of the jet collides with the surrounding interstellar medium. The shock front produces emission at longer wavelengths, starting from X-rays, but going all the way to radio. It is by studying the afterglow that we can learn about the properties of the surrounding medium, the physics of the GRB itself, and piece together the puzzle of what caused the burst in the first place.
The Third Act
Now, if the GRB came from a black hole inside of a collapsing star, there is a third act to follow. The black hole’s jet ignites runaway fusion inside the star’s interior that explodes the star from the inside out. The result is an extremely energetic supernova, sometimes called a hypernova. These long duration bursts are so energetic that they can easily outshine their host galaxies. We have detected some extremely bright bursts so far, but when GRB 22 1009a was first detected, it was without question the brightest ever seen.
The Brightest Gamma Ray Burst in Human History
The new record holder for the brightest gamma ray burst in human history is the boat. It was so bright that it actually blinded the instruments on the Fermi gamma ray telescope when its actual bright business was later reconstructed. The boat blew all of the other bursts out of the water. As a matter of fact, it was so bright that it was detected by instruments that were not even designed for this purpose.
The First Detection
The very first detection was picked up a day earlier by Voyager 1, which is now an Interstellar space. A day later, the bursts reached the inner solar system and was detected by issas, Gaia, satellite at L2 and then the integral satellite in CIS lunar space about one second later it was detected by Fermi and grb Alpha in low earth orbit. The burst even irradiated Earth’s ionosphere.
Further Detection
The bursts continued to L1, where it was detected by the ace and wind spacecraft studying the Sun and then a few minutes later it was detected at Mars by Maven and Mars Odyssey.
Once in a Lifetime Event
Something this bright doesn’t happen very often. In fact, the boat was calculated as being a nearly once in ten thousand year event. Human civilization is only about six thousand years old, so it’s quite possible that the boat is literally the brightest gamma ray burst in human history. Lucky for us, we just happen to have telescopes in space that could study it.
Mysterious Behaviour
However, the boat is behaving in ways that really don’t make sense. Bursts like these are understood to be associated with dying, massive stars, but so far the boat has not shown any evidence of a supernova.
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Investigating the Afterglow
Recently, there was an extraordinarily bright afterglow seen at X-ray wavelengths, but surprisingly quiet in radio. This has led to an investigation into why it was not brighter at radio wavelengths and why it was so bright to begin with.
The Brightest Gamma Ray Burst Ever Detected
The boat was the brightest gamma ray burst ever detected, but strangely, the boat’s radio and Afterglow were not unusually bright at all. It gets even more interesting when you look at how the radio visible and x-ray emission faded out over time.
Standard Models of Gamma Ray Bursts
In the last 50 years of studying GRBs, a number of standard models have been developed that describe how the Afterglow fades across different wavelengths. When the models were run against the boat, they matched up with the visible and x-ray emission quite well, but largely came up short on the radio.
The Boat’s Intensity
The modeling also indicated that, although the boat was the brightest ever seen, it may not have actually been all that powerful in terms of the total energy released. Rather, it may have appeared so bright because all of its energy was concentrated into a very tight beam and that beam happened to be aimed directly at Earth. It’s kind of like how a pocket laser is far more intense than a searchlight, even though it uses far less energy.
The Boat’s Origin
It also turns out that the boat originated at a redshift of 0.15 and that works out to about 1.9 billion light years away. Believe it or not, that’s actually not particularly far for a gamma ray burst.
The Strange Case of the Boat
The boat is a strange case indeed. It is a long burst, thought to come from a dying, massive star. When the afterglow of the burst fades out, the underlying light from the supernova should start to become visible. This is usually seen in the light curve of the burst, which tends to stabilize and stay bright as the supernova takes over.
However, in the case of the boat, no evidence of a supernova was detected as the afterglow faded. So what was going on?
The Plane of the Milky Way
The boat came from a direction that was almost directly in line with the plane of the Milky Way galaxy. This means its light had to pass through some 60 thousand light years of our galaxy, which is laden with dust. If a supernova really happened, then dust in our galaxy could have blocked up to 99% of its visible light.
The James Webb Space Telescope
Fortunately, longer wavelengths, like near infrared, can sidestep much of this dust. To that end, the James Webb Space Telescope captured spectra of the boat at both near and mid infrared wavelengths. This was the first time the mid infrared spectrum of a grb had ever been obtained.
No Excess Submission Detected
However, if a supernova were taking place, there should have been some excess submission or changes in web spectrum over time. Neither of those effects were detected and that might be because there was more dust at work here than just that from our galaxy alone.
The Hubble Space Telescope and the BOAT
The Hubble Space Telescope recently captured an incredible near infrared image of an extended feature emerging from a burst. This feature is actually the burst’s home Galaxy, and it appears to be a spiral seen Edge on. This means that the Supernova would have had to penetrate through its home Galaxy’s disk and then through the disk of the Milky Way before arriving at Earth. It is possible that the Supernova’s infrared signature was weakened by two galaxies worth of dust.
The Mystery of N6946bh1
In 2015, the mass of star N6946bh1 disappeared without a trace. There was no Supernova, no gamma ray burst, nothing. It is possible that this particular star was a direct collapse black hole. This could present us with a new kind of intermediate case, where a massive star’s core collapses to create a black hole which in turn kicks off a gamma ray burst. But then the black hole gets so massive, so quickly, that it reverses and consumes the subsequent Supernova.
Colliding Neutron Stars
Alternatively, there may not have been a Supernova at all, because the burst was not triggered from a collapsing massive star, but rather by a pair of colliding neutron stars. Such events trigger a shorter and less energetic gamma ray burst, but if the burst was confined to an extremely narrow Jet and aimed directly toward Earth, it would then make the burst appear bright enough to become the BOAT. Colliding neutron stars are thought to produce GRBs that are only between a few milliseconds to no more than a couple of seconds long.
The Outburst
The Outburst was initially assumed to be a collapsing massive star, as it lasted 600 seconds. However, in 2022, Jod and collaborators reported the discovery of a kilonova associated with a gamma-ray burst (GRB) that lasted nearly a minute. This would mean that the Outburst was a pair of colliding neutron stars that created a GRB that lasted up to 10 minutes longer, with an ultra-collimated jet aimed directly at Earth.
The Polarization
In order for such jets to be so tightly collimated, they would require extremely strong magnetic fields. These strong magnetic fields would polarize the emission of the initial burst, with the stronger the magnetic fields, the more polarized the light. To measure this, the Imaging X-ray Polarimetry Explorer (IXPE) was used. The IXPE found that, while the polarization was high, it was not the highest recorded for a burst.
The Rings
The Outburst was glimpsed through the plane of the Galaxy, and light passing through Galactic clouds scatter and form the Rings surrounding the burst. It is possible that these dust clouds may have reduced the polarization of the light in some way, but more observations are needed as the Afterglow fades to know what effect the dust is really having on the polarization. In total, 21 Rings were counted, and this means the burst had to pass through no fewer than 21 clouds, ranging from as close as 700 light years from Earth to as many as 61 000 light years.
The Boat GRB and the Structure of Our Galaxy
The Boat GRB, or Gamma Ray Burst, is a powerful burst of light that is visible from the other side of the galaxy. While most of the light passes directly through the geometric plane of the galaxy, some of it is at an angle of incidence of about 4 degrees, allowing for the formation of the most distant Rings above the plane of our galaxy. This begs the question: why is there dust above the galactic plane?
The Galactic Warp
It turns out that our galaxy is not perfectly flat, but rather has a warp on either end. On the far side, the warp is lifted upward, allowing enough dust to be present for the Boat GRB to pass through. This means that the Boat GRB has allowed astronomers to probe at least a portion of the structure of our galaxy. We now know where the dust clouds lie along the line of sight to the Boat, and this can tell us more about the history of our galaxy’s formation, which includes mergers and collisions with smaller galaxies.
More Observations
This past winter, the Boat disappeared behind the Sun for a couple of months, but it has since reappeared, and more observations are being conducted. There are still many questions left to be addressed and much more to be learned. A huge thanks to all Patreon supporters, who are helping to keep Launchpad Astronomy going.
Welcome Laura Sanborn
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