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Now that we know where gamma rays come from, what kind of Pandora's box has been opened?
Legend has it that the morbidly curious Pandora opened a box which released every horror imaginable. She must have made an impression, because a galaxy cluster is named after her.
What has now been unlocked is a cosmic box of questions and possibilities rather than horrors. Mysterious gamma-ray bursts (GRBs), which seem to materialize out of nowhere, were found to spawn from star-forming galaxies—like some in the Pandora cluster—but wait. What can the newly discovered origin of light more energetic than any other kind of radiation in the universe also tell us about cosmic rays, radio emissions, even dark matter?
Astrophysicist Matt Roth and his team of researchers from Australian National University have now illuminated more about gamma rays through data from NASA’s Hubble Space Telescope and the Fermi Gamma-Ray Space telescope. They recently published a study in Nature. This needed more insight into how cosmic rays, the progenitors of GRBs, move through interstellar gas, and what they encounter on the way.
“Cosmic rays are the main source of gamma-ray emission in star-forming galaxies, through collisions with other atoms in the interstellar medium,” Roth told SYFY WIRE. “To know how many gamma-rays of each energy we get, we need to know the initial energies of the cosmic rays they come from.”
Gamma rays have violent beginnings. As cosmic rays (which will eventually give birth to GRBs) rocket through clouds of potential star stuff and collide with whatever particles might be floating around, they produce subatomic particles known as a pions. These pions could have a positive or negative charge, or they could be neutral. It was the neutral ones that were of special interest to Roth because they decay the most rapidly to produce gamma rays. Positively and negatively charged pions take longer to decay, but GRBs can also come out of them.
This is why initial cosmic ray energies are necessary for revealing how many gamma rays, and how many of each type of energy, come out of a cosmic ray collision. Finding out the amount and type of gamma rays that are formed means gauging how cosmic rays make their way through galaxies. Cosmic rays are streams of charged particles, meaning that magnetic fields can deflect them. Think of it like space pinball. The more times they are deflected, the slower they travel through a galaxy, and vice versa. They can also create their own magnetic fields.
When highly excited particles cosmic rays give to a magnetic field in a phenomenon called the streaming instability, it is as if these particle streams are slowing themselves down until they run into neutral atoms that neutralize the charge and allow them to move faster again.
“Doing the computations tells us exactly how many cosmic rays at each energy go on to produce gamma-rays, and how many escape the galaxy,” said Roth. “The key piece of new understanding is that the escape (or conversion) fraction of cosmic rays is not a constant for all energies, but can depend sensitively on the energy of the cosmic ray.”
Cosmic rays and the gamma rays they set off can allow an unprecedented view into the internal structures of galaxies. When dying stars explode into supernovas, they spew an immense amount of particles that could eventually form stars again. Supernovas also generate intense shockwaves that accelerate cosmic ray particles involved in the formation of gamma rays. Electrons are among these particles, and when magnetic fields in a galactic pinball machine deflect them, they emit a type of electromagnetic radiation called synchrotron radiation.
Synchrotron radiation is emitted by electrons or other charged particles zooming in a curve at speeds close to the speed of light. The strength of the deflecting magnetic field determines the amount of synchrotron radiation, and this is where radio emissions come in. Much of the synchrotron radiation in a galaxy is observable in radio, which is why the amount of radio emissions in star-forming galaxies are directly related to electron-based cosmic rays.
Roth and his team have upgraded a model they previously created, which can use electrons in a galaxy’s cosmic rays to calculate the amount of radio emissions expected from that galaxy.
“The star formation rate sets the supernova rate, which in turn sets the amount of cosmic rays that are injected in the galaxy,” he said. “Star formation rate, combined with a galaxy’s physical size, allows us to work out the gas surface density as well as the turbulent velocity dispersion.”
So what does any of this have to do with dark matter? Getting an idea of what else is causing gamma ray emissions in the background can at least clear up the emissions whose causes have not been clearly determined. That still doesn’t mean the dark matter (which has not yet been detected) is causing them, but it could. Gamma rays are thought to emerge from particles of dark matter that crash into their antiparticles and annihilate them. If the other ways gamma rays are born are cancelled out, this leaves space for speculation on dark matter.
“Once we subtract all emission from known sources, it is the leftover excess that may yield clues to the nature of dark matter,” Roth said.
If gamma rays can tell us more about dark matter, the might be the light that (at least in a proverbial sense) finally illuminates the invisible.