Title: Supernova axions convert to gamma-rays in magnetic fields of progenitor stars
Authors: Claudio Andrea Manzari, Yujin Park, Benjamin R. Safdi, and Inbar Savoray
First Author’s Institution: Berkeley Center for Theoretical Physics, University of California, Berkeley; Theoretical Physics Group, Lawrence Berkeley National Laboratory, Berkeley
Status: Accepted to PRL [closed access]
Here on Earth, we have the privilege of studying particle acceleration and collisions in a controlled setting through experiments like the Large Hadron Collider. While scientists have achieved numerous discoveries and constraints on particle-physics theories through these experiments, there is simply no way on Earth to emulate the monumental outputs of stars and their deaths as supernovae. Instead, we usually rely on electromagnetic byproducts – i.e., photons – from particles interacting with each other to determine what exactly those particles are (or at least rule out what they are not) and infer the physics happening in these cataclysmic events.
One particle that physicists have been anticipating to observe during supernovae is the axion. This is a hypothetical particle originally conceived to solve inconsistencies in the Standard Model of particle physics. Axions were later acknowledged as a possible dark matter constituent – a catch-all term for matter we can’t observe directly through photons – due to their exceptionally low interaction rate with “normal” matter like protons and electrons. Despite the significant role axions may play in dark matter and their impact on cosmological evolution, we know very little about their properties besides some limits on their interaction rate and mass, which may be as much as 100 billion times smaller than the electron mass.
It has been hypothesized that strong magnetic fields can convert axions into gamma rays which, unlike axions themselves apparently, are observable (yay)! Normally, the galactic magnetic field is treated as the field responsible for this conversion to gamma rays. However, today’s authors evaluate whether the magnetic field of the dying star responsible for the supernova and axion production could produce detectable gamma rays through this conversion. This would happen during the stage of the supernova called the “proto-neutron star” (PNS) stage when the star’s core condenses into its final form of a neutron star.
For this evaluation, the authors consider perhaps the most popular supernova: SN1987A. In addition to being the most nearby supernova in recent history (only about 50,000 parsecs away compared to the 1,000,000 parsecs for the typical supernova we see), it is also the only supernova we have neutrino measurements for. Because of this, SN1987A has been an invaluable testbed for particle physics in extreme environments; that makes it the best event to investigate supernova-based axion production.
Before being able to estimate how much gamma-ray emission we might detect, the authors first need to estimate how many axions there are to produce the emission. For a hypothetical particle being produced in an extreme process like a supernova, this is no easy task. There are three pathways for axion production that the authors account for: 1.) photons coalescing into an axion (through a process called the Primakoff effect), 2.) charged particles like protons accelerating off of each other and emitting axions in the process (similar to the Bremsstrahlung radiation mechanism), and 3.) pions and protons converting into an axion and neutron. Understanding how many axions are produced then requires knowing how the populations of photons, protons, and pions evolve throughout the supernova.
To learn about this evolution, the authors use a supernova model that considers how properties like temperature and pressure– and the resulting particle production and acceleration– change as a function of radius and time. For these models, the authors only evaluate the first 10 seconds after the stellar material “bounces” off of the core that then heats up and explodes as the supernova; this is probably the only time when the material is hot and dense enough to produce the axions. Once the number of axions is determined (see Figure 1), the authors can figure out how many are converted into gamma rays.
The probability of an axion being converted into a gamma ray is going to depend on the PNS’s magnetic field strength and the extent of the magnetic field. Although the galactic magnetic field is about 1 billion times weaker than the star’s magnetic field (about one micro-Gauss compared to the star’s kilo-Gauss field), the relevant extent of the galactic field is about 1 billion times larger– the stellar field and galactic field probably contribute similar amounts to the conversion of axions to gamma rays.
The axion-to-photon conversion probability also depends on the coupling between axions and photons. As it turns out, the coupling between axions and photons depends on the axion’s mass: the higher the axions’ mass, the more they’re coupled to photons. This is what allows the authors to use the detection threshold of gamma rays to put limits on the axion mass! Because there was a gamma-ray telescope pointed towards SN1987A when it went off– and because it did not detect the gamma-ray flash that axion production predicts– the authors estimate that the axion mass must be less than 10,000x the electron mass. This is an observational constraint that is just an order of magnitude off from theoretical predictions!
This work demonstrates new experimental limits on the axion mass, but ultimately it is informed by limitations in both our supernova and particle theory as well as our instrumentation. To this latter point, the authors emphasize the need for an all-sky, gamma-ray telescope to make sure we observe the next supernova neighbor. They propose a system like GALAXIS: the Galactic Axion Instrument for Supernova. This would be a network of gamma-ray telescopes dedicated to monitoring the entire sky for gamma signatures of supernovae to put better constraints on axion production and dynamics. Irrespective of axions, a tool like this will be critical for effectively developing gamma-ray statistics of supernovae and other high-energy events. While theorists continue to refine their supernova and particle models, observers of all fields should be on the lookout for the possible implementation of GALAXIS.
Astrobite edited by William Lamb
Featured image credit: NASA, ESA, CSA, Mikako Matsuura (Cardiff University), Richard Arendt (NASA-GSFC, UMBC), Claes Fransson (Stockholm University), Josefin Larsson (KTH), processed by Alyssa Pagan (STScI)
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