Title: Biological radiation dose from secondary particles in a Milky Way gamma ray burst
Authors: Dimitra Atri, Adrian L. Melott, Andrew Karam
First Author’s Institution: Tata Institute of Fundamental Research, India
We all love cross-disciplinary papers, and today’s focus is a great example! High-energy astronomy, chemistry, and biology, all wrapped up into one delightful doomsday scenario: what happens to life on Earth if a gamma-ray burst points at us from within our own galaxy?
Artist’s depiction of a nearby GRB impact with Earth. Earth’s life forms are probably not too thrilled with that brown cloud of nitrogen dioxide that formed as a result of the high-energy photons interacting with the air. Image credit: NASA.
A GRB Pointed at Us Would Be a Bummer
Gamma-ray bursts (or GRBs), extraordinarily high-energy bursts of radiation and particles, are observed all over the Universe — and statistically speaking, it’s only a matter of time before one goes off within the Milky Way. Rough estimates expect one GRB within our galaxy every 1-100 million years or so.
Previous research on hypothetical GRB impacts with Earth has focused on the resulting ionization of the atmosphere and depletion of the ozone layer (which means life on the planet surface gets a UVB radiation dose several hundred times higher than what it’s getting at present, i.e. insta-sunburn + mutations and cancer + disintegration of the food chain as we know it). While this is admittedly not a trivial concern, we’ve also got to worry about something called “secondary radiation”. The incoming high-energy photons from the GRB can generate secondary particles (mostly muons) from particle showers that start at the top of the atmosphere and cascade downwards to the Earth’s surface. That means that in addition to getting hit by lots of uncomfortably-high-energy photons, we’ll also get hit by uncomfortably-high-energy muons — which has the potential to do some damage as well. Exactly how dangerous this secondary radiation would be hasn’t been carefully explored yet in previous research, according to the authors of this paper.
So let’s put UV-irradiation concerns on hold for a second and focus on the effects of high-energy muons hurtling at us.
Modeling Doom
So what goes into these calculations? The authors start by choosing models of hard-spectrum GRBs that might be typical for a galactic encounter. The energy of photons from these bursts range from a few keV to up to ~100 GeV – with ~10 GeV being what’s needed to produce muons that could make it to the ground. The number of these muons that are produced is then calculated by modeling the air showers with CORSIKA, a Monte Carlo code. The figure below shows the total number of muons that arrive at the planet surface as a function of incoming photon energy, assuming 10 million photons (each in the range of 10-100 GeV) from the GRB hit the upper atmosphere.
Total number of muons that reach Earth’s surface as a function of incoming photon energy, assuming 10 million photons with energies in the range of 10-100 GeV hit the top of Earth’s atmosphere. Figure 1 in the paper.
Now that we’ve got a particle flux, we have to figure out what effect getting hit with these muons has on life. The authors convert the flux of muons and their energies into a biological radiation dose and compare this to acceptable radiation limits for various life forms.
So Are Muons What We Should Be Worried About?
Nope.
Turns out that the radiation dose from these incoming muons on your average-sized life form (modeled as a 15-cm cube of water) is between 0.11 µSv and 0.3 mSv (Sv is short for Sievert), depending on your GRB model. For comparison, 0.1 µSv is roughly the same radiation as eating a banana, 1 Sv will produce mild radiation sickness in humans, and fatalities start at 4-5 Sv. Insects are significantly more resilient to radiation, with the fatal dose clocking in at ~100 times that for humans, and some microbes can survive exposure up to 15 kSv.
Based on the work in this paper, it looks like life on Earth is safe from high-energy-muon damage from GRBs — so all we have to worry about is that whole UVB-irradiation thing from the depletion of the ozone layer. No problem, right?
