Everyone’s excited about SN 2023ixf in M101 – the closest supernova since the last M101 supernova, SN 2011fe.

I have been interested in the possibility of an MeV-energy scale neutrino detection for this supernova from the Super-Kamiokande neutrino observatory, so I thought I would run the numbers. After all, 6.4 Mpc is far away but not *that* far away.

We can begin with the original and field-defining detection of MeV neutrinos from SN 1987A, the last naked-eye visible supernova. The former Kamiokande-II facility was running at the time that SN 1987A exploded, and detected 12 neutrinos (over 10 seconds) from that event, about half the total worldwide neutrino haul from that supernova. Those neutrino detections made SN 1987A the OG multimessenger transient, decades ahead of the recent GW 170817 and TXS 0506+056 discoveries.

SN 1987A is at a distance of 51.4 kpc in the LMC, while SN 2023ixf is 6.4 Mpc away (6400 kpc). That reduces the expected neutrino flux, but on the plus side, Super-Kamiokande is a 50 kton detector while K-II was a 3 kton detector. In addition, Super-K water is now “doped” with Gadolinium (Gd) which reduces the detector background, as we will discuss in a bit.

Direct scaling from SN 1987A by mass + distance gives 0.013 expected neutrinos in Super-K over the 10-second interval following core bounce. Half the SN neutrinos are emitted in a 2-second interval at core bounce, and the other half are emitted over the following 8 seconds as the proto-neutron star settles to its equilibrium configuration.

So that’s our number: a 1.3% chance that Super-K detected a neutrino from SN 2023ixf.

What would it mean to detect a neutrino from SN 2023ixf? And how would we know it was an SN 2023ixf neutrino rather than background? Good questions!

• Gadolinium (Gd) doping means that SN neutrino interactions in the detector now give a unique “double pulse” signature, with one flash from the initial inverse beta-decay interaction, and a second flash roughly 1 second later following neutron capture on a Gd nucleus – see Xu et al. 2016 for details.
• Thanks to that double-pulse signature, the background rate for MeV neutrinos from supernovae is much reduced. Xu+2016 estimate 1.3 to 6.7 events per year per 22.5 kton (inner volume).
• There will also be some directionality associated with the original inverse beta-decay interaction, which can be used to vet any candidate SN neutrino interaction(s).

Running the numbers, I find that if the time of core collapse can be narrowed down to a one-hour window (done! tyvm Chinese amateur astronomers!), then a single MeV neutrino interaction would be preferred over the background hypothesis by 7:1 to 40:1 odds in the absence of any directional constraint – improving appropriately if the direction is measured and consistent with the known position of SN 2023ixf. Note that background rates are well-known at this point (just not by me), so there will be no ambiguity about the actual odds if an interaction is seen.

If the time window can be narrowed down to 15 minutes, odds ratios without directionality go up to 30:1 to 120:1.

If a neutrino was detected and can be convincingly associated with SN 2023ixf, there would be multiple follow-on consequences. First, it would confirm (again!) astronomers’ model of core collapse supernovae. Second, this would probably be a useful additional constraint on neutrino oscillations. Third, we would narrow down the core-collapse time to a +/- 10-second window, which would be useful for the LIGO gravitational wave analysis.

Like we said – there’s a chance!