The neutrinos released by a supernova explosion are an excellent window into the dynamics of the supernova process for which detailed understanding is one of the remaining challenges for astrophysics. The Sudbury Neutrino Observatory (SNO) is capable of observing Galactic supernovae through the detection of neutrinos. The current supernova models have distinguishing features in their neutrino spectra and SNO's capabilities to extract energy and flavour are important tools in providing constraints on the models.
Another aspect is that the neutrinos precede the visible eruption of the mantle of the star by up to 10 hours. This potentially allows for an early alert to the astronomical community and thus for observation from the earliest moment. In order to ensure a positive alert, several neutrino detectors have formed the Supernova Early Warning System (SNEWS). At the moment, Super-K, LVD, SNO, and now AMANDA are part of the collaboration. A coincidence between these detectors can provide an alert to the astronomical community with a very high confidence.
A data analysis program to identify neutrinos from a Galactic supernova burst has been installed in the online system at SNO. The program automatically analyzes burst data, and it is anticipated that an alert to SNEWS could be issued within a few minutes.
|Sanduleak turning supernova in 1987 (photo AAO)|
|Neutrino signal in the three detectors on line at the moment of SN1987A explosion. The first neutrinos were detected at 7:35:35, 7:35:41 and 7:36:12 in Kamiokande II, IMB and Baksan, respectively. However, the clocks were not synchronized and had large uncertainties in their absolute measure of time. As a result, since the three signals probably happened within a fraction of a second, the first neutrinos in each detector are assumed to be simultaneous.|
|Supernova luminosity, Burrows' model (fig. Jaret Heise). "νμ" refers to non-electron neutrinos and anti-neutrinos.|
Moreover, an estimation of the neutrino masses could be done in the following manner. The velocity of a particle of energy E and mass m, with E >> m, is given by (with c = 1):
|v =||p||=||( E2 - m2 )½||≈ 1 -||m2||.|
|Δt[s] ≈ 0.05||m[eV]2||d[kpc]||.|
A more promising possibility is to make use of the flux cutoff expected if the supernova core becomes a black hole. In this case, the neutrino flux stops instantaneously after 1-2 s, and a much better estimate of the masses could be carried out.
SN1987A also set a limit on the electron-antineutrino lifetime. They were able to make it over 50 kpc, thus their relativistically dilated lifetime is limited in order of magnitude by γτ > 165,000 years, with γ = E/m. SNO should be able to set such a lifetime on the other neutrino flavours too.
Finally, the fact that the neutrinos arrive earlier than the photons by up to 10 hours makes possible an early alert that could allow astronomers to observe the environment near the progenitor probed by the initial stage of the core collapse, the UV/soft X-rays flash at breakout and possible unknown early effects.
|High-energy event visualized in SNO's data acquisition system. The disks are PMTs that fired, the ring pattern they form is the intersection of the Cherenkov cone with the spherical detector.|
The main reactions expected in SNO are the following:
Due to the large cross section of the first reaction listed above, almost half of the supernova neutrino events occurring in the SNO detector are expected to be associated with the charged-current reaction in light water, while one third of the total number of events are expected to participate in neutral-current reactions in the D2O. The relatively small amount of elastic scattering events, in which the recoil electron is scattered into a forward cone of about 25°, can provide information on the direction of the supernova. While all neutrino species contribute to elastic scattering, the cross section for νe is 2.4 times larger than that for νe and 5.9 to 7.3 times larger than for 'ν'μτ (where 'ν' refers to ν and ν). Finally, about 1% additional neutrino interaction events are anticipated via excitation of 16O.
|Type||Reaction||Number of events||Number of counts (MC)|
|H2O CC||νe+p → n+e+||356 e+||331|
|D2O CC||νe+d → p+p+e-||83 e-||72|
|D2O CC||νe+d → n+n+e+||106 n + 53 e+||82||138||90|
|D2O NC||'ν'eμτ+d → 'ν'eμτ+p+n||264 n||84||226||151|
|ES||'ν'eμτ+e- → 'ν'eμτ+e-||47 e-||36|
Depending on the phase, a total of 600 to 800 neutrinos is expected to be detected for a supernova at the Galactic centre (10 kpc), half of which would occur during the first 2 seconds of neutronization and thermal-pair production, and the rest within tens of seconds. The table shows a detection efficiency of 87% for CC electrons in the D2O and 32, 86 and 57% for neutrons in the D2O depending on the phase. In the last phase, the efficiency is split between 45% neutrons detected in the NCDs, and 12% on deuterons. Neutron capture on proton in H2O creates a 2.2-MeV γ, which energy is too low to be detected.
|Number of neutrinos detected in SNO versus distance of the supernova, assuming a release of 1058 particles. The SNO online threshold is 30 events in two seconds, corresponding to a total of about 60 events over a minute, thus the detector's sensitivity covers the Galaxy and spans up to half way to the Large Magellanic Cloud. Below this threshold, we would still be able to observe the events a posteriori. If a supernova explodes in the Andromeda Galaxy (M31) at ~770 kpc, SNO would have 10% chance to detect 1 neutrino. Today, the various detectors use GPS for timing and are believed to be synchronized within a second, bringing more confidence in the identification of neutrino events as coming for a same supernova.|
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