Supernova Monitoring at SNO

Introduction

On February 23, 1987, blue supergiant Sanduleak -69° 202 in the Large Magellanic Cloud turned into a supernova, now famous as SN 1987A. A posteriori, three neutrino detectors could find a burst of neutrinos in their data: Kamiokande II (11 neutrinos), IMB (8) and Baksan (5). Most of these neutrinos are thought to be electron antineutrinos.

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.

SN1987A
Sanduleak turning supernova in 1987 (photo AAO)

SN1987A neutrino signal
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.

Type-II Supernova Neutrinos

Several models describe the type-II supernova process. The neutrino spectrum is always characterized by a short peak of electron neutrinos generated during the neutronization phase carrying 1% of the total energy:

p + e^- → n + ν_e ,

and a second release of neutrinos of all flavours stemming from thermal pair processes (pair annihilation, plasmon decay, photoneutrino and Bremsstrahlung) releasing 99% of the energy:

γ + γ ↔ e⁺ + e^- → ν + ν ,
e⁺ + e^- → ν + ν ,
γ + e^- → e^- + ν + ν ,
e^- + A → A + e^- + ν + ν .

(More details.) Supernovae are expected to radiate about 3×10⁵³ ergs (3×10⁴⁶ J) in the form of neutrinos, half of this within 2 seconds, the other half within less than 1 minute.

Neutrino luminosity
Supernova luminosity, Burrows' model (fig. Jaret Heise). "ν_μ" refers to non-electron neutrinos and anti-neutrinos.

Scientific opportunities

The observation of supernova neutrinos should bring a better understanding of the the core collapse mechanism from the feature of the time and energy spectra, and constraints the supernova models. In particular, SN1987A events had a number of anomalies that could be solved or studied with more statistics and with SNO's abilities to detect all flavours.

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 )^œ ≈ 1 - m² .
E E 2E²
Thus, for a supernova at distance d, the delay of a neutrino due to its mass is, expressed in the proper units:

Δt_[s] ≈ 0.05 m_[eV]² d_[kpc] .
E_[MeV]²
Therefore, neutrinos of different energies released at the same instant should show a spread in their arrival time.

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.

Supernova detection with SNO

The Sudbury Neutrino Observatory consists of a sphere of 1,000 tonnes of heavy water surrounded by 1,700 tonnes of light water (+5,300 tonnes for shielding), 9,456 inward facing and 91 outward looking Photomultiplier Tubes (PMTs) and 400 metres of ³He Neutral Current Detectors (NCDs). The system has a capacity of 1 million supernova neutrino events.

Neutrino event in SNO
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:

charged current in H₂O (treshold 1.806 MeV):
ν_e + p → n + e⁺ ;
charged current in D₂O (thresholds 1.442 and 4.033 MeV):
ν_e + d → p + p + e^- ,
ν_e + d → n + n + e⁺ ;
neutral current in D₂O (thresholds 2.225 MeV):
ν_eμτ + d → ν_eμτ + p + n ,
ν_eμτ + d → ν_eμτ + p + n ;
elastic scattering in H₂O and D₂O:
ν_eμτ + e^- → ν_eμτ + e^- ,
ν_eμτ + e^- → ν_eμτ + e^- .

The final-state electrons and positrons are detected with the PMTs via their Cherenkov light while the neutrons are detected via three different techniques depending on the SNO phase:

Pure-D₂O phase: neutron capture on deuteron,
n + d → t + γ ,
then the 6.25-MeV γ-ray Compton-scatters on an electron, which generates Cherenkov light detected with the PMTs.
Dissolved-salt phase: neutron capture on chlorine,
n + ³⁵Cl → ³⁶Cl + k γ ,
where k = 2-3, with a total energy of about 8 MeV. Like in the previous phase, the γ-rays Compton-scatter on electrons.
NCD phase: direct neutron capture on ³He contained in the NCD proportional counters:
n + ³He → t + p .

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 ν_e is 2.4 times larger than that for ν_e 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)
D₂O Salt NCD
H₂O CC ν_e+p → n+e⁺ 356 e⁺ 331
D₂O CC ν_e+d → p+p+e^- 83 e^- 72
D₂O CC ν_e+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
Total: 909 605 803 680

Type	Reaction	Number of events	Number of counts (MC)
D₂O	Salt	NCD
H₂O CC	ν_e+p → n+e⁺	356 e⁺	331
D₂O CC	ν_e+d → p+p+e^-	83 e^-	72
D₂O CC	ν_e+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
Total:	909	605	803	680

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.

Sensitivity
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.

Identification criteria for the SNO supernova trigger

The data stream flows through dispatchers directly from the PMTs (and soon the NCDs) to the burst detection process SNOStream.

Level 1
The burst detection system reads the data stream directly from the dispatcher, and buffers events which have triggered more than 35 PMTs. When more than 30 events have been buffered in less than 2 s, the level-1 burst detection is satisfied. The background event rate above the 35-PMT threshold was measured to be 4-5 per minute.
Level 2
SNO has an automated data analysis system applying a set of instrumental background rejection cuts to the data. A burst corresponding to a supernova is expected to have at least 90% surviving events. The level-2 triggers at 35%: it alerts the SNO supernova working group by e-mail and starts the level-3 analysis. It is expected that 1% of supernova neutrino events be removed with the cuts.
Level 3
The events are reconstructed, the angular position of the potential supernova is calculated from the events' geometrical distribution. The supernova working group is e-mailed the new information, sent a text phone message, and called by phone (synthetized voice). The system keeps track of the burst history and suppresses bursts coming in high rate. In the near future, if the burst occurs during a non-calibration/maintenance run, if it is not identified as a muon, if the event distribution is homogeneous, and if the detector has not been having a high-rate of background bursts, an automatic "good" alarm would be sent to SNEWS. This process usually takes a few minutes to achieve.
Human intervention
The SNO supernova working group alerted by phone can verify the level-2 and level-3 analysis results in order to decide if the burst corresponds to a real supernova. In particular the energy and time spectrum and the geometric distribution should be easily identifiable. The detector operator is called to make sure the detector has been operating quietly. Eventually, if the signal is consistant with that of a supernova, a "confirmed" alarm would sent to SNEWS as well as directly to the astronomical community (via the SNEWS server but without waiting for a coincidence). Though improbable, if an erroneous "good" alarm had been automatically sent, it would be possible to retract it. The whole process takes 20 to 30 minutes.

More information on supernovae

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