SNEWS 2.0: a next-generation supernova early warning system for multi-messenger astronomy

The progenitor systems of type Ia supernovae and their associated explosion mechanisms remain debated. The possible progenitors of SNIae—and the observational constraints upon the various scenarios—are discussed extensively in Maoz, Mannucci and Nelemans (Maoz et al 2014) and Ruiz-Lapuente (Ruiz-Lapuente 2014). Even if we accept the canonical model of a SNIa as the disruption of a Chandrasekhar mass (1.4 M_⊙) carbon-oxygen white dwarf, many different scenarios for how the explosion proceeds can be found in the literature (Khokhlov 1991, Plewa et al 2004, Bravo and García-Senz 2006). We refer the reader to Hillebrandt et al (Hillebrandt et al 2013) for a review.

The neutrino emission from a limited number of SNIa simulations has been computed (Odrzywolek and Plewa 2011, Wright et al 2016, 2017a). Wright et al considered the most optimistic case (known as the DDT) and a more general case (their GCD case). The number of events they expect in a 374 kt water-Cherenkov detector from a SNIa at a distance of 10 kpc is of order 1 for the DDT case and 0.01 for the less optimistic GCD. A SNIa would have to be within a few kpc in order to detect tens of events but the probability the next galactic supernova is within 5 kpc is only of order 10% according to Adams et al (Adams et al 2013).

Very massive stars can explode as a PISN if they form a carbon-oxygen core in the range of 64 M_⊙ < M_CO < 133 M_⊙ (Heger and Woosley 2002). The temperatures in these cores are sufficiently high and the electron degeneracy sufficiently low that electron–positron pairs are created. The formation of the pairs softens the equation of state causing a contraction of the core triggering explosive burning of the oxygen (Barkat et al 1967, Rakavy and Shaviv 1967, Fraley 1968). The energy released is enough to unbind the entire star leaving behind no remnant. Some models of PISNe produce very large amounts of ⁵⁶Ni and PISN are candidates for some superluminous supernovae (Smith et al 2007, Gal-Yam et al 2009, Cooke et al 2012, Lunnan et al 2016).

The long-standing expectation of theorists is that only metal-free stars could remain sufficiently massive to explode as PISN (Heger and Woosley 2002). However this expectation has been challenged in recent years. (Langer et al 2007) found PISN can occur in stars with metallicities as large as Z_⊙/3 while (Georgy et al 2017) obtained the conditions for a PISN at near solar metallicities if they included surface magnetic fields. Thus, theoretically at least, a PISN in the Milky Way or one of its satellites cannot be ruled out.

The rate of PISNe is uncertain because (a) observationally we lack an unambiguous method for discriminating these kind of supernovae from the others and (b) theorists have not reached a consensus on which masses at a given metallicity produce these kinds of events. The estimate by Langer et al is for a rate of 10⁻⁴ yr⁻¹ but that could be larger by an order of magnitude if the recent revisions to the progenitor are correct.

The neutrino signals from two PISN simulations have been computed by (Wright et al 2017b). The two models they considered were a low-mass and high-mass case so that the computed signals spanned the range of possibilities. The flux at Earth from a 'small' PISN at 10 kpc was similar to the most optimistic SNIa case i.e. around 1–2 events, but for a 'large' PISN at the same distance the flux was much larger, between 50–100 events depending upon the equation of state and the neutrino mass ordering.

The neutrino emission from merging neutron stars has been computed by (Rosswog and Liebendörfer 2003) and the neutrino emission from a black-hole–neutron star merger simulation was computed by (Caballero et al 2009). In both cases the neutrino emission is similar to that from a CCSN i.e. the neutrino luminosities and mean energies are within a factor of a few of those found in core-collapse simulations), and therefore give similar event rates in detectors. However there are differences: in a core-collapse there are more neutrinos than antineutrinos emitted and the duration of the burst is of order 10 s. In a neutron star merger the opposite matter–anti-matter ratio is expected and the signal lasts for 1 s unless the supermassive neutron star can be prevented from forming a black hole. The rate of black-hole–neutron star mergers in not known precisely but the rate of neutron star–neutron star mergers can be better estimated because there exist a number of such systems in the Milky Way. (Abadie et al 2010) calculate the likely event rate to be 10⁻⁴ yr⁻¹ while (Kalogera et al 2004a, b) give the plausible range to be from 10⁻⁶ yr⁻¹ to 10⁻³ yr⁻¹.

To detect neutrinos from black-hole–neutron star and neutron star–neutron star mergers is challenging. A promising strategy is to search for neutrinos in time-coincidence with detections of mergers in gravitational waves, using a time window of, e.g. 1 s after each merger (Kyutoku and Kashiyama 2018, Lin and Lunardini 2020). This strategy reduces the backgrounds very effectively so that, in fortunate circumstances, even the detection of a single, time-coincident neutrino can be statistically significant. If alerts from Advanced LIGO (Abadie et al 2010) (which has a distance of sensitivity to mergers of about 200 Mpc) were used, a megaton water Cherenkov detector could record about 1 neutrino detection per century (Kyutoku and Kashiyama 2018). When operating in synergy with third-generation gravitational wave observatories, like the proposed Einstein Telescope (Punturo et al 2010), and Cosmic Explorer (Abbott et al 2017b) (sensitivity up to redshift z ∼ 2), the same detector could identify of up to a few neutrinos from mergers per decade, and start placing constraints on the parameters space already after a decade or so of operation (Lin and Lunardini 2020).

Large water Cherenkov detectors (WCDs), such as Super-K and the future Hyper-K, have potentially good supernova pointing capability through the anisotropic neutrino–electron elastic scattering (ES) interaction (Beacom and Vogel 1999, Tomas et al 2003). The majority of supernova neutrinos interact in WCDs through inverse beta decay (IBD). In IBD the direction of the outgoing positron is nearly random with respect to the direction of the incoming supernova neutrino. The angular distribution of measured positron directions from a large number of IBD events is necessary to detect (and possibly use) the small anisotropy in the direction of the neutrino flux.

Fortunately, a few percent of the interactions are due to elastic scatter of the supernova neutrinos from electrons. The outgoing electrons are preferentially forward-scattered and the reconstructed direction of the scattered electron is correlated to the direction of the incoming neutrino. The angular distribution of measured electron directions from ES shows a strong correlation with the direction of the neutrino flux. The magnitude of the anisotropy varies with neutrino energy and flavor. However, since the ES cross sections are small, a large detector mass is required to measure enough ES interactions for direction finding. At present WCDs cannot accurately differentiate between IBD and ES interactions, although adding increasing amounts of gadolinium to Super-K will improve this. Thus, the high ratio of IBD to ES interactions reduces the signal-to-noise ratio of the direction signal, as shown in figure 3.

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In a large WCD the direction of the neutrino flux (and therefore the supernova direction) may be found by analyzing the energies and direction vectors of the electrons/positrons, reconstructed from the Cherenkov ring for each event. For example, the current Super-K real-time supernova burst monitor performs SN direction finding using a maximum likelihood method (Abe et al 2016). The direction and energy of each event is used to calculate the likelihood of a given supernova direction and event reaction channel based on a probability density function determined from supernova neutrino flux models and Super-K MC simulations. The supernova direction angles, and other parameters, are varied until the total likelihood is maximized. The pointing accuracy for a supernova at 10 kpc is estimated using a modern supernova model to be 4.3°–5.9° (68.2% C.L.) covering all combinations of neutrino oscillations and mass orderings. This will improve to 3.3°–4.1° for the fully doped SK-Gd.

The accuracy of supernova direction finding based on the anisotropy of ES events depends on the number of events. This varies with detector volume, supernova distance, neutrino oscillations and supernova mass and neutrino emission mechanisms. The coming Mtonne-scale WCDs, such as Hyper-Kamiokande, will also have improved direction finding due to increased statistics in the ES channel. The pointing accuracy for Hyper-K is expected to be 1°–1.3° (Abe et al 2018).

The introduction of small amounts of gadolinium into the water volume of WCDs will allow accurate tagging of individual IBD events. Thus, direction finding routines could de-weight or exclude the IBD events, potentially increasing the speed and pointing accuracy. Super-K will be implementing such an upgrade in the near future.

Liquid argon time projection chambers (LArTPC) detectors have the ability to do fine-grained tracking of the final-state particles, and, like WCDs, can exploit the intrinsic directionality of anisotropic interactions in the detector. Ability to tag different interaction channels also helps. ES interactions on electrons with well-known energy dependence can be used, as well as the charged-current ν_e absorption interactions on ⁴⁰Ar. The latter have a relatively weak anisotropy but large statistics.

Unlike water Cherenkov signals, the tracked electrons have a head-tail ambiguity that results in about half of them with a fake reconstructed backwards direction. This ambiguity can be resolved statistically using sophisticated reconstruction techniques. Improvement by using the directionality of bremsstrahlung gammas, which are emitted preferentially in the electron travel direction, has been demonstrated in DUNE. Using a likelihood technique with the ensemble of electron scattering and ν_eCC events, DUNE has demonstrated about 5° pointing for a 10 kpc supernova signal (Abi et al 2020a).

Liquid scintillator and WCDs alike are mostly sensitive to IBD interactions—the major difference between the two being that such interactions are considered a background for supernova pointing in the latter while they are considered a signal in the former. Indeed, while considered isotropic at first order, the positron and neutron emitted after an IBD interaction both possess a slight energy-dependent anisotropy (Strumia and Vissani 2003). At the energies of interest for supernova neutrino detection, the positron quickly deposits its energy and therefore most of the anisotropy is carried away by the neutron, always emitted in the forward direction. Although this appears to be similar to the forward emission of an electron in ES, detecting a neutron direction is arduous in large scintillator detectors. In the vast majority of cases, only the position of the neutron capture vertex after thermalization and diffusion can be determined. For each IBD interaction, a direction vector, starting at the reconstructed positron vertex and ending at the reconstructed neutron capture vertex, can be defined. Due to the smearing caused by the neutron transport after its creation, a single IBD vector is not sufficient to efficiently reconstruct its neutrino incoming direction. However, the analysis of thousands or more of IBD interactions can help reconstruct the statistical direction of an incoming neutrino flux, as demonstrated by the CHOOZ collaboration with about 2700 events (Apollonio et al 2000).

Such an analysis can be performed to determine the expected direction of a supernova-induced neutrino flux, as shown in (Fischer et al 2015). In this study, the statistical nature of the supernova direction reconstruction through IBD anisotropy was exploited by combining the direction vectors of all IBD interactions from several liquid scintillator-based detectors, existing or proposed. While the pointing capabilities of individual existing detectors, shown in figure 4, are no match for the accuracy of Super-K, their combination, as well as the introduction of JUNO in a near future, provides non-negligible pointing information. With the addition of JUNO to the existing large liquid scintillator detectors, supernova pointing accuracy through IBD interactions could reach 12 degrees (68% confidence level (C.L)) for a supernova located 10 kpc away.

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It is worth noting that efforts are underway to extract directional information from the small amount of Cherenkov light which leads the largely isotropic scintillation light (Aberle et al 2014). The CHESS experiment found that a time resolution of 338 ± 12 ps (FWHM) was required for reasonable efficiency in separating the two light components in a mixture of LAB with 2 g L⁻¹ PPO (Caravaca et al 2017). An alternative to fast PMT's is to slow the emission of scintillation light (e.g. (Wang and Chen 2020, Biller et al 2020)), which is possible with different scintillators and fluors. Finally, the different spectra of the two components may be exploited using dichroic filters (Kaptanoglu et al 2019, 2020). Such ideas may be exploited in future liquid scintillator detectors, or upgrades of current ones.

The Super-Kamiokande detector consists in a cylindrical stainless-steel tank of 39 m diameter and 42 m height, filled with 50 kt of pure water (Fukuda et al 2003). It is optically separated into an inner detector instrumented with 11 146 Hamamatsu R3600 50 cm diameter hemispherical PMTs (facing inward and fixed at approximately 2.5 m from the wall) and an outer detector instrumented with 1885 Hamamatsu R1408 20 cm diameter hemispherical PMTs (facing outward and fixed at 2 m from the wall). For supernova detection, only events in the inner detector are used, corresponding to a mass of 32 kt.

The detector is located in the Mozumi mine in the Gifu prefecture, Japan. It lies under Mt. Ikeno (Ikenoyama) with a total of 2.700 mwe mean overburden. Data taking began in April, 1996 and has been separated (at the time of the writing) into six periods from SK-I to SK-V, plus SK-Gd starting in July, 2020. SK-Gd is the first period where gadolinium sulfate is dissolved in the tank (0.02% at first). This allows the detection of a delayed signal from neutron capture on gadolinium following IBDs.

Supernova neutrinos are mainly detected through the IBD channel (${\bar{\nu }}_{\text{e}}+p\hspace{2pt}\to \hspace{2pt}{e}^{+}+n$, ∼4–8 K events at 10 kpc) but ESs on electrons (ν + e⁻ → ν + e⁻, $\bar{\nu }+{e}^{-}\to \hspace{2pt}\bar{\nu }+{e}^{-}$) and interactions on oxygen (ν_e +¹⁶ O → e⁻ + X, ${\bar{\nu }}_{\text{e}}{+}^{\hspace{1pt}16}O\hspace{2pt}\to \hspace{2pt}{e}^{+}+X$) are also detected (with respectively ∼120–250 and ∼80–250 events at 10 kpc).

The current real-time supernova system only uses events in the SK fiducial volume (inner detector volume reduced by 2 m from the walls, leading to a fiducial volume of 22.5 kt) and with visible energy greater than 7 MeV ⁵¹ . For each selected event, a 20 s time-window is opened backward in time. The total number of selected events N_cluster in this time window is computed, as well as the variable D that identifies the dimension of the vertex position distribution (D ∈ {0, 1, 2, 3}, corresponding respectively to point-, line-, plane- and volume-like distributions) (Abe et al 2016).

If N_cluster ⩾ 60 events and D = 3, a ‛golden' warning is generated, without needing any combination with other detectors before worldwide announcement. If 25 ⩽ N_cluster < 60 events and D = 3, a ‛normal' warning is generated and is sent to Super-Kamiokande experts as well as to SNEWS for combination. ‛Golden' and ‛normal' warning correspond respectively to 100% supernova detection efficiency at the Large Magellanic Cloud and at the Small Magellanic Cloud. The direction of the supernova event can be recovered using the ESs with a typical angular resolution of 3°–5° (depending on supernova model) for an event at 10 kpc (Abe et al 2016) and is announced independently by the collaboration in a later notice.

The Hyper-Kamiokande detector will be located 8 km south of Super-Kamiokande, under the peak of Mt. Nijuugo with an overburden of 1.750 mwe. It will be conceptually very similar to the Super-Kamiokande detector, with an enlarged volume of 260 kt (217 kt fiducial for supernova), instrumented with ∼40 000 inward-facing 50 cm diameter PMTs and ∼6 700 outward-facing 20 cm diameter PMTs (Abe et al 2018).

The 50 cm PMTs for the inner detector are the newly developed Hamamatsu R12860 model, which improves the timing resolution and detection efficiency by a factor of 2 compared to the model used in Super-Kamiokande. Alternative designs are currently under consideration, which use ∼20 000 50 cm diameter PMTs, complemented with several thousand multi-PMT modules, each consisting of multiple 7.7 cm diameter PMTs. An alternative design for the outer detector, which uses a larger number of 7.7 cm diameter PMTs to increase the granularity at reduced cost, is also under development (Zsoldos 2020).

For a supernova located at 10 kpc, $\mathcal{O}$(10–100k) IBDs and $\mathcal{O}$(1k) electron scatterings are expected in the detector, allowing precise determination of the energy and time profile of the event, as well as a pointing accuracy of about 1°–1.5° (Abe et al 2018). By using the reconstructed time and energy of all events, Hyper-Kamiokande will be able to distinguish clearly between different supernova models even at distances of up to 60–100 kpc (Migenda 2020).

A special trigger for supernovae is currently being designed for the Hyper-Kamiokande DAQ system and the experiment is expected to start data taking in 2027.

The IceCube Neutrino Observatory is an array of 5160 digital optical modules (DOMs) instrumenting 1 km³ of clear Antarctic ice at the geographic South Pole (Aartsen et al 2017). The DOMs are deployed on 86 strings, with 60 DOMs per string, at depths ranging from 1.450 m to 2.450 m below the surface of the ice sheet. The strings have an average nearest-neighbor separation of 125 m, and on each string the DOMs are spaced vertically by 17 m. The detector records the Cherenkov light produced by cosmic ray muons and the charged particles created by neutrinos interacting in the ice, and uses the relative timing of direct and indirect Cherenkov photons to reconstruct the energies and arrival directions of the cosmic muons and neutrinos.

The sparse geometry of IceCube is optimized to reconstruct the arrival directions of neutrinos with energies between 10 GeV and 10 PeV (Aartsen et al 2017), so the detector cannot be used to reconstruct tracks from the $\mathcal{O}\left(10\enspace \text{MeV}\right)$ neutrinos produced in CCSNe. However, IceCube is sensitive to the positrons generated during the IBD of ${\bar{\nu }}_{\text{e}}$, which arrive over ∼10 s during the accretion and cooling phases of a CCSN (Abbasi et al 2011). The optical absorption length in the ice is >100 m, so the effective volume of each DOM is several 100 cubic meters. The positrons produced in the ice during the ${\bar{\nu }}_{\text{e}}$ burst are therefore observable as a rise in the background count rates in the DOMs over a period of 0.5 s to 10 s.

Depending on the mass of the stellar progenitor, IceCube will record 10⁵ to 10⁶ photoelectrons from a supernova located 10 kpc from Earth, enabling detailed measurements of the neutrino light curve (Abbasi et al 2011, Cross et al 2019). If the light curve has significant time structure, such as a cutoff in the ${\bar{\nu }}_{\text{e}}$ flux due to the creation of a black hole, IceCube can provide limited pointing toward the source with a resolution Δψ ≈ 10°–100°, depending on distance and model assumptions. The DOM counts can also constrain the shape and average energy the ${\bar{\nu }}_{\text{e}}$ spectral energy distribution with a resolution of ΔE/E > 25% (Köpke 2018).

Construction of IceCube finished in 2011, and since 2015 the trigger-capable live time of the detector has averaged 99.7% per year. IceCube thus provides a critical high-uptime monitor for CCSNe in the Milky Way and Magellanic Clouds. A realtime monitor tracks the photoelectron rates in the IceCube DOMs in with a resolution of 2 ms. The monitor performs a moving-average search for a collective rise in the DOM rates in sliding bins of 0.5 s, 1.5 s, 4 s, and 10 s (Abbasi et al 2011), as well as an unbinned search for significant changes in the collective count rate (Cross and BenZvi 2018). When a statistically significant increase in the DOM counts is observed, an alert is sent to SNEWS with a latency of ∼5 min. The software also buffers the full DOM waveforms in a 600 s window around the time of the alert. The buffered waveforms are not limited by the 2 ms resolution of the online system, and can be used for detailed model fits of the CCSN light curve (Köpke 2018).

The IceCube detector is currently being upgraded with two new strings instrumented with multi-anode DOMs (Ishihara 2019, Classen et al 2019), which if deployed in large numbers can improve constraints on the CCSNe neutrino average energy to ΔE/E ≈ 5% (Lozano Mariscal 2018). A build-out of the detector to 8 km³ and ∼10 000 optical modules (IceCube Gen-2), planned for the late 2020s, will more than double the photocathode area in the detector (Aartsen et al 2019).

Is a research infrastructure under construction in the Mediterranean Sea. It comprises two deep-sea neutrino detectors, ORCA, located offshore Southern France, optimized for neutrino oscillation studies and ARCA, located offshore Sicily, optimized for neutrino astronomy. They respectively focused on neutrino oscillations and astrophysics. KM3NeT detectors are arrays of multi-PMT DOMs. The two sites will be instrumented with over 6000 DOMs for a total of about 200 000 PMTs. KM3NeT DOMs are arranged in vertical detection lines of 18 DOMs, which are deployed at depths between 2500–3,400 m to form a three-dimensional array (Adrián-Martínez et al 2016).

KM3NeT sensitivity to CCSN neutrinos on the tens-of-MeV energy scale is mainly achieved through the detection of IBD interaction products in the proximity of the DOMs. A supernova neutrino burst is expected to produce an increase in the coincidence rate between different PMTs on the same optical module. The number PMTs detecting a photon in coincidence can be used to discriminate the signal from the natural backgrounds, namely bioluminescence, ⁴⁰K-dominated radioactive decays and atmospheric muons (Adrián-Martínez et al 2020).

To achieve the best detection sensitivity, a selection based on coincidences with at least six hit PMTs is defined. The background from atmospheric muons is reduced by rejecting events which are correlated over multiple DOMs. This reduces the expected background rate below 1 Hz per detection unit. With this selection, the signal expectation for the accretion phase of a CCSN exploding at 10 kpc is between 50 and 250 events for a 115-lines instrumented block. This selection is used for the sensitivity estimation and implemented in the real-time KM3NeT supernova trigger.

The KM3NeT detectors will be sensitive beyond the galactic center, reaching the coverage of ∼90%–95% of progenitors in the Galaxy. Both the online and offline searches are applied within a time window of 500 ms, covering the accretion phase. For the real-time trigger, data is sampled with a frequency of 10 Hz. The expected detection horizon for a false alert rate compatible with the SNEWS requirement varies between 13 and 26 kpc, depending on the considered flux model.

For the purpose of CCSN astrophysics studies, the signal statistics can be improved using a selection of all coincidences (two or more hit PMTs on a DOM) at the cost of a lower purity. With a complete detector at each site (one instrumented block in ORCA and two in ARCA), KM3NeT is expected to collect 20–50 000 events (see table 4). It will be able to study the neutrino light-curve for signatures of time-dependent patterns, the determination of the arrival time of the burst and the characterization of the neutrino spectrum (Colomer et al 2019).

The effective mass of a KM3NeT 115-lines instrumented block is 40–70 kt for all coincidences and 1–3 kt for coincidences with six or more hit PMTs.

The KM3NeT real-time supernova search is currently operational with a dedicated data processing pipeline, running on the already connected detection units. Alerts can be sent within 20 s from the generation of the corresponding data off-shore. The triggering capability under SNEWS requirements already reaches sensitivity to progenitors closer than 4.5–8.5 kpc. This horizon will increase in the near future with the deployment of new detection lines.

The Baksan underground scintillation telescope was one of the detectors which observed neutrinos from SN1987A (Alekseev et al 1987) and is still operational, having continuously watched for SN neutrinos from 1980 till the present (Novoseltsev et al 2020). The experiment is at a depth of 850 mwe in the Baksan Valley, Kabardino-Balkari, Russia. The experiment's 330 t of liquid scintillator is primarily sensitive to positrons produced by IBD, and is divided into two independent sub-detectors operating in coincidence with each other. The detector elements are grouped by background rate (and thus threshold), with ‛D1' having 130 t at an 8 MeV threshold and a background rate of 0.02 Hz, and ‛D2' 110 t at 10 MeV and 0.12 Hz. This arrangement further reduces the already low background rate, contributing to the detector's long-term stability and > 90% uptime. In event of a supernova, the remaining 90 t of scintillator would also record events, but this section has a background rate of 1.4 Hz so is excluded from the online SN trigger. The detector and its supernova trigger are detailed in (Novoseltsev et al 2020). A future 10 kt detector is under development, to be placed at a depth of 4800 mwe (Petkov et al 2020).

The large volume detector (LVD) is a modular 1 kt liquid scintillation detector and consists of an array of 840 counters located underground (minimal depth 3000 mwe), in the INFN Gran Sasso National Laboratory (Italy). It has been designed to study neutrinos from CCSN mainly through IBD interactions (Aglietta et al 1992). The detector modularity allows the experiment to achieve a very high duty cycle, essential in the search of unpredictable sporadic events, typically greater than 99% over more than 20 years. LVD has been in operation since 1992, after a short commissioning phase, with an active mass increasing from 300 t to 1000 t at the time of completion in 2001. The experiment is approaching the decommissioning phase planned for the end of 2020, after almost 30 years of continuous operations.

During these years, as shown in (Agafonova et al 2015), LVD has been monitoring the entire Galaxy. The number of expected neutrino events in LVD from a CCSN have been evaluated via a parameterised model based on a maximum likelihood procedure based on the SN1987A data assuming energy equipartition and normal mass hierarchy for neutrino oscillations (Pagliaroli et al 2009b). The resulting average ${\bar{\nu }}_{\text{e}}$ energy is ${\bar{E}}_{{\bar{\nu }}_{\text{e}}}=$ 14 MeV, for a E_b = 2.4 × 10⁵³ erg total energy SN. At a reference distance of 10 kpc, we expect a total of 300 events, 88% due to IBD. The burst search, i.e. the search of cluster of events in the time series of triggers, is performed on a pure statistical basis as explained in detail in (Agafonova et al 2015).

LVD is a founding member of the SNEWS project and it has been participating in the on-line operations since the beginning in 2005 together with the SuperK and SNO detectors (Antonioli et al 2004).

Is an ultra-pure liquid organic scintillation detector designed to study solar neutrinos (Alimonti et al 2009) in real time. There are two main interaction channels, namely ES on electrons and IBD, by which neutrinos are seen. In case of a supernova, observation of other processes becomes important including ES on protons and various reactions on carbon. Nevertheless it is reasonable due to low statistics to organize alarm datagram generation for SNEWS based on searching for IBD event bursts and/or point-like event excesses.

Despite the relatively small mass of the target (278 t) and therefore the quite limited event numbers from nearby supernovae compared to other modern detectors, Borexino is a rather sensitive apparatus because of very low background levels. If the Borexino SN alarm is the result of searching for an excess of point-like events, the respective mean background rate is $\mathcal{O}$(0.01 Hz) above 1 MeV. Since the alarm is generated based on the IBD analysis, the background level is even lower. For example, only 154 golden IBD candidates were found in a live time of approximately 9 years in the last geo-neutrino investigation with Borexino (Agostini et al 2020). Such low background conditions were achieved thanks to a number of features of the detector, its special location and applied methods of data analysis. To begin with, Borexino is placed in the underground laboratory (LNGS) under a thick layer of rocks that provide the shielding against cosmogenic background equivalent 3800 mwe and therefore the muon flux is decreased up to (3.432 ± 0.003) × 10⁻⁴ m⁻² s⁻¹ (Agostini et al 2019). There are no nuclear power plants in Italy and the mean weighted distance of the reactors from the LNGS site is more than 1000 km, resulting in a low antineutrino background in the detector. Other reasons for the low background rates are concentric multilayer shielding and ultra-radiopurity of all materials. The complex selection and refinement of the detector materials, accurate assembling guarantee extremely low levels of ²³⁸U and ²³²Th contamination that are less than <9.4 × 10⁻²⁰ g g⁻¹ (95% C.L.) and <5.7 × 10⁻¹⁹ g g⁻¹ (95% C.L.) respectively (Agostini et al 2020).

The light yield in Borexino is approximately 500 photoelectrons (p.e.) per 1 MeV and the energy resolution scales as $\sim 5\%/\sqrt{E\enspace \left(\text{MeV}\right)}$ (Alimonti et al 2009). Borexino is a position sensitive detector. The absolute errors of the reconstructed coordinates x, y, z for a point-like event in the target are virtually the same and depend on the energy as ∼10 cm/$\sqrt{E\enspace \left(\text{MeV}\right)}$. To classify the events, pulse shape discrimination methods are actively applied.

Since the main DAQ of the Borexino detector (‛LABEN') was designed for spectroscopy of solar neutrinos with energies in the sub-MeV energy range, a special additional data acquisition system based on flash ADCs (‛FADC') was designed to record neutrinos with energies up to ∼ 50 MeV from supernovae. Both systems are working and will continue to function until the end of the experiment.

Borexino joined the SNEWS community in 2009. Originally there were two independent modules for the LABEN data analysis and generation the alarms: the Echidna online supernova monitor and the Princeton supernova monitor. Currently only the latter is operational. The Borexino experiment is approaching its end. The data taking is planned to stop in 2021.

Kamioka liquid scintillator anti-neutrino detector (KamLAND) is an experiment to detect ${\bar{\nu }}_{\text{e}}$ signals using 1000 t of ultra pure liquid scintillator. The KamLAND detector occupies the former site of the Kamiokande experiment in the Kamioka mine (2700 mwe overburden). A spherical balloon of 13.0 m diameter is suspended in the center of the KamLAND detector and filled with 1000 t ((1171 ± 25) m³) of ultra pure LS. Scintillation photons are detected by 1325 fast 17-inch aperture Hamamatsu PMT custom-designed for KamLAND, and 554 20-inch PMTs inherited from Kamiokande. The PMT array are attached inside on the spherical stainless steel tank of 18.0 m diameter. The outside of the tank is a water Cherenkov detector for veto with approximately 3000 m³ of pure water and 140 high-QE PMTs (Ozaki and Shirai 2017). Event vertex and energy reconstruction is based on the timing and charge distributions of scintillation photons. In 2013, they are estimated to be $\sim 12\enspace \text{cm}/\sqrt{E\enspace \left(\text{MeV}\right)}$ and $6.4\enspace \%/\sqrt{E\enspace \left(\text{MeV}\right)}$, respectively (Gando et al 2013).

The main channel for supernova neutrinos in KamLAND is ${\bar{\nu }}_{\text{e}}$ via IBD. 200–300 events are expected from a galactic supernova at 10 kpc. Because of delayed coincidence measurements with temporal and spatial correlation between the positron and neutron events resulting from IBD, very low backgrounds for ${\bar{\nu }}_{\text{e}}$ are possible. KamLAND uses this channel for supernova alarms. If the online process finds two IBD events during 10 s, KamLAND will send an supernova alert to the SNEWS server.

The proton recoil (ν + p → ν + p) is one of the unique features in LS detectors (Beacom et al 2002). This channel provides a chance to study the total energy, temperature (Beacom et al 2002) and reconstruction of the ν_x spectrum (Dasgupta and Beacom 2011). Since this channel does not have background rejections like the delayed coincidence measurements, low-background measurements are required. KamLAND had performed a purification campaign from 2007 to 2009, enabling measurements of supernova neutrinos using proton recoils. In the current configuration of the KamLAND trigger, the expected number of events is about 60 with assumptions of total luminosity of 3 × 10⁵³ erg and $\langle {E}_{{\nu }_{\text{e}}}\rangle =$ 12 MeV, $\langle {E}_{{\bar{\nu }}_{\text{e}}}\rangle =$ 15 MeV, and $\langle {E}_{{\nu }_{x}}\rangle =$ 18 MeV.

It is also possible to detect neutrinos emitted before collapse (‛pre-supernova neutrinos') using IBD. KamLAND is able to detect pre-supernova neutrinos from nearby stars, such as Betelgeuse and Antares, before their cores collapse. The background rate of low energy IBD in KamLAND is typically 0.1 event/day. This strongly depends on the reactor on/off status in Japan since the main background is neutrinos emitted from reactors. Even if the expected cumulative number of IBD events from a pre-supernova in a 48 h period is only a few, the background is low enough that this is a statistically significant detection (Asakura et al 2016). With a latency of about 25 min, this detection significance is open for authorized users. If a detection significance is larger than 3σ (approximately three IBD events in the last 48 h), the KamLAND collaboration has a special meeting to discuss the result. Based on that discussion, KamLAND will send a message to The Astronomer's Telegram. The connection from KamLAND to The gamma-ray coordinates network and SNEWS 2.0 will be implemented.

KamLAND is planning new electronics and DAQ systems which will improve the latency of supernova alarms. To better measure proton recoils with a low energy threshold, the trigger threshold will be changed just after the detection of the first 10 events.

Unfortunately, supernova neutrino observations in KamLAND has a small conflict to KamLAND-Zen, an experiment to search for neutrino-less double-beta decay of ¹³⁶Xe with the existing KamLAND detector. In 2011, an inner balloon with a 3.08 m diameter was installed in the center of the KamLAND detector (Gando et al 2012). Xe-loaded LS was filled in the inner balloon. Because of double-beta decays of ¹³⁶Xe and backgrounds from the balloon materials, an additional cut for ${\bar{\nu }}_{\text{e}}$ analysis (Gando et al 2013) is applied. In 2018, an updated inner balloon with a 3.84 m diameter was installed. Improvements of the analysis methods are being studied to maximize the sensitivity to supernova neutrinos.

The Jiangmen Underground Neutrino Observatory (JUNO) is a multi-purpose neutrino experiment (An et al 2016). It primarily aims to determine the neutrino mass ordering using reactor antineutrinos. The facility is under construction in Jiangmen city, Guangdong province, China and expected to start data taking in 2022. The detector consists of a central detector, a water Cherenkov detector and a muon tracker, deployed in a laboratory 700 m underground (∼ 1800 mwe). The central detector is a 20 kt liquid organic scintillation (LS) detector with a designed energy resolution of $3\%/\sqrt{E\left(\mathrm{M}\mathrm{e}\mathrm{V}\right)}$, with a goal to reach a rather low detection energy threshold of 0.2 MeV with a conservative PMT dark noise rate of 50 kHz (Fang et al 2020), limited by the intrinsic radioactive background mainly from ¹⁴C in the LS.

Supernova detection is one of the major purposes for JUNO. Given the large fiducial mass and low energy threshold of the LS detector, JUNO is able to register all flavors of $\mathcal{O}$(10 MeV) supernova burst neutrinos with relatively high statistics. For a typical CCSN at 10 kpc, there could be ∼ 5000 events from the IBD (golden) channel, ∼ 2000 events from neutrino–proton ES, more than 300 events from neutrino–electron ES, as well as charged current and neutral current interactions of neutrinos on ¹²C nuclei expected in JUNO (Lu et al 2016). IBD events with anisotropic prompt and delayed signals could determine the supernova pointing with an uncertainty of $\mathcal{O}$(10°) (Apollonio et al 2000). By combining all these channels, JUNO has the unique opportunity to provide flux information and to reconstruct the energy spectrum of ν_x (ν_μ , ν_τ and ${\bar{\nu }}_{\mu }$, ${\bar{\nu }}_{\tau }$) supernova burst neutrinos (Li et al 2018, 2019).

For the nearby galactic CCSN within ∼ 1 kpc, JUNO is also sensitive to the $\mathcal{O}$(1 MeV) pre-supernova neutrinos via IBD and neutrino-electron ES, thanks to its low energy threshold. The IBD events could be clearly distinguished using the coincidence signals, while the neutrino–electron interaction of pre-supernova suffers from the high rate of background induced by the radioactivity of detector materials and cosmogenic isotopes. Therefore, the pre-supernova could be monitored online with the IBD candidates in JUNO, including possibly pointing information once a large number of the IBD events are accumulated (Li et al 2020). In addition, 2 GB DDR3 memory will be embedded on the global control units in the front-end electronics design of JUNO (Pedretti et al 2018), to buffer the rapid increase of events from supernova burst neutrinos from a nearby galactic CCSN.

Currently the real-time monitor strategy for both supernova burst neutrinos and pre-supernova neutrinos is under progress in JUNO, which consists of prompt monitor on hardware and DAQ monitor. The experiment is capable of providing the supernova alerts as well as the pre-supernova ones to SNEWS in the near future.

The SNO+ experiment is a liquid scintillator detector situated 2092 m underground at SNOLAB (Andringa et al 2016). It reuses much of the infrastructure of the SNO detector, which first demonstrated flavor conversion in ⁸B solar neutrinos, including the 12 m-diameter acrylic vessel (AV) containing the active material, and some 9300 PMTs arranged in a geodesic support structure with a diameter of approximately 17.8 m. The flat rock overburden provides (5890 ± 94) mwe shielding and reduces the cosmic muon flux to 63 muons per day through an 8.3 m-radius circular area.

The main goal of SNO+ is to observe neutrinoless double-β decay of the ¹³⁰Te nucleus. Tellurium will be dissolved in a cocktail of linear alkylbenzene (LAB) and 2,5-diphenyloxazole (PPO). The expected light yield is over 400 PMT hits per MeV. In addition to the new infrastructure needed to handle and purify liquid scintillator, upgrades have been made to the buffered readout electronics, cover gas system, and calibration systems using LED, laser, and radioactive sources. The trigger readout window is roughly 400 ns and is essentially deadtime-less. SNO+ began taking data with an AV filled with water in may 2017. Over the course of 2020, SNO+ is replacing the water in the AV with 780 t of scintillator, and soon after expects to load the detector with 0.5% (by mass) of tellurium, resulting in 1330 kg of the target isotope. Techniques for increasing the Te load are promising.

As with other liquid scintillator experiments, SNO+ will be sensitive to IBDs and neutrino elastic scatters off electrons. The high IBD efficiency, combined with a very low background rate, will be exploited to add to the SNEWS pre-supernova alert. In addition, the low background rate makes it possible to observe ES off protons. Though the energy deposit from such scatters is highly quenched, they may represent nearly half of the neutrinos observed from a supernova at SNO+, and are an important channel for flavor-agnostic neutral current interactions. Table 3 illustrates yields from different interaction channels for a lower, conservative light yield of 200 hits per MeV (i.e. less than half of the expected light yield given above). Efforts are also underway to investigate whether different fluors can help distinguish directional Cherenkov light (e.g. (Biller et al 2020)).

Table 3. Illustrative neutrino interaction yields, from (Andringa et al 2016), for 780 t of LAB–PPO and a supernova at 10 kpc. The supernova model releases 3 × 10⁵³ erg equally partitioned among all neutrino flavors and considers no flavor changing mechanisms. The NC νp yield will be reduced to 118.9 ± 3.4 if the trigger threshold is set at 0.2 MeV. The uncertainties quoted here include only cross section uncertainties (the Standard Model cross section uncertainty on νe ES is less than 1%).

Reaction	Yield
NC: νp → νp	429.1 ± 12.0
CC: ${\bar{\nu }}_{\text{e}}p\to {e}^{+}n$	194.7 ± 1.0
CC: ${\bar{\nu }}_{\text{e}}\enspace {\;}^{12}\mathrm{C}\to {e}^{+}\enspace {\;}^{12}{\mathrm{B}}_{\text{gs}}$	7.0 ± 0.7
CC: νe 12C → e−12Ngs	2.7 ± 0.3
NC: ν12C → ν'12C*(15.1 MeV)	43.8 ± 8.7
CC/NC: ν12C → 11C or 11B + X	2.4 ± 0.5
νe ES	13.1

The NOvA experiment consists of two segmented liquid scintillator detectors. They are functionally identical, and mainly differ by mass and location. The 300 t near detector is located at Fermilab, near Chicago, at a depth of 100 m. The 14 kt far detector is located near Ash River, Minnesota, and sits on the Earth's surface with a modest barite overburden. The detectors are composed of hollow extruded PVC cells 3.9 cm × 6.6 cm in cross-sectional area, which are connected together to form planes. The planes are glued together to form the full body of the detector, alternating between vertical and horizontal orientations to allow for reconstruction in three dimensions. The PVC cells are filled with a mixture of mineral oil and pseudocumene. Scintillation photons produced by the passage of charged particles through the cells bounce off of the highly-reflective cell walls until they are absorbed by a wavelength-shifting fiber. The fiber is looped through the length of the cell, with both ends terminating on the same channel of an avalanche photodiode.

IBD (${\bar{\nu }}_{\text{e}}+p\to {e}^{+}+n$) is the most common interaction mode for supernova neutrinos in the NOvA detectors, followed by ES on electrons, and neutral current interactions on carbon nuclei. Detection threshold is ∼ 8–15 MeV, depending on how close to the readout the energy deposition occurred, meaning many of the expected interactions will be detectable.

There are several challenges to reconstructing supernova neutrinos with NOvA. Due to its size, the far detector will see many more neutrino interactions than the near detector, but it is on the surface and subject to a cosmic ray muon rate of ∼ 148 kHz. It is easy to identify and veto the muons themselves with software, along with the Michel electrons that are often produced at the end of stopping tracks, but identifying the spallation products they leave in their wake is more difficult.

The NOvA DAQ employs a buffered readout system that allows for continuous readout of the detector. In its current configuration, the DAQ writes 45 s of continuous data to disk in the event of a supernova trigger. NOvA subscribes to SNEWS and issues a supernova trigger for any SNEWS alert. A data-driven supernova trigger has also been developed and running on both detectors since November 2017 (Acero et al 2020). This data-driven trigger removes known backgrounds and groups hits into candidate IBD interactions. A supernova will manifest as a sudden increase in the rate of IBD candidates, which then slowly returns back to baseline.

A time series of IBD candidates is constructed in real time and compared against two hypotheses, one which represents the background only and another which represents a signal superposed on the background. A log likelihood ratio is computed, which is then converted into a signal significance. Several models for the signal shape, including a flat one, have been tested for this method, and they all show similar discrimination power. With this method, NOvA can identify a potential supernova signal in a model-independent way. A trigger is issued when the signal significance exceeds a threshold, which is currently chosen to be 5.6 σ and corresponds to an SNEWS-inspired false alarm rate of one per week.

The helium and lead observatory (HALO) utilizes lead-based detection, and has been operating at SNOLAB since 2012. HALO consists of 79 t of lead, instrumented with ³He neutron counters. These counters have a very low background rate, which allows sufficient trigger discrimination for HALO to be used in the SNEWS network.

Lead-based neutrino detection is possible through charged current or neutral current interactions with lead nuclei. The charged current interactions are: ν_e + ²⁰⁸Pb → ²⁰⁷Bi + n + e⁻ and ν_e + ²⁰⁸Pb → ²⁰⁶Bi + 2n + e⁻. Their kinematic threshold energies (weighted over the isotopic abundances) are 10.2 MeV and 18.1 MeV, respectively. The neutral current interactions: ν_e + ²⁰⁸Pb → ²⁰⁷Pb + n and ν_e + ²⁰⁸Pb → ²⁰⁶Pb + 2n have weighted threshold energies 7.4 MeV and 14.4 MeV, respectively. Because these interactions have different cross sections and thresholds the number of 1n and 2n events depends of the incident neutrino energy. The neutrons that result from these interactions can travel some distance through the lead; they are thermalized by collisions with the lead and with the polyethylene moderator prior to being captured by ³He gas in the proportional counters. Lead-based neutrino detectors are sensitive primarily to electron neutrinos, because ${\bar{\nu }}_{\text{e}}$ charged-current reactions are strongly suppressed via Pauli blocking.

The relatively small lead mass of HALO limits its effectiveness for studying supernovae from distances beyond ∼ 5 kpc.

HALO-1kT is a proposed upgrade to HALO to be sited at LNGS which would leverage 1000 t of lead from the decommissioned OPERA experiment in order to create a low cost and low maintenance neutrino detector. The much larger target mass will greatly increase the sensitivity to supernova neutrinos. The lead will be instrumented with a 28 × 28 array of ³He detectors, comprising 10 000 L atm of ³He. The greatly increased target mass should allow detection of supernova at much greater distances. The major advantage of these lead-based detectors is the very low maintenance and high livetime, which could allow them to maintain continuous operation for decades. Supernova neutrino detection would be a primary mission for HALO-1kT, with the expectation of a multiple decade long uninterrupted search for supernovae.

RES-NOVA (Pattavina et al 2020) aims to obtain a large SN signal in a lead detector using a small amount of very low-background archaeological lead, allowing the lowering of the threshold and giving sensitivity to the higher-rate CEνNs interactions.

The Global argon dark matter collaboration's aim is the direct detection of WIMPs in liquid argon (LAr). Strong exclusion limits for WIMP-nucleon spin-independent interactions have already been set by the running detectors, DarkSide-50 (50 kg of LAr) (Agnes et al 2018) and DEAP-3600 (3279 kg of LAr) (Ajaj et al 2019). In order to improve the sensitivity, a future detector's target mass must be scaled up to tonnes and be extremely radiopure. Hence, the upcoming multi-tonne detectors, DarkSide-20k (DS-20k) (47 t LAr) and Argo (300 t LAr), will be built with radiopure materials including underground argon as the detection target (Agnes et al 2016), and use silicon photomultipliers rather than PMTs for photon detection. Based on an analysis done with an input supernova neutrino flux from the Garching group simulation for a 27 M_⊙ progenitor mass following the LS220K equation of state (Mirizzi et al 2016), DS-20k will be capable of observing ∼340 neutrino events after selection cuts at a distance of 10 kpc. This shows that DS-20k will be capable of observing the neutrino signal from such a CCSN with more than 5σ significance up to 40 kpc(Agnes et al 2020).

The LZ and XENON Collaborations aim for the direct detection of WIMPs, but using liquid xenon (LXe) TPCs (Akerib et al 2020a, Aprile et al 2017). The XENON1T experiment, with about 2 t of active target mass, has set the most stringent limits on WIMP cross-sections for masses above 6 GeV (Aprile et al 2018). It has also measured the half life of ¹²⁴Xe decaying via two-neutrino double electron capture, the rarest decay process to have ever been directly measured (Aprile et al 2019a). Through its lifetime, XENON1T has been directly connected with SNEWS in order to receive alarms and promptly save any related data of interest for later analysis (Aprile et al 2019b).

The future LZ and XENONnT detectors will have about 6 to 7 t of active target mass (Akerib et al 2020a, Aprile et al 2020). Detectors of this size are sensitive to extremely rare decay processes such as neutrinoless double beta decay (Akerib et al 2020b) and will be able to observe the neutrino signal from a CCSN with a 27 M_⊙ progenitor happening anywhere in the Milky Way with 5σ significance (Lang et al 2016). Such a supernova at 10 kpc will produce 350 neutrino interactions in total in the active volume of LZ (Khaitan 2018). More specifically, for a threshold of ∼3 detected electrons, the number of supernova neutrino events for the 27 M_⊙ supernova progenitor with the LS220 EoS is 17.6 events/tonne over the first 7 s post bounce (Lang et al 2016). Both the LZ and XENON collaborations are currently developing a real-time supernova trigger to be incorporated in their detectors and data framework, aiming to actively contribute to SNEWS in the near future (Khaitan 2018).

A next-generation xenon detector, such as the one proposed by the DARWIN collaboration (Aalbers et al 2016), will further increase the sensitivity to CCSN. It is expected to discern with 5σ significance such an event from a 27 M_⊙ progenitor beyond 60 kpc and possibly distinguish between different supernova models and progenitor masses for a close supernova event (Lang et al 2016, Aalbers et al 2016).

The deep underground neutrino experiment (DUNE) is a 40 kt (fiducial mass) LArTPC detector, made of four 17 kt (10 kt fiducial) modules to be constructed underground in South Dakota. There are two designs under consideration for the DUNE far detector TPCs: a ‛single-phase' design that features horizontal drift along with readout comprising one charge-collection wire plane and two induction wire plane with 5 mm pitch, and a ‛dual-phase' design that features vertical charge drift and charge amplification and collection at a top gas-phase interface. Both designs also feature scintillation photon detection. DUNE expects around 3000 ν_eCC events from a 10 kpc supernova, providing a ν_e sensitivity which complements the ${\bar{\nu }}_{\text{e}}$ ability of most other detectors. ES on electrons and ${\bar{\nu }}_{\text{e}}$CC events are also expected. NC nuclear excitations which produce deexcitation gammas are also expected, although the cross sections are currently poorly known. Preliminary studies based on full simulation and reconstruction chains indicate thresholds for efficient reconstruction of between 5–10 MeV neutrino energy and energy resolution of around 20% (Abi et al 2020a, b). The tracking capability of the TPC enables pointing to the supernova at the several degree level—see section 3.1.2.

nEXO is a proposed neutrinoless double-beta decay (0νββ) experiment which aims to employ 5 t of LXe, enriched to 90% in the target isotope ¹³⁶Xe, inside a cylindrical time-projection chamber (TPC). Both xenon scintillation light (175 nm wavelength) as well as ionization charge signals will be recorded in the detector using silicon photo-multipliers (Gallina et al 2019) and specialized charge-readout tiles (Jewell et al 2018), respectively. The simultaneous detection of scintillation and ionization signals allows for reconstruction of an event's energy, position, and multiplicity. The TPC and its cryostat will be located in a 1.5 kt water tank instrumented with up to 125 PMTs. These PMTs will detect Cherenkov radiation from traversing muons and allow for a veto to subsequent cosmogenic backgrounds. This muon veto is referred to as the nEXO outer detector. Monte Carlo work is being undertaken to optimize PMT placement, develop a trigger scheme, define water-purity requirements, and investigate background contributions from the outer detector. Details of the nEXO project and the Outer Detector are described in (Al Kharusi et al 2018); nEXO's projected sensitivity to 0νββ is discussed in (Albert et al 2018).

While being optimized for the search for 0νββ, the sizable mass of xenon and water allows for the detection of SN neutrinos from within the galaxy. The 1.5 kt of water in the outer detector is expected to have a typical response of ∼250 neutrino interactions [calculated using the GVKM model (Gava et al 2009)] for a supernova event at a distance of 10 kpc. The dominant interaction channel will be inverse-beta decay, as expected for a water Cherenkov detector. Supernova neutrinos will also interact with nEXO's xenon volume. The TPC is being optimized to detect β-like events around the ¹³⁶Xe Q-value of 2.46 MeV. Thus, ν_e charged current interactions producing excited ¹³⁶Cs will be detectable via the nucleus' gamma de-excitation. For supernovae at 2 kpc, the expected event rate of this interaction channel is ∼5 events over a few seconds. Slightly lower rates are expected for the neutral current inelastic scatters that will produce excited ¹³⁶Xe nuclei. However, the majority of supernova-neutrino interactions in the LXe will be through the CEνNs channel. Here, we expect on the order of 100 supernova neutrino interactions from a GVKM supernova at 10 kpc, as calculated using methods found in (Lang et al 2016; Abe et al 2017) and cross sections adopted from (Pirinen et al 2018). These CEνNs events are unlikely to trigger data acquisition readout on their own due to the small number of ionization electrons being generated; nevertheless, by employing a buffered readout system and receiving an SNEWS trigger, nEXO could store this data.

Because amateur astronomers will play such an important role in the optical observation of the next galactic supernova, SNEWS intends to strengthen its relationship with the global amateur astronomical community in the coming months and years. The reason is simple: astronomers can only participate if they know that SNEWS exists. This effort will involve reaching out to groups, attending conferences, and engaging with individuals on social media. Cultivating these community connections will be an ongoing process which will support the important goal of maintaining and maximizing observational readiness.

To prepare the amateur astronomical community to respond quickly and effectively to a neutrino-indicated galactic supernova, it is critical to provide training and guidance regarding how to receive and interpret SNEWS alerts, and what to do when those alerts arrive.

SNEWS maintains a public mailing list that is used to distribute alert notifications in the event of a supernova-like neutrino burst, and anyone can subscribe at the official SNEWS website (https://snews.bnl. gov/). Alert emails distributed to the list are PGP-signed for authenticity. SNEWS also plans to develop additional notification vectors in order to maximize the reach of our alerts to the community. These will likely include utilizing existing multi-messenger networks, such as the AMON mentioned in section 5.3.

In addition to connecting astronomers to the alert system, SNEWS will develop and distribute materials to teach subscribers about the information products available within the alerts and how that information will be updated as subsequent observations are made, so that amateur astronomers are able to easily and quickly identify potential supernova candidates associated with a coincident neutrino burst.

When the day finally arrives, the initial SNEWS alert may contain little or no pointing information. It will therefore be up to the professional and amateur astronomy communities to identify and report the precise right ascension and declination of the supernova as it becomes visible. Time will be of the essence; it is imperative that clear and simple reporting mechanisms exist and that observers are aware of them. SNEWS will develop and distribute training materials so that alert subscribers can familiarize themselves with the follow-up reporting process before and during a galactic supernova event.