Astron. Nachr. / AN 329, No. 3, 337 – 339 (2008) / DOI 10.1002/asna.200710934

The SuperNova Early Warning System

K. Scholberg�

Duke University Physics Dept., Box 90305, Durham, NC, USA

Received 2007 Aug 24, accepted 2007 Dec 11Published online 2008 Feb 25

Key words neutrinos – supernovae: general

A core collapse in the Milky Way will produce an enormous burst of neutrinos in detectors world-wide. Such a burst hasthe potential to provide an early warning of a supernova’s appearance. I will describe the nature of the signal, the sensitivityof current detectors, and SNEWS, the SuperNova Early Warning System, a network designed to alert astronomers as soonas possible after the detected neutrino signal.

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1 The supernova neutrino signal

When a massive star reaches the end of its life, its core col-lapses. More than 99% of the binding energy of the resultingneutron star is released in the form of neutrinos and antineu-trinos of all flavors, with energies in the tens of MeV range:the energy leaves via neutrinos because neutrinos interact soweakly that they readily leave the star. The neutrinos them-selves bring information from deep inside the core; the de-tection of such a signal will yield great insights into neutrinoproperties and core collapse physics (e.g. Raffelt 2007). Ofparticular interest for this workshop is that the timescale ofneutrino emission is a few tens of seconds, promptly aftercore collapse; the photon signal, in contrast, can take hoursor even days to emerge from the stellar envelope. Thus, thedetection of a burst of neutrinos will give astronomers anearly warning of a nearby supernova.

So far there has been only one observed instance ofcore collapse neutrino emission: for SN1987A in the LMC,two water Cherenkov detectors, IMB and Kamiokande II,observed a total of 19 neutrino interactions between them(Hirata et al. 1987; Bionta et al. 1987). Two scintillatordetector observations were also reported (Alekseev et al.1987; Aglietta et al. 1987). The water Cherenkov neutri-nos were recorded 2.5 hours before the first light was ob-served from the supernova; however the neutrino signal wasonly retrieved from data tapes after the fact. Next time wewill do better. An early observation of the supernova lightcurve turn-on will bring information about the progenitorand its environment, which can in turn feed back to neu-trino physics. The aim of SNEWS is to provide the astro-nomical community with an early warning of a supernova’soccurrence, as well as to improve global sensitivity to a su-pernova neutrino burst via inter-experiment collaboration.

� Corresponding author: schol@phy.duke.edu

2 Neutrino detectors

Because neutrinos interact so weakly, in spite of their hugeflux, enormous detectors are required to see a substantialsignal. Typically about a kiloton of target material is re-quired to observe a few hundred interactions. This limitsthe distance sensitivity of terrestrial neutrino detectors tothe Milky Way neighborhood: the largest neutrino detec-tors of the current generation have a range of a few hun-dred kpc. The typical distance to the next nearby supernovawill be 10–15 kpc (Mirizzi, Raffelt & Serpico 2006). Thenext generation of megaton-mass-scale detectors may reachMpc distance sensitivity, extending reach to Andromeda andother Local Group galaxies.

We can expect a Milky Way core collapse about every30 years. Perhaps one in six supernovae will stand out obvi-ously, and some may never become optically bright. How-ever, with current technology spanning an enormous rangeof electromagnetic wavelengths, and including gravitationalwave sensitivity, even supernovae that fizzle may be de-tectable in some channel.

A number of neutrino detectors are in existence world-wide. Typically, detectors must be underground for cosmicray shielding. Most existing supernova neutrino detectorsexploit the inverse beta decay interaction of electron an-tineutrinos on free protons, ν̄e + p → n + e+, where theenergy loss of the positron is detected, sometimes followedby neutron-capture gammas. The rate of this reaction is rel-atively large compared to other neutrino interactions in thisenergy regime. Furthermore, detectors with large numbersof free protons, such as those made of hydrocarbon or wa-ter, can be constructed cheaply. Note that predominance ofinverse beta decay as the main detection channel means thatprimary sensitivity is to electron antineutrinos, which areonly one component of the flux.

The main supernova neutrino detector types are:Scintillation detectors: such detectors consist of large vol-

umes of hydrocarbon, CnH2n. Organic scintillating ma-

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338 K. Scholberg: The SuperNova Early Warning System

terials produce light when charged particles lose en-ergy in them, and the resulting photons are picked upby photomultiplier tubes. Scintillation light is isotrop-ically emitted, so directional detection is not generallypossible even for asymmetric processes. Examples ofcurrently running scintillation detectors are LVD andBorexino in Italy, KamLAND in Japan, and Baksan inRussia.

Water Cherenkov detectors: these employ ultrapure wa-ter. Neutrino-induced charged particles move faster thanthe speed of light in water and produce Cherenkov radi-ation; as for scintillation detectors, the Cherenkov pho-tons are picked up by photomultipliers. Although mostinteractions in a water detector are inverse beta decayprocesses, a small fraction will be elastic scattering ofneutrinos from atomic electrons, ν + e− → ν + e−. Thisprocess is of particular interest because the electronsare kicked in the direction the neutrinos are traveling;because Cherenkov radiation is directional, these scat-tering interactions offer a means of knowing the direc-tion of the supernova (see Sect. 3). Super-Kamiokandein Japan is currently the only instance of a large waterCherenkov detector; with a 50 kiloton mass, it is cur-rently the most sensitive of the world’s supernova neu-trino detectors.

Long string water Cherenkov detectors: although detec-tors built of long strings of photomultiplier tubes em-bedded in water or ice are primarily designed for highenergy (> GeV) neutrinos, some are capable of observ-ing diffuse photons from inverse beta decays in ice orwater as a coincident increase of count rates in manyphotomultipliers (Halzen, Jacobsen & Zas 1995). AM-ANDA/IceCube at the South Pole has Galactic sensitiv-ity.

Other supernova neutrino detectors: other target materi-als can be used for supernova neutrino detection, anda number of novel detectors are proposed or under con-struction. Liquid argon has excellent potential for νe tag-ging, and proposed detectors based on lead or iron willhave good sensitivity to neutrino flavors other than ν̄e.See Scholberg (2007) for a review of current and futuresupernova neutrino detection.

3 Considerations for the early warning

What we want from an early warning can be summarized as“The 3 P”’s:

“Prompt”: time is of the essence for a supernova earlywarning; we are essentially racing the shock wave. Akey factor in a prompt warning is the requirement ofa coincidence between detectors, since it allows an au-tomated alert. Any individual detector’s signal requireshuman checking, which can slow things down. Auto-mated coincidence alerts on a timescale of minutes havebeen demonstrated.

“Pointing”: obviously, the better one can point to the loca-tion of the supernova, the more likely it is that the super-nova will be found promptly. Unfortunately, pointing inneutrinos (Beacom & Vogel 1998) is difficult: the bestbet will be to make use of neutrino-electron elastic scat-tering interactions, ν + e− → ν + e−, for which thekicked electron points away from the source. This is afew percent of the total signal. Super-K’s pointing willbe a few degrees for a Galactic center event. No otherexisting detectors have directional capability. Triangula-tion using timing of signals at different detectors aroundthe globe is in principle possible, but is in practice toodifficult with expected statistics. Millisecond precisionis needed, and we expect 10

3–104 interactions spread

out over tens of seconds.“Positive”: here the criterion is to have very few false alerts

(see also Sect. 5): we aim for fewer than one accidentalcoincidence per century. The coincidence requirement isessential here. We require, for a 2 out of 3 coincidence,that each individual experiment’s false alarm rate doesnot exceed about one per week.

4 SNEWS implementation and status

SNEWS, the Supernova Early Warning System (Antonioliet al. 2004), aims to address all of the above criteria. The im-plementation is relatively simple. Each individual neutrinoexperiment implements its own neutrino burst monitoringsystem, tailored to the features of that experiment, with in-teresting burst criteria defined by each experimental collab-oration. A “client” at each experiment sends out an alertdatagram if a sufficiently interesting burst of neutrino inter-action candidates is found. A “coincidence server” waits fordatagrams from each experiment’s clients. If the server findsa coincidence within 10 seconds, then it sends out an alert tothe SNEWS alert mailing list. Astronomers can sign up forthe SNEWS alert mailing list at ‘snews.bnl.gov’. SNEWShas been running in automated mode since 2004 (and in anon-automated mode since 1999).

We classify alerts as “gold” and “silver”: gold alerts godirectly to the astronomical community, whereas silver onesgo to experimenters only. A gold alert requires that a num-ber of quality checks be satisfied, and also that experimentsinvolved in the coincidence do not have a recent history ofsending alarms at an higher than usual rate. Silver alerts maybe upgraded after human checking.

Individual experiments may also use the SNEWS alertinfrastructure for human-checked (hence slower) confirmedalerts. Figure 1 shows our current flowchart, updated sinceAntonioli (2004) was published.

Our main coincidence server runs at Brookhaven Na-tional Lab and a backup server is also kept running at theU. of Bologna. The datagram protocol is SSL-encryptedTCP/IP, and server’s alert output is PGP-signed email.

The experiments currently participating in SNEWS areSuper-Kamiokande in Japan (Ikeda et al. 2007), LVD (Agli-

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Astron. Nachr. / AN (2008) 339

Fig. 1 Flowchart summarizing the sequence of events and de-cisions that determine whether an alert is GOLD, SILVER (or IN-DIVIDUAL).

etta et al. 1992) in Italy, and IceCube/AMANDA (Ahrens etal. 2002) at the South Pole. The Sudbury Neutrino Observa-tory in Canada (Virtue 2001) participated until it completedits running phase in 2006. We expect future experiments tojoin as they come online.

Amateur astronomers are an integral part of SNEWS.They have wide area viewing capability, enthusiasm and ex-pertise. In the absence of precise neutrino-derived pointinginformation, amateurs may be the first to pinpoint the super-nova location. The Sky and Telescope magazine provides anAstroAlert mailing list to reach amateur astronomers andserves as a clearinghouse for amateur responses:‘www.skyandtelescope.com/resources/proamcollab/AstroAlert.html’.

A successful amateur test was performed in February2003, for which a known fake target (the asteroid Vesta)was selected and a tagged fake alert sent via the AstroAlertlist. This demonstrated timely and accurate response fromamateurs worldwide.

It should be technically straightforward to expand theSNEWS alert output to communicate with robotic telescopenetworks. Although pointing information may be poor orunavailable, telescopes with wide fields of view may be ableto respond appropriately. We plan to implement VOEventprotocol alert output in the near future.

5 Discussion

We are often asked by astronomers why SNEWS does notturn thresholds down so as to get a higher rate of falsealerts; this would improve sensitivity, exercise the system,and keep interest high.1 The answer is primarily sociolog-

1 We have in fact performed a high rate test, as reported in Antonioli etal. (2004). However we require very low false alert rate for output to thewider community.

ical. In the community represented at this workshop, thereis high tolerance for “junk” alerts; astronomers are awashin data and the main problem is to sift the interesting in-formation from the copious noise. However, in the neutrinocommunity, because true events are so rare, there is stronginhibition against issuing any kind of false alerts. Further-more, decrease in threshold yields only modest increase insensitivity – most detectors are already sensitive to the en-tire Galaxy and moderate improvement does not bring manynew candidate stars into range. Since the dearth of datamakes it all the more urgent to gather any information onecan when the supernova actually does happen, the SNEWSnetwork’s problem becomes one of maintaining readinessduring decades-long data-less deserts. Well-tagged, well-advertised fake alerts are one way of maintaining interestand ability to react.

6 Summary

In summary, the key points of relevance for astronomers in-terested in transients are as follows:

– The neutrino signal for a core collapse event precedesits electromagnetic fireworks by hours, or perhaps tensof hours.

– The burst of neutrinos itself lasts tens of seconds.– The pointing from the neutrinos will be a few degrees

in an optimistic case. There may be no pointing infor-mation at all, or the pointing information may be not beavailable immediately.

– Currently running experiments are sensitive to a corecollapse in the Milky Way, or just beyond. The next gen-eration of detectors may reach to Mpc range.

– A few Galactic supernovae are expected per century.– SNEWS is online, and can provide an alert within min-

utes of a Galactic core collapse. Anyone may sign up forthe automated SNEWS mailing list. We hope to expandthe alert soon to VOEvent-based networks.

Acknowledgements. The author acknowledges the contributionsof the inter-experiment SNEWS working group. SNEWS is sup-ported by the U. S. National Science Foundation.

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