Neutrinos from Gamma Ray Bursts in the IceCube and ARA Era
NNeutrinos from Gamma Ray Bursts in the IceCube andARA Era
Dafne Guetta a,b a ORT Braude College, Carmiel, Israel b INAF-OAR Monteporzio Catone, Italy
Abstract
In this review I discuss the ultra-high energy neutrinos (UHEN) originatedfrom Cosmic-Rays propogation (GZK neutrinos) and from Gamma Ray Bursts(GRBs), and discuss their detectability in kilometers scale detectors like ARAand IceCube.While GZK neutrinos are expected from cosmic ray interactions on theCMB, the GRB neutrinos depend on the physics inside the sources. GRBsare predicted to emit UHEN in the prompt and in the later ’after-glow’ phase.I discuss the constraints on the hadronic component of GRBs derivedfrom the search of four years of IceCube data for a prompt neutrino fuxfrom gamma-ray bursts (GRBs) and more in general I present the results ofthe search for high-energy neutrinos interacting within the IceCube detectorbetween 2010 and 2013.
Keywords: gamma-ray:bursts, neutrino astronomy
1. Introduction
The detection of MeV neutrinos emitted from the sun and from the super-novae allowed the understanding of the physics of these astrophysical objects[1], [2]. MeV neutrino telescopes are capable of detecting neutrinos fromsources close to our galaxy up to a distances <
100 kp. The main goal of theconstruction of high energy, > ∼ eV, and is likely dominated beyond ∼ eV by extra-Galacticsources. The origin of Cosmic Rays (CR) has been a tantalizing mystery Preprint submitted to Nuclear Physics B September 25, 2018 a r X i v : . [ a s t r o - ph . H E ] M a r ver since their discovery by Hess [5] nearly a century ago. While lower en-ergy CRs of up to 10 eV are believed to originate from supernova explosionsin our galaxy [6], the source of the more energetic CR whose energies canexceed 10 eV remains unknown [7]. Although ultrahigh energy cosmic rays(UHECR) are produced throughout the Universe, those observed at Earthmust have been produced locally (within 50 Mpc) since they lose energywhile propagating through the Cosmic Microwave Background (CMB). Thisprocess (discussed by Greisen[8] and Zatsepin and Kuzmin[9] - ”GZK”) notonly causes energy loss for the primary, but also creates secondary particlesof extremely high energy [10, 11], including neutrinos above 10 eV. Thesecondary particles, particularly Ultra High Energy Neutrinos (UHEN), canbe used to explore the origins of UHECR[12]. With no electric charge, neu-trinos experience no scattering or energy loss, and so provide a probe of thesource distribution even to high redshift. Since UHECRs and the expectedGZK cutoff have been observed, the expectation of a GZK neutrino flux ison very strong footing[13]. UHE neutrinos are also likely to be created atthe acceleration sites of the UHECRs, pointing directly to the source fromearth, and providing information on the role of hadrons in the acceleration.Only Active Galactic Nuclei[14] and Gamma Ray Bursts, GRBs[15, 16,17], are believed to be capable of accelerating CRs to such enormous energies[18].Gamma Ray Bursts (GRBs) are powerful explosions, and are among thehighest-redshift point sources observed. The most common phenomenologicalinterpretation of these cosmological sources is through the so called fireballmodel [19, 20, 21]. In this model, part of the energy is carried out (e.g., froma collapsed star) by hadrons at highly-relativistic energies, some of which isdissipated internally and eventually reconverted into internal energy, whichis then radiated as γ -rays by synchrotron and inverse-Compton emission byshock-accelerated electrons. As the fireball sweeps up ambient material, itenergizes the surrounding medium through, e.g., forward shocks, which arebelieved to be responsible for the longer-wavelength afterglow emission [20].If the GRB jet comprises PeV protons, it should produce energetic neu-trinos through photon-hadron interactions. The photons for this process canbe supplied by the GRB gamma rays during its prompt phase, or duringthe afterglow phase [22, 23]. These lead to the production of charged pions,which subsequently decay to produce neutrinos. Within this picture, GRBsshould produce neutrinos with energies of ∼
100 TeV from the same regionin which the GRB photons are produced [24]. These neutrinos, if present,could be readily detected. Hence, the detectability of TeV to PeV neutrinos2epends on the presence of ( > ) PeV protons and on the efficiency at whichtheir energy is converted into neutrinos, as compared to how much of theenergy is in electrons, which is manifested primarily in the prompt GRBphoton emission.Neutrino astronomy has steadily progressed over the last half century,with successive generations of detectors achieving sensitivity to neutrinoswith increasingly higher energies. With each increase in neutrino energy, therequired detector increases in size to compensate for the dramatic decreaseof the flux with energy. The high-energy neutrinos from GRBs and the GZKneutrinos discussed above should be detected by large neutrino telescopes,such as IceCube, the Askarian Radio array (ARA) and in the future KM3NeT . All these detectors look for the Cherenkov radiation initiated by the neu-trino interactions in the ice using either optical detection (IceCube,KM3Net)or Radio Frequency (RF) detection (ARA).
2. Results from IceCube
Figure 1: Arrival angles and deposited energies of the 37 events detected by Icecube. http://km3net.org/home.php quatorial ◦ ◦ TS=2log(L/L0)
Figure 2: Arrival directions of the events in galactic coordinates. Shower-like events(median angular resolution ∼ ) are marked with + and those containing muon tracks( < ) with x. The grey line denotes the equatorial plane. Colors show the test statistic(TS) for the point source clustering test at each location. No significant clustering wasobserved. IceCube, is a Cherenkov detector [25] with photomultipliers (PMTs) atdepths between 1450 and 2450 meters in the Antarctic ice designed specifi-cally to detect neutrinos at TeV-PeV energies. Since May 2011 [26], IceCubehas been working with a full capacity of 86 strings (IC86). IceCube analysesinclude a model-independent search for GRB neutrinos[27], and for other dif-fuse and point sources. A search for high-energy neutrinos interacting withinthe IceCube detector between 2010 and 2013 provided the first evidence fora high-energy neutrino flux of extraterrestrial origin[28]. In the full 988-daysample, IceCube detected 37 events (Fig. 1) in the TeV-PeV range. A purelyatmospheric explanation for these events is strongly disfavored by their prop-erties [29]. The high galactic latitudes of many of the highest energy events(Fig. 2) suggest at least some extragalactic component. Moreover the inten-sity associated with the neutrino excess is much higher than that expected tooriginate from interaction of cosmic-ray protons with interstellar gas in theGalaxy[30] The flux, spectrum and angular distribution of the excess neu-trino signal detected by IceCube between 50 TeV and 2 PeV suggest that thesources of these neutrinos are unlikely to be (unknown) Galactic sources. Ifthe sources were galactic the events were expected to be strongly concentratedalong the galactic disk [31]. Results are consistent with an astrophysical flux4n the 100 TeV - PeV range at the level of 10 − GeV cm − s − sr − per flavor.The data are consistent with expectations for equal fluxes of all three neu-trino flavors and with isotropic arrival directions, suggesting either numerousor spatially extended sources. No evidence of neutrino emission from point-like or extended sources was found in four years of IceCube data. Searchesfor emissions from point-like and extended sources anywhere in the sky, froma pre-defined candidate source list and from stacked source catalogs all re-turned results consistent with the background-only hypothesis. 90% C.L.upper limits on the muon neutrino fluxes for models from a variety of sourceshave been calculated and compared to predictions[32]. The coincidence ofthe excess, E ν φ ν = 3 . ± . − GeV/cm sr s, with the Waxman-Bahcall(WB) bound, E ν φ W B = 3 . ± . − GeV/cm sr s, is probably a clue tothe origin of IceCubes neutrinos. The most natural explanation of this co-incidence is that both the neutrino excess and the ultra-high energy, > eV, cosmic-ray (UHECR) flux are produced by the same population of cos-mologically distributed sources [31]. The coincidence with the WB flux alsosupport the hypothesis that the detected neutrinos have an extra-Galacticorigin. The ν µ : ν e : ν τ = 1 : 1 : 1 flavor ratio is consistent with that expectedfor neutrinos originating from pion decay in cosmologically distant sources,for which oscillations modify the original 1 : 2 : 0 ratio to a 1 : 1 : 1 ratio[33]. In order to identify the sources of UHE neutrinos a multiwavelengthanalysis is needed.
3. Neutrinos from Gamma-Ray-Bursts
The high-energy neutrinos from GRBs should be detected by large neu-trino telescopes, such as IceCube and in the future KM3NeT . Since GRBneutrino events need to be correlated both in time and in direction withthe gamma-rays, they are sought after in small angular and short time win-dows. Therefore the search for neutrino events should be coordinated withthe gamma-ray telescopes that provide the GRB trigger. In this context,IceCube has recently developed a powerful model-independent analysis toolfor neutrinos detection, which is coincident in direction and in time to within1,000 seconds with GRB flares reported by the gamma ray satellites. Ice-Cube reported no detection of any GRB-associated neutrino in a data set http://km3net.org/home.php Neutrino Energy (GeV) -9 -8 E Φ ν ( G e V c m − s − s r − ) Waxman & BahcallIC40 limitIC40 Guetta et al.IC40+59 Combined limitIC40+59 Guetta et al. -1 E F ν ( G e V c m − ) Figure 3: Comparison of results to predictions based on observed gamma-ray spectra. Thesummed flux predictions normalized to gamma-ray spectra[36, 24] is shown in dashed lines;pay attention that the cosmic ray normalized Waxman-Bahcall flux [22] shown here[34]for reference is not correct and should be a factor 10 smaller. taken from April 2008 to May 2010 [34]; None of the high energy neutrinosreported for the next two years [35] is GRB-associated either, and as far aswe know no neutrino event has been associated with any GRB to date. Thisnon-detection is in conflict with earlier models [22, 36, 24, 37, 38], all of whichpredicted the detection of approximately ten GRB neutrinos by IceCube dur-ing this period. Those earlier estimates were largely calibrated based on thefireball hypothesis, and were motivated by the assumption that UHECRs areproduced primarily by GRBs. As shown in Fig.3 the IceCube results thusappear to rule out GRBs as the main sources of UHECRs [37, 34]. Thisimplies either that GRBs do not have the ( > )PeV protons, hypothesized inthe fireball model, or that the efficiency of neutrino production from theseprotons is much lower than what have been estimated [39, 40, 41]. Recentlythe Icecube collaboration set the most stringent limits yet on GRB neutrinoproduction using four years of IceCube data[42]. They constrain parts of theparameter space relevant to the production of UHECRs in the latest models.Because of the very low on-time and on-source background rate, the analysisgrows more sensitive almost linearly with time.Recently Yacobi et al.2014[43] have used the data from the GRB Monitor(GBM) on board Fermi to calibrate the photon (representing electrons) en-ergy content of the GRB jet. Subsequently, they have compared this with theupper limit on proton (turned pion) energy content, given the non-detection6
90 −60 −30 0 30 60 9010 −8 −6 −4 −2 N ν f e / f π GRB Declination [deg] α ν =2 α ν = α γ obs Figure 4: Expected number of ν µ from 250 GBM detected GRBs, which have a measuredspectral slope, as a function of declination, and factored by the unknown electron to pionenergy ratio f e /f π . The neutrino spectral slope α ν is assumed to be that of the photons α γ . The results for α ν = 2 are reported for comparison[43] of GRB neutrinos and derived a constraint on the fraction of energy thatgoes into hadrons to the one that goes into electrons, the ratio f π /f e . Theresults for each individual GRB are plotted in Fig.4, which shows manyGRBs yielding high values of N ν ( f e /f π ), and thus tightening the upper limiton the f π /f e by more than two orders of magnitude. Considering all theGBM sample Yacobi et al. 2014 found that the lack of detected neutrinosfrom Fermi/GBM GRBs since 2008 points to f π /f e (cid:46) .
24 with a 95% CL.The obtained value of f π /f e (cid:46) . f e ≈ f π ≈ . f Had ≤
1, i.e. all LAT fluence ishadronic (via pair-photon cascades). Using the typical ratio F GBM /F LAT ≈
10 [44] Yacobi et al. 2014 constrain the typical GRB hadronic fraction to be f π /f e (cid:46) .
4. GZK neutrinos and ARA
Although ultrahigh energy cosmic rays (UHECR) likely originate through-out the Universe, those observed at Earth must be produced locally since suchUHECR lose energy while propagating through the CMB ( pγ − > nπ + − >nµ + ν µ , etc.) producing GZK neutrinos. The detection of UHE GZK neutri-nos is an experimental challenge at the frontier of neutrino astronomy, whichhas progressed over the last half century.The flux of GZK neutrinos incident on earth ranges from ∼ . / km / year[49] to ∼ / km / year [50],depending on the model considered. This flux canbe detected in dense, RF-transparent media such as ice via the Askaryaneffect[51]. There is approximately one UHE neutrino interaction occurringin each km in the ice per year. The long RF attenuation length in theAntarctic ice allows a more efficient area coverage that makes it possible toconstruct detectors of order tens to hundreds of km , and several small-scalepioneering efforts to develop this approach exist[52, 53, 54]. Two years ago, atwo-phased experiment named ARA (Askaryan Radio Array) was initiated,designed to ultimately accumulate hundreds of GZK neutrinos[55]. The goalof Phase 1 of ARA (2010-2016) is to make the first definitive observationof cosmogenic neutrinos; Phase 2 of ARA (37 stations) would then accu-mulate the statistics necessary to carry out an expanded astrophysics andparticle-physics science program. The primary goal of ARA is to discoverGZK neutrinos and to establish the spectrum.8 igure 5: The limits placed compared with the projected ARA37 trigger-level sensitivityand results from other experiments. The detection of GZK neutrinos from ARA will allow to understand theorigin of the UHECR cutoff confirmed by the recent data of Auger and thenature of the UHECR composition.
5. Acknowledgements
I Thank the Scientific Organization Committee for having invited me togive this plenary talk. This research is supported by a grant from the U.S.Israel Binational Science Foundation.