Elsevier

Astroparticle Physics

Volume 86, January 2017, Pages 11-17
Astroparticle Physics

Borexino’s search for low-energy neutrino and antineutrino signals correlated with gamma-ray bursts

https://doi.org/10.1016/j.astropartphys.2016.10.004Get rights and content

Abstract

A search for neutrino and antineutrino events correlated with 2350 gamma-ray bursts (GRBs) is performed with Borexino data collected between December 2007 and November 2015. No statistically significant excess over background is observed. We look for electron antineutrinos (ν¯e) that inverse beta decay on protons with energies from 1.8  MeV to 15  MeV and set the best limit on the neutrino fluence from GRBs below 8  MeV. The signals from neutrinos and antineutrinos from GRBs that scatter on electrons are also searched for, a detection channel made possible by the particularly radio-pure scintillator of Borexino. We obtain currently the best limits on the neutrino fluence of all flavors and species below 7  MeV. Finally, time correlations between GRBs and bursts of events are investigated. Our analysis combines two semi-independent data acquisition systems for the first time: the primary Borexino readout optimized for solar neutrino physics up to a few MeV, and a fast waveform digitizer system tuned for events above 1 MeV.

Introduction

Gamma ray bursts (GRBs) are among the most energetic events known in the Universe, with a typical apparent energy release of 1054 erg (or ∼ 1 solar mass), assuming isotropic emission of energy. The average rate of observed GRBs is about 1 event per day from the entire sky. The observer-frame duration of gamma ray emission in the MeV range can be less than 2  s (for a smaller sub-class of so called short GRBs) or, more typically, is of the order of 10 to 100 s. Longer afterglows in X-rays, optical, and radio wavelengths are also observed. The measured redshifts of optical afterglows (z=0.01..8.2) allow to attribute GRBs as extra-galactic events, whose progenitors lie at cosmological distances.

Currently, there is no universally accepted model of GRBs. However, the long GRBs are usually linked to the rotating cores of very massive stars collapsed into neutron stars (NS) or black holes (BH) [1], [2]. Short GRBs can result from binary mergers of NS + NS or NS + BH [2]. These models usually assume that neutrino cooling dominates over the electromagnetic one with neutrino energies in the MeV range [3]. In these models, the energy emitted in the form of MeV thermal neutrinos is of the order of one Solar mass (Mc2 ≈ 2 · 1054 ergs) while the energy released in gamma quanta is up to ∼ 100 times less [4], [5]. Even for the nearest GRBs with z ∼ 0.01 (for the standard ΛCDM cosmological model [6], this comoving distance is ∼ 1026 cm or 30 Mpc), the expected fluence of low-energy neutrinos at Earth is equal to 105106cm2, which is too small to be observed by current detectors. However, there exist other models for the origin of GRBs, such as cusps of superconducting cosmic strings [7] which can better explain some peculiarities of GRBs [8]. Some of these models are predicting the fluence of MeV-range neutrinos to be up to 1010 times larger than that of gamma quanta. The neutrino fluence in these models can be estimated as [4]: Φν=108cm2(ηγ1010)1(Eν100MeV)1×(Fγ106erg·cm2),where ηγ is the ratio of photon and neutrino fluences, Fγ is the observed gamma ray energy fluence of the GRB. Thus, for a typical GRB with gamma energy release of 3 × 1051 erg at redshift z=2 (Fγ=106erg·cm2), the predicted fluence of neutrinos with energy of ∼ 10 MeV is ∼ 108109cm2. For comparison, the observed flux of solar neutrinos is about 6·1010cm2·s1, of geo-antineutrinos is about 5·106cm2·s1 [9]. It demonstrates that the sensitivity level of existing neutrino detectors in the MeV range is close to the fluxes expected in several GRB models, if one uses a big set of GRBs. Hereinafter, the energy of neutrino refers to the observed energy; the emitted energy has to be multiplied by factor 1+z which is not important due to large uncertainties in models predictions.

Production of TeV and PeV neutrinos by protons accelerated by the plasma shock wave of GRBs was discussed [10], [11] and such high-energy neutrinos were searched for by AMANDA [12], ANITA [13], ANTARES [14], [15], Baikal [16], IceCube  [17], [18], and SuperKamiokande [19], but no signal was found. The searches for GRB neutrinos in the MeV energy range have been performed by four experiments: SuperKamiokande [19], SNO [20], KamLAND [21], and BUST [22].

The SuperKamiokande collaboration searched for electron and muon neutrinos and antineutrinos in the energy range of 7–80 MeV. The SNO collaboration searched for electron neutrinos, electron antineutrinos, and for (anti)neutrinos of non-electron flavors in the range of 5–13 MeV. The KamLAND collaboration set upper limits on electron antineutrino fluence associated with GRBs with known redshift, in the energy ranges of 7.5–100 MeV and (after the Japanese nuclear reactors were switched off in 2011) of 1.8–100 MeV. The BUST (Baksan Underground Scintillation Telescope) was sensitive to electron neutrinos and antineutrinos in the energy interval of 20–100 MeV. None of these four experiments found any correlation between GRBs and neutrino events in their detectors.

In this paper, we present a search for possible correlations between GRBs and (anti)neutrino events for all neutrino flavours in the Borexino detector.

Section snippets

The Borexino detector

Borexino is a liquid scintillator detector placed in the Hall C of the underground Laboratory Nazionali del Gran Sasso (LNGS) in central Italy. The radiopurity of this unsegmented detector has reached unprecedented levels through the techniques described in [23]. This was fundamental in reaching the primary goal of Borexino, the real time spectroscopy of solar neutrinos below 1 MeV [24], [25]. The detector design [26] is based on the principle of graded shielding, with the inner scintillator

GRB and time window selection

We have used the GRB database compiled by the IceCube collaboration [29], which considers the data from several satellites such as SWIFT, Fermi, INTEGRAL, AGILE, Suzaki, and Konus/WIND. This database contains information about GRB’s position, time of the detection, duration, energy spectrum, intensity, and, when available, the redshift. We underline that the latter is available only for about 10% of GRBs. In the period of interest from December 2007 (December 2009) to November 2015, 2350 (1813)

Analysis and results

We have analyzed Borexino data acquired between December 2007 and November 2015. The two semi-independent DAQ systems used in this analysis, the primary and the FADC one, have separate triggers and can operate individually, independently from each other. Different analysis approaches, as it will be specified below, have used data from the two systems in different ways: either individually or in a combined way. In some cases the presence of the data from both systems was required, in others the

Conclusions

We have performed a search for time correlation between gamma ray bursts and events detected by Borexino and associated with neutrinos and antineutrinos reactions – the inverse beta decay reaction on protons and neutrino-electron elastic scattering. We have also searched for correlations of GRBs with short bursts of events in Borexino. The analysis was performed with data of two semi-independent data acquisition systems: the primary DAQ, optimized for events up to a few MeV, and a Flash ADC

Acknowledgments

The Borexino program is made possible by funding from INFN (Italy); the NSF (U.S.); BMBF, DFG, HGF, and MPI (Germany); RFBR: Grants No. 14-22-03031, No. 15-02-02117, and No. 16-29-13014 (Russia); NCN Poland (Grant No. UMO-2013/10/E/ST2/00180). We acknowledge the generous support and hospitality of the Laboratori Nazionali del Gran Sasso (LNGS).

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