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Probing neutrino emission at GeV energies from compact binary mergers with the IceCube Neutrino Observatory

The advent of multi-messenger astronomy has allowed for new types of source searches by neutrino detectors. We present the results of the first search for 0.5 - 5 GeV astrophysical neutrinos emitted from all compact binary mergers, i.e., binary black hole, neutron star black, mass gap and binary neutron star mergers, detected by the LIGO and Virgo interferometers during their three first runs of observation. We use an innovative approach that lowers the energy threshold from ~10 GeV to ~0.5 GeV and searches for an excess of GeV-scale events during astrophysical transient events. No significant excess was found from the studied mergers, and there is currently no hint of a population of GeV neutrino emitters found in the IceCube data.


Abstract
The advent of multi-messenger astronomy has allowed for new types of source searches by neutrino detectors. We present the results of the first search for 0.5 -5 GeV astrophysical neutrinos emitted from all compact binary mergers, i.e., binary black hole, neutron star black, mass gap and binary neutron star mergers, detected by the LIGO and Virgo interferometers during their three first runs of observation. We use an innovative approach that lowers the energy threshold from ∼10 GeV to ∼0.5 GeV and searches for an excess of GeV-scale events during astrophysical transient events. No significant excess was found from the studied mergers, and there is currently no hint of a population of GeV neutrino emitters found in the IceCube data.
The first direct detection of gravitational waves by the LIGO interferometers [1] followed by three successful observing runs by the LIGO and Virgo collaborations (LVC) [2][3][4], have brought new opportunities in multi-messenger astronomy. While the first observed object was a binary black hole merger (BBH) [1], other compact binary merger types have since been reported: binary neutron star (BNS) mergers [5,6], neutron star-black hole (NSBH) mergers [4,7], and mergers involving compact objects with mass between the heaviest known neutron stars and the lightest known black holes ("Mass-gap") [4,7]. The astroparticle and astrophysics communities have carried out follow-up observations for every gravitational wave candidate reported by LIGO and Virgo [8][9][10][11][12][13][14]. Identifying a potential counterpart in electromagnetic waves and/or neutrinos can provide a multi-messenger picture of the studied object and can help constrain physical parameters of the sources, such as their surrounding density, understand acceleration mechanisms, and place constraints on fundamental physics, as has been done for the binary neutron star merger of August 17th, 2017 [5].
High-energy neutrino observatories have so far searched for neutrino counterparts in the TeV-PeV range, where astrophysical neutrinos emerge above the background of atmospheric neutrinos created by the interaction of cosmic rays with atmospheric nuclei [12,14]. The GeV energy domain has not yet been extensively explored in terms of astrophysical neutrino observations. Only a few searches reported upper limits in the single and multi-GeV range [15][16][17][18] [19], optimized for neutrinos in the MeV -GeV range, was the only neutrino detector providing limits on the astrophysical neutrino flux in the GeV regime. It has set, among others, limits for the first detected gravitational wave events, GW150914 and GW151226, and GW170817, the binary neutron star merger [15,16].
Part of the population detected by the LIGO and Virgo instruments may be associated with at least a fraction of the electromagnetically-detected gamma-ray bursts (GRBs) [20].
In such scenario, the neutrino production is expected to come from different processes depending on the considered energy range. While TeV neutrinos are predicted as a consequence of the internal shocks in the prompt emission phase of GRBs [21], GeV neutrinos could be produced by collisions of neutrons and protons following their decoupling during the acceleration phase [22]. Besides offering evidence of hadronic acceleration mechanisms, the detection of GeV neutrinos from GRBs would also constitute a probe of the target density surrounding the astrophysical object, allowing better constraints of the environments, acceleration processes, and progenitors of these phenomena [23][24][25].
In this paper, we report on a search for 0.5 -5 GeV neutrinos, using the IceCube Neutrino Observatory (IceCube), from compact binary mergers detected by the LIGO and Virgo interferometers during the first three runs of observations [3,4,7]. Searching for GeV neutrinos with IceCube is challenging for several reasons. First, the atmospheric neutrino background dominates over most astrophysical sources at this energy. As we focus on transient events, we have used the timing coincidence with the gravitational wave detection to integrate the background only over short time windows. Secondly, no reconstruction of the incoming neutrino direction is feasible in IceCube at this energy, so we searched for an increase in the number of GeV events compared with the off-time region. We describe hereafter the use of IceCube for GeV neutrino detection as well as the searches that were carried out and their results. A discussion of the results, including a comparison of existing limits on the neutrino flux emitted from these objects is then presented. We conclude the paper with the prospects for similar searches in the coming decade.

RANGE
The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector installed in the ice at the geographic South Pole between depths of 1450 m and 2450 m [26]. It includes 5160 digital optical modules (DOMs) distributed along 86 strings deployed within the detector volume. This includes a lower energy infill detector, the DeepCore subarray, which includes 8 densely instrumented strings with smaller vertical spacing between its optical modules (7 m versus 17 m) and smaller horizontal spacing between its strings (72 m on average versus 125 m) [27]. When a neutrino interacts inside or in the surrounding medium of the detector, the subsequent electromagnetic and/or hadronic cascade produces Cherenkov photons that can be detected by one or more DOMs.
While IceCube was originally optimized to detect TeV neutrinos, the Collaboration has previously demonstrated the ability to extend the sensitivity to a wider energy range by the use of DeepCore, as it allows searches for neutrino interactions down to 5.6 GeV (see e.g., [28]). Besides a higher density of optical modules, a softer trigger condition has been implemented in DeepCore to improve its sensitivity to lower neutrino energies. A detailed description of the event selection developed in view of extracting GeV neutrino events from the IceCube data and its inherent systematic uncertainties is presented in [29]. A GeV neutrino interaction produces a cascade and, depending on the interaction channel, also a short track, emitting light close to the interaction vertex, resulting in a small number of causally connected DOMs. The rejection of high-energy i.e., > 5 GeV, events originating from atmospheric muons or neutrinos, produced by the interaction of cosmic rays with atmospheric nuclei, is done by placing strong upper constraints on the number of optical modules triggered by the interactions. The main obstacle is identifying GeV neutrino interactions in IceCube among a background due to random DOM noise, which also trigger the detector with a pattern similar to that expected from a low-energy neutrino.
A detailed simulation of DOM noise hits in the detector helps to estimate the potential contamination of accidental triggers, hereafter referred to as "noise events". These include uncorrelated thermal noise, uncorrelated radioactive noise, and correlated scintillation noise [30][31][32]. About 6 Hz of noise events that were part of the initial 1400 Hz of data satisfies the constraints on the number of triggered optical modules and constitutes the 9 dominant contribution in the event sample. By requiring the distance between every pair of hits divided by the time separation between them to be consistent with the speed of light in ice, the selection ensures causality between pairs of hits. Applying this causality condition reduces the rate of detector noise events from 6 Hz to 0.2 Hz. Additional variables sensitive to the morphology of the events help to create a cleaner GeV neutrino sample. Among these, cuts on the estimated depth of the interaction point and the charge located around it as well as the total charge of the event reduce the data rate down to 0.02 Hz, as described in [29]. More than 40% of the neutrino interactions between 0.5 GeV and 5 GeV generated with GENIE 2.8.6 [33] following a generic E −2 spectrum pass this selection with respect to those passing the online trigger and are considered in the analysis as GeV event candidates.
The final rate, being dominated by remaining noise events, is larger than the expectation of atmospheric neutrinos, estimated to occur at the mHz level. The all-interaction-channel effective areas for electron and muon neutrinos passing the described selection is shown in any potential detector effects causing several events to happen at a specific location. In order to further ensure data quality, a visual inspection of the events in the time window of interest was performed to validate the unblinded sample. The data recorded during each of the 72 gravitational wave candidates reported by LIGO and Virgo during their three runs of observations pass our data quality checks, with the exception of GW190513 205428 and GW190731 140936, for which the data taking in IceCube was not stable enough to provide a satisfactory GeV neutrino candidate sample.
We carried out two different GeV neutrino searches using the obtained data sample: a search for a prompt signal and a search in an extended time window. Derivation of the isotropic-equivalent energy E iso upper limit for the 3 s search for

GW170817
A convenient way to compare the obtained limit with other constraints derived for similar compact binary mergers is to convert it into a limit on the isotropic-equivalent energy E iso .
This variable represents the energy flux at Earth multiplied by 4π times the square of the distance between the Earth and the source. Figure 3 shows a comparison of the all-flavor limit obtained on E iso for GW170817 with similar constraints obtained by Super-Kamiokande in the 1.6 to 10 8 GeV energy range, when focusing on upward-going muons, and 100 MeV to 10 GeV for the all-flavor search [19]; the ANTARES [12] and IceCube [14] limits in the ∼10 2 to ∼10 11 GeV energy range are also displayed. All analyses have assumed neutrino production at the source following a power law with a spectral index of -2. While the limit obtained with the analysis presented in this paper is less stringent than some of the other constraints, it probes a lower energy range and thus different production mechanisms, as previously described. We note that the E iso measured by the gamma-ray detection made by Fermi-GBM stands 5 orders of magnitude below the best constraints set on the timeintegrated neutrino energy [12,14], as shown in Fig. 3b.

CONCLUSION AND DISCUSSION
We have presented the first search for a 0.5 -5 GeV neutrino counterpart to compact binary mergers using the IceCube Neutrino Observatory. The main results are: • The search in an extended time window around the compact binary merger time [t − 500 s, t + 500 s] led to no significant neutrino excess; • For BNS, NSBH, and Mass-gap events, a neutrino search for prompt emission was carried out in the [t, t + 3s] time window and no significant excess was found; (blue shaded (all-flavor) and orange shaded (muon neutrino) areas) and using high-energy neutrinos (ANTARES [12] (dashed blue line, all-flavor) and IceCube HE [14] (orange, muon neutrinos)). A time window of 3 s was used here, while 1000 s were integrated to obtain the constraints using high-energy neutrinos and Super-Kamiokande data. While Super-Kamiokande could in principle detect neutrinos in the PeV energy range, its effective area is significantly lower at these energies than that of large neutrino telescopes.
• Upper limits on the 0.5 -5 GeV neutrino flux per binary compact merger were determined to be 1.2 × 10 4 and 3.9 × 10 4 neutrinos cm −2 for the 3 s and the 1000 s time windows, respectively; • Our all-flavor upper limit on E iso for the BNS of August 17th, 2017 improves the constraints in the GeV energy range for all-flavor neutrino searches compared to existing limits set by Super-Kamiokande and represents complementary information to highenergy neutrino searches from ANTARES and IceCube, and gamma-ray observations made by Fermi-GBM; • No source population is detected when performing a shape analysis of the merger vs.
background-only event distributions, considering the number of neutrino candidate events per burst integrated over 1000 s time window.
This work validates the innovative approach required to allow IceCube to be sensitive in the 0.5 -5 GeV energy range and confirms the feasibility to perform follow-up searches in this energy range. This approach will be used to carry out searches for neutrino counterparts of relevant astrophysical phenomena detected via gravitational or electromagnetic waves.
The main limitation of this search comes from the minimal trigger requirement in Ice-Cube, i.e., 3 DOMs with coincident pulses within 2.5 µs, which prevents some GeV neutrino interactions from being recorded. A separate data stream in IceCube, called HitSpooling [26], saves every single hit occurring in the detector, independent of the implemented trigger conditions. Sub-threshold neutrino interactions, lost in regular IceCube data, can be saved and studied. Using such a data stream would directly result in an increase of the sensitivity in the energy range studied in this paper. HitSpool data have significantly larger size than regular IceCube data, and cannot therefore be continuously saved. To take advantage of this additional data stream and the increased sensitivity it offers, we have created an alert system that saves HitSpool data based on the initial GCN circulars sent by the LIGO and Virgo collaborations. The system has successfully saved data for the alerts sent during their third run of observation and the data analysis is currently ongoing.
The current construction of KM3NeT/ORCA in the Mediterranean Sea [35,36], and the deployment of the IceCube Upgrade [37] within IceCube, may improve the current detection capabilities between 0.5 and 5 GeV. These detectors will be equipped with multianode sensors allowing event directional reconstruction and better noise rejection. This will lead to a lower detection threshold and improved sensitivity. The increasing sensitivity of the gravitational wave interferometers will lead to a larger population to probe using GeV neutrino interactions, improving global constraints on the physics of gravitational wave sources.

ACKNOWLEDGMENTS
The IceCube collaboration acknowledges the significant contributions to this manuscript from Gwenhaël de Wasseige. We gratefully acknowledge the support from the following agencies and institutes: USA -U.S. National Science Foundation-Office of Polar Programs, U.S.