The search for sources of high-energy astrophysical neutrinos can be significantly advanced through a multi-messenger approach, which seeks to detect the gamma rays that accompany neutrinos as they are produced at their sources. Multi-messenger observations have so far provided the first evidence for a neutrino source, illustrated by the joint detection of the flaring blazar TXS 0506+056 in highenergy (HE, E > 1 GeV) and very-high-energy (VHE, E > 100 GeV) gamma rays in coincidence with the high-energy neutrino IceCube-170922A, identified by IceCube. Imaging atmospheric Cherenkov telescopes (IACTs), namely FACT, H.E.S.S., MAGIC, and VERITAS, continue to conduct extensive neutrino target-of-opportunity follow-up programs. These programs have two components: followup observations of single astrophysical neutrino candidate events (such as IceCube-170922A), and observation of known gamma-ray sources after the identification of a cluster of neutrino events by IceCube. Here we present a comprehensive analysis of follow-up observations of high-energy neutrino events observed by the four IACTs between September 2017 (after the IceCube-170922A event) and January 2021. Our study found no associations between gamma-ray sources and the observed neutrino events. We provide a detailed overview of each neutrino event and its potential counterparts. Furthermore, a joint analysis of all IACT data is included, yielding combined upper limits on the VHE gamma-ray flux.
Figure 1: Skymap in equatorial coordinates showing IceCube alert positions in the period from September 2017 to January 2021. Alerts followed up by IACTs are shown in color (according to the alert type), and those not followed up are shown in gray. Letters indicate which IACTs participated in the observations (F - FACT, H - H.E.S.S., M - MAGIC, V - VERITAS). The latitude band between two dashed orange lines and two dashed red lines indicate regions of the sky that are potentially observable at zenith angles less than 45◦ from the northern (FACT, MAGIC, VERITAS) and southern (H.E.S.S.) IACTs, respectively. The light cyan band represents the overlapping visibility window for instruments in both hemispheres around the celestial equator, where the IceCube sensitivity to neutrinos in the ∼ 100 TeV energy range is at its best.
Figure 2: Delay times plotted against exposure times for IACT follow-up observations of neutrino alerts in the period from September 2017 to January 2021. The delay is calculated from the neutrino event arrival time (single events) or the flare threshold-crossing time (clusters) up to the start of the IACT observation. Observations performed with delays of less than 100 s or total exposures longer than 4 hours are labeled by alert names. The marker color represents the IACT performing the observation while the marker type represents the alert type.
Figure 3: 1ES 1312-423 MWL SED showing both archival data and observations obtained during the period following the GFU neutrino alert (March 12th-13th, 2019).
Figure 4: SEDs for the counterparts of the GFU-cluster alerts mentioned in the text. They comprise IACT ULs and simultaneous MWL data, together with archival data provided for comparison.
Figure 5: Combined differential-flux upper limits at 95% C.L. for sources observed by multiple IACTs.
Figure 6: Integral VHE γ-ray flux upper-limit maps derived from VERITAS (a, b, c), MAGIC (d), and H.E.S.S. (e, f, g). The two white lines denote the 50% and 90% containment contours of the IceCube event localizations (for panels (c) and (d), only the 50% containment contours are shown). The energy thresholds used to derive the upper-limit maps are 350, 200, 138, 150, 307, 530, and 326 GeV for panels (a) through (g), respectively.
Figure 7: SEDs for the potential counterparts of the single high-energy neutrino alerts. They comprise IACT ULs and simultaneous MWL data, together with archival data (from ASI ASDC, Stratta et al. 2011) provided for comparison.
Figure 8: Model comparison of the SED for blazar PKS 1502+106, potentially associated with the single high-energy neutrino alert IC-190730A (Section 6.4). Two different models are compared with IACT ULs and simultaneous MWL data, together with archival data (from ASI ASDC, Stratta et al. 2011).
