arXiv subjects

Nobuhiro Shimizu

Publications and source records attributed to Nobuhiro Shimizu.

7 recordsLinked to original sources

arXiv2026

Deep Optical Follow-up Observations to IceCube Cosmic Neutrinos: a case for IC230724A with Subaru/HSC and prospects with Rubin/LSST

IceCube has been detecting cosmic high-energy neutrinos for more than 10 years, but their major sources are still under debate. To identify them, IceCube is issuing neutrino alerts, which enable us to perform electromagnetic follow-up observations. In this paper, we present our Subaru/HSC deep optical follow-up observations down to 25.5 mag to a well-localized neutrino event, IceCube 230724A. We conduct a dedicated analysis with extensive evaluation of background rates and true positive rates adopting the blind analysis policy to identify or disfavor tidal disruption events (TDEs) as cosmic neutrino sources. Our analysis found no TDE candidate in the region of interest. Rubin/LSST survey will enable us to constrain their fractional contribution to the cosmic high-energy neutrino background, either $\lesssim 60\%$ or $\gtrsim30\%$ for non-detection and detection, respectively, if Rubin covers the error regions of 10 neutrino events.

astro-ph.HE↗

arXiv2025

Relaxation of time-variable neutron-loaded relativistic jets across the photosphere and their GeV-TeV neutrino counterparts

Both observational and theoretical studies indicate that the central engine of a gamma-ray burst (GRB) is intrinsically time-variable, implying jet inhomogeneity. A jet with an inhomogeneous Lorentz factor distribution develops internal shocks both below and above the photosphere, relaxing toward homologous expansion. Below the photosphere, neutrons, whose mean free paths are much longer than those of charged particles, play an essential role in the dissipation process. Using neutron-inclusive shell simulations with initial conditions based on the collapsar scenario, we link the statistical inhomogeneity of the jet at the breakout of the progenitor to the dissipation that occurs inside and outside the photosphere, and calculate the GeV-TeV neutrino counterpart originated from inelastic neutron-proton interactions consistently with the prompt gamma-ray emission. We find that the peak energy of the GeV-TeV neutrinos is in 10-30 GeV irrespective to the baryon loading factor of the jet, with the high-energy tail extending into the TeV range as the amplitude of the time variability becomes stronger. When gamma-ray emission is efficient as in typical GRBs (i.e., the gamma-ray radiation efficiency with respect to the total jet power is approximately 100%, the radiative efficiency of GeV-TeV neutrinos remains 0.1-10%. By contrast, when the gamma-ray radiation efficiency is relatively low (< 10%) for jets where a large fraction of the energy is dissipated below the photosphere, the neutrino efficiency can increase up to 20%. This suggests that GRBs with relatively low gamma-ray luminosities, as well as X-ray-rich transients, can be promising targets for ongoing and future GeV-TeV neutrino transient searches.

astro-ph.HE↗

arXiv2025

The First Search for Optical Transient as a Counterpart of a Month-timescale IceCube Neutrino Multiplet Event

Optical transients with timescale of months, such as supernovae (SNe) and tidal disruption events (TDEs), are candidates of high-energy neutrino sources. Multiple neutrino detections from the same direction within a month timescale provide a unique opportunity to identify such optical counterparts in the nearby Universe. In this work, we conduct archival search for the optical counterpart of an IceCube triplet event using the data of Zwicky Transient Facility. We develop a dedicated alert filtering system and validate the performance by following a blind analysis method. Applying this filtering system to the data after the detections of the IceCube triplet event, we find no transient candidates within the localization area. Assuming that the IceCube triplet event originates from an astrophysical source, we constrain parameters of optical transient, a peak luminosity and a decay timescale, using a simple signal model that is motivated by TDEs and superluminous SNe (SLSNe). Assuming the case with no time lag between neutrino detections and optical peak, almost entire parameter space of the known TDEs and SLSNe would be constrained. To give constraints on transients with a rapidly evolving light curve, quick follow-up observations for future neutrino multiplet events are crucial.

astro-ph.HE↗

arXiv2022

Identifying High Energy Neutrino Transients by Neutrino Multiplet-Triggered Followups

Transient sources such as supernovae (SNe) and tidal disruption events are candidates of high energy neutrino sources. However, SNe commonly occur in the universe and a chance coincidence of their detection with a neutrino signal cannot be avoided, which may lead to a challenge of claiming their association with neutrino emission. In order to overcome this difficulty, we propose a search for $\sim10-100$ TeV neutrino multiple events within a timescale of $\sim 30$ days coming from the same direction, called neutrino multiplets. We show that demanding multiplet detection by a $\sim 1$ km$^3$ neutrino telescope limits distances of detectable neutrino sources, which enables us to identify source counterparts by multiwavelength observations owing to the substantially reduced rate of the chance coincidence detection of transients. We apply our results by constructing a feasible strategy for optical followup observations and demonstrate that wide-field optical telescopes with a $\gtrsim4$ m dish should be capable of identifying a transient associated with a neutrino multiplet. We also present the resultant sensitivity of multiplet neutrino detection as a function of the released energy of neutrinos and burst rate density. A model of neutrino transient sources with an emission energy greater than ${\rm a~few}\times 10^{51}$erg and a burst rate rarer than ${\rm a~few}\times 10^{-8}\ {\rm Mpc}^{-3}\ {\rm yr}^{-1}$ is constrained by the null detection of multiplets by a $\sim 1$km$^3$ scale neutrino telescope. This already disfavors the canonical high-luminosity gamma ray bursts and jetted tidal disruption events as major sources in the TeV-energy neutrino sky.

astro-ph.HE↗

arXiv2021

Performance studies for a next-generation optical sensor for IceCube-Gen2

We present performance studies of a segmented optical module for the IceCube-Gen2 detector. Based on the experience gained in sensor development for the IceCube Upgrade, the new sensor will consist of up to eighteen 4 inch PMTs housed in a transparent pressure vessel, providing homogeneous angular coverage. The use of custom molded optical `gel pads' around the PMTs enhances the photon capture rate via total internal reflection at the gel-air interface. This contribution presents simulation studies of various sensor, PMT, and gel pad geometries aimed at optimizing the sensitivity of the optical module in the face of confined space and harsh environmental conditions at the South Pole.

astro-ph.IM↗

arXiv2021

Performance of the D-Egg Optical Sensor for the IceCube Upgrade

New optical sensors called the "D-Egg" have been developed for cost-effective instrumentation for the IceCube Upgrade. With two 8-inch high quantum efficient photomultiplier tubes (PMTs), they offer increased effective photocathode area while retaining as much of the successful IceCube Digital Optical Module design as possible. Mass production of D-Eggs has started in 2020. By the end of 2021, there will be 310 D-Eggs produced with 288 deployed in the IceCube Upgrade. The D-Egg readout system uses advanced technologies in electronics and computing power. Each of the two PMT signals is digitised using ultra-low-power 14-bit ADCs with a sampling frequency of 240 megaSPS, enabling seamless and lossless event recording from single-photon signals to signals exceeding 200 PE within 10 nanosecond, as well as flexible event triggering. In this paper, we report the single photon detection performance as well as the multiple photon recording capability of D-Eggs from the mass production line which have been evaluated with the built-in data acquisition system.

physics.ins-det↗

arXiv2021

A next-generation optical sensor for IceCube-Gen2

For the in-ice component of the next generation neutrino observatory at the South Pole, IceCube-Gen2, a new sensor module is being developed, which is an evolution of the D-Egg and mDOM sensors developed for the IceCube Upgrade. The sensor design features up to 18 4-inch PMTs distributed homogeneously in a borosilicate glass pressure vessel. Challenges arise for the mechanical design from the tight constraints on the bore hole diameter (which will be 2 inches smaller than for IceCube Upgrade) and from the close packing of the PMTs. The electronics design must meet the space constraints posed by the mechanical design as well as the power consumption and cost considerations from over 10,000 optical modules being deployed. This contribution presents forward-looking solutions to these design considerations. Prototype modules will be installed and integrated in the IceCube Upgrade.

astro-ph.IM↗