arXiv subjects

Sam Hori

Publications and source records attributed to Sam Hori.

7 recordsLinked to original sources

arXiv2026

Characterizing Gamma-Radio Delayed Flaring Activity from Blazars

Flaring activity from the jets of active galactic nuclei has been studied for several decades, closely related to the loading and evolution of the jet. In this work, we focus on the sub-hundred parsec jet region, well traced by non-thermal radio and gamma-ray emission. Only in recent years have light curves capturing the decade-long behavior of such sources become available for a large ensemble of objects. While previous studies have focused on a direct correlation or few-month lag between gamma-ray and radio activity, recent neutrino-bright blazars observed by the IceCube Neutrino Observatory present multi-year delays between initial gamma-ray activity and subsequent radio flares. In this work, we search for similar-timescale correlations between Fermi-LAT gamma-ray data and RATAN-600 radio data from ~100 blazars. We consider two gamma-ray bands, 100 MeV-1 GeV and 1 GeV-500 GeV, as well as the integral band, to compare correlations between potential opaque and unabsorbed regions of the jet. Gaussian process modeling is used for smooth light curve function prediction. We also analyze morphological AGN core data from the MOJAVE survey, forming a sub-selection to better illustrate potential dependence on location. In the broader selection, several sources exhibit delayed flares on the order of 1-3 years. In the stacked analysis, we find the highest correlation for a radio delay on the order of 180 days. The stacked correlation resulting from the MOJAVE sub-selection corresponds to a slightly smaller time lag. Delayed radio flares or extended radio emission appear to be notable features within the general blazar population.

astro-ph.HE↗

arXiv2025

A multi-wavelength view of the multi-messenger sources NGC 1068 and PKS 1502+1061

Multi-messenger astronomy offers a powerful approach to studying high-energy radiative processes in astrophysical sources. A notable example was seen in 2017, when the IceCube Neutrino Observatory detected a high-energy neutrino event that was found to coincide with a gamma-ray flare from a blazar. Since then, numerous multi-messenger studies combining neutrino and photon data have been conducted, yet the origin of neutrinos from active galactic nuclei (AGN) remains uncertain. In this work, we present the results of an X-ray observing program targeting two AGNs, NGC 1068 and PKS 1502+106. The multi-wavelength dataset includes new observations from NICER and NuSTAR from the observing proposal along with gamma-ray data collected using Fermi-LAT, and one archival observation from Chandra. Additionally, we derive the neutrino fluxes for both AGNs using ten years of IceCube data and neutrino spectra predicted by theoretical models. These results demonstrate the value of combining multi-messenger data in building and constraining theoretical models. They also highlight the importance of testing model predictions against observational data to refine measurements of both the neutrino flux and spectral shape.

astro-ph.HE↗

arXiv2025

Multi-Energy and Multi-Sample Searches for Neutrinos from GW Events

The IceCube Neutrino Observatory at the South Pole detects neutrinos of astrophysical origin via their interactions with ice. The main array is optimized for the detection of neutrinos with energies above 1 TeV. A much smaller infill array, known as IceCube DeepCore, extends the sensitivity down to a few GeV. Neutrinos observed in both parts of the detector are used for astrophysical-source searches with multiple messengers. We present two analyses that follow up archival gravitational wave (GW) events from runs O1 through O3 of LIGO/Virgo/KAGRA. The first analysis uses two neutrino datasets: one with high-energy tracks and another consisting of low-energy tracks and cascades. These two neutrino datasets were previously used independently to follow-up GW events. In the analysis presented here, a combined likelihood search is performed using both datasets to search for neutrinos coincident with the GW events across a wide energy range, from a few GeV to several PeV. The second analysis, for the first time, uses a neutrino-induced cascade sample with events of energy above ~1 TeV for searches of coincident neutrino-GW emission. We present results from both analyses and discuss prospects for conducting these analyses in real time.

astro-ph.HE↗

arXiv2025

Low-latency neutrino follow-up combining diverse IceCube selections

Neutrino observations are crucial for multi-messenger astronomy, but currently limited by effective area and atmospheric background. However, while other telescopes must point their limited field of view, IceCube can perform full-sky realtime follow-up of astrophysical transient sources with high uptime. For this, IceCube uses its Fast Response Analysis (FRA), which can provide results from an unbinned maximum likelihood analysis within tens of minutes of an astrophysical transient. Besides manually selected transient candidates, it also routinely scans areas of the sky compatible with gravitational wave alerts from LIGO/Virgo/KAGRA and the IceCube event singlets most likely to originate from an astrophysical source. Currently the analysis uses TeV muon neutrino candidate events whose signature permits especially precise angular reconstruction, selected and reconstructed at the South Pole and transmitted with low-latency over satellite. Recently, different event selections are also being included in IceCube analyses. These efforts include the follow-up of gravitational wave events with GeV neutrinos detected by IceCube-DeepCore and the observation of the Galactic plane with cascade events produced by all neutrino flavors. If made available on a day-scale latency, these event selections can also be used in FRAs. Moreso, multiple event samples can be combined in a Fast Response Analysis that is sensitive to a broader energy range of a neutrino transient spectrum and ensures the inclusion of all neutrino flavors. We present the analysis method and technical aspects of such an extension of the existing framework. This includes a proposed new pipeline allowing the inclusion of the more computationally-intensive reconstruction methods used by the aforementioned event selections. The extension is validated using example analyses implemented in this framework. (abbreviated)

astro-ph.HE↗

arXiv2025

Results from IceCube Follow-up of Nearby Supernova SN2023ixf

Core-collapse supernovae are of particular interest in multi-messenger astronomy due to their potential to accelerate cosmic rays and produce high-energy neutrinos. One such supernova is the recent SN2023ixf located in M101 (the Pinwheel Galaxy). It is the closest (6.4 Mpc) and brightest (B band magnitude 10.8) core-collapse supernova in nearly a decade. This supernova likely had a progenitor surrounded by dense circumstellar material which, during the supernova, may have produced neutrinos when ejecta collided with the material. I will present results of a follow-up of this supernova using data collected from the IceCube Neutrino Observatory located at the South Pole. We obtain results consistent with background expectations with time-integrated energy flux ($E^2 dN/dE$) upper limits of 0.35 GeV/cm$^2$ for a 32-day time window and 0.44 GeV/cm$^2$ for a 4-day time window, both at 90% confidence level for an $E^{-2}$ power law. These correspond to values of $2.7 \times 10^{48}$ erg for the 32-day time window and $3.5 \times 10^{48}$ erg for the 4-day time window at the supernova.

astro-ph.HE↗

arXiv2025

IceCube population constraints on neutrino emission by Fermi-LAT detected active galactic nuclei

Gamma-ray-bright active galactic nuclei (AGN) have been one of the most promising source classes of high-energy astrophysical neutrinos detected by IceCube. The first evidence of an IceCube point source was a blazar detected by the Fermi Large Area Telescope (LAT), TXS ~0506+056. Previous analyses have ruled out GeV-bright blazars as the predominant contributor to the high-energy astrophysical neutrino flux under simple correlation assumptions about the relationship between the fluxes of gamma rays and neutrinos. We present results from a more general and more sensitive search for correlation between neutrinos and GeV-selected AGN using improvements in the IceCube statistical methods and 13 years of data. We detect no correlation and set stringent constraints on neutrino emission by populations of GeV-detected AGN. These include constraints on the neutrino emission from subclasses of GeV-detected AGN, including BL Lacs, flat-spectrum radio quasars (FSRQ) and non-blazar AGN, using stacking analyses testing a variety of hypothesized relationships between neutrino and gamma-ray flux. We also present results from an analysis that is sensitive to a wider range of relationships between the gamma-ray and neutrino signal.

astro-ph.HE↗

arXiv2023

Searches for IceCube Neutrinos Coincident with Gravitational Wave Events

Searches for neutrinos from gravitational wave events have been performed utilizing the wide energy range of the IceCube Neutrino Observatory. We discuss results from these searches during the third observing run (O3) of the advanced LIGO and Virgo detectors, including a low-latency follow-up of public candidate alert events in O3, an archival search on high-energy track data, and a low-energy search employing IceCube-DeepCore. The dataset of high-energy tracks is mainly sensitive to muon neutrinos, while the low energy dataset is sensitive to neutrinos of all flavors. In all of these searches, we present upper limits on the neutrino flux and isotropic equivalent energy emitted in neutrinos. We also discuss future plans for additional searches, including extending the low-latency follow-up to the next observing run of the LIGO-Virgo-KAGRA detectors (O4) and analysis of gravitational wave (GW) events using a high-energy cascade dataset, which are produced by electron neutrino charged-current interactions and neutral-current interactions from neutrinos of all flavors.

astro-ph.HE↗