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1. Search for point sources of ultra-high-energy photons with the Telescope Array surface detector

   https://doi.org/10.1093/mnras/stz3618  

   Abbasi, R.U. ( February 2020 , Monthly notices of the Royal Astronomical Society)

   Ultra-high-energy (UHE) photons are an important tool for studying the high-energy Universe. A plausible source of photons with exa-eV (EeV) energy is provided by UHE cosmic rays (UHECRs) undergoing the Greisen–Zatsepin–Kuzmin process (Greisen 1966; Zatsepin & Kuzmin 1966) or pair production process (Blumenthal 1970) on a cosmic background radiation. In this context, the EeV photons can be a probe of both UHECR mass composition and the distribution of their sources (Gelmini, Kalashev & Semikoz 2008; Hooper, Taylor & Sarkar 2011). At the same time, the possible flux of photons produced by UHE protons in the vicinity of their sources by pion photoproduction or inelastic nuclear collisions would be noticeable only for relatively near sources, as the attenuation length of UHE photons is smaller than that of UHE protons; see, for example, Bhattacharjee & Sigl (2000) for a review. There also exists a class of so-called top-down models of UHECR generation that efficiently produce the UHE photons, for instance by the decay of heavy dark-matter particles (Berezinsky, Kachelriess & Vilenkin 1997; Kuzmin & Rubakov 1998) or by the radiation from cosmic strings (Berezinsky, Blasi & Vilenkin 1998). The search for the UHE photons was shown to be the most sensitive method of indirect detection of heavy dark matter (Kalashev & Kuznetsov 2016, 2017; Kuznetsov 2017; Kachelriess, Kalashev & Kuznetsov 2018; Alcantara, Anchordoqui & Soriano 2019). Another fundamental physics scenario that could be tested with UHE photons (Fairbairn, Rashba & Troitsky 2011) is the photon mixing with axion-like particles (Raffelt & Stodolsky 1988), which could be responsible for the correlation of UHECR events with BL Lac type objects observed by the High Resolution Fly’s Eye (HiRes) experiment (Gorbunov et al. 2004; Abbasi et al. 2006). In most of these scenarios, a clustering of photon arrival directions, rather than diffuse distribution, is expected, so point-source searches can be a suitable test for photon - axion-like particle mixing models. Finally, UHE photons could also be used as a probe for the models of Lorentz-invariance violation (Coleman & Glashow 1999; Galaverni & Sigl 2008; Maccione, Liberati & Sigl 2010; Rubtsov, Satunin & Sibiryakov 2012, 2014). The Telescope Array (TA; Tokuno et al. 2012; Abu-Zayyad et al. 2013c) is the largest cosmic ray experiment in the Northern Hemisphere. It is located at 39.3° N, 112.9° W in Utah, USA. The observatory includes a surface detector array (SD) and 38 fluorescence telescopes grouped into three stations. The SD consists of 507 stations that contain plastic scintillators, each with an area of 3 m2 (SD stations). The stations are placed in the square grid with 1.2 km spacing and cover an area of ∼700 km2. The TA SD is capable of detecting extensive air showers (EASs) in the atmosphere caused by cosmic particles of EeV and higher energies. The TA SD has been operating since 2008 May. A hadron-induced EAS significantly differs from an EAS induced by a photon because the depth of the shower maximum Xmax for a photon shower is larger, and a photon shower contains fewer muons and has a more curved front (see Risse & Homola 2007 for a review). The TA SD stations are sensitive to both muon and electromagnetic components of the shower and therefore can be triggered by both hadron-induced and photon-induced EAS events. In the present study, we use 9 yr of TA SD data for a blind search for point sources of UHE photons. We utilize the statistics of the SD data, which benefit from a high duty cycle. The full Monte Carlo (MC) simulation of proton-induced and photon-induced EAS events allows us to perform the photon search up to the highest accessible energies, E ≳ 1020 eV. As the main tool for the present photon search, we use a multivariate analysis based on a number of SD parameters that make it possible to distinguish between photon and hadron primaries. While searches for diffuse UHE photons were performed by several EAS experiments, including Haverah Park (Ave et al. 2000), AGASA (Shinozaki et al. 2002; Risse et al. 2005), Yakutsk (Rubtsov et al. 2006; Glushkov et al. 2007, 2010), Pierre Auger (Abraham et al. 2007, 2008a; Bleve 2016; Aab et al. 2017c) and TA (Abu-Zayyad et al. 2013b; Abbasi et al. 2019a), the search for point sources of UHE photons has been done only by the Pierre Auger Observatory (Aab et al. 2014, 2017a). The latter searches were based on hybrid data and were limited to the 1017.3 < E < 1018.5 eV energy range. In the present paper, we use the TA SD data alone. We perform the searches in five energy ranges: E > 1018, E > 1018.5, E > 1019, E > 1019.5 and E > 1020 eV. We find no significant evidence of photon point sources in all energy ranges and we set the point-source flux upper limits from each direction in the TA field of view (FOV). The search for unspecified neutral particles was also previously performed by the TA (Abbasi et al. 2015). The limit on the point-source flux of neutral particles obtained in that work is close to the present photon point-source flux limits. 

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2. Validation of standardized data formats and tools for ground-level particle-based gamma-ray observatories

   https://doi.org/10.1051/0004-6361/202243527  

   Albert, A. ; Alfaro, R. ; Arteaga-Velázquez, J. C. ; Ayala Solares, H. A. ; Babu, R. ; Belmont-Moreno, E. ; Brisbois, C. ; Caballero-Mora, K. S. ; Capistrán, T. ; Carramiñana, A. ; et al ( November 2022 , Astronomy & Astrophysics)

   Context. Ground-based γ-ray astronomy is still a rather young field of research, with strong historical connections to particle physics. This is why most observations are conducted by experiments with proprietary data and analysis software, as is usual in the particle physics field. However, in recent years, this paradigm has been slowly shifting toward the development and use of open-source data formats and tools, driven by upcoming observatories such as the Cherenkov Telescope Array (CTA). In this context, a community-driven, shared data format (the gamma-astro-data-format , or GADF) and analysis tools such as Gammapy and ctools have been developed. So far, these efforts have been led by the Imaging Atmospheric Cherenkov Telescope community, leaving out other types of ground-based γ -ray instruments. Aims. We aim to show that the data from ground particle arrays, such as the High-Altitude Water Cherenkov (HAWC) observatory, are also compatible with the GADF and can thus be fully analyzed using the related tools, in this case, Gammapy. Methods. We reproduced several published HAWC results using Gammapy and data products compliant with GADF standard. We also illustrate the capabilities of the shared format and tools by producing a joint fit of the Crab spectrum including data from six different γ -ray experiments. Results. We find excellent agreement with the reference results, a powerful confirmation of both the published results and the tools involved. Conclusions. The data from particle detector arrays such as the HAWC observatory can be adapted to the GADF and thus analyzed with Gammapy. A common data format and shared analysis tools allow multi-instrument joint analysis and effective data sharing. To emphasize this, a sample of Crab nebula event lists is made public with this paper. Because of the complementary nature of pointing and wide-field instruments, this synergy will be distinctly beneficial for the joint scientific exploitation of future observatories such as the Southern Wide-field Gamma-ray Observatory and CTA. 

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3. IceCube Search for Neutrinos Coincident with Gravitational Wave Events from LIGO/Virgo Run O3

   https://doi.org/10.3847/1538-4357/aca5fc  

   Abbasi, R. ; Ackermann, M. ; Adams, J. ; Aggarwal, N. ; Aguilar, J. A. ; Ahlers, M. ; Ahrens, M. ; Alameddine, J. M. ; Alves, A. A. ; Amin, N. M. ; et al ( February 2023 , The Astrophysical Journal)

   Abstract Using data from the IceCube Neutrino Observatory, we searched for high-energy neutrino emission from the gravitational-wave events detected by the advanced LIGO and Virgo detectors during their third observing run. We did a low-latency follow-up on the public candidate events released during the detectors’ third observing run and an archival search on the 80 confident events reported in the GWTC-2.1 and GWTC-3 catalogs. An extended search was also conducted for neutrino emission on longer timescales from neutron star containing mergers. Follow-up searches on the candidate optical counterpart of GW190521 were also conducted. We used two methods; an unbinned maximum likelihood analysis and a Bayesian analysis using astrophysical priors, both of which were previously used to search for high-energy neutrino emission from gravitational-wave events. No significant neutrino emission was observed by any analysis, and upper limits were placed on the time-integrated neutrino flux as well as the total isotropic equivalent energy emitted in high-energy neutrinos. 

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4. Hybrid cosmic ray measurements using the IceAct telescopes in coincidence with the IceCube and IceTop detectors

   Larissa Paul, Matthias Plum ( July 2021 , Proceedings of the 37th International Cosmic Ray Conference)

   null (Ed.)

   IceAct is a proposed surface array of compact (50 cm diameter) and cost-effective Imaging Air Cherenkov Telescopes installed at the site of the IceCube Neutrino Observatory at the geographic South Pole. Since January 2019, two IceAct telescope demonstrators, featuring 61 silicon pho- tomultiplier (SiPM) pixels have been taking data in the center of the IceTop surface array during the austral winter. We present the first analysis of hybrid cosmic ray events detected by the IceAct imaging air-Cherenkov telescopes in coincidence with the IceCube Neutrino Observatory, includ- ing the IceTop surface array and the IceCube in-ice array. By featuring an energy threshold of about 10 TeV and a wide field-of-view, the IceAct telescopes show promising capabilities of im- proving current cosmic ray composition studies: measuring the Cherenkov light emissions in the atmosphere adds new information about the shower development not accessible with the current detectors, enabling significantly better primary particle type discrimination on a statistical basis. The hybrid measurement also allows for detailed feasibility studies of detector cross-calibration and of cosmic ray veto capabilities for neutrino analyses. We present the performance of the telescopes, the results from the analysis of two years of data, and an outlook of a hybrid simulation for a future telescope array. 

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5. Search for Astrophysical Neutrinos from 1FLE Blazars with IceCube

   https://doi.org/10.3847/1538-4357/ac8de4  

   Abbasi, R. ; Ackermann, M. ; Adams, J. ; Aguilar, J. A. ; Ahlers, M. ; Ahrens, M. ; Alameddine, J. M. ; Alves, A. A. ; Amin, N. M. ; Andeen, K. ; et al ( October 2022 , The Astrophysical Journal)

   Abstract The majority of astrophysical neutrinos have undetermined origins. The IceCube Neutrino Observatory has observed astrophysical neutrinos but has not yet identified their sources. Blazars are promising source candidates, but previous searches for neutrino emission from populations of blazars detected in ≳GeV gamma rays have not observed any significant neutrino excess. Recent findings in multimessenger astronomy indicate that high-energy photons, coproduced with high-energy neutrinos, are likely to be absorbed and reemitted at lower energies. Thus, lower-energy photons may be better indicators of TeV–PeV neutrino production. This paper presents the first time-integrated stacking search for astrophysical neutrino emission from MeV-detected blazars in the first Fermi Large Area Telescope low energy (1FLE) catalog using ten years of IceCube muon–neutrino data. The results of this analysis are found to be consistent with a background-only hypothesis. Assuming an E −2 neutrino spectrum and proportionality between the blazars MeV gamma-ray fluxes and TeV–PeV neutrino flux, the upper limit on the 1FLE blazar energy-scaled neutrino flux is determined to be 1.64 × 10 −12 TeV cm −2 s −1 at 90% confidence level. This upper limit is approximately 1% of IceCube’s diffuse muon–neutrino flux measurement. 

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