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M. Chikawa

Publications and source records attributed to M. Chikawa.

At least 37 records · Page 2Linked to original sources
arXiv2021

Evidence for Declination Dependence of the Ultrahigh Energy Cosmic Ray Spectrum in the Northern Hemisphere

Telescope Array (TA) is the largest experiment in the Northern Hemisphere studying ultrahigh energy cosmic rays. TA measurements of the cosmic ray spectrum using the surface detector have the best statistical power in the experiment, and observe the ankle of the spectrum and the high energy cutoff. When the data are divided into two declination bands, above and below 24.8 degrees, the cutoff appears at $10^{19.64 \pm 0.04}$ ($10^{19.84 \pm 0.02}$) eV in the lower (higher) band, an energy difference of 58\%. The global significance of the difference is 4.3 standard deviations. The lack of an instrumental cause of this difference implies it is astrophysical in nature.

astro-ph.HE↗
arXiv2021

Indications of a Cosmic Ray Source in the Perseus-Pisces Supercluster

The Telescope Array Collaboration has observed an excess of events with $E \ge 10^{19.4} ~{\rm eV}$ in the data which is centered at (RA, dec) = ($19^\circ$, $35^\circ$). This is near the center of the Perseus-Pisces supercluster (PPSC). The PPSC is about $70 ~{\rm Mpc}$ distant and is the closest supercluster in the Northern Hemisphere (other than the Virgo supercluster of which we are a part). A Li-Ma oversampling analysis with $20^\circ$-radius circles indicates an excess in the arrival direction of events with a local significance of about 4 standard deviations. The probability of having such excess close to the PPSC by chance is estimated to be 3.5 standard deviations. This result indicates that a cosmic ray source likely exists in that supercluster.

astro-ph.HE↗
arXiv2021

Surface detectors of the TAx4 experiment

Telescope Array (TA) is the largest ultrahigh energy cosmic-ray (UHECR) observatory in the Northern Hemisphere. It explores the origin of UHECRs by measuring their energy spectrum, arrival-direction distribution, and mass composition using a surface detector (SD) array covering approximately 700 km$^2$ and fluorescence detector (FD) stations. TA has found evidence for a cluster of cosmic rays with energies greater than 57 EeV. In order to confirm this evidence with more data, it is necessary to increase the data collection rate.We have begun building an expansion of TA that we call TAx4. In this paper, we explain the motivation, design, technical features, and expected performance of the TAx4 SD. We also present TAx4's current status and examples of the data that have already been collected.

astro-ph.IM↗
arXiv2021

Sensitivity of the Cherenkov Telescope Array to a dark matter signal from the Galactic centre

We provide an updated assessment of the power of the Cherenkov Telescope Array (CTA) to search for thermally produced dark matter at the TeV scale, via the associated gamma-ray signal from pair-annihilating dark matter particles in the region around the Galactic centre. We find that CTA will open a new window of discovery potential, significantly extending the range of robustly testable models given a standard cuspy profile of the dark matter density distribution. Importantly, even for a cored profile, the projected sensitivity of CTA will be sufficient to probe various well-motivated models of thermally produced dark matter at the TeV scale. This is due to CTA's unprecedented sensitivity, angular and energy resolutions, and the planned observational strategy. The survey of the inner Galaxy will cover a much larger region than corresponding previous observational campaigns with imaging atmospheric Cherenkov telescopes. CTA will map with unprecedented precision the large-scale diffuse emission in high-energy gamma rays, constituting a background for dark matter searches for which we adopt state-of-the-art models based on current data. Throughout our analysis, we use up-to-date event reconstruction Monte Carlo tools developed by the CTA consortium, and pay special attention to quantifying the level of instrumental systematic uncertainties, as well as background template systematic errors, required to probe thermally produced dark matter at these energies. "Full likelihood tables complementing our analysis are provided here [ https://doi.org/10.5281/zenodo.4057987 ]"

astro-ph.HE↗
arXiv2020

Observations of the Origin of Downward Terrestrial Gamma-Ray Flashes

In this paper we report the first close, high-resolution observations of downward-directed terrestrial gamma-ray flashes (TGFs) detected by the large-area Telescope Array cosmic ray observatory, obtained in conjunction with broadband VHF interferometer and fast electric field change measurements of the parent discharge. The results show that the TGFs occur during strong initial breakdown pulses (IBPs) in the first few milliseconds of negative cloud-to-ground and low-altitude intracloud flashes, and that the IBPs are produced by a newly-identified streamer-based discharge process called fast negative breakdown. The observations indicate the relativistic runaway electron avalanches (RREAs) responsible for producing the TGFs are initiated by embedded spark-like transient conducting events (TCEs) within the fast streamer system, and potentially also by individual fast streamers themselves. The TCEs are inferred to be the cause of impulsive sub-pulses that are characteristic features of classic IBP sferics. Additional development of the avalanches would be facilitated by the enhanced electric field ahead of the advancing front of the fast negative breakdown. In addition to showing the nature of IBPs and their enigmatic sub-pulses, the observations also provide a possible explanation for the unsolved question of how the streamer to leader transition occurs during the initial negative breakdown, namely as a result of strong currents flowing in the final stage of successive IBPs, extending backward through both the IBP itself and the negative streamer breakdown preceding the IBP.

physics.ao-ph↗
arXiv2020

Search for Large-scale Anisotropy on Arrival Directions of Ultra-high-energy Cosmic Rays Observed with the Telescope Array Experiment

Motivated by the detection of a significant dipole structure in the arrival directions of ultrahigh-energy cosmic rays above 8 EeV reported by the Pierre Auger Observatory (Auger), we search for a large-scale anisotropy using data collected with the surface detector array of the Telescope Array Experiment (TA). With 11 years of TA data, a dipole structure in a projection of the right ascension is fitted with an amplitude of 3.3+- 1.9% and a phase of 131 +- 33 degrees. The corresponding 99% confidence-level upper limit on the amplitude is 7.3%. At the current level of statistics, the fitted result is compatible with both an isotropic distribution and the dipole structure reported by Auger.

astro-ph.HE↗
arXiv2020

Evidence for a Supergalactic Structure of Magnetic Deflection Multiplets of Ultra-High Energy Cosmic Rays

Evidence for a large-scale supergalactic cosmic ray multiplet (arrival directions correlated with energy) structure is reported for ultra-high energy cosmic ray (UHECR) energies above 10$^{19}$ eV using seven years of data from the Telescope Array (TA) surface detector and updated to 10 years. Previous energy-position correlation studies have made assumptions regarding magnetic field shapes and strength, and UHECR composition. Here the assumption tested is that, since the supergalactic plane is a fit to the average matter density of the local Large Scale Structure (LSS), UHECR sources and intervening extragalactic magnetic fields are correlated with this plane. This supergalactic deflection hypothesis is tested by the entire field-of-view (FOV) behavior of the strength of intermediate-scale energy-angle correlations. These multiplets are measured in spherical cap section bins (wedges) of the FOV to account for coherent and random magnetic fields. The structure found is consistent with supergalactic deflection, the previously published energy spectrum anisotropy results of TA (the hotspot and coldspot), and toy-model simulations of a supergalactic magnetic sheet. The seven year data post-trial significance of this supergalactic structure of multiplets appearing by chance, on an isotropic sky, is found by Monte Carlo simulation to be 4.2$σ$. The ten years of data post-trial significance is 4.1$σ$. Furthermore, the starburst galaxy M82 is shown to be a possible source of the TA Hotspot, and an estimate of the supergalactic magnetic field using UHECR measurements is presented.

astro-ph.HE↗
arXiv2020

Measurement of the Proton-Air Cross Section with Telescope Array's Black Rock Mesa and Long Ridge Fluorescence Detectors, and Surface Array in Hybrid Mode

Ultra high energy cosmic rays provide the highest known energy source in the universe to measure proton cross sections. Though conditions for collecting such data are less controlled than an accelerator environment, current generation cosmic ray observatories have large enough exposures to collect significant statistics for a reliable measurement for energies above what can be attained in the lab. Cosmic ray measurements of cross section use atmospheric calorimetry to measure depth of air shower maximum ($X_{\mathrm{max}}$), which is related to the primary particle's energy and mass. The tail of the $X_{\mathrm{max}}$ distribution is assumed to be dominated by showers generated by protons, allowing measurement of the inelastic proton-air cross section. In this work the proton-air inelastic cross section measurement, $σ^{\mathrm{inel}}_{\mathrm{p-air}}$, using data observed by Telescope Array's Black Rock Mesa and Long Ridge fluorescence detectors and surface detector array in hybrid mode is presented. $σ^{\mathrm{inel}}_{\mathrm{p-air}}$ is observed to be $520.1 \pm 35.8$[Stat.] $^{+25.0}_{-40}$[Sys.]~mb at $\sqrt{s} = 73$ TeV. The total proton-proton cross section is subsequently inferred from Glauber formalism and is found to be $σ^{\mathrm{tot}}_{\mathrm{pp}} = 139.4 ^{+23.4}_{-21.3}$ [Stat.]$ ^{+15.0}_{-24.0}$[Sys.]~mb.

astro-ph.HE↗
arXiv2020

Search for Ultra-High-Energy Neutrinos with the Telescope Array Surface Detector

We present an upper limit on the flux of ultra-high-energy down-going neutrinos for $E > 10^{18}\ \mbox{eV}$ derived with the nine years of data collected by the Telescope Array surface detector (05-11-2008 -- 05-10-2017). The method is based on the multivariate analysis technique, so-called Boosted Decision Trees (BDT). Proton-neutrino classifier is built upon 16 observables related to both the properties of the shower front and the lateral distribution function.

astro-ph.HE↗
arXiv2020

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

The surface detector (SD) of the Telescope Array (TA) experiment allows one to indirectly detect photons with energies of order $10^{18}$ eV and higher and to separate photons from the cosmic-ray background. In this paper we present the results of a blind search for point sources of ultra-high energy (UHE) photons in the Northern sky using the TA SD data. The photon-induced extensive air showers (EAS) are separated from the hadron-induced EAS background by means of a multivariate classifier based upon 16 parameters that characterize the air shower events. No significant evidence for the photon point sources is found. The upper limits are set on the flux of photons from each particular direction in the sky within the TA field of view, according to the experiment's angular resolution for photons. Average 95% C.L. upper limits for the point-source flux of photons with energies greater than $10^{18}$, $10^{18.5}$, $10^{19}$, $10^{19.5}$ and $10^{20}$ eV are $0.094$, $0.029$, $0.010$, $0.0073$ and $0.0058$ km$^{-2}$yr$^{-1}$ respectively. For the energies higher than $10^{18.5}$ eV, the photon point-source limits are set for the first time. Numerical results for each given direction in each energy range are provided as a supplement to this paper.

astro-ph.HE↗
arXiv2019

Constraints on the diffuse photon flux with energies above $10^{18}$ eV using the surface detector of the Telescope Array experiment

We present the results of the search for ultra-high-energy photons with nine years of data from the Telescope Array surface detector. A multivariate classifier is built upon 16 reconstructed parameters of the extensive air shower. These parameters are related to the curvature and the width of the shower front, the steepness of the lateral distribution function, and the timing parameters of the waveforms sensitive to the shower muon content. A total number of two photon candidates found in the search is fully compatible with the expected background. The $95\%\,$CL limits on the diffuse flux of the photons with energies greater than $10^{18.0}$, $10^{18.5}$, $10^{19.0}$, $10^{19.5}$ and $10^{20.0}$ eV are set at the level of $0.067$, $0.012$, $0.0036$, $0.0013$, $0.0013~\mbox{km}^{-2}\mbox{yr}^{-1}\mbox{sr}^{-1}$ correspondingly.

astro-ph.HE↗
arXiv2019

Mass composition of ultra-high-energy cosmic rays with the Telescope Array Surface Detector Data

The results on ultra-high-energy cosmic rays (UHECR) mass composition obtained with the Telescope Array surface detector are presented. The analysis employs the boosted decision tree (BDT) multivariate analysis built upon 14 observables related to both the properties of the shower front and the lateral distribution function. The multivariate classifier is trained with Monte-Carlo sets of events induced by the primary protons and iron. An average atomic mass of UHECR is presented for energies $10^{18.0}-10^{20.0}\ \mbox{eV}$. The average atomic mass of primary particles shows no significant energy dependence and corresponds to $\langle \ln A \rangle = 2.0 \pm 0.1 (stat.) \pm 0.44 (syst.)$. The result is compared to the mass composition obtained by the Telescope Array with $\mbox{X}_{\mbox{max}}$ technique along with the results of other experiments. Possible systematic errors of the method are discussed.

astro-ph.HE↗
arXiv2018

Testing a reported correlation between arrival directions of ultrahigh-energy cosmic rays and a flux pattern from nearby starburst galaxies using Telescope Array data

The Pierre Auger Collaboration (Auger) recently reported a correlation between the arrival directions of cosmic rays with energies above 39 EeV and the flux pattern of 23 nearby starburst galaxies (SBGs). In this Letter, we tested the same hypothesis using cosmic rays detected by the Telescope Array experiment (TA) in the 9-year period from May 2008 to May 2017. Unlike the Auger analysis, we did not optimize the parameter values but kept them fixed to the best-fit values found by Auger, namely 9.7% for the anisotropic fraction of cosmic rays assumed to originate from the SBGs in the list and 12.9° for the angular scale of the correlations. The energy threshold we adopted is 43 EeV, corresponding to 39 EeV in Auger when taking into account the energy-scale difference between two experiments. We find that the TA data is compatible with isotropy to within 1.1σ and with the Auger result to within 1.4σ, meaning that it is not capable to discriminate between these two hypotheses.

astro-ph.HE↗
arXiv2018

First observations of speed of light tracks by a fluorescence detector looking down on the atmosphere

EUSO-Balloon is a pathfinder mission for the Extreme Universe Space Observatory onboard the Japanese Experiment Module (JEM-EUSO). It was launched on the moonless night of the 25$^{th}$ of August 2014 from Timmins, Canada. The flight ended successfully after maintaining the target altitude of 38 km for five hours. One part of the mission was a 2.5 hour underflight using a helicopter equipped with three UV light sources (LED, xenon flasher and laser) to perform an inflight calibration and examine the detectors capability to measure tracks moving at the speed of light. We describe the helicopter laser system and details of the underflight as well as how the laser tracks were recorded and found in the data. These are the first recorded laser tracks measured from a fluorescence detector looking down on the atmosphere. Finally, we present a first reconstruction of the direction of the laser tracks relative to the detector.

astro-ph.IM↗
arXiv2018

Gamma-ray Showers Observed at Ground Level in Coincidence With Downward Lightning Leaders

Bursts of gamma ray showers have been observed in coincidence with downward propagating negative leaders in lightning flashes by the Telescope Array Surface Detector (TASD). The TASD is a 700~square kilometer cosmic ray observatory located in southwestern Utah, U.S.A. In data collected between 2014 and 2016, correlated observations showing the structure and temporal development of three shower-producing flashes were obtained with a 3D lightning mapping array, and electric field change measurements were obtained for an additional seven flashes, in both cases co-located with the TASD. National Lightning Detection Network (NLDN) information was also used throughout. The showers arrived in a sequence of 2--5 short-duration ($\le$10~$μ$s) bursts over time intervals of several hundred microseconds, and originated at an altitude of $\simeq$3--5 kilometers above ground level during the first 1--2 ms of downward negative leader breakdown at the beginning of cloud-to-ground lightning flashes. The shower footprints, associated waveforms and the effect of atmospheric propagation indicate that the showers consist primarily of downward-beamed gamma radiation. This has been supported by GEANT simulation studies, which indicate primary source fluxes of $\simeq$$10^{12}$--$10^{14}$ photons for $16^{\circ}$ half-angle beams. We conclude that the showers are terrestrial gamma-ray flashes (TGFs), similar to those observed by satellites, but that the ground-based observations are more representative of the temporal source activity and are also more sensitive than satellite observations, which detect only the most powerful TGFs.

physics.ao-ph↗
arXiv2018

Depth of Ultra High Energy Cosmic Ray Induced Air Shower Maxima Measured by the Telescope Array Black Rock and Long Ridge FADC Fluorescence Detectors and Surface Array in Hybrid Mode

The Telescope Array observatory utilizes fluorescence detectors and surface detectors to observe air showers produced by ultra high energy cosmic rays in the Earth's atmosphere. Cosmic ray events observed in this way are termed hybrid data. The depth of air shower maximum is related to the mass of the primary particle that generates the shower. This paper reports on shower maxima data collected over 8.5 years using the Black Rock Mesa and Long Ridge fluorescence detectors in conjunction with the array of surface detectors. We compare the means and standard deviations of the observed $X_{\mathrm{max}}$ distributions with Monte Carlo $X_{\mathrm{max}}$ distributions of unmixed protons, helium, nitrogen, and iron, all generated using the QGSJet~II-04 hadronic model. We also perform an unbinned maximum likelihood test of the observed data, which is subjected to variable systematic shifting of the data $X_{\mathrm{max}}$ distributions to allow us to test the full distributions, and compare them to the Monte Carlo to see which elements are not compatible with the observed data. For all energy bins, QGSJet~II-04 protons are found to be compatible with Telescope Array hybrid data at the 95% confidence level after some systematic $X_{\mathrm{max}}$ shifting of the data. Three other QGSJet~II-04 elements are found to be compatible using the same test procedure in an energy range limited to the highest energies where data statistics are sparse.

astro-ph.HE↗
arXiv2018

Study of muons from ultra-high energy cosmic ray air showers measured with the Telescope Array experiment

One of the uncertainties in interpretation of ultra-high energy cosmic ray (UHECR) data comes from the hadronic interaction models used for air shower Monte Carlo (MC) simulations. The number of muons observed at the ground from UHECR-induced air showers is expected to depend upon the hadronic interaction model. One may therefore test the hadronic interaction models by comparing the measured number of muons with the MC prediction. In this paper, we present the results of studies of muon densities in UHE extensive air showers obtained by analyzing the signal of surface detector stations which should have high $\it{muon \, purity}$. The muon purity of a station will depend on both the inclination of the shower and the relative position of the station. In 7 years' data from the Telescope Array experiment, we find that the number of particles observed for signals with an expected muon purity of $\sim$65% at a lateral distance of 2000 m from the shower core is $1.72 \pm 0.10{\rm (stat.)} \pm 0.37 {\rm (syst.)}$ times larger than the MC prediction value using the QGSJET II-03 model for proton-induced showers. A similar effect is also seen in comparisons with other hadronic models such as QGSJET II-04, which shows a $1.67 \pm 0.10 \pm 0.36$ excess. We also studied the dependence of these excesses on lateral distances and found a slower decrease of the lateral distribution of muons in the data as compared to the MC, causing larger discrepancy at larger lateral distances.

astro-ph.HE↗