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P. Sokolsky

Publications and source records attributed to P. Sokolsky.

At least 19 recordsLinked to original sources
arXiv2024

Observation of Declination Dependence in the Cosmic Ray Energy Spectrum

We report on an observation of the difference between northern and southern skies of the ultrahigh energy cosmic ray energy spectrum with a significance of ${\sim}8σ$. We use measurements from the two largest experiments$\unicode{x2014}$the Telescope Array observing the northern hemisphere and the Pierre Auger Observatory viewing the southern hemisphere. Since the comparison of two measurements from different observatories introduces the issue of possible systematic differences between detectors and analyses, we validate the methodology of the comparison by examining the region of the sky where the apertures of the two observatories overlap. Although the spectra differ in this region, we find that there is only a $1.8σ$ difference between the spectrum measurements when anisotropic regions are removed and a fiducial cut in the aperture is applied.

astro-ph.HE↗
arXiv2021

The Unreasonable Effectiveness of the Air-Fluorescence Technique in Determining the EAS Shower Maximum

We review all existing air-fluorescence measurements of the elongation rate of extensive air showers (slope of mean EAS shower maximum (Xmax) vs log of shower energy E) above 1017 eV. We find remarkable agreement for all current and historic experiments over a 30 year period for the energy range from 1017 to 3x1018 eV. The mean elongation rate in this energy interval is near 80 gm/cm2/decade Above this energy, experiments in the Northern hemisphere are in good agreement with an average elongation rate of 48 +/- 10 gm/cm2/decade while Southern hemisphere experiments have a flatter elongation rate of 26 +/- 2 gm/cm2/decade We point out that, given the agreement at lower energies, possible systematic reasons for this difference are unlikely. Given this, the world elongation rate data alone may indicate a composition difference of UHECR in the Northern and Southern hemisphere and thus a diversity of UHECR sources in the Northern and Southern sky.

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

EUSO-TA ground based fluorescence detector: analysis of the detected events

EUSO-TA is a ground-based florescence detector built to validate the design of an ultra-high energy cosmic ray fluorescence detector to be operated in space. EUSO-TA detected the first air shower events with the technology developed within the JEM-EUSO program. It operates at the Telescope Array (TA) site in Utah, USA. With the external trigger provided by the Black Rock Mesa fluorescence detectors of Telescope Array (TA-FDs), EUSO-TA observed nine ultra-high energy cosmic ray events and several laser events from the Central Laser Facility of Telescope Array and portable lasers like the JEM-EUSO Global Light System prototype. The reconstruction parameters of the cosmic ray events which crossed the EUSO-TA field of view (both detected and not detected by EUSO-TA), were provided by the Telescope Array Collaboration. As the TA-FDs have a wider field of view than EUSO-TA ($\sim$30 times larger), they allow the cosmic ray energy reconstruction based on the observation of most of the extensive air-shower profiles, including the shower maximum, while EUSO-TA only observes a portion of the showers, usually far from the maximum. For this reason, the energy of the cosmic rays corresponding to the EUSO-TA signals appear lower than the actual ones. In this contribution, the analysis of the cosmic-ray events detected with EUSO-TA is discussed.

astro-ph.HE↗
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↗
arXiv2017

Search for Anisotropy in the Ultra High Energy Cosmic Ray Spectrum using the Telescope Array Surface Detector

The Telescope Array (TA) experiment is located in the western desert of Utah, USA, and observes ultra high energy cosmic rays (UHECRs) in the Northern hemisphere. At the highest energies, $E>10$~EeV, the shape of cosmic ray energy spectrum may carry an imprint of the source density distribution along the line of sight different in different directions of the sky. In this study, we search for such directional variations in the shape of the energy spectrum using events observed with the Telescope Array's surface detector. We divide the TA field of view into two nearly equal-exposure regions: the "on-source" region which we define as $\pm 30^\circ$ of the supergalactic plane containing mostly nearby structures, and the complementary "off-source" region where the sources are further away on average. We compare the UHECR spectra in these regions by fitting them to the broken power law and comparing the resulting parameters. We find that the off-source spectrum has an earlier break at highest energies. The chance probability to obtain such or larger difference in statistically equivalent distributions is estimated as $6.2\pm1.1\times10^{-4}$ ($3.2σ$) by a Monte-Carlo simulation. The observed difference in spectra is in a reasonable quantitative agreement with a simplified model that assumes that the UHECR sources trace the galaxy distribution from the 2MRS catalogue, primary particles are protons and the magnetic deflections can be neglected.

astro-ph.HE↗
arXiv2017

Past, Present and Future of UHECR Observations

Great advances have been made in the study of ultra-high energy cosmic rays (UHECR) in the past two decades. These include the discovery of the spectral cut-off near 5 x 10^19 eV and complex structure at lower energies, as well as increasingly precise information about the composition of cosmic rays as a function of energy. Important improvements in techniques, including extensive surface detector arrays and high resolution air fluorescence detectors, have been instrumental in facilitating this progress. We discuss the status of the field, including the open questions about the nature of spectral structure, systematic issues related to our understanding of composition, and emerging evidence for anisotropy at the highest energies. We review prospects for upgraded and future observatories including Telescope Array, Pierre Auger and JEM-EUSO and other space-based proposals, and discuss promising new technologies based on radio emission from extensive air showers produced by UHECR.

astro-ph.HE↗
arXiv2016

Search for EeV Protons of Galactic Origin

Cosmic rays in the energy range $10^{18.0}$ - $10^{18.5}$ eV are thought to have a light, probably protonic, composition. To study their origin one can search for anisotropy in their arrival directions. Extragalactic cosmic rays should be isotropic, but galactic cosmic rays of this type should be seen mostly along the galactic plane, and there should be a shortage of events coming from directions near the galactic anticenter. This is due to the fact that, under the influence of the galactic magnetic field, the transition from ballistic to diffusive behavior is well advanced, and this qualitative picture persists over the whole energy range. Guided by models of the galactic magnetic field that indicate that the enhancement along the galactic plane should have a standard deviation of about 20$^\circ$ in galactic latitude, and the deficit in the galactic anticenter direction should have a standard deviation of about 50$^\circ$ in galactic longitude, we use the data of the Telescope Array surface detector in $10^{18.0}$ to $10^{18.5}$ eV energy range to search for these effects. The data are isotropic. Neither an enhancement along the galactic plane nor a deficit in the galactic anticenter direction is found. Using these data we place an upper limit on the fraction of EeV cosmic rays of galactic origin at 1.3% at 95% confidence level.

astro-ph.HE↗
arXiv2016

First Upper Limits on the Radar Cross Section of Cosmic-Ray Induced Extensive Air Showers

TARA (Telescope Array Radar) is a cosmic ray radar detection experiment colocated with Telescope Array, the conventional surface scintillation detector (SD) and fluorescence telescope detector (FD) near Delta, Utah, U.S.A. The TARA detector combines a 40 kW, 54.1 MHz VHF transmitter and high-gain transmitting antenna which broadcasts the radar carrier over the SD array and within the FD field of view, towards a 250 MS/s DAQ receiver. TARA has been collecting data since 2013 with the primary goal of observing the radar signatures of extensive air showers (EAS). Simulations indicate that echoes are expected to be short in duration (~10 microseconds) and exhibit rapidly changing frequency, with rates on the order of 1 MHz/microsecond. The EAS radar cross-section (RCS) is currently unknown although it is the subject of over 70 years of speculation. A novel signal search technique is described in which the expected radar echo of a particular air shower is used as a matched filter template and compared to waveforms obtained by triggering the radar DAQ using the Telescope Array fluorescence detector. No evidence for the scattering of radio frequency radiation by EAS is obtained to date. We report the first quantitative RCS upper limits using EAS that triggered the Telescope Array Fluorescence Detector.

astro-ph.IM↗
arXiv2015

Measurement of the Proton-Air Cross Section with Telescope Array's Middle Drum Detector and Surface Array in Hybrid Mode

In this work we are reporting on the measurement of the proton-air inelastic cross section $σ^{\rm inel}_{\rm p-air}$ using the Telescope Array (TA) detector. Based on the measurement of the $σ^{\rm inel}_{\rm p-air}$ the proton-proton cross section $σ_{\rm p-p}$ value is also determined at $\sqrt{s} = 95_{-8}^{+5}$ TeV. Detecting cosmic ray events at ultra high energies with Telescope Array enables us to study this fundamental parameter that we are otherwise unable to access with particle accelerators. The data used in this report is the hybrid events observed by the Middle Drum fluorescence detector together with the surface array detector collected over five years. The value of the $σ^{\rm inel}_{\rm p-air}$ is found to be equal to $567.0 \pm 70.5 [{\rm Stat.}] ^{+29}_{-25} [{\rm Sys.}]$ mb. The total proton-proton cross section is subsequently inferred from Glauber Formalism and Block, Halzen and Stanev QCD inspired fit and is found to be equal to $170_{-44}^{+48} [{\rm Stat.}] _{-17}^{+19} [{\rm Sys.}] $mb.

astro-ph.HE↗
arXiv2015

Report of the Working Group on the Composition of Ultra High Energy Cosmic Rays

For the first time a proper comparison of the average depth of shower maximum ($X_{\rm max}$) published by the Pierre Auger and Telescope Array Observatories is presented. The $X_{\rm max}$ distributions measured by the Pierre Auger Observatory were fit using simulated events initiated by four primaries (proton, helium, nitrogen and iron). The primary abundances which best describe the Auger data were simulated through the Telescope Array (TA) Middle Drum (MD) fluorescence and surface detector array. The simulated events were analyzed by the TA Collaboration using the same procedure as applied to their data. The result is a simulated version of the Auger data as it would be observed by TA. This analysis allows a direct comparison of the evolution of $\langle X_{\rm max} \rangle$ with energy of both data sets. The $\langle X_{\rm max} \rangle$ measured by TA-MD is consistent with a preliminary simulation of the Auger data through the TA detector and the average difference between the two data sets was found to be $(2.9 \pm 2.7\;(\text{stat.}) \pm 18\;(\text{syst.}))~\text{g/cm}^2$.

astro-ph.HE↗
arXiv2015

A Northern Sky Survey for Point-Like Sources of EeV Neutral Particles with the Telescope Array Experiment

We report on the search for steady point-like sources of neutral particles around 10$^{18}$ eV between 2008 May and 2013 May with the scintillator surface detector of the Telescope Array experiment. We found overall no significant point-like excess above 0.5 EeV in the northern sky. Subsequently, we also searched for coincidence with the Fermi bright Galactic sources. No significant coincidence was found within the statistical uncertainty. Hence, we set an upper limit on the neutron flux that corresponds to an averaged flux of 0.07 km$^{-2}$ yr$^{-1}$ for $E>1$ EeV in the northern sky at the 95% confidence level. This is the most stringent flux upper limit in a northern sky survey assuming point-like sources. The upper limit at the 95% confidence level on the neutron flux from Cygnus X-3 is also set to 0.2 km$^{-2}$ yr$^{-1}$ for $E>0.5$ EeV. This is an order of magnitude lower than previous flux measurements.

astro-ph.HE↗
arXiv2014

Study of Ultra-High Energy Cosmic Ray Composition Using Telescope Array's Middle Drum Detector and Surface Array in Hybrid Mode

Previous measurements of the composition of Ultra-High Energy Cosmic Rays(UHECRs) made by the High Resolution Fly's Eye(HiRes) and Pierre Auger Observatory(PAO) are seemingly contradictory, but utilize different detection methods, as HiRes was a stereo detector and PAO is a hybrid detector. The five year Telescope Array(TA) Middle Drum hybrid composition measurement is similar in some, but not all, respects in methodology to PAO, and good agreement is evident between data and a light, largely protonic, composition when comparing the measurements to predictions obtained with the QGSJetII-03 and QGSJet-01c models. These models are also in agreement with previous HiRes stereo measurements, confirming the equivalence of the stereo and hybrid methods. The data is incompatible with a pure iron composition, for all models examined, over the available range of energies. The elongation rate and mean values of Xmax are in good agreement with Pierre Auger Observatory data. This analysis is presented using two methods: data cuts using simple geometrical variables and a new pattern recognition technique.

astro-ph.HE↗
arXiv2014

Telescope Array Radar (TARA) Observatory for Ultra-High Energy Cosmic Rays

Construction was completed during summer 2013 on the Telescope Array RAdar (TARA) bi-static radar observatory for Ultra-High Energy Cosmic Rays (UHECR). TARA is co-located with the Telescope Array, the largest "conventional" cosmic ray detector in the Northern Hemisphere, in radio-quiet Western Utah. TARA employs an 8 MW Effective Radiated Power (ERP) VHF transmitter and smart receiver system based on a 250 MS/s data acquisition system in an effort to detect the scatter of sounding radiation by UHECR-induced atmospheric ionization. TARA seeks to demonstrate bi-static radar as a useful new remote sensing technique for UHECRs, extending their detection aperture far beyond what is accessible by conventional means. In this report, we describe the design and performance of the TARA transmitter and receiver systems.

astro-ph.IM↗
arXiv2014

Indications of Intermediate-Scale Anisotropy of Cosmic Rays with Energy Greater Than 57 EeV in the Northern Sky Measured with the Surface Detector of the Telescope Array Experiment

We have searched for intermediate-scale anisotropy in the arrival directions of ultrahigh-energy cosmic rays with energies above 57~EeV in the northern sky using data collected over a 5 year period by the surface detector of the Telescope Array experiment. We report on a cluster of events that we call the hotspot, found by oversampling using 20$^\circ$-radius circles. The hotspot has a Li-Ma statistical significance of 5.1$σ$, and is centered at R.A.=146.7$^{\circ}$, Dec.=43.2$^{\circ}$. The position of the hotspot is about 19$^{\circ}$ off of the supergalactic plane. The probability of a cluster of events of 5.1$σ$ significance, appearing by chance in an isotropic cosmic-ray sky, is estimated to be 3.7$\times$10$^{-4}$ (3.4$σ$).

astro-ph.HE↗