arXiv subjects

Z. Cao

Publications and source records attributed to Z. Cao.

At least 55 records · Page 3Linked to original sources

arXiv2015

The Knee of the Cosmic Hydrogen and Helium Spectrum below 1 PeV Measured by ARGO-YBJ and a Cherenkov Telescope of LHAASO

The measurement of cosmic ray energy spectra, in particular for individual species, is an essential approach in finding their origin. Locating the "knees" of the spectra is an important part of the approach and has yet to be achieved. Here we report a measurement of the mixed Hydrogen and Helium spectrum using the combination of the ARGO-YBJ experiment and of a prototype Cherenkov telescope for the LHAASO experiment. A knee feature at 640+/-87 TeV, with a clear steepening of the spectrum, is observed. This gives fundamental inputs to galactic cosmic ray acceleration models.

astro-ph.HE↗

arXiv2014

Identification of the TeV Gamma-ray Source ARGO J2031+4157 with the Cygnus Cocoon

The extended TeV gamma-ray source ARGO J2031+4157 (or MGRO J2031+41) is positionally consistent with the Cygnus Cocoon discovered by $Fermi$-LAT at GeV energies in the Cygnus superbubble. Reanalyzing the ARGO-YBJ data collected from November 2007 to January 2013, the angular extension and energy spectrum of ARGO J2031+4157 are evaluated. After subtracting the contribution of the overlapping TeV sources, the ARGO-YBJ excess map is fitted with a two-dimensional Gaussian function in a square region of $10^{\circ}\times 10^{\circ}$, finding a source extension $σ_{ext}$= 1$^{\circ}$.8$\pm$0$^{\circ}$.5. The observed differential energy spectrum is $dN/dE =(2.5\pm0.4) \times 10^{-11}(E/1 TeV)^{-2.6\pm0.3}$ photons cm$^{-2}$ s$^{-1}$ TeV$^{-1}$, in the energy range 0.2-10 TeV. The angular extension is consistent with that of the Cygnus Cocoon as measured by $Fermi$-LAT, and the spectrum also shows a good connection with the one measured in the 1-100 GeV energy range. These features suggest to identify ARGO J2031+4157 as the counterpart of the Cygnus Cocoon at TeV energies. The Cygnus Cocoon, located in the star-forming region of Cygnus X, is interpreted as a cocoon of freshly accelerated cosmic rays related to the Cygnus superbubble. The spectral similarity with Supernova Remnants indicates that the particle acceleration inside a superbubble is similar to that in a SNR. The spectral measurements from 1 GeV to 10 TeV allows for the first time to determine the possible spectrum slope of the underlying particle distribution. A hadronic model is adopted to explain the spectral energy distribution.

astro-ph.HE↗

arXiv2014

Energy Spectrum of Cosmic Protons and Helium Nuclei by a Hybrid Measurement at 4300 m a.s.l

The energy spectrum of cosmic Hydrogen and Helium nuclei has been measured, below the so-called "knee", by using a hybrid experiment with a wide field-of-view Cherenkov telescope and the Resistive Plate Chamber (RPC) array of the ARGO-YBJ experiment at 4300 m above sea level. The Hydrogen and Helium nuclei have been well separated from other cosmic ray components by using a multi-parameter technique. A highly uniform energy resolution of about 25% is achieved throughout the whole energy range (100 TeV - 700 TeV). The observed energy spectrum is compatible with a single power law with index gamma=-2.63+/-0.06.

astro-ph.HE↗

arXiv2014

Phase modulation parallel optical delay detector for microwave angle-of-arrival measurement with accuracy monitored

A novel phase modulation parallel optical delay detector is proposed for microwave angle-of-arrival (AOA) measurement with accuracy monitored by using only one dual-electrode Mach-Zenhder modulator. A theoretical model is built up to analyze the proposed system including measurement accuracy monitoring. The spatial delay measurement is translated into the phase shift between two replicas of a microwave signal. Thanks to the accuracy monitoring, the phase shifts from 5° to 165° are measured with less than 3.1° measurement error.

physics.optics↗

arXiv2013

Photonic Angle-of-arrival Measurement of a Microwave Signal with Range Selectivity and SNR Enhancement

A novel parallel optical delay detector (PODD) is proposed for angle-of-arrival (AOA) measurement of a microwave signal with selective measurement range and signal-to-noise (SNR) enhancement. The PODD is experimentally demonstrated by using a dual parallel Mach-Zehnder modulator. The noise robustness enhancement is experimentally investigated. Based on the proposed PODD, the measurement error for the full range can be less than 0.02 radian (0.71% of full measured range).

physics.optics↗

arXiv2013

TeV gamma-ray survey of the Northern sky using the ARGO-YBJ detector

The ARGO-YBJ detector is an extensive air shower array that has been used to monitor the northern $γ$-ray sky at energies above 0.3 TeV from 2007 November to 2013 January. In this paper, we present the results of a sky survey in the declination band from $-10^{\circ}$ to $70^{\circ}$, using data recorded over the past five years. With an integrated sensitivity ranging from 0.24 to $\sim$1 Crab units depending on the declination, six sources have been detected with a statistical significance greater than 5 standard deviations. Several excesses are also reported as potential $γ$-ray emitters. The features of each source are presented and discussed. Additionally, $95\%$ confidence level upper limits of the flux from the investigated sky region are shown. Specific upper limits for 663 GeV $γ$-ray AGNs inside the ARGO-YBJ field of view are reported. The effect of the absorption of $γ$-rays due to the interaction with extragalactic background light is estimated.

astro-ph.HE↗

arXiv2013

Observation of TeV gamma-rays from the unidentified source HESS J1841-055 with the ARGO-YBJ experiment

We report the observation of a very high energy γ-ray source, whose position is coincident with HESS J1841-055. This source has been observed for 4.5 years by the ARGO-YBJ experiment from November 2007 to July 2012. Its emission is detected with a statistical significance of 5.3 standard deviations. Parameterizing the source shape with a two-dimensional Gaussian function we estimate an extension σ=(0.40(+0.32,-0.22}) degree, consistent with the HESS measurement. The observed energy spectrum is dN/dE =(9.0-+1.6) x 10^{-13}(E/5 TeV)^{-2.32-+0.23} photons cm^{-2} s^{-1} TeV^{-1}, in the energy range 0.9-50 TeV. The integral γ-ray flux above 1 TeV is 1.3-+0.4 Crab units, which is 3.2-+1.0 times the flux derived by HESS. The differences in the flux determination between HESS and ARGO-YBJ, and possible counterparts at other wavelengths are discussed.

astro-ph.HE↗

arXiv2012

Long-term Monitoring on Mrk 501 for Its VHE gamma Emission and a Flare in October 2011

As one of the brightest active blazars in both X-ray and very high energy $γ$-ray bands, Mrk 501 is very useful for physics associated with jets from AGNs. The ARGO-YBJ experiment is monitoring it for $γ$-rays above 0.3 TeV since November 2007. Starting from October 2011 the largest flare since 2005 is observed, which lasts to about April 2012. In this paper, a detailed analysis is reported. During the brightest $γ$-rays flaring episodes from October 17 to November 22, 2011, an excess of the event rate over 6 $σ$ is detected by ARGO-YBJ in the direction of Mrk 501, corresponding to an increase of the $γ$-ray flux above 1 TeV by a factor of 6.6$\pm$2.2 from its steady emission. In particular, the $γ$-ray flux above 8 TeV is detected with a significance better than 4 $σ$. Based on time-dependent synchrotron self-Compton (SSC) processes, the broad-band energy spectrum is interpreted as the emission from an electron energy distribution parameterized with a single power-law function with an exponential cutoff at its high energy end. The average spectral energy distribution for the steady emission is well described by this simple one-zone SSC model. However, the detection of $γ$-rays above 8 TeV during the flare challenges this model due to the hardness of the spectra. Correlations between X-rays and $γ$-rays are also investigated.

astro-ph.HE↗

arXiv2012

Observation of TeV gamma rays from the Cygnus region with the ARGO-YBJ experiment

We report the observation of TeV gamma-rays from the Cygnus region using the ARGO-YBJ data collected from 2007 November to 2011 August. Several TeV sources are located in this region including the two bright extended MGRO J2019+37 and MGRO J2031+41. According to the Milagro data set, at 20 TeV MGRO J2019+37 is the most significant source apart from the Crab Nebula. No signal from MGRO J2019+37 is detected by the ARGO-YBJ experiment, and the derived flux upper limits at 90% confidence level for all the events above 600 GeV with medium energy of 3 TeV are lower than the Milagro flux, implying that the source might be variable and hard to be identified as a pulsar wind nebula. The only statistically significant (6.4 standard deviations) gamma-ray signal is found from MGRO J2031+41, with a flux consistent with the measurement by Milagro.

astro-ph.HE↗

arXiv2011

Properties and Performance of Two Wide Field of View Cherenkov/Fluorescence Telescope Array Prototypes

A wide field of view Cherenkov/fluorescence telescope array is one of the main components of the Large High Altitude Air Shower Observatory project. To serve as Cherenkov and fluorescence detectors, a flexible and mobile design is adopted for easy reconfiguring of the telescope array. Two prototype telescopes have been constructed and successfully run at the site of the ARGO-YBJ experiment in Tibet. The features and performance of the telescopes are presented.

physics.ins-det↗

arXiv2011

TeV gamma-ray survey of the northern sky using the ARGO-YBJ experiment

The ARGO-YBJ experiment is an extensive air shower array with full coverage RPC detectors located at Yangbajing (4300 m asl, Tibet, China). It is operated with high duty cycle (>86%) and a large field of view ($\sim$ 2sr). It continuously monitors the entire overhead sky at $γ$-ray energies above 0.1 TeV. In the talk, we will present the result of the northern sky survey (between declinations of -10$^{\circ}$ and 70$^{\circ}$) from an analysis of ~4 years of the ARGO-YBJ data (between July 2006 and February 2011). There are four known TeV sources observed with significance greater than 5 S.D.. The significance from Crab Nebula is more than 16 S.D.. 90% confidence level upper limits to the flux from all directions in the sky are also presented, which vary from 0.09 to 0.53 Crab unit for Crab-like point sources.

astro-ph.HE↗

arXiv2011

Long-term monitoring of the TeV emission from Mrk 421 with the ARGO-YBJ experiment

ARGO-YBJ is an air shower detector array with a fully covered layer of resistive plate chambers. It is operated with a high duty cycle and a large field of view. It continuously monitors the northern sky at energies above 0.3 TeV. In this paper, we report a long-term monitoring of Mrk 421 over the period from 2007 November to 2010 February. This source was observed by the satellite-borne experiments Rossi X-ray Timing Explorer and Swift in the X-ray band. Mrk 421 was especially active in the first half of 2008. Many flares are observed in both X-ray and gamma-ray bands simultaneously. The gamma-ray flux observed by ARGO-YBJ has a clear correlation with the X-ray flux. No lag between the X-ray and gamma-ray photons longer than 1 day is found. The evolution of the spectral energy distribution is investigated by measuring spectral indices at four different flux levels. Hardening of the spectra is observed in both X-ray and gamma-ray bands. The gamma-ray flux increases quadratically with the simultaneously measured X-ray flux. All these observational results strongly favor the synchrotron self-Compton process as the underlying radiative mechanism.

astro-ph.HE↗

arXiv2011

Mean Interplanetary Magnetic Field Measurement Using the ARGO-YBJ Experiment

The sun blocks cosmic ray particles from outside the solar system, forming a detectable shadow in the sky map of cosmic rays detected by the ARGO-YBJ experiment in Tibet. Because the cosmic ray particles are positive charged, the magnetic field between the sun and the earth deflects them from straight trajectories and results in a shift of the shadow from the true location of the sun. Here we show that the shift measures the intensity of the field which is transported by the solar wind from the sun to the earth.

astro-ph.SR↗

arXiv2010

Geometry and optics calibration of WFCTA prototype telescopes using star light

The Large High Altitude Air Shower Observatory project is proposed to study high energy gamma ray astronomy ( 40 GeV-1 PeV ) and cosmic ray physics ( 20 TeV-1 EeV ). The wide field of view Cherenkov telescope array, as a component of the LHAASO project, will be used to study energy spectrum and compositions of cosmic ray by measuring the total Cherenkov light generated by air showers and shower maximum depth. Two prototype telescopes have been in operation since 2008. The pointing accuracy of each telescope is crucial to the direction reconstruction of the primary particles. On the other hand the primary energy reconstruction relies on the shape of the Cherenkov image on the camera and the unrecorded photons due to the imperfect connections between photomultiplier tubes. UV bright stars are used as point-like objects to calibrate the pointing and to study the optical properties of the camera, the spot size and the fractions of unrecorded photons in the insensitive areas of the camera.

astro-ph.IM↗

arXiv2010

Indications of Proton-Dominated Cosmic Ray Composition above 1.6 EeV

We report studies of ultra-high energy cosmic ray composition via analysis of depth of airshower maximum (Xmax), for airshower events collected by the High Resolution Fly's Eye (HiRes) observatory. The HiRes data are consistent with a constant elongation rate d /d(log(E)) of 47.9 +- 6.0 (stat.) +- 3.2 (syst.)g/cm^2/decade for energies between 1.6 EeV and 63 EeV, and are consistent with a predominantly protonic composition of cosmic rays when interpreted via the QGSJET01 and QGSJET-II high-energy hadronic interaction models. These measurements constrain models in which the galactic-to-extragalactic transition is the cause of the energy spectrum "ankle' at 4 EeV.

astro-ph.HE↗

arXiv2010

Analysis of large-scale anisotropy of ultra-high energy cosmic rays in HiRes data

Stereo data collected by the HiRes experiment over a six year period are examined for large-scale anisotropy related to the inhomogeneous distribution of matter in the nearby Universe. We consider the generic case of small cosmic-ray deflections and a large number of sources tracing the matter distribution. In this matter tracer model the expected cosmic ray flux depends essentially on a single free parameter, the typical deflection angle theta. We find that the HiRes data with threshold energies of 40 EeV and 57 EeV are incompatible with the matter tracer model at a 95% confidence level unless theta is larger than 10 degrees and are compatible with an isotropic flux. The data set above 10 EeV is compatible with both the matter tracer model and an isotropic flux.

astro-ph.HE↗

arXiv2009

Simulation of the cosmic ray tau neutrino telescope (CRTNT) experiment

A tau lepton can be produced in a charged current interaction by cosmic ray tau neutrino with material inside a mountain. If it escapes from the mountain, it will decay and initiate a shower in the air, which can be detected by an air shower fluorescence/Cherenkov light detector. Designed according to such a principle, the Cosmic Ray Tau Neutrino Telescope (CRTNT) experiment, located at the foothill of Mt. Balikun in Xinjiang, China, will search for very high-energy cosmic tau neutrinos from energetic astrophysical sources by detecting those showers. This paper describes a Monte Carlo simulation for a detection of tau neutrino events by the CRTNT experiment. Ultra-high-energy cosmic ray events are also simulated to estimate the potential contamination. With the CRTNT experiment composed of four detector stations, each covering 64 by 14 degrees field of view, the expected event rates are 28.6, 21.9 and 4.7 per year assuming AGN neutrino flux according to Semikoz et. al. 2004, MPR AGN jet model and SDSS AGN core model, respectively. Null detection of such tau event by the CRTNT experiment in one year could set 90% C.L. upper limit at 19.9 (eV^-1 cm^-2 s^-1 sr^-1) for E^-2 neutrino spectrum.

hep-ex↗

arXiv2009

Measurement of the Flux of Ultra High Energy Cosmic Rays by the Stereo Technique

The High Resolution Fly's Eye experiment has measured the flux of ultrahigh energy cosmic rays using the stereoscopic air fluorescence technique. The HiRes experiment consists of two detectors that observe cosmic ray showers via the fluorescence light they emit. HiRes data can be analyzed in monocular mode, where each detector is treated separately, or in stereoscopic mode where they are considered together. Using the monocular mode the HiRes collaboration measured the cosmic ray spectrum and made the first observation of the Greisen-Zatsepin-Kuzmin cutoff. In this paper we present the cosmic ray spectrum measured by the stereoscopic technique. Good agreement is found with the monocular spectrum in all details.

astro-ph.HE↗