The Influence of the Sun and Moon on the Observation of Very High Energy Gamma-ray Sources Using EAS Arrays

With great advance of ground-based extensive air shower arrays,such as LHAASO and HAWC,many very high energy(VHE)gamma-ray sources have been discovered and are being monitored regardless of the day and the night.Hence,the Sun and Moon would have some impacts on the observation of gamma-ray sources,which have not been taken into account in previous analysis.In this paper,the influence of the Sun and Moon on the observation of very high energy gamma-ray sources when they are near the line of sight of the Sun or Moon is estimated.The tracks of all the known VHE sources are scanned and several VHE sources are found to be very close to the line of sight of the Sun or Moon during some period.The absorption of very high energy gamma rays by sunlight is estimated with detailed method and some useful conclusions are achieved.The main influence is the block of the Sun and Moon on gamma rays and the shadow on the cosmic ray background.The influence is investigated considering the detector angular resolution and some strategies on data analysis are proposed to avoid the underestimation of the gamma-ray emission.

Simulating gamma-ray production from cosmic rays interacting with the solar atmosphere in the presence of coronal magnetic fields

Cosmic rays can interact with the solar atmosphere and produce a slew of secondary messengers,making the Sun a bright gamma-ray source in the sky.Detailed observations with Fermi-LAT have shown that these interactions must be strongly affected by solar magnetic fields in order to produce a wide range of observational features,such as a high flux and hard spectrum.However,the detailed mechanisms behind these features are still a mystery.In this study,we tackle this problem by performing particle-interaction simulations in the solar atmosphere in the presence of coronal magnetic fields using the potential field source surface(PFSS)model.We find that low-energy(~GeV)gamma-ray production is significantly enhanced by the coronal magnetic fields,but the enhancement decreases rapidly with energy.The enhancement directly correlates with the production of gamma rays with large deviation angles relative to the input cosmic-ray direction.We conclude that coronal magnetic fields are essential for correctly modeling solar disk gamma rays below 10 GeV,but above that,the effect of coronal magnetic fields diminishes.Other magnetic field structures are needed to explain the high-energy disk emission.

Monitoring the daily variation of Sun-Earth magnetic fields using galactic cosmic rays

The interplanetary magnetic field(IMF)between the Sun and Earth is an extension of the solar magnetic field carried by the solar wind into interplanetary space.Monitoring variations in the IMF upstream of the Earth would provide very important information for the prediction of space weather effects,such as effects of solar storms and the solar wind,on human activity.In this study,the IMF between the Sun and Earth was measured daily for the first time using a cosmicray observatory.Cosmic rays mainly consist of charged particles that are deflected as they pass through a magnetic field.

Strong constraints on Lorentz violation using new γ-ray observations around PeV

The tiny modification of dispersion relation induced by Lorentz violation(LV)is an essential topic in quantum gravity(QG)theories,which can be magnified into significant effects when dealing with astrophysical observations at high energies and long propagation distances.LV would lead to photon decay at high energies;therefore,observations of high-energy photons could constrain LV or even QG theories.The Large High Altitude Air Shower Observatory(LHAASO)is the most sensitive gamma-array instrument currently operating above 100 TeV.Recently,LHAASO reported the detection of 12 sources above 100 TeV with maximum photon energy exceeding 1 PeV.According to these observations,the most stringent restriction is achieved in this study,i.e.,limiting the LV energy scale to 1.7×10^(33) eV,which is over 139,000 times that of the Planck energy,and achieving an improvement of approximately 1.9 orders of magnitude over previous limits.