Solar Gamma Rays and Measurements with ARGOYBJ Zhe

Solar Gamma Rays and Measurements with ARGO-YBJ Zhe Li, S. Z. Chen, H. H. He On behalf of the ARGO-YBJ collaboration Institute of High Energy Physics, CAS 1

Outline • Introduction • Upper limit of Te. V γ rays from Sun with ARGO-YBJ • Theoretical result and expectation with LHAASO • Conclusion 2

1. 1 γ emission from the Sun l The highest energy of γ rays observed from a solar flare is ＜ 10 Ge. V l High energy γ rays from the solar region are produced mainly by two distinct processes: ü IC of cosmic-ray electrons on solar photons IC component ü hadronic interaction of cosmic rays with solar atmosphere (photosphere and chromosphere) Solar disk component Sun features: Solar Atmosphere Point like steady γ-ray source can be detected on the Earth 3

1. 2 High energy γ rays from the Sun >0. 5 Ge. V (2008 -08 to 2010 -02) (γ-ray distribution from CR interaction with sunlight, 2006, SLAC) Ø IC component appears as an extended halo around the Sun even at larger elongation angles； Ø Solar-disk component mainly coming from 1°solid angle around the Sun； 1 2011，Fermi 4

1. 3 Significance of the Solar gamma-ray Observation üEstimate the cosmic ray distribution when they pass through the solar system (determine the electron spectrum across the inner solar system, make accurate predictions of the proton spectrum in the close proximity to the Sun); ü will allow to study the deep atmospheric layers of the Sun ü important to analyze other gamma-ray sources in the universe ü implications for solar physics, cosmic-ray physics, and new physics … 5

1. 4 Recent related research on Solar-disk γ-rays Ep<ET=3 Te. V 1991, D. Seckel spectral index ≈2. 4 (mean value) Theoretical result (10 Me. V-100 Ge. V) l So far the first detailed theoretical study of γ emission from interactions of CR protons in the solar atmosphere; l “Naive” model gives an upper bound. Absorption of the Sun is not considered and ignore any effect due to solar magnetic fields or the IMF; l “Nominal” model gives a lower limit. This model considers CR diffusion in the IMF and corona; EGRET results ( 100 -300 Me. V, >300 Me. V) Data Model 2013，Orlando, et al. Compared with “Naive” model 6

1. 4 Recent related research on Solar-disk γ rays Fermi results ( 0. 1 -10 Ge. V & 1 -100 Ge. V) 2011, Abdo et al. Fermi l 1. 5 y data collection l Energy range : 0. 1 -10 Ge. V l Spectral index: ≈2. 11± 0. 73 (power law) 2016, Kenny et al. l 6 y data collection l Energy range : 1 -100 Ge. V l Spectral index: ≈2. 3 (power law) 7

1. 5 Some expectations of solar-disk γ ray detection Ø theoretical model should be investigated above 10 Ge. V； Ø To expand solar gamma-ray observations into the Te. V range and beyond, large ground-based experiments are required； Ø the Sun is a new and promising source for large water-Cherenkov gamma-ray telescopes, such as HAWC and LHAASO; Ø either a detection or an upper limit from the large ground-based experiments can provide valuable information on γ-ray production from the Sun expanding solar γ-ray observations into Te. V range and beyond. 2016, Kenny et al. 8

2. 1 ARGO-YBJ experiment and data analysis The ARGO-YBJ observatory (Tibet, China at 4300 m a. s. l. ) Real observation time is 7471. 9 h ü Can detect γ-rays and cosmic-rays up to Pe. V energies; ü Can observe γ-rays from the Sun during most of days in one year; Event selection: ① the number of fired pads is more than 20; ② zenith angle θ is less than 50 o; ③ distance between the shower core position and the carpet center is less than 100 m; ④ the time spread of the shower front in the conical fit is less than 100 ns. 9

2. 1 ARGO-YBJ experiment and data analysis uthe events with reconstructed energy from 0. 32 Te. V to 10 Te. V are divided into 6 energy bins. Six energy-proxy bins and simulated ARGO-YBJ angular resolution for CRs and gamma-rays u The correlation between the reconstructed energy and the primary energy for γ-rays 10

2. 1 ARGO-YBJ experiment and data analysis Significance maps for six energy-proxy bins from 0. 32 Te. V to 10 Te. V. l The deficit distribution of CR forms a distinct Sun shadow in each map; l The Solar-disk γ-rays are electrically neutral and they spread by a straight line which can provide a positive signal distribution at (0 o, 0 o); No significant positive signal of solar-disk γ-rays was found from ARGO-YBJ data. 11

2. 2 Upper limit of Te. V γ rays from Sun with ARGO-YBJ The differences between angular distribution of CRs and solar-disk γ-rays supplies a prospect to calculate the upper limit of solar-disk γ-ray flux! Cosmic rays Blocked by the Sun; the shadow is displaced by strong and variable magnetic field of Sun and IMF Solar-disk γ-rays spread along a straight line Centered at (0°, 0°) An important constraint: the total background around Sun ( ~0. 26◦) Take the upper limit with 90% C. L. of solar background as a limitation to restrict the negative source intensity. 12

2. 2 Upper limit of Te. V γ-rays from Sun with ARGO-YBJ Fitting method: Non: number of events in each bin Nb: number of background in each bin S_: number of negative Sun shadow events in each bin S+: number of positive solar γ-rays in each bin 90% C. L. Upper limit: 13

2. 2 Upper limit of Te. V γ-rays from Sun with ARGO-YBJ Fermi 1. 5 y data: Index ≈2. 11± 0. 73 y r a n i lim ARGO-YBJ 90%C. L. Fermi 6 y data: Index ≈2. 3 e r P LHAASO Sensitivity 14

3. 1 Preliminary calculation for Te. V Solar-disk γ rays Principle：p+p γ Calculation range：<3. 8 R 0； R 0: solar radius 6. 96× 105 km Assumption: Model region R Ignore the influence of Solar magnetic field to CR; Ignore the absorption of Sun backlight to high energy γ-rays； Ignore the diffusion effect of IMF and corona to CR； Proton flux in primary cosmic rays near the Earth is adopted; Abdo et al. 2009 Theoretical model： p-p cross section Proton number distribution Proton spectrum in solar atmosphere γ-ray spectrum produced by p-p 15

3. 2 Preliminary calculation: Proton spectrum 1 Ge. V-1. 8 Te. V 2015，AMS 2. 5 -250 Te. V Ecut=1 Pe. V 2011，Cream 16

3. 2 Preliminary calculation: Solar atmosphere Below photosphere Above photosphere Column density 50 g/cm 2 521. 5 km below solar limb edge； 1991, Seckel. etal 1966, Marvin L. White. 17

3. 3 Theoretical results Fermi 1. 5 y data: Index ≈2. 11± 0. 73 y r a n i lim e r P ARGO-YBJ 90%C. L. HAWC sensitivity This work Fermi 6 y data: Index ≈2. 3 LHAASO Sensitivity 18

Conclusions u For the first time, we have obtained the upper limit of solar-disk γ-ray flux at 0. 3 Te. V-10 Te. V by analyzing more than 5 years data of the ARGO-YBJ experiment; u A preliminary theoretical calculation gives an expectation of solar-disk γray flux up to Pe. V energies； u LHAASO may detect high energy gamma rays from solar-disk up to the 100 Te. V range. 19