1. On behalf of the ARGO-YBJ collaboration
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

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

3. 1.1 γ emission from the Sun
The highest energy of γ rays observed from a solar flare is ＜10GeV
High energy γ rays from the solar region are produced mainly by two distinct processes:
IC of cosmic-ray electrons on solar photonsIC component
hadronic interaction of cosmic rays with solar atmosphere (photosphere and chromosphere) Solar disk component
Sun
Point like
steady γ-ray source
can be detected on the Earth
features:
Solar Atmosphere

4. 1.2 High energy γ rays from the Sun
>0.5 GeV
( to )
(γ-ray distribution from CR interaction with sunlight,2006,SLAC)
IC component appears as an extended halo around the Sun even at larger elongation angles；
Solar-disk component mainly coming from 1°solid angle around the Sun； 1
2011，Fermi

5. 1.3 Significance of the Solar gamma-ray Observation
Estimate 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);
will allow to study the deep atmospheric layers of the Sun
important to analyze other gamma-ray sources in the universe
implications for solar physics, cosmic-ray physics, and new physics …

6. 1.4 Recent related research on Solar-disk γ-rays
Theoretical result (10MeV-100GeV)
Ep<ET=3TeV
So far the first detailed theoretical study of γ emission from interactions of CR protons in the solar atmosphere;
“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;
“Nominal” model gives a lower limit. This model considers CR diffusion in the IMF and corona;
1991,D.Seckel
spectral index ≈2.4
(mean value)
EGRET results ( MeV,>300MeV)
Data
Model
2013，Orlando,et al.
Compared with “Naive” model

7. 1.4 Recent related research on Solar-disk γ rays
Fermi results ( GeV & 1-100GeV)
2016,Kenny et al.
2011, Abdo et al. Fermi
1.5y data collection
Energy range : GeV
Spectral index:≈2.11±0.73 (power law)
6y data collection
Energy range : GeV
Spectral index:≈2.3 (power law)

8. 1.5 Some expectations of solar-disk γ ray detection
theoretical model should be investigated above 10GeV；
To expand solar gamma-ray observations into the TeV range and beyond, large ground-based experiments are required；
the Sun is a new and promising source for large water-Cherenkov gamma-ray telescopes, such as HAWC and LHAASO;
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 TeV range and beyond.
2016,Kenny et al.

9. 2.1 ARGO-YBJ experiment and data analysis
Real observation time is h
The ARGO-YBJ observatory
(Tibet, China at 4300 m a.s.l.)
Can detect γ-rays and cosmic-rays up to PeV energies;
Can observe γ-rays from the Sun during most of days in one year;
Event selection:
the number of fired pads is more than 20;
zenith angle θ is less than 50o;
distance between the shower core position and the carpet center is less than 100 m;
the time spread of the shower front in the conical fit is
less than 100ns .

10. 2.1 ARGO-YBJ experiment and data analysis
the events with reconstructed energy from 0.32 TeV to 10 TeV are divided into 6 energy bins .
Six energy-proxy bins and simulated ARGO-YBJ angular resolution for CRs and gamma-rays
The correlation between the reconstructed energy and the primary energy for γ-rays

11. 2.1 ARGO-YBJ experiment and data analysis
Significance maps for six energy-proxy bins from 0.32 TeV to 10 TeV.
The deficit distribution of CR forms a distinct Sun shadow in each map;
The Solar-disk γ-rays are electrically neutral and they spread by a straight line which can provide a positive signal distribution at (0o, 0o);
No significant positive signal of solar-disk γ-rays was found from ARGO-YBJ data.

12. Cosmic rays Solar-disk γ-rays
2.2 Upper limit of TeV 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!
Blocked by the Sun; the shadow is displaced by strong and variable magnetic field of Sun and IMF
Cosmic rays
spread along a straight line
Solar-disk γ-rays
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 .

13. 2.2 Upper limit of TeV 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:

14. Preliminary 2.2 Upper limit of TeV γ-rays from Sun with ARGO-YBJ
Fermi 1.5y data:
Index ≈2.11±0.73
ARGO-YBJ 90%C.L.
Preliminary
Fermi 6y data:
Index ≈2.3
LHAASO Sensitivity

15. 3.1 Preliminary calculation for TeV Solar-disk γ rays
Abdo et al.2009
Principle：p+pγ
Calculation range：<3.8R0；
Assumption:
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;
R0:solar radius 6.96×105 km
Model region R
Theoretical model：
Proton number distribution
in solar atmosphere
Proton spectrum
p-p cross section
γ-ray spectrum produced by p-p

16. 3.2 Preliminary calculation: Proton spectrum
2015，AMS
2011，Cream
1GeV-1.8TeV
TeV
Ecut=1PeV

17. 3.2 Preliminary calculation:
Solar atmosphere
Column density 50g/cm2  521.5km below solar limb edge；
Below photosphere
Above photosphere
1966,Marvin L. White.
1991,Seckel.etal

18. Preliminary 3.3 Theoretical results Fermi 1.5y data: Index ≈2.11±0.73
ARGO-YBJ 90%C.L.
HAWC sensitivity
This work
Fermi 6y data:
Index ≈2.3
LHAASO Sensitivity

19. Conclusions
For the first time, we have obtained the upper limit of solar-disk γ-ray flux at 0.3TeV-10TeV by analyzing more than 5 years data of the ARGO- YBJ experiment;
A preliminary theoretical calculation gives an expectation of solar-disk γ- ray flux up to PeV energies；
LHAASO may detect high energy gamma rays from solar-disk up to the 100TeV range.