Extended solar emission - an analysis of the EGRET data
Microsoft Word - sun.doc
Extended Solar Emission – an Analysis of the EGRET Data 
Elena Orlando, Dirk Petry and Andrew Strong 
Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, D-85741 Garching, Germany 
Abstract. The Sun was recently predicted to be an extended source of gamma-ray emission, produced by inverse-
Compton scattering of cosmic-ray electrons with the solar radiation. The emission was predicted to contribute to the 
diffuse extragalactic background even at large angular distances from the Sun. While this emission is expected to be 
readily detectable in future by GLAST, the situation for available EGRET data is more challenging. We present a detailed 
study of the EGRET database, using a time dependent analysis, accounting for the effect of the emission from 3C 279, the 
moon, and other sources, which interfere with the solar signal. The technique has been tested on the moon signal, with 
results consistent with previous work. We find clear evidence for emission from the Sun and its vicinity. The 
observations are compared with our model for the extended emission.  
Keywords: Cosmic rays, gamma-ray emission, Sun, EGRET. 
PACS: 95.85, 96.50, 96.60  
THE EXTENDED SOLAR EMISSION MODEL 
The heliosphere has been studied as an extended source of gamma-ray emission, produced by inverse-Compton 
scattering of cosmic-ray electrons with the solar photon field [1,2]. For this analysis our model [1] has been 
improved using the modulated electron spectrum at all distances following [2] instead of the measured local electron 
spectrum, and using the anisotropic inverse-Compton scattering formalism [3].   
ANALYSIS OF THE EGRET DATA 
We analyzed the EGRET data using the code developed for the moving target Earth [4] and adding necessary 
features (solar and lunar ephemerides, occultation, background point source trace calculations). The diffuse 
background was reduced by excluding the Galactic plane. Otherwise all available exposure within mission phase 1-3 
was used. When the Sun passed by other gamma-ray sources (moon, 3C 279 and several quasars), these sources 
were included in the analysis.  Details will be given in [5]. 
We fitted the data in the Sun-centred system using a multi-parameter likelihood fitting technique, leaving as free 
parameters the solar extended inverse-Compton flux from the model, the solar disk flux from pion decay [7], a 
uniform background, and the flux of 3C279, the dominant background point source.  The moon flux was determined 
from moon-centred fits and the 3EG source fluxes were fixed at their catalogue values. All components were 
convolved with the energy-dependent EGRET PSF.  The region used for fitting is a circle of radius 10º centred on 
the Sun.  Since the interesting parameters are solar disk source and extended emission, the likelihood is maximized 
over the other components.  In order to verify our method, we checked that the fluxes of the Crab Nebula, 3C 279, 
and in particular the moon [6] were reproduced.  
Results 
The log-likelihood ratio for E >100 MeV is displayed in Fig.1 as a function of solar disk flux and extended flux, 
compared with the model prediction of solar inverse-Compton flux for modulation parameter 500 MV at 1 AU. The 
solar emission is detected with 5.3σ significance. There is evidence for the extension of the emission at a level of 
2.7σ; the maximum log L indicates an extended component with a flux compatible with the IC model. The total flux 
from the Sun is more than expected for the disk source [7], so this is clear evidence for the IC emission even without 
the proof of extension. We find that the measured extended flux is fully consistent with the model. Figure 2 shows 
the fitted model counts of the main components and the total including uniform background. 
In future work we will perform a detailed spectral analysis, refine the analysis of systematic errors, and study 
different models for the modulated electron spectrum. This is important for future missions such as GLAST and for 
studying solar modulation. 
FIGURE 1.  Log Likelihood above 100 MeV as function of  the solar disk flux and extended solar flux, relative to point at (0,0). 
The level of our predicted IC model flux and the predicted disk flux [7] are shown.   
FIGURE 2.  Fitted model counts of the main components centered on the Sun. From left to right: Sun disk, Sun IC, moon,  
3C 279, and the total predicted counts including uniform background.  The colors show the counts/pixel, for 0.5°µ 0.5° pixels.  
REFERENCES 
1. E. Orlando and A. W. Strong, Ap&SS in press, astro-ph/0607563 (2006). 
2. I. V. Moskalenko et al., ApJL 652 (2006) L65-L68. 
3. I. V. Moskalenko and A. W. Strong, ApJ, 528, 357 (2000). 
4. D. Petry, AIP Conf. Proc., 745, 709 (2005). 
5. D. Petry, E. Orlando and A.W. Strong, in prep. (2007). 
6.  D.J. Thompson et al., Journal of Geophys. Res. 102 (A7), 14735 (1997). 
7. D. Seckel et al., ApJ 382, 652 (1991). 
