A model of the Earth's gamma-ray emission for GLAST derived from EGRET data. Dirk Petry (UMBC, NASA/GSFC) DC2 Meeting GSFC June 27-29, 2005

Transcription

1 A model of the Earth's gamma-ray emission for GLAST derived from EGRET data (UMBC, NASA/GSFC) DC2 Meeting GSFC June 27-29,

2 Gamma-ray emission from the Earth's atmosphere Incoming cosmic rays Cosmic rays above cut off rigidity ( 15 GV) penetrate atmosphere and cause airshowers. Grazing incidence showers produce forward colimated gamma radiation which can escape atmosphere. Gamma rays from secondaries with large transverse momentum or from low energy pions can escape "upwards". Observer 2

3 First measurement of HE gamma emission of the Earth from space made by SAS-2 Nov June 1973: photons above 35 MeV (Thompson et al., 1981) The next mission taking Earth data flew almost 20 years later: Operational for 9.1 years ( ) circular orbit at approx. 420 km altitude, inclination 28.5º energy range 35 MeV 30 GeV mostly pointed observations => Earth in field of view > 50 % of elapsed time! In spite of special Earth avoidance trigger modes, ca. 60 % of the gold plated EGRET photon events come from zenith angles > 105º (3 million events) 3

4 Data Analysis Results (excerpt) Example of a flux map at MeV Spectral index as a function of zenith angle 5E 5 2E 4 1.5E 3 3E 3 6E 3 2E 2 cm 2 s 1 sr 1 See also astro ph/

5 Flux maps in three energy bands + composite: 2d maps, rgb plots 5

6 Summary of the model properties in Earth (flux v8r20): Bright rim with Gaussian profile and exponential tail towards Nadir Spectrum hardest (1.7) at rim peak, softer both towards Nadir and Zenith, softest at Nadir (2.4) Exponential cutoff at ca. 3 GeV (AZ independent rim and central component), 30 GeV (AZ dependent rim component), consistent with the available data above 10 GeV from EGRET Entire EGRET dataset contains 675 events with E > 10 GeV and zenith angle > 105 deg, only 4 events of these have a zenith angle > 120 deg. The most energetic event has 120 GeV. The energy resolution of EGRET at 10 GeV is 25% FWHM. A detailed calculation predicts that one would expect a spill over of about 20 % with a spectrum like the nominal one measured below 10 GeV. I.e. the model should predict roughly 540 events above 10 GeV. The model parameters were adjusted to fulfill this condition. 6

7 Summary of the model properties in Earth (flux v8r20) contnd.: Rim peak position is energy independent at the geometrical horizon Detailed study showed that the previously assumed energy dependent position of the rim peak can be entirely explained by the PSF. Rim width (std. dev.) = 25 % of angular width of atmosphere Azimuthal variation to a very good approximation symmetric around West (no North South effect) East West ratio first increasing with energy, peaking at around 600 MeV, then decreasing again (implicit property of the model) Altitude dependence consistent with expectations from geometry 7

8 Altitude dependence: Dedicated study of EGRET data confirms: Altitude dependence as one would expect from purely geometrical considerations, i.e. surface brightness constant, intensity decreases as the solid angle. In addition all zenith angle related parameters scale with the Horizon ZA = 90º+acos(R E /R O ) GLAST Earth Change in altitude changes horizon ZA 8

9 Differential flux maps generated from the model (old version before flux v8r20) 9

10 Differential flux maps generated from the model (new, improved version, analytical) 12 Jan 05 10

11 Event maps generated from the model (new version, actual Monte Carlo Photons, ideal detector) 11

12 Spectrum generated by Earth in flux v8r20 (all directions) Number of generated photons log(e/mev) 12

13 Summary New model version has improved correction for EGRET PSF and an improved spectrum above 10 GeV based on all available EGRET data. Total flux has essentially not changed, only the spatial distribution and the spectrum at the highest energies Above 10 GeV, the model is really an intelligient guess ; the EGRET data essentially only gives an integral flux above 10 GeV For the energy range up to 350 GeV and for the present pursposes of GLAST, this model should be sufficient (as already mentioned above: in its entire lifetime, EGRET only detected a single photon above 100 GeV from the Earth). But I will keep working to improve the knowledge of the Earth's spectrum beyond 350 GeV. 13