Analysis of the modulation in the first harmonic of the right ascension distribution of cosmic rays detected at the Pierre Auger Observatory

Transcription

1 Analysis of the modulation in the first harmonic of the right ascension distribution of cosmic rays detected at the Pierre Auger Observatory H. Lyberis 1 for the Pierre Auger Collaboration 2 1 Université Paris VII Denis diderot,paris, France Università degli Studi di Torino, Torino, Italia Institut de Physique Nucléaire, Orsay, France 2 Observatorio Pierre Auger, Av. San Martin Norte 304, (5613) Malargüe, Mendoza, Argentina Full author list at: 08/15/2011

2 Detector and data set The Pierre Auger Observatory The SD array: 1660 water-cherenkov detectors covering an area of ~3000 km 2 on a triangular grid with 1.5 km spacing Unitary cell : active detector with six active neighbours Good quality of event reconstruction and a robust estimation of the exposure The dataset 3-fold trigger Median energy : 3 x 17 ev Rate : 1/min data from 1 st Jan to 31 st Dec. 20 corresponding exposure 20,905 km 2 sr yr 2

3 Methods I/III Rayleigh analysis (E>1EeV) Dipolar modulation of experimental origin 1 - Exposure modulation 2 - Energy correction due to weather effects 1- { 2- Conversion of the shower size into energy taking into account the atmospheric (P,ρ) to a reference one (P0,ρ0) 3

4 Methods II/III Analysis at the solar frequency above 1EeV The decoupling between frequencies is now observable after 7 yrs. Spurious sideband effect is proportional to the solar amplitude Amplitude [in %] 6 4 Solar No correction Energy correction + Exposure correction Isotropic expectation no correction: ~4% with energy correction: ~3% add exposure correction : ~1% 2 Anti-sidereal Sidereal Close to the statistical noise Figure Frequency [cycles/year] Good control of the exposure and the weather effects above 1EeV 4

5 Methods III/III For energies E<1EeV, weather effects affect the detection efficiency in a larger extent East-West method: (see ref astro-ph/ , ApJ 2011 in press, poster #1145) Difference between the counting rate from East/West is related to the RA modulation : Compute the Fourier coefficients: RA amplitude and phase of the dipolar modulation N.B. : the factor is the conversion from dipole amplitude to RA amplitude to convert the phase to the one of the first harmonic analysis, we need to add 90 5

6 sidereal frequency Differential: Amplitude 1-1 Energy bin: -2 E<8 EeV : below energy resolution E>8 EeV : stat. to detect signal at 2% level (~ 5,000 evts) Combined probability to come from isotropic distribution : ~ 45% Cumulative: No further evidence Amplitude % upper bounds for iso. expectation East/West analysis Rayleigh analysis E [EeV] Figure 2 East/West analysis Rayleigh analysis 99% upper bounds for iso. expectation Figure 3 E th [EeV] 6

7 Upper limits on the amplitude Measurmet depends on: latitude of the experiment range of θ Comparison with other experiments Equatorial dipole d EAS-TOP KASCADE A Grande AGASA Auger C-G XGal Gal Comparison with models: A/S: drift motion due to Breg Gal: diffuse motion due to Bturb C-G XGal: -4 Figure 4 14 S Energy [ev] 20 motion of the Galaxy with respect to the CMB reference frame - supposed to be the frame of extragalactic isotropy 7

8 Amplitude vs Phase p.d.f s=0 s=1% p.d.f. 0.6 s=0 s=1% Figure Amplitude in case of a real underlying anisotropy a consistency of the phase measurement in ordered energy intervals is expected with lower statistics than that required for the amplitude to significantly stand out of the background noise* 0 Figure ![ ] Power [in %] Test on phases Test on amplitudes Likelihood test in adjacent energy bins MC with : - 30,000 evts in each bin - s = 1% Phase is ~ 2.5 times more sensitive than the amplitude Figure N bins 8 *see J. Linsley, Phys. Rev. Lett.,1975, 34

9 Phase of the first harmonic Phase [ ] Galactic Anti-center 12h 18h 0 0h 270 Galactic Center 6h Figure 8 East/West analysis Rayleigh analysis E [EeV] 12h Phase measurement: not randomly distributed smooth transition in RA from ~ 270 to ~ 90 Likelihood test (a posteriori): Pchance ~ -3 9

10 conclusion Amplitude measurement No significant signal Phase measurement Smooth transition from ~ 270 to ~ 90 In the future Lower energy threshold now available in Auger with the infill array also see ref: Astropart. Phys. 34 : (2011)