52° Congresso SAIt 2008 Raffaella Bonino* for the Pierre Auger Collaboration ( * ) IFSI – INFN – Università di Torino

08/05/2008SAIt 2008 Raffaella Bonino Outline extra-galactic The origin of the highest energies cosmic rays (>10 19 eV) is expected to be extra-galactic What are these extra-galactic sources?  search for correlations transition Somewhere downwards in the spectrum, the transition from galactic to extra-gal. must occur Where?  study of large scale anisotropies (change in the large scale angular distribution) Galactic Center The Galactic Center is one of the most interesting galactic target  look for localized excesses of CRs in the GC region at ~ eV Required tools: knowledge of the angular resolution of the Surface Detector  angular reconstruction and timing uncertainty model

08/05/2008SAIt 2008 Raffaella Bonino Auger Surface Detector

08/05/2008SAIt 2008 Raffaella Bonino SD arrival directions arrival times and angular resolution Especially for the analysis of small scale anisotropy a good angular resolution and detector stability are required The angular resolution is strictly dependent on the accuracy in the arrival time measurement of the particles in the tanks The arrival direction is measured from the delays among the hit tanks

08/05/2008SAIt 2008 Raffaella Bonino “Start Time” It should correspond to the arrival time of the shower front to the detectors It’s identified with the arrival time of the first particle detected ⇒ the first bin above a fixed threshold in a 2 or 3-fold coincidence Start Time

08/05/2008SAIt 2008 Raffaella Bonino SD angular resolution Computed on an event by event basis: →  and  derived from the fit of the arrival time of the first particle on the tank Based on: Parabolic shower front model Semi-empirical timing uncertainty model Angular resolution ≡ angular radius that would contain 68% of the showers coming from a point-like source

08/05/2008SAIt 2008 Raffaella Bonino SD angular resolution  Comparison with hybrid reconstruction (   ~0.6°) confirms the SD-only result SD-onlyHybrid

Large scale anisotropy studies

08/05/2008SAIt 2008 Raffaella Bonino Overview Objective: galactic: %-level modulation (models of gal. propagation) CR’s origin at ~10 18 eV extra-gal.: no structure except for a CMB-dipole (~0.6%) at higher energies: GZK cut-off → sources → anisotropy Difficulties: control of spurious modulations sky exposure instabilities due to atmospheric and instrumental effects not constant acceptance 3 complementary analysis in the EeV (=10 18 eV) range (5·10 5 events)

08/05/2008SAIt 2008 Raffaella Bonino Auger results Results from the search for large- scale patterns: no modulation in RA 95% c.l. upper limit = 1.4% 95% c.l. upper limit = 1.4% for 1 < E < 3 EeV Exposure-independent cross- checks confirm the lack of significant pattern The AGASA 4% modulation is not confirmed (but the observed regions of the sky are different)

The Galactic Center region

08/05/2008SAIt 2008 Raffaella Bonino Why the Galactic Center? GC contains a super-massive black hole → possible candidate to accelerate CR It passes only 6° away from the AUGER zenith Claims in the past from other experiments of large excesses in GC region: SUGAR SUGAR: excess = 85% E = eV ( ,  ) = (274°,-22°) AGASA AGASA: excess = 22% E = eV ( ,  ) = (280°,-17°) H.E.S.S. detected a TeV  -ray source close to Sagittarius A*

08/05/2008SAIt 2008 Raffaella Bonino Auger results as extended source In our analysis GC treated both as point-like source Data set divided into 2 energy bands : 0.1 < E < 1 EeV  1 < E < 10 EeV  Conclusions: No significant CRs flux excess in both energy ranges Distribution of Li-Ma overdensity significances consistent with isotropic sky

08/05/2008SAIt 2008 Raffaella Bonino

08/05/2008SAIt 2008 Raffaella Bonino UHECR sources Sizes and magnetic field strengths of astronomical objects that are possible candidates as CR sources AGN Radio Galaxies Hillas plot BUT… GZK cut- off : only nearby sources (within ~ Mpc) should contribute to the flux above E GZK = 6·10 19 eV

08/05/2008SAIt 2008 Raffaella Bonino Ultra High Energy Cosmic Rays Particles with E ~ eV exist and have been detected what are and where do they come from? Complementary studies: Energy spectrum and composition Origin → study of anisotropy in arrival directions: LARGE SCALE: transition from galactic to extra-galactic origin = change in the large scale angular distribution because of different mechanisms of propagation SMALL SCALE: above 5·10 19 eV cosmic rays are only slightly deflected (2°-3°) by magnetic fields → direct way to search for UHECR sources If sources are nearby and not uniformly distributed, an anisotropic arrival directions distribution is expected (“clustering”)

08/05/2008SAIt 2008 Raffaella Bonino SD angular resolution Computed on an event by event basis: →  and  derived from the fit of the arrival time of the first particle on the tank Based on: Parabolic shower front model [C.Bonifazi et al., astro-ph ] Semi-empirical timing uncertainty model [C.Bonifazi et al., astro-ph ] Space-angle uncertainty computed from   and   : Angular resolution ≡ angular radius that would contain 68% of the showers coming from a point-like source:

08/05/2008SAIt 2008 Raffaella Bonino 1 st method - Fourier time analysis Spectral analysis on the « modified time » of the events : t mod =t+RA-LST f sid f sol f antisid All energies Δf Results: no signal in sidereal frequency solar modulation of 3.2% due to atmospheric effects analysis repeated in 3 different energy ranges → no significant sid. modulation At sidereal frequency, recover the RA modulation Frequency resolution Δf ~1/T acquis Compare the sidereal signal to: - the antisidereal signal - the average noise in other freq.

08/05/2008SAIt 2008 Raffaella Bonino Characteristics: Differential method Allows to remove direction-independent phenomena (i.e. atmospheric and not-constant acceptance effects) 2 nd method - East-West method where I(t) = physical CR intensity First harmonic analysis on (E-W)  amplitude of the sidereal modulation of I(t)

08/05/2008SAIt 2008 Raffaella Bonino Results (using the whole data set: E median ~ 6· eV) : solar modulation: reduced from 4.2% to 0.8% (±0.4%) sidereal modulation: (0.7±0.4)% corresponding to P Rayleigh =24% antisidereal modulation = (0.5±0.4)% 95% c.l. upper limit = 1.4% 95% c.l. upper limit = 1.4% amplitudes 95% c.l. upper limits Scan in energy (4.2 ± 0.4)%(0.8 ± 0.4)% 2 nd method - East-West method

08/05/2008SAIt 2008 Raffaella Bonino 3 rd method - Exposure-based study Residual modulation 1 < E < 3 EeV A scan in energy has been done  no significant excess Energy (EeV) nb events - (expected noise) sid. ampl. – (phase) antisid. ampl. upper limits 95% c.l. 1 < E < (0.7%)0.74% (330º)0.6%1.4% 3 < E < (2.0%)1.2% (100º)1.1%3.2% E > (4.7%)3.5% (70º)4.2%8.6% Measure the residual modulation after dividing by the exposure - modulations compatible with the statistical + systematic errors Upper limit derived from MC, taking into account the measured modulation Results:

08/05/2008SAIt 2008 Raffaella Bonino Large scale anisotropy 1 st method 2 nd method 3 rd method All energies f sid f sol f sid 95% c.l. upper limits amplitudes Scan in energy

Correlation of UHECR with nearby extra-gal. objects

08/05/2008SAIt 2008 Raffaella Bonino What are AGN ? Active Galactic Nuclei: galaxies hosting central black holes that feed on gas and stars and may eject vast plasma jets into intergalactic space Different names → unified scheme 

08/05/2008SAIt 2008 Raffaella Bonino Other possible UHECR sources BL – Lacs = subclass of blazars, active galaxies with beamed emission from a relativistic jet aligned toward our line of sight → potential sources of UHECRs AUGER results: AGASA, Yakutsk HiRes AUGER doesn’t support correlations reported by AGASA, Yakutsk and HiRes data no excess from an extended search