28 th International Cosmic Ray Conference, Tsukuba, Japan John Belz (University of Montana)

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John Belz University of Montana

Anisotropy Studies of Ultra-High Energy Cosmic Rays Using Monocular Data

Collected by the High-Resolution Fly’s Eye (HiRes)

28th International Cosmic Ray Conference,

Tsukuba, Japan

John Belz (University of Montana)

for J. Bellido, B. Dawson, C. Jui, M. Kirn, B. Stokes

and the HiRes Collaboration

To be presented:

• Upper limits on pointlike sources (> 1018.5 eV)

• Search for global dipole enhancement(> 1018.5 eV)

• Clustering studies (> 1019.5 eV)

Pointlike Source Searches (Full Sky)

•Consider E > 1018.5 eV monocular dataset.

•Compare with “isotropic” time-shuffled datasets.

•Use simulated sources to evaluate sensitivities.

•Flux upper limits determined by integrating exposure-corrected events.

Interpreting Variances

•Choose “search circle” and count bins with variance above some particular value. •Use artificial (pointlike MC) “sources” to tune circle size and variance cuts.•Seek 90% signal reconstruction with <10% false-positive probability.

Simulated 25-Event Source

Pointlike Source Flux Upper Limit

• With HiRes monocular dataset (1,525 events, E > 1018.5 eV), we rule out the existence of northern hemisphere pointlike sources of greater than 25 events at 90% confidence level.

• < 0.33 events/km2 *Yr (90% c.l.)

• < 1.1 x 10-18 events/cm2*sec (90% c.l.)

Pointlike Source Upper Limit (Cygnus X-3)

• Cygnus X-3 chosen as a priori candidate:– G. Cassiday et al., Phys. Rev. Lett 62 383 (1989).

– M. Lawrence et al., Phys. Rev Lett 63 1121 (1989).

– M. Teshima et al., Phys. Rev. Lett. 64 1628 (1990).

• Use simulated sources to evaluate sensitivities.• Flux upper limits determined by integrating

exposure-corrected events.

>1018 eV Skymap in CYG-X3 Region

HiRes Data w/expected background:

w/ background + 2 source:

w/background + 4 source:

Flux Upper Limit (Cygnus X-3)

• 90% confidence level flux upper limit for pointlike source > 1018 eV (~3,700 events):– < 1.2 x 10-18 events/cm2*sec (4 events)

• Compare previous results (E > 5x1017 eV):– (20 +- 6) x 10-18 events/cm2*sec (Fly’s Eye)

– (18 +- 7) x 10-18 events/cm2*sec (Akeno)

– < 4x10-18 events/cm2*sec (neutrons) (Haverah Park)

– < 8x10-18 events/cm2*sec (gamma rays) (Haverah Park)

Global Dipole Searches

• Galactic Center — possible effects observed by AGASA and Fly’s Eye

• CEN-A — suggested as a potential source of a dipole effect (Farrar and Piran)

• M87 — possible weak effects (Biermann)

Dipole source models:• Arrival directions of cosmic rays possess a

number density n=1+cosis the angle w.r.t. dipole source).

galactic dipole model, galactic dipole model,

Dipole Functions for an Isotropic Source Model and HiRes-I Data

Isotropic source model HiRes Data

The Galactic Center: 0.005+/-0.05590% Confidence Interval: [-0.085,0.090]

Centaurus A: 0.005+/-0.06590% Confidence Interval: [-0.090,0.085]

M87: 0.010+/-0.04590% Confidence Interval: [-0.080,0.070]

(HiRes-1 Monocular, 1525 events > 1018.5 eV)

HiRes-I Monocular Data, E > 1019.5 eV

Sensitivity to small-scale clustering above 1019.5 eV

• “Autocorrelation” (opening angle distribution) Null Result

HiRes-I Autocorrelation Function Simulated dataset with clustering

But how sensitive is HiRes-I?

HiRes-I Autocorrelation Results

• HiRes-I-like experiment should be able to distinguish seven doublets from background 90% of the time.

• Current data set (52 northern-hemisphere events above 1019.5 eV) contains < 4 doublets (90% c.l.)

ConclusionsHiRes-I Anisotropy searches have yielded null results: • Flux upper limit of 0.33 events/km2 *Yr (90% c.l.) on pointlike

sources with E > 1018.5 eV in northern hemisphere

• Flux upper limit of 1.2 x 10-18 events/cm2*sec (90% c.l.) on pointlike sources with E > 1018 eV centered at Cygnus X-3

• Dipole analyses consistent with isotropy for sources at galactic center, Centaurus A, M-87

• No clustering observed in highest energy events. Upper limit of 4 doublets (90% c.l.) in HiRes-I monocular dataset.

28 th International Cosmic Ray Conference, Tsukuba, Japan John Belz (University of Montana)

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