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

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### 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

University of Montana

To be presented:

- Upper limits on pointlike sources (> 1018.5 eV)
- Search for global dipole enhancement(> 1018.5 eV)
- Clustering studies (> 1019.5 eV)

University of Montana

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.

University of Montana

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.

University of Montana

Simulated 25-Event Source

University of Montana

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.)

University of Montana

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.

University of Montana

>1018 eV Skymap in CYG-X3 Region

University of Montana

expected background:

w/ background + 2 source:

w/background + 4 source:

University of Montana

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)

University of Montana

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)

University of Montana

Dipole source models:

- Arrival directions of cosmic rays possess a number density n=1+acosq (q is the angle w.r.t. dipole source).

galactic dipole model, a=1

galactic dipole model, a=-1

University of Montana

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

Isotropic source model

HiRes Data

University of Montana

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

University of Montana

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

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

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

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

University of Montana

HiRes-I Monocular Data, E > 1019.5 eV

University of Montana

Sensitivity to small-scale clustering above 1019.5 eV

- “Autocorrelation” (opening angle distribution)

Null Result

Simulated dataset with clustering

HiRes-I Autocorrelation Function

University of Montana

But how sensitive is HiRes-I?

University of Montana

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.)

University of Montana

Conclusions

HiRes-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.

University of Montana

Other HiRes Anisotropy Presentations:

- Ben Stokes, Using Fractal Dimensionality in the Search for Anisotropy of Ultra-High Energy Cosmic Rays, HE: Poster Session 2.
- Chad Finley, Small-Scale Anisotropy Studies of the Highest Energy Cosmic Rays Observed in Stereo by HiRes, HE-1.3, 16.

University of Montana

University of Montana

AGASA Autocorrelation Function for Events above 40 EeV

University of Montana

University of Montana

Flux Upper Limit (Cygnus X-3)

- 90% confidence level flux upper limit for pointlike source > 1018 eV (~3,700 events):
- < 5.4 x 10-18 events/cm2*sec (18 events)
- < 1.2 x 10-18 events/cm2*sec (4 events)
- 90% confidence level flux upper limit for pointlike source > ~5x1017 eV (~5k events) at Cyg X-3:
- < 3.7 x 10-18 events/cm2*sec (16 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)

University of Montana

Compare events in 1.5 degree circle with isotropic background:

Determine 90% c.l. upper limit:

University of Montana

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