Neutrino Point Source Search Strategies for AMANDA-II and Results from 2005

J. Braun, A. Karle, T. Montaruli

For the IceCubeCollaboration

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Motivation

• No extraterrestrial high energy neutrino source yet observed– Need larger detectors and more sophisticated

analysis methods

• Current searches: Use event direction (time)

• Construct a search to fully utilize– Event energy estimation– Event direction– Detector angular resolution

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• Model the detector point spread function as a 2D Gaussian

Method

• Energy correlated observable: Number of hit channels

• For each event at position xi, assign a probability of belonging to a source at xo

with spectral index

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• A ±5o declination band centered on the source location contains N total events and is modeled as a mixture of signal and background in a likelihood function:

• The Background PDF Bi depends on• Minimize –log(L) with respect to number of signal events and spectral index• Significance is measured by the ratio of best likelihood and background-only likelihood:

Method

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Discovery Potential

90% of sources with the given E-2 flux are detectedat 5 (3) significance

90% Chance of Discovery

35% less events requiredfor 5 detection at = 42.5o

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Sensitivity

90% confidence level sensitivity to E-2 spectra

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Results for 2005

• 1.8 billion raw events recorded in 2005

• 6001 events survive quality cuts, 887 above = 10o

• Search in range -5o < < 82o

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2005 All-Sky Results

69 out of 100 sky maps randomized in RA contain an excess of at least 3.6

Largest Excess: 3.6=48o , =2.75h

Log10(p-value)

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Results for Candidate Sources

Object RAo Dec.o P-value 90

Mrk 421 166.1 38.2 ~1 5.9

Mrk 501 253.5 39.8 0.184 18.1

Cyg. X-1 299.6 35.2 0.414 12.9

Cyg. X-3 308.1 41.0 0.458 11.0

NGC 1275 50.0 41.5 0.064 22.4

Crab 83.6 22.0 ~1 9.24

MGRO J2109+37 304.8 36.8 0.152 20.1

E2 < 90.10-11 TeV cm-2 s-1

• The method is applied separately to a list of 26 candidate sources to enhance discovery potential

• No striking excesses found

• Most significant source is NGC 1275 (Perseus A), p = 0.064– 3 events near source

location (~1.5 expected)

– 80% chance of randomly observing this significance from a source on the list

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What if a Source is Observed?

• Suppose Markarian 421 produces 8 signal events in AMANDA following an E-2 spectrum and the background is well described by atmospheric neutrinos

• >50% probability of 5 detection, assuming no trial penalty.

• Minimization of with respect to signal strength and spectral index yields an estimate of both parameters

• Approximate as a 2 distribution with two degrees of freedom

)ˆ/log(2 LL

)log(L8 E-2 Signal Events

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What if a Source is Observed?

• Suppose Markarian 421 produces 8 signal events in AMANDA following an E-2 spectrum and the background is well described by atmospheric neutrinos

• >50% probability of 5 detection, assuming no trial penalty.

• Minimization of with respect to signal strength and spectral index yields an estimate of both parameters

• Approximate as a 2 distribution with two degrees of freedom

)ˆ/log(2 LL

)log(L50 E-2 Signal Events

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Benefit of Likelihood Method

• Other benefits:

– Position confidence contours can be drawn similarly using information from the likelihood function

– Data from detectors with different angular resolution can be combined more efficiently

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Conclusions

• New methods enhance point source sensitivity and discovery potential by 30%

• Adding an energy correlated observable additionally allows estimation of signal spectral index in the case of discovery

• No significant excess observed in 2005