1. J. Braun, A. Karle, T. Montaruli
Neutrino Point Source Search Strategies for AMANDA-II and Results from 2005
J. Braun, A. Karle, T. Montaruli
For the IceCube
Collaboration

2. 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 direction
Detector angular resolution
Event energy estimation

3. 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 g
Model the detector point spread function as a 2D Gaussian

4. Method
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:

5. Discovery Potential Simulate a neutrino source at a given declination
90% of sources with the given E-2 flux are detected
at the stated significance

6. Sensitivity
90% confidence level sensitivity to E-2 spectra

7. Results for 2005 1.8 billion raw events recorded in 2005
6001 events survive quality cuts, 887 above d = 10o
Search in range -5o < d < 82o

8. 2005 All-Sky Results Log10(p-value)
69 out of 100 sky maps randomized in RA contain an excess of at least 3.6s
Largest Excess: 3.6s
d=48o , a=2.75h
Log10(p-value)
Need to mention how significances are evaluated again

9. Results for Candidate Sources
The method is applied separately to a list of 26 candidate sources to enhance discovery potential
Object
RAo
Dec.o
P-value
m90
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
9.24
MGRO J
304.8
36.8
0.152
20.1
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
Limits for 2005 ONLY!!!
E2F < m TeV cm-2 s-1

10. Spectral Index Estimation
Likelihood function depends on source spectral index
Minimization of –Log(L) with respect to signal strength and spectral index yields an estimate of both parameters

11. Example: Mrk 421
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 5s detection, assuming no trial penalty.
Approximate as a c2 distribution with two degrees of freedom
Evaluating likelihood over a grid of spectral index and signal strength yields simultaneous confidence contours

12. 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

13. 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

14. Extra Slide - 50 Signal Events

15. Extra Slide – Effective Area