Searches for Astrophysical Neutrinos in Coincidence with Gamma Ray Bursts

1. Searches for Astrophysical Neutrinos in Coincidence with Gamma Ray Bursts Erik Strahler University of Wisconsin-Madison 1/28/2009

2. Outline • Motivation • Neutrinos from Gamma Ray Bursts • Detection • AMANDA-II Analysis • IceCube Analysis • Outlook

3. Why look for Neutrinos? Neutrinos from GRBs

4. The Cosmic Ray Connection

5. Gamma Ray Bursts • Isotropic • Highly Variable • Extremely Energetic • Particle acceleration via relativistically expanding shock fronts • New theories with recent evidence of bright optical emission from GRB080319B

6. GRB030329 Neutrinos from GRBs • Gamma emission from GRBs follows broken power law • Protons interact with this spectra to produce neutrinos • Further break at high energy due to pion syncrotron losses • Also Precursor Emission.

7. Swift Satellite • Launched in November 2004 • BAT (Burst Alert Telescope) • 1.4 sr field-of-view ~100 bursts / yr. • Slew within 20-75s • 0.3-5.0 arcsec positioning • XRT, UVOT • Afterglow measurements

8. Neutrino Detection in IceCube -Cherenkov radiation emitted by muon -Optical sensors record arrival time of photons for track reconstruction

9. dfasdfsadf Detector • 19 strings • 677 optical modules • Operated 2000-2007 • Now integrated with IceCube

10. Reconstruction • Tracks / Cascades reconstructed based on Cherenkov photon arrival times and intensities. Better Pointing Resolution Better Energy Resolution Better Background Rejection

11. AMANDA-II Analysis • Previous searches for neutrino-induced muons (Achterberg A. et al. 2008, ApJ, 674, 357) and cascades (Achterberg A. et al. 2007, ApJ, 664, 397) saw no events and set upper limits on GRB neutrino fluxes • 2005-2006 data sample contains 85 northern hemisphere bursts that pass all detector stability criteria. • Largest sample in the post-BATSE era.

12. Downgoing Muons GRB Neutrinos Atmospheric Neutrinos Atmospheric Neutrinos GRB Neutrinos Downgoing Muons Detection Challenges • Down-going muons from CR showers misreconstructed as up-going • Particularly coincident muons from independent showers • Must reject with tight quality cuts • Up-going atmospheric neutrinos from CR showers on other side of Earth • Softer energy spectrum than GRB signal • Isotropically distributed

13. GRB Advantages • Time, Position of predicted signal known. • Drastic reduction in background rate • Use of data for background estimation • Removes many sources of systematic error • Reduces reliance on simulation

14. Initial upgoing event selection Angular Offset from GRB Position (degrees) Initial upgoing event selection Initial upgoing event selection Data Downgoing Muons Atm. Neutrinos GRB Neutrinos Data Downgoing Muons Atm. Neutrinos GRB Neutrinos Data Downgoing Muons Atm. Neutrinos GRB Neutrinos Angular Resolution (degrees) Event Selection • Angular offset from GRB position • Angular resolution of tracks • Timing coincidence with gamma emission • Likelihood ratio of upgoing reconstruction vs. downgoing reconstruction

15. Preliminary Cut Optimization • Optimize for Discovery • 5s observation in 90% of experiments • Search for best cuts for summed contribution of all GRBs • Tight cuts to remove the aggregate background Preliminary

16. Preliminary Results Expected Limit Observed 0 events in the GRB emission windows survive final cuts 05-06 Limit: 14.7 * Prompt flux Preliminary Combined Limit: 1.1 * Prompt flux

17. GRBs with IceCube • Analysis of 22 String Configuration (4/07 – 4/08) • 41 Northern Sky GRBs that pass quality data quality considerations • Incorporate individual burst information • Unbinned analysis, using likelihood ratio test, and incorporating event energy

18. Preliminary Preliminary Event Selection • Looser event selection with ~5000 neutrinos. (~1 expected on-time) • Likelihood tests of tracks • Error estimate of likelihood minimum • Number of direct hits

19. Likelihood Method Signal PDF: Likelihood function: Null hypothesis: Likelihood Ratio: Background PDF from data Maximize LLH ratio by varying ns

20. Sensitivity Test statistic distribution for 100M null hypothesis trials Discovery Potential: 2.2 * Prompt flux

21. IC-22 Results • Test statistic consistant with the null hypothesis for both prompt and precursor emission • Checks with looser cuts reveal no signal-like events • Preliminary limits of 4.6 * prompt flux, 4.9 * precursor flux. • Very preliminary combination with AMANDA-II results places the prompt limit below the predicted signal. • Note: IceCube is much more efficient at background rejection and will improve with construction.

22. Systematics • Remove many sources of error by using off-time data to estimate the background • Ice Properties • Optical Module Efficiency • Seasonal Atmospheric Variation • Studies are Ongoing

23. Conclusions and Outlook • No neutrinos from GRBs yet, but limits are at WB level • Work ongoing on southern sky GRBs • 40 string IceCube configuration online from April 2008 • Already as large as full detector on long axis • GRB analysis framework in develpoment • Data run complete in April 2009 • 19 additional strings deployed this season. • Fermi GST adds significant observation opportunities • 80 string IceCube only a few years away. Expect 1-10 GRB neutrinos per year with full detector • See signal neutrinos very soon!