A Search for Astrophysical Neutrinos in Coincidence with Gamma Ray Bursts

Transcription

1 A Search for Astrophysical Neutrinos in Coincidence with Gamma Ray Bursts Preliminary Examination Erik Strahler UW Madison 4/5/2006 Major Advisor: Albrecht Karle

2 Outline Gamma Ray Bursts Neutrino Physics The AMANDA detector Analysis Strategy The Future 2

3 GRBs : A History Nuclear tensions led to the Vela satellite system Discovery in 1969 of short duration, highly energetic bursts of gamma rays coming from space 70 s and 80 s a Dark Age for GRB physics Many theories without observational backing 3

4 BATSE : A Breakthrough The Burst And Transient Source Experiment 9 year mission aboard CGRO ( ) With 1/2 sky coverage, detected 1 GRB/day (2704) Isotropic distribution implies extragalactic origins BATSE Current Catalog,

5 Burst Distribution 2 classes of GRBs Ref: Brennan Hughey, from BATSE 4B Catalogue Long GRBs associated with type Ic SN Ref: Mike Stamatikos 5

6 Temporal Variability 6

7 Swift : A New Era Launched in November 2004 BAT (Burst Alert Telescope) 2 sr field of view 100 bursts / yr. Slew within 20 75s arcsec positioning XRT, UVOT Afterglow measurements Redshifts 7

8 Swift slews towards a GRB Coded Mask is transparent to >150 KeV photons 8

9 So what are GRBs? Very high energies over short time scales Known to be cosmological Huge energies ( ergs) Rapid temporal variability, δτ,10 ms Optical afterglow 9

10 Compactness Problem δτ 10-2 s implies Rs = cδτ 3000 km γ s may pair produce if E1E2 > me c 2 τ γγ f pστ FD 2 = Rs2 me c 2 Given typical GRB parameters τγγ 1013 But we see photons (and they re non-thermal)! Let s make it relativistic: E Eobs / Γ, Rs Γ2cδτ With Γ 102, gives τγγ < 1 10

11 What is the Compact Object? Progenitor is optically thick to observation Binary Merger Neutron star accretion on to a Black Hole Stellar Collapse Association with Type Ic SN 11

12 Fireball Model Relativistically expanding plasma of e+, e, γ Initially optically thick Γ increases with time, until τγγ < 1 Kinetic Energy dissipated via shocks Internal between shells with varying Γ External with ISM 12

13 Shocks Central Engine emits shells of plasma with varying relativistic bulk Lorentz factors A faster shell can overtake a slower one and produce a shock Dissipated energy can be used to accelerate particles or create magnetic fields Accounts for rapid variability in GRB light curve External shocks with the ISM are likely the source of afterglow emission 13

14 Particle Acceleration n2, e2 n1, e1 Frame Shock Upstream Material Downstream Material Conserve m, p, E across Shock Front Velocities isotropically distributed in own rest frames Energy gain crossing shock in either direction We are in the mildly-relativistic regime (Γ 1 to 10) n = n1 ( 4Γsh + 3) 4n1Γsh Create equipartition fractions εe, εb to hold our 2 e2 = 4Γsh2 e1 complete ignorance of the microphysics 2 Γsh2 = ( Γrel + 1) / 2 B = 8πε B e2 Blandford and McKee, Mon. Not. RAS, 180: ,

15 Energy Gain Crossing relativistic shocks is complicated: Ef Ei Γ 2 sh 1 Ef Ei 2 Relativistic particles scattered by magnetic fields In upstream region, a small change in p allows the shock to re overtake Require highly turbulent downstream region to reverse direction and enable repeated shock crossing 15

16 Particle Spectrum Convolve energy gain with escape probability Gives rise to a power law spectrum p NdE E de with ln(1 /(1 Pesc )) p = 1+ ln( E f / Ei ) Solved for the Γsh >> 1 case p Pretty close to regular Fermi 16

17 Gamma Emission Accelerated electrons in high magnetic fields lead to gamma ray emission GRB Synchrotron Inverse Compton (Synchrotron Self Compton) Broken power law spectrum Ref: Mike Stamatikos 17

18 Evidence for Jets Afterglow emission exhibits breaks in the power law Beamed jet emission drops to Γ 1 from decceleration on circumburst matter No longer forward beamed in direction of emission Ref: Lipkin et al Lower energy bursts, but need more of them 18

19 Outline Gamma Ray Bursts Neutrino Physics The AMANDA detector Analysis Strategy The Future 19

20 What are Neutrinos? Neutral lepton Weakly interacting Very low mass Three Flavors Oscillations 20

21 Why look for them? Direct pointing Magnetic fields Absorption Probe Hadronic acceleration No ν production in purely electromagnetic processes Hints at the source of the highest energy Cosmic Rays 21

22 Signal Hypothesis Precursor neutrinos from interactions on thermal photons within stellar envelope Prompt neutrinos from interactions on GRB photons Late neutrinos from interactions on afterglow emission Ref: Razzaque, Mészáros, and Waxman, astro-ph/

23 Prompt Signal Assume shock accelerated protons with same spectrum as electrons ( E-2) Reasonable if GRBs are the source of the highest energy CRs Produce pions in photomeson reactions with fireball γ-rays p + γ (n) + π ± µ + ν µ e + ν e + ν µ + ν µ (n ) + π 0 γ + γ A fraction fπ ~ 0.20 of the proton energy is given to the pion Eν ~ 0.05 Ep Yields neutrino spectrum with break at ~105 GeV Further break at ~107 GeV due to π synchrotron losses 23

24 Neutrino Spectrum γ spectrum from GRBs fitted by Band function: α Eγ E E0 γ A e 100 KeV Nγ ( E ) = α β (α β ) E0 ( β α ) A 100 KeV e Eγ ε γb Eγ 100 KeV ε γb (α β ) E0 β Eγ ε γb ν spectrum tracks above from resonance considerations: ( Eν / ενb ) β 1 for Eν < ενb 2 dnν Eν Aν ( Eν / ενb ) α 1 for ενb < Eν ενs deν ( E / ε b ) α 1 ( E / ε s )-2 for E ε s ν ν ν ν ν ν 24

25 The Ugly Details Observed Quantities Calculated Quantities Peak photon energy Energy partition fractions Photon spectral indices Jet break time Variability time Duration Fluence Redshift Photon break energy Luminosity Lorentz factor Proton efficiency Beaming angle Neutrino break energy Pion synchrotron break Flux normalization GRB

26 Oscillations Produced at source in ratio νe:νµ:ντ of 1:2:0 Long baseline (cosmological scale) gives rise to a flavor ratio at Earth of 1:1:1 τ appearance experiment At high energies, π may decay, but µ may not Occurs above 1015 ev Gives ratio at source of 0:1:0 Results in ratio of 1:2:2 at Earth 26

27 Interactions with Matter σν is very small, but MEarth is very large Sometimes, a ν will interact with an Ice nucleus and produce a charged lepton (e, µ, τ) Lepton scatters within 1 above TeV energies (highly forward boosted) Conveniently less than our detector resolution 27

28 Obligatory Feynman Diagrams Charged Current νµ Charged Current νe,τ µ Tracks W N X e,τ W X N Neutral Current Cascades νe,µ,τ Z N νe,µ,τ X 28

29 Cascades Hadronic EM Electromagnetic γ s via Bremsstrahlung e+e- via pair production Hadronic More complex due to Nuclei Many π s, K s π0 γγ interactions make shower more EM like 29

30 Cherenkov Effect moving charged particles disrupt local EM field emitted photons constructively interfere Simulated Cherenkov Radiation in IceCube 41 in Ice Cherenkov Radiation in a Research Reactor 30

31 Backgrounds Downgoing Muons (>300 GeV) Atmosphere Created by CR showers Factor of 106 above signal Earth blocks upgoing muons Atmospheric Neutrinos Mostly νµ resulting from π and K decay π ν µ + µ Follow E-3.7 π + ν + µ + µ spectrum K (π 0 ) + ν µ + µ K + (π 0 ) + ν µ + µ + Earth Ref: Brennan Hughey 31

32 Outline Gamma Ray Bursts Neutrino Physics The AMANDA detector Analysis Strategy The Future 32

33 South Pole Physics y wa Ski Geographic South Pole Dome AMANDA-II Detector Location Amundsen-Scott South Pole Station 33

34 Why here? Large, clear, homogeneous Cherenkov medium Depth removes much of downgoing muon background Blocks upgoing muons entirely Quiet ( 1 khz noise rate) Compare to 100 khz rate in ANTARES Long absorption length Balanced by short scattering length 34

35 The Antarctic Muon And Neutrino Detector Array 1.5 km under the ice 200m diameter, 500m height AMANDA II Configuration 677 Optical Modules 19 Strings Send signal to surface Trigger on 24 events within 2.5 µs 35

36 Optical Modules Breakout Downward pointing PMT Graffiti Cable to surface sends data and provides OM with power Pressure Sphere 36

37 Reconstruction Tracks / Cascades reconstructed based on Cherenkov photon arrival times and intensities. Better Pointing Resolution Better Energy Resolution Better Background Rejection 37

38 Outline Gamma Ray Bursts Neutrino Physics The AMANDA detector Analysis Strategy The Future 38

39 2004/2005 Burst Sample Well localized, Swift triggered bursts 106 GRBs (59 northern sky) Durations from 0.04 to 520 seconds 27 with measured redshifts (0.225 to 6.2) Addendum: HETE II and INTEGRAL bursts 52 additional GRBs 9 also seen by the Swift XRT / UVOT 39

40 The Beauty of Transients Use on source, off time data to determine the background No messy background Monte Carlo Short durations imply very low background count rates from the very start A background free search 40

41 Options Diffuse Treat every burst as average Add them all up Discrete Treat each burst individually Semi Discrete Are cuts sensitive to burst parameters? [experiment] Use individual burst parameters to set limits [physics] 41

42 Blindness Extract 2 hour intervals centered on each trigger Blind a window around the burst 10 min 2 hour GRB Trigger 42

43 Detector Stability Remove bursts that occur during periods of high noise rate, i.e. bad runs or bad files. Due to construction, testing, VLF interference, etc. Check off time interval around each burst Seconds of Day Detector Rate dt between events 43

44 Simulated Signal Generate signal Monte Carlo for each burst ANIS (creation, earth propagation, interaction) MMC/PTD (ice propagation, scattering, detection) AmaSim (detector effects, OM sensitivity, etc.) Reconstruct as if data Reweight to Flux calculated from GRB measurements Want to simulate all flavors, neutral and charged currents This waits on AmaSim update + Photonics 44

45 Low Level Filtering Crosstalk Cleaning Electronic correspondence between optical modules Creates false hits that skew reconstruction Select events that pass the cascade trigger or have a reconstructed angle of >70 degrees Rejected Region So far, so good for a muon track or cascade analysis 45

46 Cascade Analysis Cascade Likelihood Muon Hypothesis θ > 70 g oin g up ing go n w do Rapid removal of downgoing muon background Bkgd Rejection: 99.1% Sig Retention: 66.9% Ref: Ignacio Taboada, 2000 Cascade Analysis Off-Time Background Signal MC 46

47 Cascade Optimization Cascade Likelihood Ref: Ignacio Taboada, 2000 Cascade Analysis Cascade Energy Off-Time Background Signal MC 47

48 Muon Analysis Angle between reconstructed track and GRB position Bkgd Rejection: 99% Sig Retention: 86% Off-Time Background Signal MC Ref: Mike Stamatikos, GRB Analysis 48

49 Significance Determine background rate, λ, after cuts Probability that seeing n events is due to background fluctuations is given by Poisson statistics: (λδt ) n e λδt P(n ) = n! where δt is the burst duration 49

50 Sensitivity WB νµ Flux Prediction: 3 x 10-8 GeV / cm2 s sr (after oscillations) 1.5 x 10-8 GeV / cm2 s sr Sensitivity : 4.23 x 10-8 GeV / cm2 s sr Sensitivity: (same cuts) 2.53 x 10-8 GeV / cm2 s sr 50

51 Outline Gamma Ray Bursts Neutrino Physics The AMANDA detector Analysis Strategy The Future 51

52 Let s Build a Bigger One IceCube Construction: (status February 2006) In-Ice Array: Number of strings: Optical Sensors: Depth: Instrumented Volume: 80 (9) 4800 (540) m 0.9 km3 Surface Array (IceTop): 160 (32) tanks (2 per in-ice string) 2 DOMs per tank Total 320 DOMs (64) 52

53 Online Analysis Receive notification from GCN Network Tell AMANDA / IceCube to send data around burst time over satellite Use standard methodology to filter, set cut levels, and unblind Set limits based on user specified input model 53

54 Satellites Swift continues its mission We continue to analyze Swift bursts GLAST (Gamma ray Large Area Space Telescope) Scheduled for 2007 launch Possibly too late for me 54

55 The Next 2 Years of My Life Process data and burst localized MC Determine feasibility of Semi Discrete approach Cut parameters vs. zenith, burst energy. Others? Optimize Analysis Cuts Determine 16 parameters for each of 160 bursts Set limits on neutrino flux from GRBs for different models Do it all again for IceCube! 55