Searching for the Origin of Cosmic Rays with IceCube

Transcription

1 Searching for the Origin of Cosmic Rays with IceCube Stefan Westerhoff University of Wisconsin-Madison Fall 2010 Physics Seminar Wichita State University September 29,

2 2

3 3

4 Outline Cosmic Rays and Their Acceleration The IceCube Detector Results: Neutrino Point Source Search and Moon Shadow Search for Anisotropies in Cosmic Ray Arrival Directions with IceCube 4

5 Cosmic Rays Energy Spectrum Cosmic ray energy spectrum is nonthermal: Energy distribution has no characteristic temperature. Source energy is given to a relatively small number of particles. Energies of the nonthermal Universe (up to ev) are well beyond the capabilities of thermal emission processes. Knee The origin of cosmic rays at energies above GeV is unknown - no astrophysical object has ever been definitively identified as an accelerator of high energy nucleons. 5

6 Cosmic Rays Energy Spectrum Accessible to experiment: Energy spectrum. Chemical composition. Arrival directions. Astronomy with charged particles? Protons and nuclei are charged and therefore subject to deflection in Galactic and intergalactic magnetic fields (of unknown strength)! Knee R 1 kpc E EeV 1 B G Z 6

7 Acceleration Mechanism A possible acceleration mechanism was suggested in 1949 by Enrico Fermi. Particles are accelerated by collisions against moving magnetic fields. Enrico Fermi ( ) 7

8 Acceleration Mechanism Fermi showed that particle acceleration is possible when particles are reflected by moving shock waves. The process is called shock acceleration. Enrico Fermi ( ) 8

9 Source Candidates? In Fermi's cosmic ray shock accelerator, protons or heavier nuclei speed up by bouncing off moving magnetic clouds in space - just like a tennis ball is faster after it bounces off a wall moving towards the observer. incoming cosmic ray magnetic field Shock acceleration is a tedious process - the particles gain energy over many (10 7 or more) collisions. Where do we find moving shocks in the Universe? outgoing cosmic ray Moving shock front 9

10 Cas A supernova remnant in X-rays shock fronts 10 Fermi acceleration when particles cross high B-fields

11 Cosmic Particle Accelerators Supernovae can account for cosmic rays with energies up to ~10 16 ev. Observed energy density of galactic cosmic rays: ~ erg/cm 3 Supernova remnants: erg every 30 years: ~ erg/cm 3 Supernova remnants provide the environment and energy to explain the galactic cosmic rays But no direct evidence so far Cas A, courtesy Chandra (NASA) 11

12 Cosmic Particle Accelerators Supernovae cannot account for cosmic rays with energies above ~10 16 ev. Above this energy, cosmic rays must be accelerated in more extreme sources, most likely outside of our Galaxy. Active Galactic Nuclei are possible sources: they consist of a supermassive black hole, an accretion disk, and two jets in which shocks move outward. 12

13 13

14 Cosmic Rays A cosmic ray standard model Solar Galactic Origin up to ~10 16 ev Supernova Remnants, indirect evidence (H.E.S.S.) Extragalactic origin above ~10 18 ev Active Galactic Nuclei. Gamma Ray Bursts. Galactic Knee Transition from Galactic to extragalactic above ev. Extragalactic 14

15 Neutrino Production Cosmic rays are not the only messenger from high energy sources - where there are cosmic rays, there are also neutrinos Neutrinos are the ideal messenger particle: Neutrinos propagate in a straight line and are not easily absorbed - they can get away from the source! However, they are not easily absorbed in detectors, either we need km 3 size detectors! 15

16 Requirements for a Neutrino Detector Large detector volumes (of order ~ km 3) if we want to detect a few neutrinos from astrophysical sources per year Neutrinos must interact near or in the detector and produce a particle that can be detected, for example a muon. The detector must be shielded from the enormous background of atmospheric muons, so it needs to be deep below some absorbing material. At the same time, the absorbing material must allow for detection of particles created by neutrinos. A possibility is to have neutrinos interact near the detector and observe the charged particles created by this interaction via their Cherenkov light. In that case, the detector material must be transparent for optical light and dense enough to enable interaction of neutrinos 16

17 Blue light travels 200+ meters in ice 17

18 Amundsen-Scott South Pole Station 18

19 19

20 Where are we? runway South Pole IceCube 20

21 80 Stations, each with: 86 strings 1.5 km km deep typically 125 m spacing between strings 60 Modules per string 1 km billion tons of instrumented volume 21

22 22

23 Digital Optical Module electronics photomultiplier tube 23

24 IceCube 25 m E m 45 m

25 IceCube Concept Shielded and transparent medium Lattice of photomultipliers 25

26 IceCube Concept Neutrino travels through the Earth. 26

27 IceCube Concept Infrequently, a neutrino causes a nuclear interaction in the ice and produces a muon. 27

28 IceCube Concept The muon produces Cherenkov light, and photomultipliers capture and map the light. 28

29 IceCube Concept The muon produces Cherenkov light, and photomultipliers capture and map the light. 29

30 Event Display Number of hit modules: 148 est. angular error:

31 Stockholm University Uppsala University University of Alberta University of Oxford EPFL, Lausanne U. of West Indies, Barbados Univ. of Canterbury, Christchurch The IceCube Collaboration Institutions, ~250 members

32 32 A photomultiplier starts its journey to 2500 m

33 IceCube Status 33

34 IceCube Status 34

35 IceCube Status 35

36 IceTop Surface Array IceCube 40-string Data ~ 1000 TeV cosmic ray muon bundle July 24 Chad Finley - Oskar Klein Centre - Stockholm University 36

37 IceCube 40-string Data ~ 300 TeV cosmic ray muon bundle July 24 Chad Finley - Oskar Klein Centre - Stockholm University 37

38 IceCube 40-string Data ~ 1 TeV neutrino-induced muon July 24 Chad Finley - Oskar Klein Centre - Stockholm University 38

39 Neutrino Spectrum 39

40 Cosmic Ray Background Southern Sky Background: Atmospheric muons from cosmic rays. Cosmic ray air showers produce muons and neutrinos reaching the detector. In one year, IceCube detects: billions of downgoing muons. thousands of neutrino-induced muons. Northern Sky Background: Atmospheric neutrinos from cosmic rays. 40

41 IceCube 40-String ( ) +60 Northern sky (ν bkg) preliminary events: up-going (neutrino candidates) down-going (high energy muons) 41 Southern sky (µ bkg) Median PSF for E -2 spectrum: 0.8 in northern sky 0.6 in southern sky

42 IceCube 40-String ( ) +60 Northern sky (ν bkg) preliminary -60 Southern sky (µ bkg) Hottest location in the all-sky search is: R.a. = 115, Dec. = % of simulated background sets (scrambling data in r.a.) have an equal or greater excess occurring by chance. Not significant. 42

43 Moon Shadow Cosmic rays blocked by the moon lead to a deficit in the distribution of downward going muons in the detector. 43

44 Moon Shadow Unbinned likelihood method using the arrival direction error of individual events. preliminary Expected number of shadowed events: 5732 Observed number at moon position:

45 Cosmic Rays in IceCube IceCube is also a cosmic ray detector: by measuring downward going muons from air showers, IceCube can study the arrival direction distribution of cosmic rays in the energy range ~10 TeV to several 100 TeV and produce a cosmic ray sky map of the southern sky. Muons produced in downward going cosmic ray air showers are detected by IceCube at a rate of > 1000 Hz. cosmic ray air shower The median energy of the cosmic ray primaries is 20 TeV, and the median angular resolution is 3 (not to be confused with the resolution for neutrinos in IceCube!) muons from air showers 45

46 Large-Scale Anisotropies Cosmic ray showers observed in IceCube are about 1 to 100 TeV in energy. An arrival direction anisotropy in charged cosmic rays is not expected at these energies due to interactions with the Galactic magnetic field. Predicted Larmor radius: 0.1 pc. Nevertheless, there have been several observations of large-scale, part-per-mille anisotropies in cosmic ray arrival directions between 0.1 and 100 TeV. Tibet: M. Amenomori et al., Science 314 (2006) 439. Milagro: A. Abdo et al., Astrophys. J. 698 (2009) 212. Milagro ARGO-YBJ 46 equatorial coordinates

47 Large-Scale Anisotropies All previous detections of large-scale anisotropies have been in the northern hemisphere. IceCube is the first detector able to measure anisotropies of the southern sky. IceCube records several cosmic ray events per year, which makes it possible to study anisotropies in the cosmic ray arrival direction distribution at the 10-4 level and less. Large-scale anisotropies (with an extent greater than about 40 in right ascension) can be caused by large scale and local magnetic field configurations, the heliosphere (up to around TeV), discrete diffuse cosmic ray sources (at higher energies), the motion of the Earth. 47

48 Dipole Anisotropies Due to Earth s Motion Large scale anisotropies can be caused by the motion of the Earth through the cosmic ray rest frame: The intensity of cosmic rays should be higher coming from the direction in which Earth is moving, causing a dipole anisotropy if we choose a coordinate frame where the direction of Earth s motion is fixed. Earth s motion through space is complex and a superposition of several motions. Two dipole anisotropies due to Earth s motion have been postulated: A solar dipole anisotropy due the Earth s motion around the Sun. This anisotropy has been observed (Milagro, IceCube, ). The Compton-Getting Effect due to the motion of the solar system around the Galactic center. In certain scenarios, this effect should be strong enough to be within reach of modern detectors. 48

49 Solar Dipole Effect This effect caused by Earth s motion around the Sun is apparent when the arrival directions are plotted in a frame where solar time (or universal time UT) is used to calculate the sky coordinates. The expected anisotropy is of order v = km/s 49

50 Solar Dipole Effect IceCube observes the solar dipole effect at the expected strength corresponding to the Earth s velocity about the Sun of v = km/s. degrees from the Sun Note that fixed signals in universal time UT will be washed out in sidereal time over the course of a year, so the solar dipole effect will not be visible in a skymap of cosmic ray arrival directions in equatorial coordinates. 50

51 Compton-Getting Effect If galactic cosmic rays do not co-rotate with us about the Galactic center, the Galactic motion of the solar system create a dipole anisotropy in equatorial coordinates. Motion of cosmic ray plasma is not known but assuming cosmic rays are at rest with respect to the galactic center, we should observe a dipole of 0.35%, inclined relative to the equatorial plane. I I 2 v c cos 2.7 cosmic ray spectral index v 220 km s speed equatorial coordinates 51

52 Analysis Method Any method to detect large scale anisotropies like the Compton-Getting dipole needs to be sensitive to anisotropies at the 10-4 level, in the presence of much stronger variations due to diurnal and seasonal variations of atmospheric conditions, non-uniform detector exposure to different regions of the sky due to gaps in the detector uptime and an uneven run selection due to quality cuts, asymmetries in the detector geometry. IC22 IC40 52

53 Harmonic Analysis Relative intensity of the cosmic ray event rate in equatorial coordinates: for each declination belt of width 3, the plot shows the number of events relative to the average number of events in the belt. equatorial coordinates IceCube-22 Median energy: 20 TeV R. Abbasi et al. (IceCube Collaboration), Astrophys. J. 718 (2010) L194 53

54 Harmonic Analysis In the harmonic analysis, the right ascension distribution of the data is fitted with a first- and second-order harmonic function of the form 2 A i cos(i( i )) B i 1 A 1 ( ) 10 4 A 2 ( ) o 2.6 o 3.8 o o 4.0 o 7.5 o CG minimum χ 2 / dof = 19/22 CG maximum The anisotropy is not a pure dipole and does not have the right phase to be explained by the Compton-Getting effect. There may be a Compton-Getting component, but it seems to be overwhelmed by other effects. 54

55 Analysis Technique To produce a skymap of the southern sky, we adopt a method commonly used in large field-of-view gamma ray detectors. A signal map is made based on the arrival direction of each event, and a background map is created from the data itself. The cosmic ray background N exp can be estimated from the data using several (equivalent) methods. N exp (ra, ) E(ha, ) R(t) (ha,ra,t) dt d E probability that an event comes from angular element d R event rate (as a function of time t) 1 if event is in ra and bin under consideration, 0 otherwise This equation can be integrated numerically by randomly assigning detected event times with local arrival directions. This procedure compensates for any event rate variations and ensures that the background events have the same arrival direction distribution in local coordinates as the data. 55

56 Analysis Technique The uncertainty in the background estimate can be reduced by generating a large number (~20) of simulated background maps. The method assumes that detector conditions (i.e. the local arrival direction distribution) remain constant. Typically, background maps are created for time periods of order several hours ( time scrambling period ) to make sure this condition holds (in IceCube, the detector is stable on much longer times scales). The length of the time interval used for the background estimate determines the maximum size of features we are sensitive to. For example, with a 4 hour scrambling period, we are insensitive to features of more than around 4 15º = 60. A 24 hour scrambling period will show all features. By choosing shorter time scrambling periods, we can filter large-scale structure. 56

57 Skymap 1º 1º (HEALPIX) skymap for 24 hour scrambling period shows dipole - bins are statistically independent. When searching for anisotropies on larger scales, we can smooth over larger regions to improve the sensitivity to larger features. 57

58 Large-Scale Anisotropy preliminary 20º smoothing 58

59 Relative Intensity 59

60 Comparison to Northern Hemisphere Milagro equatorial coordinates 60

61 Anisotropy on Smaller Scales Several experiments have discovered anisotropies on scales of about 10. Milagro Median energy: 1 TeV Milagro: >10σ hotspots seen with 7 years of data (amplitude is a few 10-4 ). Also observed by Tibet ASγ and ARGO-YBJ. Origin: magnetic funneling? Diffusion from nearby SNR? ARGO-YBJ Median Energy: 2 TeV IceCube can search for similar anisotropies in the southern sky. equatorial coordinates 61

62 Angular Power Spectrum Information on the scale of anisotropies in skymaps (multipoles, regular structure, and other features) can be obtained by calculating the angular power spectrum. The angular power spectrum from a map is basically a simple Fourier transform, decomposing the map of the sky into spherical harmonics. WMAP IceCube does not have full sky coverage. The absence of the northern hemisphere will distort the power spectrum since additional Fourier components will be needed to zero the northern sky. 62

63 Angular Power Spectrum Results of a power spectrum analysis for the true IC40 data map (red) and the map in anti-sidereal time (AST), which is non-physical and should (and does) not show a signal. Angular scale [ ] C l grey scales: 1 and 2 error bands for isotropic maps 3 Multipole l 63

64 Skymap 24-hour 64

65 Skymap 12-hour 65

66 Skymap 8-hour 66

67 Skymap 6-hour 67

68 Skymap 4-hour 68

69 Skymap 3-hour 69

70 Skymap 2-hour 70

71 Skymaps 2-hour 3-hour 4-hour 6-hour 8-hour 12-hour 24-hour 71

72 IceCube Skymap Significance map (signal over expected background) of the southern sky as observed by IceCube. The map is smoothed using a 20 radius to increase sensitivity to larger features, so neighboring bins are highly correlated. The skymap shows a broad excess region around right ascension 120, with an equally strong deficit around right ascension 220. preliminary IceCube-40 equatorial coordinates pre-trial 72 significance [ ]

73 IceCube Skymap Significance map (signal over expected background) of the southern sky as observed by IceCube. The map is smoothed using a 20 radius to increase sensitivity to larger features, so neighboring bins are highly correlated. The skymap shows a broad excess region around right ascension 120, with an equally strong deficit around right ascension 220. preliminary IceCube-40 Vela (RA 128.6, DEC ) equatorial coordinates pre-trial 73 significance [ ]

74 IceCube Skymap Methods that determine the background from the data have systematic errors in the presence of strong signals. Since events from the source bin are used to estimate the background, the background is overestimated. This can be especially problematic for strong diffuse signals. preliminary IceCube-40 equatorial coordinates pre-trial 74 significance [ ]

75 Significance versus Smoothing The significance of the hot spot is a pre-trial significance: Li&Ma significance The location and the extent of the excesses were determined by examining the data. In particular, the smoothing radius of 20 maximizes the significance of the hot spot and was chosen a posteriori for that reason. smoothing angle [ ] However, the data used in this analysis (IC40) is only a fraction of the data already recorded. The parameters that optimize the signal in IC40 (source location and smoothing angle) are a priori for the analysis of data from IC59 and beyond. 75

76 IceCube 22 Skymap As a first indication that the signal is consistent over time, we analyze the smaller IC22 data set using the same parameters. The main features are identical, with smaller significance due to the smaller size of the data set. equatorial coordinates 76

77 IC22 IC40 77

78 Systematic Check: Seasonal Variations? A constant signal in universal time (a constant daily variation) will wash out in sidereal time for any integer number of years. Seasonal variations do not wash out over the year and can cause a signal in sidereal time. As an important systematic check, we use anti-sidereal time to estimate the systematic error on the signal due to seasonal variations. Anti-sidereal time is a nonphysical time frame created by flipping the sign in the transformation from universal time to sidereal time. No signal is expected in this time frame, but systematic distortions due to seasonal effects will show up in this frame also. 78

79 Consistency Check: Anti-Sidereal Time There is no significant signal in the skymap if anti-sidereal time is used instead of sidereal time for the transformation from local to equatorial coordinates. preliminary pseudo equatorial (using anti-sidereal time) 79

80 IceCube and Milagro equatorial coordinates 10 smoothing 10 TeV 20 TeV Equatorial Coordinates 80

81 IceCube and Milagro equatorial coordinates 10 smoothing 10 TeV 20 TeV Equatorial Coordinates 81

82 Origin of the Anisotropies? Do the hot spots indicate the location of nearby cosmic ray accelerator? Protons? Problem: Larmor radius of a 10 TeV proton in a 2 G magnetic field is pc. Possible solutions: Non-standard cosmic ray diffusion? Vela Geminga Drury & Aharonian, Astropart. Phys. 29 (2008) 420 Non-standard cosmic ray beam? equatorial coordinates Malkov et al.,arxiv: Neutrons? Problem: decay length of 10 TeV neutron is 0.1 pc. 82

83 Origin of the Anisotropies? The hot spots might indicate the structure of local magnetic fields. Heliosphere? Future studies can shed some light on these issues Study of the energy dependence of the anisotropy: With the 59 string data, the energy dependence of the signal can be studied. However, the energy resolution of IceCube for downgoing muons is poor, and the muon energy is also only a poor indicator of the cosmic ray energy. Study of the time dependence of the anisotropy: Large-scale anisotropy appears stable in Tibet data during the 23rd solar cycle anisotropy does not depend on solar activity. Amenomori et al., Astrophys. J. 711 (2010) 119 Full sky coverage with IceCube (south) and HAWC (north). 83

84 Summary (I) The IceCube detector at the South Pole is nearing completion. The last strings will be installed during the 2010/2011 drilling season. IceCube is a neutrino detector covering the northern and (with less sensitivity due to the much larger background) the southern hemisphere. IceCube has been taking data for several years. No significant neutrino point sources have been detected so far. IceCube observes the shadow of the Moon. IceCube records several downgoing cosmic ray events per year. This allows for a study of anisotropies in the cosmic ray arrival direction distribution in the southern hemisphere at the 10-4 level and less. 84

85 Summary (II) IceCube sees a large-scale anisotropy in cosmic ray arrival directions in the southern sky. The anisotropy is not a pure dipole, so the postulated Compton- Getting effect is at best a part of the observed anisotropy. The IceCube skymap also shows a broad (~20 ) excess region around right ascension 120, with an equally strong deficit around right ascension 220. Anisotropies of similar strength have been observed by experiments in the northern hemisphere. The origin of these anisotropies is currently not known. In the near future, IceCube can study whether the anisotropy persists at higher energy (>100 TeV). 85

86 Thanks Segev BenZvi Simona Toscano Rasha Abbasi Paolo Desiati Marcos Santander (power spectrum) Chad Finley (neutrino point source search) Laura Gladstone, Jan Blumenthal (moon shadow) 86