Search for PeV Gamma Rays with IceTop and IceCube Zach Griffith and - - PowerPoint PPT Presentation

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Search for PeV Gamma Rays with IceTop and IceCube Zach Griffith and - - PowerPoint PPT Presentation

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Search for PeV Gamma Rays with IceTop and IceCube Zach Griffith and Hershal Pandya The IceCube Collaboration TeVPA 2017 7 August 2017 Columbus, Ohio Outline - Motivation - Event Selection - Results Point Source Analyses All-sky scan

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SLIDE 1

Search for PeV Gamma Rays with IceTop and IceCube

Zach Griffith and Hershal Pandya

The IceCube Collaboration TeVPA 2017 7 August 2017 Columbus, Ohio

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SLIDE 2

Search for PeV Gamma Rays | TeVPA 2017 | 7 August 2017

  • Motivation
  • Event Selection
  • Results
  • Point Source Analyses

✦ All-sky scan ✦ Search for correlation with known TeV gamma-ray sources (H.E.S.S.) ✦ Search for correlations with high-energy IceCube neutrinos

  • Search for excess of PeV gamma rays from the Galactic

plane

Outline

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SLIDE 3

Search for PeV Gamma Rays | TeVPA 2017 | 7 August 2017

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100 101 102 103 104

Energy [TeV]

10−21 10−20 10−19 10−18 10−17 10−16 10−15 10−14 10−13 10−12

dN/dE [cm−2s−1TeV−1]

IceCube Preliminary

HESS J1427-608 Best Fit Best Fit (Extrapolated) IceCube 5 year 90% Upper Limit

Motivation

IceCube is the most sensitive southern hemisphere experiment to PeV gamma rays

arxiv:0712.1173

IceCube can test whether the spectra of known TeV sources extend up to PeV energies without a cut-off

H.E.S.S. sources in the IceCube Field of View

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SLIDE 4

Search for PeV Gamma Rays | TeVPA 2017 | 7 August 2017

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Analysis Region

Observation of PeV Gamma-rays can indicate galactic origin of some of the IceCube neutrinos

Motivation

arxiv:1510.05223 Physical Review D 90, 023010 (2014)

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SLIDE 5

Search for PeV Gamma Rays | TeVPA 2017 | 7 August 2017

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  • Gamma-ray air showers have ~10 times

fewer muons, less local fluctuations, and younger shower age compared to cosmic-ray showers

  • IceTop measures:
  • Shower energy
  • Shower core and direction
  • Peripheral GeV muons
  • Lateral distributions of energy and

timing

  • IceCube measures:
  • Thoroughgoing (>300 GeV) muons

from shower core

Gamma Hadron Discrimination

Data Candidate

IceTop IceCube

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SLIDE 6

Search for PeV Gamma Rays | TeVPA 2017 | 7 August 2017

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μ

Log(Lateral Distance from Shower Axis / m) Log(Lateral Distance from Shower Axis / m) Log(Tank Charge / VEM) Log(Tank Charge / VEM)

IceTop Gamma/Hadron Discrimination

LLHR = Log10 L( (Qi Ri)

{ }| Hγ )

L( (Qi Ri)

{ }| HCR)

! " # # $ % & &; i =1to162 IT Tanks Plots from Hershal Pandya

Gamma Ray Simulation Cosmic Ray Data

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SLIDE 7

Search for PeV Gamma Rays | TeVPA 2017 | 7 August 2017

  • Features
  • IceCube Charge
  • IceTop LLHRatio
  • log10(S125) (IceTop energy proxy)
  • sin(Declination)
  • IceCube Containment
  • Point source selection
  • Classifiers constructed with gamma

simulation weighted to relatively hard (E-2.0) and soft (E-2.7) spectral bounds

  • Diffuse galactic plane selection
  • Classifier constructed with softer

(E-3.0) spectrum

Selection

Random Forest Classifier

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6.0 6.5 7.0 7.5 8.0

log(E/GeV)

10−4 10−3 10−2 10−1 100

Passing Fraction

Data Gamma Ray MC (E−2.0 weighted)

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SLIDE 8

Search for PeV Gamma Rays | TeVPA 2017 | 7 August 2017

  • Implemented with 5 years of full detector

data

  • Test over entire sky using standard

unbinned likelihood method

  • with energy weights
  • fitting to spectral index
  • Significance of hottest spot determined by

background trial comparison

  • hottest spot post-trial p-value: 77%

All-Sky Scan

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Right Ascension Declination

10 20 30 40 50

Test Statistic

100 101 102 103 104

Trials

IceCube Preliminary

Observed TS 1 σ 2 σ 3 σ Scrambled Trials

  • δ = -63.5°
  • 𝛅 = 3.21
  • α = 28.5°
  • ns = 11.22

Hottest Spot

South Pole Projection

Flux: 1.75 × 10-19 GeV-1cm-2s-1

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SLIDE 9

Search for PeV Gamma Rays | TeVPA 2017 | 7 August 2017

80 75 70 65 60 55

Declination ()

1021 1020 1019 1018 1017

Flux at 1 PeV [cm2s1TeV1]

IceCube Preliminary

E2.0 Sensitivity E2.0 Discovery Potential E2.7 Sensitivity E2.7 Discovery Potential Extrapolated H.E.S.S. Sources

Point Source Sensitivity

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H.E.S.S. Gal. Plane Survey Unexplored Sky

  • H.E.S.S. sources shown assume optimistic scenario:
  • no break in the fitted power law at TeV energies
  • no absorption
  • Sensitivity:
  • 90% C.L. upper

limit

  • Discovery Potential:
  • 5σ detection

50% of the time

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SLIDE 10

Search for PeV Gamma Rays | TeVPA 2017 | 7 August 2017

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H.E.S.S. Sources

Source pre-trial p-value HESS J1356-645 >0.50 HESS J1507-622 0.28 SNR G292.2-00.5 0.39 Kookaburra (Rabbit) 0.35 HESS J1458-608 0.20 HESS J1427-608 0.11 Kookaburra (PWN) >0.50 SNR 6318.2+00.1 >0.50 MSH 15-52 0.47 HESS J1018-589 B 0.09 HESS J1018-589 A 0.12 HESS J1503-582 0.24 HESS J1026-582 0.04 Westerlund 2 0.07 SNR G327.1-01.1 >0.50

  • 15 total in FOV with no evidence of a cutoff at TeV

energies

  • Hottest individual H.E.S.S. source: H.E.S.S.

J1026-582

  • post-trial p-value of hottest source: 45%
  • Stacked likelihood test also performed with catalog
  • stacking test p-value: 5%
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SLIDE 11

Search for PeV Gamma Rays | TeVPA 2017 | 7 August 2017

  • Unidentified source with confirmed

counterpart at GeV energies by Fermi (Guo et al. 2016)

  • Well fit to E-2 over 4 orders of magnitude

(GeV-TeV) with no cut-off, unique among known TeV sources

  • IceCube limit is first at PeV energies

100 101 102 103 104

Energy [TeV]

10−21 10−20 10−19 10−18 10−17 10−16 10−15 10−14 10−13 10−12

dN/dE [cm−2s−1TeV−1]

IceCube Preliminary

HESS J1427-608 Best Fit Best Fit (Extrapolated) IceCube 5 year 90% Upper Limit

H.E.S.S. J1427-608

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arxiv:0712.1173

IceCube Preliminary

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SLIDE 12

Search for PeV Gamma Rays | TeVPA 2017 | 7 August 2017

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IceCube High Energy Starting Event (HESE) Neutrinos

  • Pure sample of neutrino

events with strong astrophysical evidence

  • Using 4-year sample,

where 11 total events have a reconstructed direction within 1σ of FOV

  • Event types:
  • Cascades: good energy resolution, poor angular resolution
  • Tracks: poor energy resolution, good angular resolution

IceCube Preliminary

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SLIDE 13

Search for PeV Gamma Rays | TeVPA 2017 | 7 August 2017

  • Cascades
  • Broad spans in declination require

convolution of detector acceptance - use template method

  • Spectral index fixed at 4-yr best fit
  • f HESE data: E-2.58
  • p-value > 49%
  • Tracks
  • Vertical location of track limits

right ascension only scrambling

  • For <5° zenith, scramble in right

ascension and declination

  • p-value > 71%

IceCube High Energy Starting Event (HESE) Neutrinos

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IceCube Preliminary IceCube Preliminary

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SLIDE 14

Search for PeV Gamma Rays | TeVPA 2017 | 7 August 2017

  • Signal PDF constructed from the

pion decay component of the Fermi-LAT diffuse emission model

  • Fermi template is multiplied with

detector acceptance and convolved with point spread function of reconstructed events

  • Unbinned maximum likelihood

analysis performed with the likelihood function described above

  • No significant evidence found to

exclude null hypothesis

Diffuse Galactic Plane Template Analysis

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Signal PDF

Signal PDF Signal subtracted background PDF generated from observed data

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SLIDE 15

Search for PeV Gamma Rays | TeVPA 2017 | 7 August 2017

  • Template analysis yields normalization A to

spectral energy distribution of the angular- integrated flux from the entire field of view

  • We compare with CASA-MIA, the previous

IceCube analysis, and models by comparing “scaled angular-integrated” fluxes:

Galactic Plane Upper Limit

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Scaled Angular-Integrated Flux

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SLIDE 16

Search for PeV Gamma Rays | TeVPA 2017 | 7 August 2017

  • No evidence for PeV gamma-ray sources was

found in 5 years of IceCube/IceTop data

  • Point Source Analyses

✦ All-sky scan ✦ Search for correlation with known TeV gamma-ray sources (H.E.S.S.) ✦ Search for correlations with high-energy IceCube neutrinos

  • Search for excess of PeV gamma rays from the Galactic

plane

  • Limits set most stringent yet to PeV gamma-rays in

southern hemisphere

Conclusion

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SLIDE 17

Thank you!

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SLIDE 18

Backup Slides

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SLIDE 19

Search for PeV Gamma Rays | TeVPA 2017 | 7 August 2017

  • IC40 Analysis:
  • IceCube used as veto
  • No point sources were found
  • Upper limit on the diffuse gamma-

ray flux from the Galactic Plane in the energy range 1.2 - 6.0 PeV

  • Current Analysis Details:
  • 5 years of IC86 data
  • Energy range of ~1 PeV - 100 PeV
  • Random forest for event selection
  • Cleaned and optimized In-Ice

charge

  • Composition sensitive IceTop LLH

ratio

Comparison to Prior Analysis

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IC40 Upper Limit

arxiv:1309.4077v3

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SLIDE 20

Search for PeV Gamma Rays | TeVPA 2017 | 7 August 2017

SLCs

  • The IC-40 analysis required 0 HLC

hits in IceCube.

  • This analysis uses the total charge

in IceCube as a separating feature, a sum of:

IceCube Charge

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✦ Charge in HLC hits

  • Keep pulses with 3.5 𝜈s < tpulse-tITtrigger < 11.5 𝜈s for events with no IceCube

trigger

  • Clean with SeededRT for events with IceCube trigger

✦ Charge in SLC hits

  • within 130 meters of reconstructed track
  • Top 16 DOMs
  • 1.8 µs time window
  • Starting Time = (4.8 µs + depth/c)/cos(Zenith)
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SLIDE 21

Search for PeV Gamma Rays | TeVPA 2017 | 7 August 2017

  • Implemented using scikit-learn
  • Data used for background
  • 10% of each year dedicated as burn sample for training
  • Gamma simulation used for signal
  • 80% used for training, remaining 20% kept for final sensitivity
  • 5-fold cross-validation used for hyper-parameter optimization
  • Overtraining tested with KS test and validation curves of training

and testing samples

Random Forest Classifier

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Parameter Setting min_samples_split 2 min_samples_leaf 1 min_weight_fraction_leaf max_leaf_nodes None max_depth 8 n_estimators 1000