Search for Astronomical Neutrino from Blazar TXS0506+056 in

Super-Kamiokande

Kaito Hagiwara (Okayama Univ.) For Super-Kamiokande Collaboration

ICRC2019 2019.Jul.27

Introduction

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A high-energy neutrino (~290 TeV) was detected by IceCube on 22 September 2017. It was coincident in direction and time with a γ-ray flare from the blazar TXS0506+056. IceCube also had observed neutrino events from that direction at 2015 without a γ-ray flare.

TXS 0506+056Distance ～1.75 Gpc (5.7×109 ly)

Right Ascension (R.A.) 77.3582°

Declination (Dec.) +5.69314°

350− 300− 250− 200− 150− 100− 50− 0R.A.

80−60−40−20−020406080

Dec.

R.A. = 77.36° Dec = 5.69°

350 300 250 200 150 100 50 0

http://science.sciencemag.org/content/361/6398/147.long

I would like to search the neutrino event came from that direction in

Super-Kamiokande

Super-Kamiokande (SK)

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41.4

m

39.3 m

50000 tons ofWater Cherenkov detector

Charged particle

Cherenkov light

~1km

~3km ~2km(2700 mwe)

Kamioka mine Japan

ID

OD

Phase Period Fiducial vol. # of PMTs

SK-I Apr.1996 - Jul.2001

22.5 kton

11146 (40%)

SK-II Oct.2002 - Oct.2005 5182 (20%)SK-III Jun.2006 - Sep.2008

11129 (40%)SK-IV Sep.2008 - May.2018

SK-V Feb.2019-

Dataset from Apr.1996 to Feb.2018 was used.

The Super-Kamiokande is a large water Cherenkov detector which is located at 1000 m underground in the Kamioka-mine, Japan. The SK detector constructed a cylindrical

tank has a 50 kilotonne pure water as neutrino target. The tank is separated into an inner detector volume and an outer detector volume.

Category of Neutrino Event

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The event that all daughter particles stop within the ID is categorized as fully-contained (FC). If any of the produced particles exit to the OD, it is categorized as partially-contained (PC). The energetic muon-neutrinos interacting with the

rock around the SK create muons which enter the ID through OD. Only the muons with upward momentum are categorized as upward-going muon (UPMU)

because down-going muon events of this kind could not be distinguished from cosmic muons.

FC PC UPMU

Astronomical Neutrino Search @ SK

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Since SK and IceCube have different energy ranges that can be observed, we search for neutrinos focusing on low energy (MeV-GeV) regions. If the Blazar emits neutrinos over the whole observation period of SK, the neutrino flux is higher than the other direction. If the Blazar emits a large amount of neutrinos in a short time (coincident with gamma-ray flare), we can see a time-dependent significant signal. If no significant signal is seen against the expected background, an upper limit can be determined.

Angular Resolution

!6Threshold Energy [MeV]

0 2000 4000 6000 8000 10000

[%]

Num

ber o

f Tot

al E

vent

sth

rθ ≤ θ

Num

ber o

f Eve

nt w

ith

0102030405060708090

100SK0 MCSK-All period

Neutrino Direction

Reco

nstru

cted

Direc

tion θ

From MC simulation, the angle between the true neutrino direction and reconstructed direction was calculated for point source search. As a results, 68% of all events are included in the search cone (10°

for FC and PC, 5° for UPMU) for energy cut.

FC, PC UPMUθthr : 10° 5°

Preliminary

1σ (68%) Level EnergyFC 5.1 GeVPC 1.8 GeV

UPMU 0 GeV

68%

Event RateThe event rate was

compared with at off-source region. The

errors for mean value of event rate without

Blazar direction are calculated by RMS.

The errors for Blazar direction are calculated

by statistics. No significant signal was

found.

Event Rate [/ Live Time Day]

Mean Value of Event Rate w/o

Blazar Direction

Blazar Direction

FC 0.0027±0.0006 0.0030±0.0007PC 0.0039±0.0006 0.0049±0.0009

UPMU 0.0025±0.0006 0.0034±0.0008

Right Ascension [degree]0 50 100 150 200 250 300 350

Cou

nts [

/with

in c

ircle

]0123456789

10-310× Event Rate for SK-0 (Dec.=5.693140)Event Rate for SK-0 (Dec.=5.693140)

TXS0506+056

Number of Events within search coneLive Time [day]

—-

: FC—-

: PC—-

: UPMU

Event Rate (SK-All)

Preliminary

MC and Experimental Data

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・ : FCx : PC+ : UPMU

The expected background was estimated by MC. The observed events inside a search

cone agree with expected background within 0.7σ for FC, 1.1σ for PC, and 1.2σ for

UPMU.

Declination [degree]-80-60-40-20 0 20 40 60 80

Num

ber o

f Eve

nts i

n Se

arch

Con

e

0

5

10

15

20

25

30

35FC

Declination [degree]-80-60-40-20 0 20 40 60 80

Num

ber o

f Eve

nts i

n Se

arch

Con

e

0

5

10

15

20

25

30

35PC

Declination [degree]-80-60-40-20 0 20 40 60 80

Num

ber o

f Eve

nts i

n Se

arch

Con

e

0

5

10

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35UPMU

Preliminary

90% Confidence LevelThe number of 90%

Confidence Level (C.L.) was determined from the

number of observed events against the

expected number of background events using a

Poisson distribution.

0 5 10 15 20 25 30 35 40 45 50λ

00.010.020.030.040.050.060.070.080.09

TMath::Poisson(20,x)

90%

NB

N90

10%

Background Events

(NB)

Observed Events

(N)

Events of 90% Confidence Level

(N90)

FC 15.2014 18 10.186

PC 22.9062 29 14.623

UPMU 14.5687 20 12.688

Declination [degree]-80-60-40-20 0 20 40 60 80

Num

ber o

f Eve

nts i

n Se

arch

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e

0

5

10

15

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25

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35FC

Declination [degree]-80-60-40-20 0 20 40 60 80

Num

ber o

f Eve

nts i

n Se

arch

Con

e

0

5

10

15

20

25

30

35PC

Declination [degree]-80-60-40-20 0 20 40 60 80

Num

ber o

f Eve

nts i

n Se

arch

Con

e

0

5

10

15

20

25

30

35UPMU

!9Prelim

inary

Preliminary

Search for Time-Dependent Excess by KS-test

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If there is a relationship between the gamma-ray flare of the blazar and emission of neutrino, the neutrino flux should increase depending

on time. The P-value of KS-test was calculated. The results were 61.08%, 41.17%, and 98.33% for FC, PC, and UPMU events. It can

not be said that it does not agree with the stable event rate (P-value

Summary

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The event rate focused on the blazar direction was constant with other direction. From MC data, the background events depended on declination was estimated. The experimental data agrees with the expected background within 0.7σ for FC, 1.1σ for PC, and 1.2σ for UPMU events. From the results of the Kolmogorov-Smirnov test, 
no significant temporal event excess was found from blazer direction.

A high-energy neutrino detected by IceCube was coincident in direction and time with a γ-ray flare from the blazar TXS0506.