宇宙ニュートリノ望遠鏡
IceCubeによる新しいアラート
マルチプレットチャンネルの開発

清水 信宏 (千葉大)for the IceCube collaboration
JPS2023 Autumn (Sep. 18th)

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Picture: Y. Makino

IceCube experiment
Neutrino telescope operated in Antarctica

Digital Optical
Module (DOM)・5160 optical modules are deployed in ice 1 ㎦・Cherenkov light from charged particles produced byneutrino interaction is detected by DOM

PMT

JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)2

High energy cosmic 𝜈 BG
Our universe is filled with high energy 𝝂 !
IceCube collaboration
PRL. 124, 051103 (2020)“Diffuse” flux→ The origin of 𝜈 is not specified= integrated whole the direction
𝟏𝟎−𝟖Energy flux
𝐸 2 𝜙𝜈 ∼ 𝟏𝟎−𝟖𝟏𝟎𝟎 𝐓𝐞𝐕

𝐸100 TeV[GeV⋅ cm−2 ⋅ s −1 ⋅ sr −1 ]IceCube collaboration
APJ. 928 50 (2022)
JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)3

Neutrinos flux characterization𝝓𝝂 ∝ ℰ𝜈 × 𝜌𝜈(+ evolution)

𝜈-flux
Rate density [Mpc −3 ⋅ yr −1 ]𝜈 emission energy [erg]Scenarios of the origin of diffuse neutrinos
Bright (high ℰ𝜈 ) & Rare (small 𝜌𝜈 )Dim (small ℰ𝜈 ) & Generous (high 𝜌𝜈 )JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)

4Diffuse energy flux on (ℰ𝜈 , 𝜌𝜈 ) plane𝐸 2 𝜙𝜈 ∼ 10−8 (100 TeV)

[Condition]
Consistency with the diffuse flux
Evolution:
star formation[GeV⋅ cm−2 ⋅ s −1 ⋅ sr −1]

𝑬𝟐𝟏𝟎𝟎 TeV 𝝓𝝂𝝁+ത𝝂𝝁

Inconsistent(too large)

Inconsistent(too small)ℰ𝜈 [erg]

JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)5Diffuse energy flux on (ℰ𝜈 , 𝜌𝜈 ) plane𝐸 2 𝜙𝜈 ∼ 10−8 (100 TeV)

[Source characterization][GeV⋅ cm−2 ⋅ s −1 ⋅ sr −1]

𝑬𝟐𝟏𝟎𝟎 TeV 𝝓𝝂𝝁+ത𝝂𝝁
Dim & Generous

Bright & Rare
Evolution:star formationℰ𝜈 [erg]

JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)6𝜈-multiplet as a probe of bright sources
What is multiplet?
𝑵 ≥ 𝟐 coincident 𝜈-signals
in Δ𝑇 from the same direction
In terms of rate𝝓𝝂 = 𝑹signlet ∝ ℰ𝜈

𝝂𝝂

× 𝜌𝜈

𝑹doublet ∝ ℰ𝜈 𝟐 × 𝜌𝜈𝑹triplet ∝ ℰ𝜈 𝟑 × 𝜌𝜈Multiplets are sensitive to
Bright (high ℰ𝜈 ) & Rare (small 𝜌𝜈 ) sources
JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)

7Expected number of 𝜈-Multiplet signal
𝜙 ∝ 𝐸 −2.3

Evolution:star formation

Bright & rare

Δ𝑇: 30 daysℰ𝜈 [erg]
This is a pure number of detections.
In reality, we need to further select multiplets against backgrounds.
JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)

8Multiplet timing window Set Δ𝑇 = 30 days.

→ Various sources can have time scale of month!

Blazar flare
TDE
SN + circumstellar material

Open sensitivity
to various sources.

Low-L GRB
High-L GRB

minhourdaymonth year

𝜈 emission
time scale Technical challenge⚫ Long Δ𝑇 → large # of backgrounds.
⚫ Previous multiplet study was 100 sec→ Accommodate 30 days with an improved method of BG rejection
JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)

9Selection of multiplet signal• Major background source → atmospheric neutrinos• Focus only on doublets and triplets• Construct a test statistic 𝜦 from signal and background likelihoods: 𝓛𝒔𝒊𝒈 , 𝓛𝒃𝒈[Doublet case]

Energy PDF

Rate

Spatial PDF

𝟐doubletℒ sig= 𝑅sig 𝑛𝑜𝑏𝑠

1 𝑑𝑁sig 𝐸𝑜𝑏𝑠
1 𝑑𝑁sig 𝑛𝑜𝑏𝑠 ; 𝑛𝑠𝑖𝑔ෑ⋅𝑁sig𝑑𝐸𝑁sig𝑑Ω
𝑖=1𝟐

doubletℒ bg= 𝑅bg 𝑛𝑜𝑏𝑠

1 𝑑𝑁bg 𝐸𝑜𝑏𝑠
1 𝑑𝑁bg 𝑛𝑜𝑏𝑠
ෑ⋅𝑁bg
𝑑𝐸𝑁bg𝑑Ω𝑖=1

Λdoublet = 2log

doubletℒsig

In the case ofDoubleselectiondoubletℒbg

Select most signal-like 𝜈 combinations using 𝜦
JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)

10Search for multiplets in 12 years data
Performed a multiplet search with the new method.

Item
DatasetDuration
Livetime
Δ𝑇

ValuesNorthern track* (DEC > −5∘ )2011-May to 2022-Dec.
11.4 years
30 daysdoublet𝑁expected

1.2 × 107

triplet𝑁expected

2.8 × 104

*Common dataset used for realtime alert→ this method will be used also for multiplet alert
JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)

11Test statistic 𝜦 distribution
Doubletsignalatmospheric 𝝂doublet
𝑁expected= 1.2 × 107

𝜙 ∝ 𝐸 −2.3

Assumedsignalℰ𝜈 [erg]

More signal like
JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)

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Result (12 years data)
DoubletJPS 2023 Autumn Nobu Shimizu (Chiba Univ.)

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Result (12 years data)
TripletJPS 2023 Autumn Nobu Shimizu (Chiba Univ.)14

Is this consistent with background?
The observed test statistic 𝜦 is converted to false alarm rate (FAR) [1/year].

FAR (𝜦) → expected rate to observe higher 𝜦 only from backgrounds.

1/11.4 yr (total livetime)

As a whole dataset, the observation was consistent with BG-only hypothesis.

Global p-value = 0.14 (1.1σ)
JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)15

What can we interpret?
Using the largest 𝜦, we scanned the consistent region of (ℰ𝜈 , 𝜌𝜈 ).
Assumed several spectral indices 𝝓 ∝ 𝑬−𝜸
Constraint at 90% C.L.
Δ𝑇 = 30 days

ExcludedEg., for 𝜸 = −𝟐 case,At ℰ𝜈 = 1052 [erg]𝜌𝜈 > 3 × 10−8 [Mpc −3 ⋅ yr −1 ]is excluded at 90% C.L.

ℰ𝜈 [erg]
JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)16

What do the rare multiplets look like?

Nentries/bin

1/11.4 yr(total livetime)Though global p-value=0.14 was 1.1σ, significant multiplets are still interesting.
With the criteria of FAR<1/11.4 [yr −1 ], two multiplets were found.
JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)

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The most significant multiplet
Type: Doublet, (RA, DEC)=(110.05 deg, 11.05 deg)
Energy: E=(6 PeV, 6 TeV), Δ𝑇 = 5.8 days,local p-value=3.2 × 10−7 , FAR= 1/29 [1/yr]

2014 June 11th (04:54)①JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)

2014 June 5th (09:03)②18

2nd significant multiplet
Type: Triplet, (RA, DEC)=(0.58 deg, -0.35 deg)
Energy: E=(4 TeV, 30 TeV, 20 TeV), Δ𝑇 = 16.4 days,local p-value=7.4 × 10−7 , FAR= 1/13 [1/yr]
2020 June 27th (15:45)①

2020 June 11th (05:22)②

2020 June 15th (10:58)③
JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)

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Hunt the origin of the 𝜈-source

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Use multiplet signals to hunt bright & rare 𝜈-sources
Strategy: issue a public multiplet alert to other telescopes

IceCube𝝂
Subaru

LIGO

TDE

KAGRA

JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)

CTAKanataWhy multiplet alert?

𝝓 ∝ 𝑬−𝟐𝒛 distribution of multiplet from𝝂-source and supernovae➢ multiplet signal gives bias on close sources (𝒛 < 𝟎. 𝟏)① close sources are easy to observe with small telescopes② discriminate irrelevant transients such as SNe.
➢ Higher angular resolution than the usual singlet signal (∼ 1∘ )Δ𝜓 ~ 0.3∘ at 90% containment → useful for optical telescope
JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)21

Summary and plan of the alert operation
 Multiplet signal is a probe of bright & rare 𝝂-sources 12 years archival data was analyzed⚫ As a dataset, the observed multiplets were consistent with BG.

⚫ Two observed multiplets satisfied FAR<1/livetime.
⚫ Applied constraint on the 𝜈-source parameter Intensively working towards the operation of new multiplet alert!
⚫ Aiming for the start of the operation early in next year
We are happy about your cooperation of future follow-up!
JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)22

Backup

JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)

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Diffuse energy flux on (ℰ𝜈 , 𝜌𝜈 ) plane[Source candidates?]

𝐸 2 𝜙𝜈 ∼ 10−8 (100 TeV)
[GeV⋅ cm−2 ⋅ s −1 ⋅ sr −1]

𝑬𝟐𝟏𝟎𝟎 TeV 𝝓𝝂𝝁+ത𝝂𝝁

SNeLow Lum.
GRB
High Lum.
GRB
TDE

Evolution:
star formationℰ𝜈 [erg]

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JPS 2023 Autumn Nobu Shimizu (ChibaUniv.)

Doublet and triplet in skymap

Equatorial coordinate

Galactic coordinate

JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)25

Distribution of energy and declination
Observed doublet
Observed triplet

Astrophysical neutrino

Atmospheric neutrino

JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)

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Binomial tests
𝑁

Binomial p-value

𝑃 𝑘 = ෍𝑚=𝑘𝑁
𝑝𝑘𝑚 1 − 𝑝𝑘 𝑁−𝑚𝑚

Global p-value = 0.14

JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)27

TS distribution••••

𝑑𝑁/𝑑𝐸 ∝ 𝐸 −𝛾
sig𝑁det = 𝟐𝑇max = 30 days
Timing distribution of signal → uniform in 𝑇max

All-skyΛall−skyThisworkΛdoublet

BG
𝛾 = 2.0
𝛾 = 2.3
𝛾 = 2.728*the lowest bin is underflow(for triplet, no associationof three tracks)

Thiswork
Λtriplet

The new proposed method shows higher separation capability between BG andsig
signal than the all-sky method (for such small 𝑵det ).
JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)Detection Efficiency of signal
Efficiency: fraction of successive trials which exceeds Λ-threshold at FAR=1/year.

1 trial : addition of 𝑁𝑑𝑒𝑡 signal events into 30 days BG events
To unite both Λdoublet and Λtriplet , we define TS-score:
𝑇𝑆 score = max −log10 FARdoublet Λdoublet , −log10 FARtriplet Λtriplet

passprob (𝑁𝑑𝑒𝑡 ) =For small number of 𝑁det ,the new method shows O(100) higher efficiency to signal.
JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)

𝑁trial𝑁trial29

Constraint of 𝜈-source parameter
If we do not have any 𝜈-source (BG-only scenario)

𝐸 2 𝜙diff = 10−7 [GeV ⋅ cm−2 ⋅ s −1 ⋅ sr −1 ]𝐸 2 𝜙diff = 10−9 [GeV ⋅ cm−2 ⋅ s −1 ⋅ sr −1 ]𝛾 = 2.3
𝛾 = 2.7𝛾 = 1.5
𝛾 = 2.0

p-value = 10%

JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)

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Information to be sent in the alert Basic: time, RA, DEC, FAR, angular uncertainty
 Doublet/Triplet
 z-distribution, allowed region in (ℰ𝜈 , 𝜌𝜈 ) plane Matching information with a galaxy catalog (GLADE+)

JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)

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Discovery potential
This becomes more apparent if we relax the requirement of signal efficiency.

𝑑𝑁∝ 𝐸 −2.0𝑑𝐸𝑑𝑁∝ 𝐸 −2.0𝑑𝐸relaxed
5𝜎 discovery potential:Signal strength: 5σ of BG-only hypothesis
Signal efficiency 𝟎. 𝟎𝟓
Signal efficiency 𝟎. 𝟓

Our proposed method is more optimized to “tasty” events, and shows further
high performance when the requirement on the signal efficiency is loosened.
JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)

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Fermi catalog
0.28∘

 ID=1FBQS J0001-0011

JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)

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JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)

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JPS 2023 Autumn Nobu Shimizu (Chiba Univ.)

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