Search for GeV Gamma‐Ray Counterparts of Gravitational Wave Events with CALET/CAL

CALET/CALによる重⼒波
イベントのガンマ線対応
天体探索
森 正樹
⽴命館⼤学理⼯学部物理科学科

Masaki Mori for the CALET collaboration
マルチメッセンジャー天⽂学研究会2018, 千葉⼤学ハドロン宇宙国際研究センター、March 26‐27, 2018

Multimessenger astronomy 2018, Chiba University

CALET Collaboration

CALET System Overview Launched Aug.2015(CAL)(*) JEM stands for Japan Experiment Module

CALET‐CAL Detector
Fully active thick calorimeter (30X0) optimized for electron
spectrum measurements well into TeV region
448mmCharge Detector

CHD

plastic scintillator hodoscope,absolute charge measurement(including charge zero)

ImagingCalorimeter

IMC

SciFi + tungsten plate (3X0),
reconstruction of arrival direction
and initial shower development

Total Absorption
Calorimeter

1TeV electron shower isfully contained in TASCTASCPWO hodoscope (27X0),
energy measurements and particle
identification

CALET‐CAL Shower Imaging Capability (MC)positronelectronpion••

Proton rejection power of 105 can be achieved by taking advantage ofshower imaging capability of IMC and TASC
Angular resolution of ~0.2o for 10GeV gamma rays

Overview of CALET Trigger System
Y. Asaoka et al., Astroparticle Phys. 100, 29 (2018)

ISS Orbit and CALET Operations

Y. Asaoka et al., Astroparticle Phys. 100, 29 (2018)

CAL Limit Calculation Procedure
Gamma‐ray Event
Selection

Effective Area
Calculation

Exposure Calculation

Upper Limit
Calculation

Check Overlap with
LIGO Localization Map

Lower energy is important
Seff as a function of zenith
and azimuth

Seff(zen,azi) Tlive projected tosky event by event along theISS (CALET) orbit
If no gamma‐rays are observed

Validation ofSensitivity with
Observation of
Diffuse/Point SourcesGamma Ray Event Selection= Electron Selection Cut + Gamma‐ray ID Cut w/ Lower Energy Extension

gamma‐ray

100 GeV Event Examples
electron

Charge Z=0proton

Charge Z=1

Electromagnetic Shower

Hadron Shower

well contained, constant shower development

larger spread

Gamma Ray Event Selection= Electron Selection Cut + Gamma‐ray ID Cut w/ Lower Energy Extension
1. Geometry Condition‐ CHD‐Top to TASC
1st layer (2cm margin)

2. Pre selection‐ Offline trigger‐ Shower concentration‐ Shower starting point

3. Track quality cut‐ Track hits >2‐ matching w/ TASC
An example of gamma‐ray event candidate in flight data
(reconstructed primary energy ~5GeV)

4. Electromagnetic
shower selection‐ shower shape

5. Gamma‐ray ID‐ CHD/IMC‐veto(combination ofloose cuts)
6. FOV cut

Gamma Ray Event Selection= Electron Selection Cut + Gamma‐ray ID Cut w/ Lower Energy Extension
1. Geometry Condition‐ CHD‐Top to TASC
1st layer (2cm margin)

2. Pre selection‐ Offline trigger‐ Shower concentration‐ Shower starting point

3. Track quality cut‐ Track hits >2‐ matching w/ TASC

4. Electromagnetic
shower selection
To maximize the field of view (FOV), the requirements
on acceptance condition was loosened as much as
possible compared to electron analysis. However,penetration of CHD paddle by shower axis is required
to ensure charge zero selection.

‐ shower shape

5. Gamma‐ray ID‐ CHD/IMC‐veto(combination ofloose cuts)
6. FOV cut

Gamma Ray Event Selection= Electron Selection Cut + Gamma‐ray ID Cut w/ Lower Energy Extension
1. Geometry Condition“K‐cut”
FE : fractional energy deposit of TASC‐Y6 relative tototal TASC deposit
RE : second moment of lateral energy deposit
distribution relative to shower axis [cm]

‐ CHD‐Top to TASC
1st layer (2cm margin)

2. Pre selection‐ Offline trigger‐ Shower concentration‐ Shower starting point

3. Track quality cut‐ Track hits >2‐ matching w/ TASCcut

4. Electromagnetic
shower selection‐ shower shape

5. Gamma‐ray ID

O. Adriani et al., PRL 119, 181101 (2017) supplemental material‐ CHD/IMC‐veto(combination ofloose cuts)
6. FOV cut

Gamma Ray Event Selection= Electron Selection Cut + Gamma‐ray ID Cut w/ Lower Energy Extension
1. Geometry Condition‐ CHD‐Top to TASC
1st layer (2cm margin)

CHD hit Y

2. Pre selection
ElectronMCProtonMC

‐rayMC‐ Offline trigger‐ Shower concentration‐ Shower starting point

3. Track quality cut‐ Track hits >2‐ matching w/ TASC

reject Z 1 region

CHD hit X

4. Electromagnetic
shower selection‐ shower shape

5. Gamma‐ray ID‐ CHD/IMC‐veto(combination ofloose cuts)
6. FOV cut

Gamma Ray Event Selection= Electron Selection Cut + Gamma‐ray ID Cut w/ Lower Energy Extension
It was found that secondary gamma‐ray produced in ISS
structures are dominant source of background

1. Geometry Condition

LE Trigger

2. Pre selection‐ CHD‐Top to TASC
1st layer (2cm margin)‐ Offline trigger‐ Shower concentration‐ Shower starting point60o45o

3. Track quality cut‐ Track hits >2‐ matching w/ TASC

4. Electromagnetic
shower selection‐ shower shape
Gamma‐ray candidates
in CALET FOV

Fish‐eye view of CALET FOV

By removing Black parts, it is possible to reject majority ofsuch background. More sophisticated rejection method isunder development.

5. Gamma‐ray ID‐ CHD‐veto
6. FOV cut

Effective Area and Sensitivity

Preliminary
O. Adriani et al., in prep.

Effective area is estimated as a function of incident angle (dx/dz, dy/dz) and energy.
Maximum effective area is achieved at around 5 GeV, but lower energy is more
important for steep spectrum like E‐2.
dy/dz

3‐10GeV averagedx/dz

Mostly axially symmetric except
for FOV cut

Effective area as a function of energy. Four
representing zenith angle ranges are shown.
15

CALET Sky Map w/ LE‐ Trigger (E>1GeV)
While exposure is not
uniform, we haveclearly identified thegalactic plane andbright GeV sources.
151102‐180131

Equatorial coordinate

Galactic coordinate

Point Source Spectra: Sensitivity Validation
CALET Preliminary
N. Cannady et al., in prep.

The observed point source spectra are well consistent with
Fermi‐LAT’s parameterizations. Therefore, it was found thatcurrent selection criteria has a validated sensitivity and canbe used to set limit on GW counterpart flux.

N. Cannady et al., in prep.

Angular resolution
68% containment angle [deg]• Containment angles by Monte Carlo simulation
Np : number of points usedfor track reconstruction

yExample: Np=5, 1GeV

Np = 34657x18

CALET

Gravitational‐wave events by LIGO/VirgoGW Counterpart Search with CALET

Publishedin ApJ Letters

GW 151226 [B. P. Abbott et al., PRL 116 (2016) 241103]• GW trigger Time: 2015/12/26 3:38:53.647 UT‐ gravitational‐wave signal produced by the coalescenceof two stellar‐mass black holes at a luminosity distanceof ~440Mpc.

CALET Observation• CGBM HV‐on (3:20 – 3:40 UT)‐ No on‐board trigger• CAL: low‐energy gamma‐ray mode (> 1GeV) 3:30‐3:43UTAnalysis update

CALET Preliminary
N. Cannady et al., in prep.

• EM Track … developed and used extensively for electron analysis• CC Track … developed specifically for low energy gamma‐rays

90% CL Upper limit for GW151226
Preliminary
Counterpart Search
O. Adriani et al., in prep.
NO event remained after applying all the selection criteria.

T0: 1451101134 => CROSS
T0‐525 to T0+221 sec => CYAN

+180°

−180°

Contour:
GW localization
significance Map
1‐10 GeV

Background contamination is negligible in such a short time period.

22

90% CL Upper limit for GW151226
Preliminary
counterpart search
O. Adriani et al., in prep.

+180°

Contour:
GW localization
significance Map−180°

UL = 9.3x10‐8 erg cm‐2sec‐1@ 15%

1‐10 GeV

CALET observation constrains at least some portion of LIGO probability.

23

90% CL Upper limit for GW170104
Preliminary
counterpart search
O. Adriani et al., in prep.

Contour:
GW localization
significance Map+180°

UL = 6.4x10‐6 erg cm‐2sec‐1
@ 30%

−180°

10‐100 GeV

CALET observation constrains at least some portion of LIGO probability.

24

90% CL Upper limit for GW170608
Preliminary
counterpart search
O. Adriani et al., in prep.

Contour:
GW localization
significance Map
+180°

−180°

10‐100 GeV

CALET observation was out of localized region of LIGO event.

25

90% CL Upper limit for GW170814
Preliminary
counterpart search
O. Adriani et al., in prep.

+180°

Contour:
GW localization
significance Map−180°

10‐100 GeV

CALET observation was out of localized region of LIGO event.

26

90% CL Upper limit for GW170817
Preliminary
counterpart search
O. Adriani et al., in prep.

−180°+180°Contour:
GW localization
significance Map
10‐100 GeV

CALET observation was out of localized region of LIGO event.

27

CALET Sensitivity to GeV Gamma‐Rays
Short GRBs accompanied by GeV gamma‐ray emissions could be
detected by CALET‐CAL given the closeness of GW candidates.
GRB 09510
scaled toz=0.09

Assumed Spectrum:   E‐2 t ‐1

Preliminary:
O. Adriani etal., in prep.

Summary & Prospects

Preliminary
O. Adriani et al., in prep.

1.

CALET was successfully launched on Aug. 19, 2015, and the detector is being
very stable for observation since Oct. 13, 2015.

2.

As a result of GW151226 counterpart search in GeV gamma‐rays, CALET‐CALobservation constrains 15% of LIGO localization map by 90% upper limit fluxof 9.3x10‐8 erg cm‐2sec‐1 (1‐10GeV).

3.

GeV gamma‐ray counterpart of other GW events during O1&O2 have been
performed and limits are set if there are overlap between our FOV andLIGO/Virgo localization map.

4.

Its sensitivity was validated with diffuse and point‐source observations.

5.

Due to closeness of GW candidates, FOV coverage is more important thandeepness of counterpart search assuming on‐axis short GRBs as candidates.

6.

Automated pipeline to search for gamma‐ray transient was also developedand is being implemented.
Transient objects such as GW counterparts and GRBs, as well as flaring
point sources will be monitored.

This work is partially supported by JSPS Grant‐in‐Aid for Scientific Research (S)Number 26220708, JSPS Grant‐in‐Aid for Scientific Research (B) Number 17H02901,
JSPS Grant‐in‐Aid for Scientific Research (C) Number 16K05382.

https://fermi.gsfc.nasa.gov/

GeV sky• Agile• CALET/CAL• DAMPE (Wukong)