1. CALET-CALによる ガンマ線観測初期解析
早大理工研，早大先進理工A，CRESST/USRA/GSFCB, 神奈川大工C，JAXAD,青学大理工E, 名大ISEEF
浅岡陽一，鳥居祥二，小澤俊介A ，山口優幸A, 竹本翔一A, 赤池陽水B，清水雄輝C ，田村忠久C ， 中平聡志D, 吉田篤正E, 坂本貴紀E, 川久保雄太E, 山岡和貴F, 他CALETチーム
2016/9/21 秋の物理学会 @ 宮崎大学
21pSP-2

2. Objectives of CALET Gamma-Ray Observation
Transient Object
GRB
Gravitational wave
Other transients
Diffuse Component => High energy region
Galactic
Extragalactic
Point Source
Calibration of pointing accuracy
Confirmation of angular resolution
Cross check of energy measurements/efficiency

3. Objectives of CALET Gamma-Ray Observation
Transient Object
GRB
Gravitational wave
Other transients
Diffuse Component => High energy region
Galactic
Extragalactic
Point Source
Calibration of pointing accuracy
Confirmation of angular resolution
Cross check of energy measurements/efficiency
Statistics is the KEY

4. GW Counterpart Search with CALET
Accepted for publication in ApJ Letters
GW [B. P. Abbott et al., PRL 116 (2016) ]
GW trigger Time: 2015/12/26 3:38: UT
gravitational-wave signal produced by the coalescence of two stellar-mass black holes at a luminosity distance of ~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

5. GW Counterpart Search with CALET
Accepted for publication in ApJ Letters
CGBM Upper Limits:
(23aSP-1 by Kawakubo)
1.0x10-6 erg cm-2 s-1 33%
1.8x10-6 erg cm-2 s-1 49%
LaBr3(Ce)
BGO
Assuming the distance of 440 Mpc, the upper limit in luminosity is 3-5 x 1049 erg s-1 (typical mean luminosity of s-GRB is 1.6 x 1051 erg s-1).

6. CAL Limit Calculation Procedure
Gamma-ray Event Selection
Lower energy is important
Validation of Sensitivity with Observation of Diffuse Component
Effective Area and Sensitivity Calculation
Seff as a function of zenith and azimuth
Exposure Calculation
Seff(zen,azi) Tlive projected to sky event by event along CALET trajectory
Upper Limit Calculation
If no gamma-rays are observed
Check Overlap with LIGO Localization Map

7. Gamma Ray Event Selection
= Electron Selection Cut + Gamma-ray ID Cut w/ Lower Energy Extension
1. Geometry Condition
- CHD-Top to TASC Bottom (415cm2)
2. Pre selection
- Offline trigger
- Shower concentration
- Shower starting point
3. Track quality cut
- Track hits >2
- matching w/ TASC
4. Electromagnetic shower selection
- shower shape
5. Gamma-ray ID
- CHD-veto
For quick result, not fully optimized, yet.

8. Gamma Ray Event Selection
= Electron Selection Cut + Gamma-ray ID Cut w/ Lower Energy Extension
1. Geometry Condition
- CHD-Top to TASC Bottom (415cm2)
2. Pre selection
- Offline trigger
- Shower concentration
- Shower starting point
3. Track quality cut
- Track hits >2
- matching w/ TASC
4. Electromagnetic shower selection
- shower shape
5. Gamma-ray ID
- CHD-veto
ALL VETO
(E<8GeV)
⇒ reject

9. Effective Area and Sensitivity
Effective are is estimated as a function of incident angle (dx/dz, dy/dz) and energy.
Maximum effective area is achieved at around 10 GeV, but lower energy is more important for steep spectrum like E-2.
3-30 GeV
average
dy/dz
mostly axially symmetric
dx/dz
Effective area as a function of energy. Three representing zenith angle ranges are shown
Maximum sensitivity of 4x10-5 erg/cm2 per Δ(ln E) was achieved at around 1 GeV for E-2 spectrum

10. Diffuse Gamma-Ray Observation
Purpose: Sensitivity validation & BG estimation
Data set: from to (232 days)
Observation Mode: Low Energy Gamma-Ray Trigger
+180°
－180°
Black Point: Gamma-ray
event candidates
Color Map: Exposure
in GeV in cm2sec

11. Diffuse Gamma-Ray Observation
Purpose: Sensitivity validation & BG estimation
Data set: from to (232 days)
Observation Mode: Low Energy Gamma-Ray Trigger
+180°
－180°
Black Point: Gamma-ray
event candidates
exposure is limited in low latitude region, it covers a half of galactic plane and Geminga+Crab region

12. Diffuse Gamma-Ray Observation
Purpose: Sensitivity validation & BG estimation
Data set: from to (232 days)
Observation Mode: Low Energy Gamma-Ray Trigger
Geminga
+180°
－180°
cygnus region => exposure difference
Crab
Filtered with point spread functions to see point sources
Geminga and Crab are clearly identified.

13. Projection to Galactic Latitude (|l|<80deg)
And comparison with diffuse radiation model
BG is estimated from |b|>20deg
as 2.4 x 10-7 event/(cm2sec sr)
by assuming isotropic contamination of cosmic-rays in galactic coordinates.
Considering the contribution from point sources, it will be consistent with expectation.

14. Projection to Galactic Latitude (|l|<80deg)
And comparison with Fermi-LAT’s observation
Fermi-LAT Data:
2015/10/8-2016/6/02
(using weekly files)
Fermi Data: 2015/10/8-2016/6/02 (using weekly files)
0.1GeV--1000GeV を各decade 10bin (計40bin)
Considering the contribution from point sources, it was actually consistent with expectation. Therefore, it was found that current selection criteria has a validated sensitivity and can be used to set limit on GW counterpart flux.

15. Integrated Exposure for GW151226 counterpart search
Assuming E-2 spectrum, exposure was integrated over 1 to 100GeV.
T0: => RED
T0-525 to T0+221 sec => CYAN
+180°
－180°
1-100GeV

16. 90% CL Upper limit for GW151226 counterpart search
NO event remained after applying all the selection criteria.
T0: => RED
T0-525 to T0+221 sec => CYAN
+180°
－180°
1-100GeV
Background contamination is negligible: ~0.035 event

17. 90% CL Upper limit for GW151226 counterpart search
Contour:
GW localization significance Map
+180°
－180°
UL = 2x10-7 erg cm-2sec-1
@ 15%
1-100GeV
CALET observation constrains at least some portion of LIGO probability.

18. Summary & Conclusion
GW counterpart in GeV gamma-rays are searched with CAL in low-energy gamma-ray trigger mode and no gamma-ray event candidates were observed.
The sensitivity of CAL instrument was validated with diffuse gamma-ray observation. At the same time, background rate was estimated to be negligible for GW search.
Exposure and resultant flux upper limit are derived. There’s overlap with LIGO localization map and CALET observation constrains 15% of its region by 90% upper limit flux of 2x10-7 erg cm-2sec-1 .
Due to closeness of GW candidates, FOV coverage is more important than deepness of counterpart search assuming typical short GRBs as candidates.

19. 232days with High Energy Trigger
Prospects
Continues observation of transient objects
Analysis flow is established through the analysis of GW counterpart.
We’re ready for LIGO/Virgo O2 and KAGRA’s alert.
Detailed analysis of point source and diffuse components
optimizing analysis for each target
accumulating more and more statistics
Exposure = 2x108cm2sec
232days with High Energy Trigger
VERY PRELIMINARY

20. backup

21. [5] Resultant Flux Upper Limit for E-2 spectrum
The sky region containing 15% of LIGO probability is constrained by flux upper limit of ~2x10-7 erg cm-2sec-1 in 1-100GeV.
1-100GeV

22. s-GRB w/ known redshift
energy range: 1GeV~100GeV =>
limit worsen by 1 order of magnitude assuming E-2 spectrum

23. expectation from Fermi-LAT data