SMILE-2+: Balloon observation of electron-positron annihilation line

gamma-ray in the galactic center region

T. Takemura1, T. Tanimori1, A. Takada1, Y. Mizumura1, K. Yoshikawa1, Y. Nakamura1,

M. Abe1, K. Onozaka1, K. Saito1, S. Komura1, T. Kishimoto1, T. Taniguchi1, H. Kubo1,

S. Kurosawa2, K. Miuchi3, K. Hamaguchi4, T. Sawano5, M. Kozai6

1 Kyoto University, 2 Tohoku University, 3 Kobe University,

4 University of Maryland, 5 Kanazawa University, 6 ISAS/JAXA

MeV gamma-ray Astronomy

SN2014J

Sky image @1.8 MeV (26Al)

Sky image @511 keV

847 keV4.7σ

1238 keV4.3σ

30°

30°

Problem of MeV gamma-ray telescopes

COMPTEL

Event circles

INTEGRAL/SPI

Statically imaging

COMPTEL and SPI cannot completely reject huge background created by interaction of cosmic-ray and a satellite bodyT.Siegert (MPE,Garching)

On orbit around the earth

TOF spectrum von Balloos+, SPIE (2014)Ground calibration

COMPTEL

INTEGRAL/SPI

Energy spectrum (SN2014J)

sky

How MeV gamma-ray telescope defeats BG

+ ele

ctron

-trackin

g

Sig: signal BG: background Aeff: effective area θ: Point Spread Function (PSF) (50% included)

If BG dominated

Significance

∝𝑨𝒆𝒇𝒇 ∗ 𝑺𝒊𝒈

𝜽 𝑨𝒆𝒇𝒇 ∗ 𝑩𝑮

ARM: accuracy of the scattering angle

ARM is used as 𝜽 on Compton camera

𝜽 is not ARM but 2D PSF

e-γ’

γ

Measured by SMILE ETCC

with e- tracking

Measured by SMILE ETCC

without e- tracking

γ’

γ

Conventional Compton Camera

Electron-Tracking Compton Camera

deposited energy

Electron-Tracking Compton Camera (ETCC)

Reconstruct Compton scattering event by event completely

=>Well-defined 2D PSF

➢Gaseous Time Projection Chamber (TPC)

Tracker: 3-D track and energy of electrons

2D imaging (x,y) + Drift Time (z) =>3D

➢Pixel Scintillator Arrays

Absorber: absorption position and

energy of scattered gamma-ray

➢Back ground rejection◆Two-Dimensional PSF

◆Particle identify (PID)

with dE/dX

◆Compton Kinematical test with a

➢Without heavy VETO➢Large FOV (~ 3 str)

= track length

dE/dX map

GSO:Ce

Bottom 2 R.L.

Side 1 R.L.

Sub-MeV/MeV gamma-ray Imaging Loaded-on-balloon Experiment

➢We obtained diffuse cosmic/atmospheric gamma-ray spectra

➢Success in background reduction with PID

SMILE

All sky survey with an ETCC loaded on a satellite

SMILE-3

SMILE-2+ 1-day flight @ Alice Springs (Apr. 7th 2018)

SMILE-I @ Sanriku (Sep. 1st 2006)

➢Certification of imaging spectroscopy in MeV Astrophysics

➢observation of bright objects (Crab & Galactic Center)

Effective area a few cm2

PSF (50% included) ~ 10 deg @ 662 keV

➢Scientific observation loaded on a long duration balloon

with several times better sensitivities of COMPTEL

SMILE-2: Only ground test

A. Takada+. ApJ,2011

T. Tanimori+. ApJ,2015

Requirement

(detect 5s )

~ sub-mCrab sensitivity

radius

Tracks of charged particles in flight

SMILE-2+Aim : certificate imaging spectroscopy of ETCC using celestial objects

Targets : e± annihilation line from the galactic center region / Crab nebula

©google

Flight duration ~29 hourslevel flight ~26 hours

SMILE-2+ balloon, Flight Model, ETCC

gondora

Analysis method of ETCC Fully-contained e- event

100 keV – 1 MeVEscaped e- event1 MeV – 10 MeV

Escaped e- event is especially important for several MeV gamma rays Analysis is on going !!

In this presentation, we show fully-contained e- events

Th energy spectrum

1

10−1

10−2

100 1000 keV

[cm2]Effective area

Th imaging ~ 10 kBq

SMILE-2+Ground Calibration Welding rods

0 1000 2000

2x10-4

1x10-3

keV

6x10-4

2.6 MeV

Time variation of detection rate of gamma rays

Event selection Spectra of flight observation104

103

102

0 400 800 1200Energy [keV]

Level flight 4/7 8:45- 4/8 4:05 Livetime 5.8 x 104 sec Raw 1.7 x 107 events

Kinematic test

Enhancement looks consistent with the observation ratio of the cosmic diffuse gamma-ray and galactic diffuse gamma-ray of about 3:1

mostly diffuse cosmic gamma-rays due to the consistency to its expected rate and following reason

TPC discharged

4/7 9:00-13:00 (ACST)

OFF time

Galactic coordinate

Excess gamma rays from Galactic Center

An excess of ~ 511 keV & diffuse components in G.C. is observed

gam

ma-r

ay

candid

ate

rate

[events

/sec] OFF Time ON-A ON-B ON-C

Excess : 11.5 σ

・COSI 2016 balloon experiment with telescope in this energy band

∼1.4×10-3 ph/cm2/sec9σ (the same energy range)

Quick check :10−3 - 10−2 events/cm²/sec3σ-5σ @ 460 – 560 keV

・SMILE-2+

Comparison with COSI

Assuming the ETCC has the same observation time of COSI:

3σ-5σ ×1.5 × 106 sec

6.4 ×103sec= 45σ - 77σ

ETCC can improve the significance dramatically.

Carolyn Kierans(29th International Texas Symposium)

The analysis of escaped e- greatly improves the detection of gamma-rays > 1 MeV

∼1.5×106 sec

500 520480 keV

Flu

x(1

0-3

γ/s

/cm

2/k

eV)

0

0.1

0.2

0.3

SMILE-2+ COSI

Eff. Area (511 keV)

~1.5 cm2 ~10 cm2

△E/E 12 % 0.5 %

VETO nothing exit

Sig/BG ~ 1/1 ~ 1/60

Next Work26Al (1.809 MeV)

candidates

AGB stars

Wolf-Rayet stars

Type II supernova

Outflow of novae

SMILE-3 unravel source of 26Al !!

SMILE-3

Effective area > 10 cm2

@511 keV

Flight duration ~30 days

PSF ~5 deg @1 MeV

~3 deg @3 MeV

Sky image @1.8 MeV

Expected sky image with SMILE-3

Sensitivity

The sensitivity of SMILE-3 is 5 times better than that of COMPTEL

Summary• For the first time, we demonstrated the excellent ability of

proper imaging spectroscopy for MeV gamma-ray astronomy

• We succeeded in the balloon flight in Australia in April 2018.

- The level flight continued during 26 hours at altitude of 38.4-40.5 km.

- Observation time - Galactic Center Region ~8 hours

• Success of bright object detection in the Quick Check

- Galactic Center Region 5σ in 511 keV band

11σ in continuum component

• ETCC has well-defined PSF and the strong capability of background reduction

=> We successfully reduced almost all the backgrounds which

hampered the advance of MeV gamma-ray astronomy

• We are analyzing the SMILE-2+ data in detail.

• In the future, the observation of an ETCC satellite will reach the sensitivity of

sub-mCrab.