地上ガンマ線望遠鏡で挑む
ガンマ線バーストのマルチメッセンジャー観測
寺内 健太

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Gamma-ray Burst (GRB)• Extremely energetic emission from relativistic jet• Isotropic gamma-ray energy: typically, Eiso > 1052 erg• Prompt: series of short pulses• Afterglow: power-law decay with duration of days to weeks
PromptAfterglow

PromptMAGIC Collaboration (2019)

Afterglow

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Multi-messenger Observation
This talk’s content
CTA North

LST-1

MAGIC

Upcoming next generation instrument

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Origin of Neutrino Diffuse Flux
Low-Luminosity GRB (LLGRB): GRBs with isotropic energy Eiso < 1050 erg
IceCube-Gen2 Collaboration (2020a)

IceCube-Gen2 measurement can distinguish whether
LLGRB is the origin of IceCube diffuse neutrino flux4

Insights from VHE Observation• Inverse Compton radiation gives us info on:• Energy release in VHE range that we have overlooked• (Amplified) Magnetic field• Together with multi-wavelength data, one can obtain more accurate
kinetic jet energy of initial afterglow phase Ek• Combined with prompt energy release, one can derive more accurate
radiation efficiency of prompt emission• Essential to investigate the prompt emission mechanism• Relates to total proton energy after internal shock dissipation5

Less Luminous VHE GRBs
So far ground-based telescopes have (marginally) detected
two faint GRBs (on the boundary between GRB and LLGRB)★GRB190829A• Eiso = 1.8 × 1050 erg• Redshift z = 0.078• H.E.S.S. detected VHE emission• Very low radiation efficiency ofprompt emission (0.12 %)D. Xu et al. ApJ 776 98 (2013)cf.) Salafia et al. (2022)★GRB201015A• Eiso = 1.1 × 1050 erg• Redshift z = 0.426• MAGIC observed and reported a hintof signal• MAGIC paper in prep.
(K. Terauchi)★ GRB180720B (z = 0.65)★ GRB201216C (z = 1.1)
★ GRB190829A (z = 0.078)★ GRB190114C (z = 0.42)★ GRB221009A (z = 0.15)★ GRB160821B (z = 0.16; short)6Less Luminous VHE GRBs
So far ground-based telescopes have (marginally) detected
two faint GRBs (on the boundary between GRB and LLGRB)★GRB190829A• Eiso = 1.8 × 1050 erg• Redshift z = 0.078• H.E.S.S. detected VHE emission• Very low radiation efficiency ofprompt emission (0.12 %)D. Xu et al. ApJ 776 98 (2013)cf.) Salafia et al. (2022)★GRB201015A• Eiso = 1.1 × 1050 erg• Redshift z = 0.426• MAGIC observed and reported a hintof signal• MAGIC paper in prep.
(K. Terauchi)
Further VHE observation will reveal the properties of LLGRB
which are still largely unknown

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Future Prospect: CTA North/LST• >104 times better integral sensitivity than Fermi-LATin few tens of GeV• Suitable for GRB follow-up• Best sensitivity in VHE range
CTA North• Suitable for observing LLGRB which
faint signal is typically expected• Low energy (tens of GeV) sensitivity
is essential to avoid EBL absorption
LST-1

LST-1

MAGIC

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Alert Follow-up Strategy: Tiling Observation• Position error of Fermi-GBM and gravitational wave (GW) alerts
are often large• 5 - 15 deg (GBM position notice), 3.2 – 32 deg (GW, O3)• Some approach is necessary to increase the number of GRB
detection• “Tiling” observation is one of the ways to tackle this problem• Real time analysis is essential for the alert follow-up• Currently under development

Tiling observation:
Observe in a tiled way

2nd

3rd

4th

1st

5th

LST FOV(~ few deg)

7th

6th

Error circle(e.g., GBM, GW)

9New Detection Method• New method for calculating detection
significances• Use likelihood ratio test
• Take into account the temporal
information of gamma-ray events• Assume signal from a source isdecaying in power-law• Use a priori info of GRBs
(especially the ones detected in VHE)

K. Terauchi(JPS Meeting 2023 Spring)

Time-dep Li&Ma(new method)

Li&Ma(standard)• Will be implemented in real time analysis
in the future5.25 σ → 6.58 σ(Sensitivity improvement ofabout 25 %)10

Summary• Low Luminosity GRB (LLGRB) is a subclass of GRB with small isotropic
energy (Eiso < 1050 erg)
• LLGRB is a good target for multi-messenger astronomy• Future observation by IceCube (Gen2) will help us determine
whether LLGRB is the origin of neutrino diffuse flux
• Future VHE gamma-ray observation by CTA (especially LST) will
provide us info (e.g. radiation efficiency) on LLGRB• Several strategies for future VHE observation of (LL)GRBs• Tiling observation for the alerts with large position uncertainty• New technique for calculating detection significance of gamma-raysignal11