Observation of GRBs at tens of GeV
with a full-coverage air shower array
at 5000-6000 m elevation
Zhaoyang Feng (Speaker), Yiqing Guo, Yi Zhang, Hong Lu, Hongbo Hu
Institute of High Energy Physics, CAS, Beijing, China
Tianlu Chen
Tibet University, Lhasa, China
ICRC2011, Beijing, China
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Gamma-ray bursts:
the most violent explosions in the universe
Compact Star Merger
Massive Star Core Collapse
2
The GRB field
 Still a relatively young field, many open questions:
--Classification(how many physically distinct types?)
--Progenitors(massive stars vs. compact stars; others?)
--Central engine( black hole, magnetar, quark star?)
--Ejecta composition (baryonic vs.magnetic?)
--Energy dissipation mechanism (shock vs.magnetic reconnection)
--Particle acceleraton & radiaton mechanisms (synchrotron, inverse Compton,quasi-thermal)
--Afterglow physics (medium interaction vs. long-term engine activity)
 Due to their elusive feature, GRBs are still not fully
observationally uncovered in all the temporal and spectral
regimes
 Major advance is made whenever a new temporal or spectral
window is unveiled
---Bing Zhang
3
Why Study GRBs at Very High Energy (GeV-TeV)?

A new temporal window

Need to understand acceleration mechanisms , energetics, and therefore constrain the progenitors and
jet feeding mechanism

Constrain local environment characteristics: Doppler factor, seed populations, photon density, B field,
acceleration and cooling timescales, …

Understanding progenitor then leads to an understanding of cosmology & stellar evolution required to
support progenitor population

Extragalactic background light induced absorption (EBL absorption) of high energy photons

Potential ultra high energy cosmic ray sources

Limits on Lorentz Invariance Violation
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Gamma-Ray Telescopes
Pair Production Telescopes
EGRET/Fermi
Atmospheric Cherenkov
Telescopes
HESS/VERITAS/MAGIC
Particle Detection Arrays
Milagro/HAWC
0.1 - 100 GeV
Space-Based (small area)
Background free
Large Aperture/High Duty Cycle
50 GeV - 100 TeV
Large Area
Excellent background rejection
100 GeV - 100 TeV
Large Area
Good background rejection
Large Aperture & Duty Cycle
Small acceptance: difficult to
extend to high energies
Small FOV, small duty circle,
not fast slew speed
Low altitude, High threshold
5
Let’s go to 5000-6000 m altitude, with
 Lower energy threshold
 Better energy resolution
 Better angular resolution
Cosmological GAmma rays Observatory (CGAO)
Observation of GRBs at tens of GeV
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Candidate sites( please refer to next talk #1348）
•
•
A site survey team (7 physicists) investigating 7 candidate sites at June 2011.
Two excellent sites are found
 Pumajiangtang Township (~5100m): suitable for HAWC-like detector
 Sheka(4300-5400m): Weather is good for IACTs
YangBaJing
Sheka
Pumajiangta
ng Township
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HAWC-like CGAO @5100m(Pumajiangtang Township)
Preliminary configuration in MC study:
Instrumented Area:
150m*150m = 22,500m2
841 PMTs (29x29)
Single layer with 4m depth

5.0m spacing

Trigger Condition: Nhit > 10
Trigger ~42 kHz
4m

5m
Sincerely thank HAWC collaboration for allowing us to use HAWC simulation and
reconstruction code
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Effective Area
Very preliminary!
5100m CGAO:
~ 5-6x effective area VS 4100m
~ 100x effective area VS Fermi-LAT @40GeV
~ 1000x effective area VS Fermi-LAT @80GeV
9
Sensitivity as function of spectral index and cutoff
Very preliminary!
Simulated GRB:
T = 1 s
Zenith = 20 deg
Power law spectrum with energy
cutoff
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Sensitivity with different Z
Very preliminary!
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Another possibility: CGAO (IACTs) at Sheka
（4300m-5400m), A new 5@5 project ?
Traditional IACTs, Magic, HESS,…, but with very
fast slew speed: 5-10o/s?
or
GAW, Gamma Air Watch – a new generation of IACTs
with large field of view?
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In summary, we proposed a
“Cosmological GAmma rays Observatory ”
Observation of GRBs at tens of GeV at 50006000 m elevation in Tibet
* Nature of GRBs (central engine, radiation mechanisms, et al.)
* Ultra high energy cosmic ray sources
* Cosmology, EBL absorption
* Lorentz Invariance Violation
...
Also an excellent telescope for detection and study of
gamma ray sources
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Thank you!
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Fermi-LAT GRB catalog
•
•
•
•
•
•
•
PRELIMINARY
~30 GRB have been seen by LAT
above 100 MeV;
Both long (>2 sec) and short (<2
sec) bursts have been seen;
Some bursts are only visible in
LAT Low Energy events;
Most of the bursts show highenergy emission afterglow and
delayed high-energy onset;
Constraint: lower limit of bulk
Lorentz factor of the colliding
shells: ~1000;
Some bursts have an extra
spectral component (a different
mechanism at high energy?);
These short, distant and bright
flashes can be used as tools to
probe basic physics…
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GeV GRBs observation
 EGRET: detected 4 GeV photos. For GRB940217, 2
photons with energies 3 GeV, and one photon with energy
18 GeV 90 minutes later.
 Fermi-LAT: detected 32 GRBs in three years’ run, six of
which have gamma-rays with energies up to tens of GeV.
 Ground-based experiments: Tibet ASgamma experiment,
ARGO-YBJ ,Milagro,Pierre Auger Observatory, Wipple,
MAGIC, HESS, VERITAS without positive result.
only the prototype of Milagro (Milagrito) reported a possible
detection of signals associated with GRB970417 with 3
confidence level.
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Limits on Lorentz Invariance Violation

Some QG models violate Lorentz invariance: vph(Eph) ≠ c
2

n 






E ph
E ph
E ph  1 n E ph
2 2
2 


c pph  E ph 1


...
,
v


c
1




ph

 M QG,1c 2 M QG,2c 2 

pph
2 M QG,n c 2  




A high-energy photon Eh would arrive after (or possibly before
in some models) a low-energy photon El emitted together
 GRB080916C: highest energy photon (13 GeV) arrived 16.5 s
after low-energy photons started arriving (=the GRB trigger)
 a conservative lower limit: MQG,1 > (1.50±0.20)×1018 GeV/c2

Pulsar
GRB
(Kaaret 99) (Ellis 06)
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1015 1.8x10 0.9x10 1016
AGN
(Biller 98)
GRB
AGN
(Boggs 04) (Albert 08)
4x1016 1017 1.8x1017 0.2x1018
GRB080916C
1018
1.5x1018
Planck mass
min MQG
(GeV/c2)
1019 1.2x1019
(Jacob & Piran 2008)
n = 1,2 for linear and quadratic Lorentz invariance violation, respectively
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Moderate Angular and energy Resolution
Very preliminary!
Energy resolution is under studied
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