Search for emission from Gamma Ray Bursts with the ARGOYBJ detector

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Search for emission from Gamma Ray Bursts with
the ARGO-YBJ detector
Tristano Di Girolamo
Universita Federico II and INFN, Napoli, Italy
for the ARGO-YBJ Collaboration
ECRS, September 9, 2008, Koice
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The ARGO-YBJ experiment

• Collaboration between
• Istituto Nazionale di Fisica Nucleare (INFN)
  Italy
• Chinese Academy of Science (CAS)
• Site YangBaJing Cosmic Ray Laboratory (Tibet,
  P.R. of China), 4300 m a.s.l.

Site Coordinates longitude 90 31 50 E,
latitude 30 06 38 N
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Physics Goals

• g-ray Astronomy search for Galactic and
  extragalactic point
• sources with a large field of view (2 sr) and
  a duty cycle
• ?100, at an energy threshold of a few
  hundreds of GeV
• Diffuse g-Rays
• from the Galactic plane and SuperNova Remnants
• Gamma Ray Burst (GRB) physics
• in the full GeV TeV energy range
• Cosmic Ray physics
• spectrum and composition up to ? 103 TeV
• anti-p / p ratio at energy ? TeV
• Sun and Heliosphere physics
• with an energy threshold ? 10 GeV

through the observation of Extensive Air Showers
(EASs) produced in the atmosphere by g-rays and
primary nuclei
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Detector layout
time resolution 1 ns space resolution strip
99 m
74 m
8 Strips (6.5 x 62 cm2) for each Pad
10 Pads (56 x 62 cm2) for each RPC
1 CLUSTER 12 RPCs
(5.7 ? 7.6 m2)
78 m
111 m
Single layer of Resistive Plate Chambers (RPCs)
with a full coverage (92 active surface) of a
large area (5600 m2) sampling guard ring (6700
m2 in total) 0.5 cm of lead converter
? detection of small showers (low energy
threshold)
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Experiment Hall
CLUSTER
RPC
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Data Acquisition
The detector carpet is connected to two different
DAQ systems, working independently
Shower Mode for each event the location and
timing of each detected particle is recorded,
allowing the reconstruction of the lateral
distribution and of the arrival direction (see
also poster by D. Martello with results in ?-ray
astronomy)
Scaler Mode the counting rate of each CLUSTER is
measured every 0.5 s, with very poor information
on both the space distribution and the arrival
direction of the detected particles
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Scaler Mode DAQ

• For each CLUSTER 4 scalers record every 0.5 s
  the rates
• of counts 1, 2, 3 and 4 in a time window of
  150 ns
• Measured rates are, respectively, 40 kHz, 2
  kHz,
• 300 Hz and 120 Hz
• Counting rates for a given multiplicity are then
  obtained
• with the relation ni n?i n?i1 (i1,2,3)
• The energy threshold for photons is about 1 GeV,
  lower
• than the highest energies detected by
  satellite experiments
• The modular structure of the detector allowed to
  collect
• data during the different mounting phases
• During data taking, the detector active area
  increased
• from 693 to 6628 m2

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Physics Goals in Scaler Mode

• GRBs search for the high energy tail and
  spectral cutoff
• Solar physics Forbush Decreases and Ground
  Level
• Enhancements in correlation with neutron
  monitors
• Correlation with changes of the Atmospheric
  Electric Field
• (thunderstorms)

I will focus on the search for emission from GRBs
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GRB High Energy Tails ?

• EGRET detected emission above 1 GeV from 3 GRBs,
  with photons up to
• 18 GeV (GRB940217, about 90 minutes after the
  burst start)
• Hints (3 ?) of emission at higher energies from
  GRB970417a (E gt 650 GeV,
• Milagrito) and GRB920925c (E gt 20 TeV, HEGRA
  AIROBICC)
• The Tibet Air Shower array found an indication
  of 10 TeV emission in a
• stacked analysis of 57 bursts
• Many theoretical models predict a significant
  high energy tail for GRBs
• High energy ?-rays are absorbed by the
  Extragalactic Background
• Light (EBL), whose amount however is not yet
  well known.
• The ?-ray extinction increases with the GRB
  redshift z.
• Recent obsevation of blazar 3C279 (z 0.536)
  with MAGIC at E gt 80 GeV !
• The GeV TeV energy range may reveal the
  spectral cutoff of GRBs,
• constraining emission models

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Search Triggered by Satellites

• Search started since the first GRB detection by
  Swift
• on December 17, 2004 (as a Follow-Up
  instrument)
• Selection of GRBs with zenith angle ? lt45
• Search for an excess in the scaler counts in
  coincidence
• with the satellite detection
• In case of no signal evidence, calculation of
  the fluence
• upper limit (at 99 confidence level)
• Search for a signal from stacked GRBs
• (both in phase and time)

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GRB Sample

• Total number of GRBs analyzed 39
• With known redshift 8
• Long duration GRBs (gt 2s) 34
• Short duration GRBs ( 2s) 5

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Distribution of the Significances
mean ? 0.26
N
The distribution of the statistical significances
is compared with a standard normal distribution
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Upper Limits in the 1?100 GeV range
Fluence upper limits obtained extrapolating the
keV power law spectra measured by satellites
Fluence upper limits obtained assuming a
differential index 2.5
Red triangles represent GRBs with known redshift,
for the others z 1 is assumed to consider
extragalactic absorption (Kneiske et al. 2004)
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Upper Limits to the Cutoff Energy
An upper limit on the GRB cutoff energy is given
by the intersection of the fluence upper limit,
as a function of the cutoff energy, with the
extrapolation of the fluence measured by
satellites
If the spectra of these GRBs extended up to Ecut
with the index measured by satellites, a 99 c.l.
signal would have been produced
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GRBs Stacked in Phase
The 33 GRBs with T90 ? 5 s (0.5 s ? 10 bins) have
been added up in phase scaling their total
duration, in order to search for a possible
cumulative high energy emission at a certain
phase of the low energy burst
The resulting significances for the 10 phase
bins show that there is no evidence of emission
at a certain phase
The overall significance of the GRBs stacked in
phase (adding up all the 10 bins) with respect to
background fluctuations is 0.36 ?
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GRBs Stacked in Time
The data of the first ?t seconds (?t 0.5, 1, 2,
5, 10, 20, 50, 100, 200) after T0 for all the
GRBs have been added up, in order to search for
a possible cumulative high energy emission with
a fixed duration after the low energy trigger T0
(the 9 bins are not independent)
The resulting significances for the 9 time bins
show that there is no evidence of emission for a
certain ?t
The overall significance of the GRBs stacked in
time (taking into account the dependency of the 9
bins) with respect to background fluctuations is
0.27 ?
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Search for GRBs in Shower Mode

• 20 GRBs analyzed between July 2006 and July 2008
  
• No significant excess found for any of them
• 99 c.l. upper limits to the fluence determined
  assuming
• a power law spectrum with differential index
  2.0
• Values down to ?10-5 erg/cm2 in the 10 GeV?1 TeV
  range
• More details about this analysis in Chen et al.
• (Proc. Nanjing GRB Conference, June 2008)

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Conclusions

• Until now, the search for emission from GRBs
  with
• the ARGO-YBJ detector has given no positive
  result
• The simple scaler mode has shown a good
• sensitivity, with fluence upper limits down
  to
• ?10-5 erg/cm2 in the 1?100 GeV energy range
• The directional capability of the shower mode at
  
• energies of a few hundreds of GeV allows to
  study
• GRBs in the whole 1 GeV ? 1 TeV energy range