Diapositiva 1

1
P. Bernardini - Università and INFN, Lecce
(Italy) June 24, 2009 XXIèmes Rencontres de
Blois Windows on the universe
VHE g-astronomy observations with the ARGO-YBJ
detector
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• Outline
• Detector features and performance
• Cosmic rays and Moon shadow
• Gamma astronomy
• Search for Gamma Ray Bursts

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The YangBaJing Cosmic Ray Observatory (Tibet,
China) Altitude 4300 m a.s.l. Longitude
90 31 50 East Latitude 30 06 38 North
Astrophysical Radiation with Ground-based Observat
ory at YangBaJing
Tibet AS-?
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YangBaJing (Tibet, China) High Altitude Cosmic
Ray Observatory (4300 m a.s.l.) Longitude 90
31 50 East Latitude 30 06 38 North
Astrophysical Radiation with Ground-based Observat
ory at YangBaJing

• VHE g-Ray Astronomy
• Gamma Ray Bursts
• Cosmic Ray Physics

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YangBaJing (Tibet, Cina) Longitudine 90 31
50 East Latitudine 30 06 38 North
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12 RPC 1 cluster ( 5.7 7.6 m2 )
8 Strips 1 Pad (56 62 cm2)
99 m
74 m
10 Pads 1 RPC (2.80 1.25 m2)
78 m
111 m

• Layer of Resistive Plate Chambers (RPC)
• Active area central carpet ? 5600 m2
• sampling guard-ring ? 1000 m2
• Data taking since July 2006 with the
  central carpet
• since November 2007 with the guard-ring
• Installation of analog charge read-out in
  progress
• dynamical range up to 104 TeV

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Analog read-out
Read-out of the charge induced on Big
Pads (other side with respect to strip read-out)
The analog signal of half chamber is recorded
and digitised in order to avoid saturation up to
104 particles/m2 Dynamical range for protons by
means of pads, strips and big pads 1 - 104 TeV
3500 particles
35 m
31 m
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Operation modes Shower mode Trigger
number of fired pads (Npad) within 420 ns on
the central carpet for Npad ³ 20, rate
3.6 kHz ( 220 GBytes/day) Detection of
Extensive Air Showers (direction, size, core
) Aims cosmic-ray physics (threshold 1
TeV) VHE g-astronomy (threshold 300 GeV)
gamma-ray bursts Scaler mode counting rates
( ³ 1, ³ 2, ³ 3, ³ 4 coincidences) for each
cluster Aims detector and environment
monitor flaring phenomena ( gamma ray bursts,
solar flares) with a threshold of few GeV
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Operation modes Shower mode Trigger
number of fired pads (Npad) within 420 ns on
the central carpet for Npad ³ 20, rate
3.6 kHz ( 220 GBytes/day) Detection of
Extensive Air Showers (direction, size, core
) Aims cosmic-ray physics (threshold 1
TeV) VHE g-astronomy (threshold 300 GeV)
gamma-ray bursts Scaler mode counting rates
( ³ 1, ³ 2, ³ 3, ³ 4 coincidences) for each
cluster Aims detector and environment
monitor flaring phenomena ( gamma ray bursts,
solar flares) with a threshold of few GeV
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Shower mode Space pixel single strip ( 762
cm2) Time pixel pad ( 5662 cm2) is the
OR of 8 strips, with a resolution of 1.8 ns
The number of pixels, the time resolution and the
full coverage of the central carpet allow
to reconstruct the shower with unprecedented
details
The detailed shower topology is a possible
tool for g/h discrimination
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Cosmic rays
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High space/time granularity allows unprecedeted
studies on the EAS phenomenology (different
topologies and time structures)
Conical shape
Same trigger two showers
Conical shape in small shower
High energy
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Studies in progress on the shower time structure
shower curvature
shower thickness
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Flux attenuation and p-Air cross section
Measurement of the flux attenuation L for fixed
energies and shower ages
k is determined by simulations, selecting energy
and age ranges by means of the actual
experimental observables (number of fired strips,
hit density, lateral profile)
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Data selection a) shower size on the detector
(strip multiplicity Nstrip) b) core reconstructed
in a fiducial area (64 x 64 m2) c) constraints on
strip density ( gt 0.2 m-2 within R70 ) and
shower extension (R70 lt 30 m) Nstrip is used to
get subsamples with different energy

• Studied and taken into account
• variation of the atmospheric
• pressure
• contribution of heavy primaries

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Proton-Air cross section measurement
arXiv0904.4198
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Total p-p cross section (inferred by means of the
Glauber theory)
arXiv0904.4198

• No data from accelerators available at these
  energies
• The log2(s) asymptotic behaviour is favoured

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Measurement of attenuation length (L) and p-Air
cross section
arXiv0904.4198
submitted to Phys. Rev. D
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The shadow of the Moon
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The shadow of the Moon

• A deficit in the cosmic ray flux is expected
• from the Moon direction. Many items are
• connected
• angular resolution ( width of the deficit )
• pointing accuracy ( position of the deficit)
• energy calibration ( the westward
• deflection due to the geomagnetic field
• depends on the energy of cosmic rays )
• proton/antiproton ratio (antiprotons are
  deflected eastward)

Moon diameter 0.5
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2006-2008 data, with the cut NSTRIP gt 60 Until
November 2007 installation and debug
operations, low duty-cycle Since December 2007
stable data taking with high duty-cycle
ang. res.
The deficit surface is the convolution of the
Point Spread Function of the detector and the
widespread Moon disc
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Selected sample (1-year data) December 2007
December 2008 NSTRIP gt 40 .and. ? lt 50 130
x 109 analyzed events Observation time 1350
hrs Source visibility time 1500 hrs On-source
duty-cycle 90 Signal statistical
significance 32 s.d.
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Looking for an antiproton signal
Deficit on the South-North axis
Deficit on the East-West axis
A likelihood method is applied to estimate the
upper limit of the antiproton flux at 2 TeV
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Upper limit on the anti-proton flux looking for
an anti-shadow (East shift)
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Looking for an antiproton signal (East deficit)
Deficit on the East-West axis
A likelihood method is applied to estimate the
upper limit of the antiproton flux
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The Sun shadow
Exploring the solar magnetic field The deficit
significance and position are correlated with the
sunspots, even in a quiet period (2006-2008)
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g-astronomy
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• The technique of EAS detection allows a
  duty-cycle
• limited only by maintenance
• Duty-cycle 100
• The field of view is limited only by the
  atmosphere
• thickness. Requiring zenith angle lt 40
• Field of View 2 sr
• Continuous monitoring of the sky in the
  declination
• band
• -10lt ? lt70
• Presently ARGO-YBJ is the only wide-field-of-view
  
• g-telescope able to detect AGN TeV flares on a
  few days

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CR excesses in the sky map
? 0.06
Smoothing radius 5
? 0.1
Data collected in 424 days
NPAD gt 40 Proton median energy ? 2 TeV
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ARGO-YBJ
MILAGRO
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Sky map after correction
Mrk 421 (8 s)
Smoothing radius 1.3
Crab (7 s)
Data collected in 424 days
NPAD gt 40 Gamma median energy ? 0.6-2 TeV
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Excesses distribution
mean value (3.6 ? 0.8 ) 10-3 r m s (1.0 ?
5) 10-4
Crab Mrk 421
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Crab Nebula energy spectrum
dN/dE (3.73 ? 0.80) 10-11 E2.67?0.25 g cm2
s1 TeV1
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Mrk 421
X-ray data by ASM/RXTE
TEST DATA
ARGO-YBJ REGULAR DATA TAKING
July-August 2006
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Mrk 421 X-ray data by ASM/RXTE
TEST DATA
ARGO-YBJ REGULAR DATA TAKING
July-August 2006
2006
2007
2008
NPAD gt 40 q lt 40
Mrk 421 - July-August 2006 ARGO-YBJ test data
collected during the x-ray flare Observation
time ? 109 hours Flux ? 4 Crab units
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Mrk 421 - July-August 2006 ARGO-YBJ test data
collected during the x-ray flare
Excess distribution
NPAD gt 40 q lt 40
Mrk 421
Standard deviations
Observation time ? 109 hours Flux ? 4 Crab units
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Mrk 421 2008 activity
ARGO-YBJ NPAD gt 100
ASM/RXTE
g-x correlation coefficient r 0.64
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Mrk 421 spectrum days 41 180, 2008
Integral flux (Eg gt 1 TeV) (4.9?2.0) 10-11 g
cm2 s1 2 Crab units
Primack et al, AIP Conference Proceedings 745
(2005) 23
Spectrum EBL absorption dN/dE (7.5 ? 1.7)
10-11 E2.51?0.29 e-t(E) g cm2 s1 TeV1
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Mrk 421 - June 2008 Flare
June 5-7 3.0 s
June 11-13 4.2 s
ARGO-YBJ 3-day mean
ASM/RXTE 1-day mean
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The June 2008 flare observed from optical to TeV
energies (12 decades)-- optical R-band
(GASP-WEBT)-- UV band (UVOT)-- soft x-rays
(ASM/RXTE and SWIFT)-- hard x-rays (SWIFT and
AGILE)-- gamma rays (AGILE)-- VHE gamma-rays
(Veritas and Magic)
GASP-WEBT (optical R-band)
RXTE-ASM (soft x-rays) 2-12 keV
SuperAGILE 20-60 keV
SWIFT-BAT 15-50 keV
AGILE (gt 100 MeV) MAGIC VERITAS (gt
400 GeV)
No VHE Cerenkov data after June 8
Donnarumma et al. ApJ 691 (2009) L13
41
SEDs for June 2008 flares Donnarumma et al, ApJ
691 (2009) L13 the variability is due to the
hardening/softening of the electron energy
distribution, not to the increase/decrease of the
particle density
June 5-7 June 11-13
predicted flux
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Mrk 421 observed by ARGO-YBJ in continuous data
taking
June 11-13 significance 4.2 s flux ? 7 Crab units
VHE gamma predicted fluxes
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SEDs for June 2008 flares
ARGO-YBJ
June 5-7 June 11-13
ARGO-YBJ measurements in agreement with the model
by Donnarumma et al
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Excesses distribution for 135 sources (115
Fermi-LAT, 44 TeV sources)
86 extragalactic sources lt ns gt -0.02 ? 0.12
49 galactic sources lt ns gt 0.45 ? 0.14
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Search for Gamma Ray Burst
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• Scaler mode the counting rates ( ?1, ?2, ?3, ?4
  counts
• on each cluster) are measured each 0.5 s
• Search for sudden increases of the counting rate
  in coincidence
• with GRBs observed by satellites
• Energy range of the scaler mode search 1 - 100
  GeV
• December 2004 April 2009
• 66 GRBs in the ARGO-YBJ Field of View (q lt 40)
• 11 with known redshift
• 59 long duration GRBs (gt 2s)
• 7 short duration GRBs ( 2s)
• Look for coincidences

The larger sample of GRBs studied on ground in
the GeV energy region
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Fluence Upper Limits in the 1?100 GeV rangefor
GRBs with unknown redshift
99 c.l.
99 c.l.
Extrapolating the keV spectra measured by
satellites
Assuming 2.5 as spectral index
z1 is assumed to consider extragalactic
absorption (Kneiske et al. 2004)
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Fluence Upper Limits in the 1?100 GeV rangefor
GRBs with known redshift
99 c.l.
The spectral indexes are assumed in the range
from the value measured by satellites to 2.5
(only this latter case is considered for Cutoff
Power Law spectra)
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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 energy, with the
extrapolation of the fluence measured by
satellites
99 c.l.
The spectra of these GRBs do not extend beyond
Ecut (with the index measured by satellites)
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No signal detected (1-100 GeV) Fluence Upper
Limits of the order of 10-5 - 10-4 erg
cm-2 Upper Limits on the GRB cutoff
energy Intersection of the fluence upper limit
with the extrapolation of the fluence measured
by the satellites These limits are a
significant test for competing GRB models
extrapolation
upper limit
just published ApJ 699 (2009) 1281
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Conclusions

• ARGO-YBJ detector (central carpet guard ring)
  is taking data
• since November 2007 (duty-cycle gt 90)
• Studies on Cosmic Rays are going on (p-p cross
  section,
• anisotropies, shower profile, limit on
  antiproton flux )
• First results on g-astronomy (mainly 1-year
  data)
• - angular resolution as expected (Moon shadow)
• - limits on 1-100 GeV fluence from GRBs
• - Crab Nebula g-spectrum in agreement with
• other measurements
• - continuous monitor of Markarian 421, flares
  observed
• in 2006 and 2008, VHE g-flux correlated with
  x-emission
• Studies to increase the sensitivity are in
  progress
• (data quality, g-hadron separation)
• VHE g-sky survey is going on