Search for emission from Gamma Ray Bursts with the ARGO-YBJ detector

Tristano Di Girolamo

Universita` “Federico II” and INFN, Napoli, Italy

ECRS, September 9, 2008, Košice

for the ARGO-YBJ Collaboration

The ARGO-YBJ experimentThe ARGO-YBJ experiment• Collaboration between: Collaboration between:

Istituto Nazionale di Fisica Nucleare (INFN) – ItalyIstituto Nazionale di Fisica Nucleare (INFN) – Italy Chinese Academy of Science (CAS)Chinese Academy of Science (CAS)

• Site:Site: YangBaJingYangBaJing Cosmic Ray Laboratory (Tibet, P.R. of China), 4300 m a.s.l.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

Physics Physics GGoalsoals-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 -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• proton-air cross section (talk by A. Surdo)

Sun and Heliosphere physics with an energy threshold 10 GeV

through the observation of Extensive Air Showers (EASs) produced in the atmosphere by -rays and primary nuclei

Detector layoutDetector layout

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

time resolution ~1 nsspace resolution = strip

10 Pads (56 x 62 cm2)for each RPC

8 Strips (6.5 x 62 cm2) for each Pad

1 CLUSTER = 12 RPCs

78 m

111 m

99 m

74 m

(5.7 7.6 m2)

detection of small showers (low energy threshold)

Experiment Hall

CLUSTER

RPC

RPC also with analog charge readout

Data AcquisitionData Acquisition

The detector carpet is connected to two different DAQ systems, working independently:

Shower Mode:for each event the locationand timing of each detectedparticle is recorded, allowingthe reconstruction of thelateral distribution and of the arrival direction(see also poster by D. Martello with results in -ray astronomy)

Scaler Mode:the counting rate of eachCLUSTER is measured every 0.5 s, with very poor information on both the space distribution and the arrival direction of the detected particles

Scaler Mode DAQScaler 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 = ni – ni+1 (i=1,2,3)• 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 the collection of data during the different mounting phases• During data taking, the detector active area increased from 693 to 6628 m2

Physics Goals in Scaler ModePhysics Goals in Scaler Mode

I will focus on the search for emission from GRBs

GRBs:GRBs: ssearchearch for the for the high energy tail and high energy tail and spectral spectral cutoffcutoff

Solar Solar physicphysics: Forbush Decreases and Gs: Forbush Decreases and Ground round LLevel evel

EEnhancementsnhancements in correlation with neutron monitorsin correlation with neutron monitors

Correlation with changes of the Atmospheric Electric FieldCorrelation with changes of the Atmospheric Electric Field

(thunderstorms)(thunderstorms)

GRB High Energy Tails ?GRB High Energy Tails ? EGRET detected emission above 1 GeV from 3 GRBs, with photons up to EGRET detected emission above 1 GeV from 3 GRBs, with photons up to

18 GeV (GRB940217, about 90 minutes after the burst start)18 GeV (GRB940217, about 90 minutes after the burst start)

Hints (~3 Hints (~3 ) of emission at higher energies from GRB970417a ) of emission at higher energies from GRB970417a (E > 650 (E > 650

GeV, GeV,

Milagrito) and GRB920925c (E > 20 TeV, HEGRA AIROBICC)Milagrito) and GRB920925c (E > 20 TeV, HEGRA AIROBICC)

The Tibet Air Shower array found an indication of 10 TeV emission in a The Tibet Air Shower array found an indication of 10 TeV emission in a

stacked analysis of 57 burstsstacked analysis of 57 bursts

Many theoretical models predict a significant high energy tail for GRBsMany theoretical models predict a significant high energy tail for GRBs

High energy High energy -rays are absorbed by the Extragalactic Background -rays are absorbed by the Extragalactic Background

Light Light (EBL), whose amount however is not yet well known. (EBL), whose amount however is not yet well known.

The The -ray extinction increases with the GRB redshift z.-ray extinction increases with the GRB redshift z.

Recent obsevation of blazar 3C279 (z = 0.536) with MAGIC at E > 80 GeV !Recent obsevation of blazar 3C279 (z = 0.536) with MAGIC at E > 80 GeV !

The GeV The GeV – TeV energy range may reveal the spectral cutoff of GRBs, – TeV energy range may reveal the spectral cutoff of GRBs,

constraining emission modelsconstraining emission models

Search Triggered by SatellitesSearch Triggered by Satellites

Search started sinceSearch started since the first GRB detection by the first GRB detection by SwiftSwift

on December 17, 2004 (as a Follow-Up instrument)on December 17, 2004 (as a Follow-Up instrument)

Selection of GRBs with zenith angle Selection of GRBs with zenith angle <45 <45°°

Search for aSearch for an n excess excess in the scaler counts in coincidence in the scaler counts in coincidence

with the satellite detection (T90)with the satellite detection (T90)

In case of no signal evidence, calculation of the fIn case of no signal evidence, calculation of the fluence luence

upper limit upper limit (at 99% confidence level)(at 99% confidence level)

Search for a signal from stacked GRBs Search for a signal from stacked GRBs

(both in phase and time) (both in phase and time)

GRB SampleGRB Sample

Total number of GRBs analyzed: 39

With known redshift: 8

Long duration GRBs (> 2s): 34

Short duration GRBs (≤ 2s): 5

Distribution of the SignificancesDistribution of the Significances

The distribution of the statistical significances is compared with a standard normal distribution

mean = 0.26

N

Upper Limits in the 1Upper Limits in the 1100 GeV range100 GeV range

Fluence upper limits obtained extrapolating the keV power lawspectra 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)

Upper Limits to the Cutoff EnergyUpper 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

extrapolation

upper limit

GRBs Stacked in PhaseGRBs Stacked in Phase

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

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

GRBs Stacked in TimeGRBs Stacked in Time

The resulting significances for the 9 time bins show that there is no evidence of emission for a certain t

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 overall significance ofthe GRBs stacked in time (taking into account the dependency of the 9 bins) with respect to background fluctuations is 0.27

Search for GRBs in Shower ModeSearch 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 (considering absorption by EBL when redshift available)

• Values down to 10-5 erg/cm2 in the 10 GeV1 TeV range

• More details about this analysis in Chen et al. (Proc. Nanjing GRB Conference, June 2008)

ConclusionsConclusions

Until now, the search for emission from GRBs with Until now, the search for emission from GRBs with

the ARGO-YBJ detector has given no positive result the ARGO-YBJ detector has given no positive result

The simple scaler mode has shown a good The simple scaler mode has shown a good

sensitivity, with fluence upper limits down to sensitivity, with fluence upper limits down to

1010-5-5 erg/cm erg/cm22 in the 1 in the 1100 GeV energy range 100 GeV energy range

The directional capability of the shower mode at The directional capability of the shower mode at

energies of a few hundreds of GeV allows to study energies of a few hundreds of GeV allows to study

GRBs in the whole 1 GeV GRBs in the whole 1 GeV 1 TeV energy range 1 TeV energy range