Searc h for emission from G amma R ay B urst s with the ARGO-YBJ detector

1. 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, Košice

2. The ARGO-YBJ experiment • Collaboration between: • Istituto Nazionale di Fisica Nucleare (INFN) – Italy • Chinese Academy of Science (CAS) • Site:YangBaJingCosmic Ray Laboratory (Tibet, P.R. of China), 4300 m a.s.l. Site Coordinates: longitude 90° 31’ 50” E, latitude 30° 06’ 38” N

3. 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  103TeV • 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 g-rays and primary nuclei

4. 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)

5. Experiment Hall CLUSTER RPC RPC also with analog charge readout

6. 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

7. 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i+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

8. Physics Goals in Scaler Mode • GRBs:search for the high energy tail and spectral cutoff • Solar physics: Forbush Decreases and Ground Level • Enhancementsin correlation with neutron monitors • Correlation with changes of the Atmospheric Electric Field • (thunderstorms) I will focus on the search for emission from GRBs

9. 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 > 650 GeV, • Milagrito) and GRB920925c (E > 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 > 80 GeV ! • The GeV – TeV energy range may reveal the spectral cutoff of GRBs, • constraining emission models

10. Search Triggered by Satellites • Search started sincethe first GRB detection by Swift • on December 17, 2004 (as a Follow-Up instrument) • Selection of GRBs with zenith angle  <45° • Search for an excess in the scaler counts in coincidence • with the satellite detection (T90) • 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)

11. GRB Sample • Total number of GRBs analyzed: 39 • With known redshift: 8 • Long duration GRBs (> 2s): 34 • Short duration GRBs (≤ 2s): 5

12. Distribution of the Significances mean  = 0.26 N The distribution of the statistical significances is compared with a standard normal distribution

13. 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)

14. extrapolation upper limit 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

15. 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 

16. 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 

17. 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 (considering absorption by EBL when redshift available) • 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)

18. 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