X.-X. Li, H.-H. He, F.-R. Zhu, S.-Z. Chen on behalf of the ARGO-YBJ collaboration Institute of High Energy Physics Nanjing GRB Conference,Nanjing, China, 2008, June Search for GeV Gamma-Ray Bursts with the ARGO-YBJ Detector in Scaler Mode

Outline 1. Introduction 2. ARGO search for GeV GRBs coincident with satellite (such as Swift) in scaler mode 3.Preliminary results 4.Conclusions

1. Introduction Gamma-ray bursts are short-lived bursts of gamma rays, the most energetic form of light. They are flashes of gamma rays emanating from seemingly random places in deep space at random times. Isotropic, but inhomogeneous. Isotropy is an indicator of the cosmological origin of GRBs. Spatial distribution Bimodal distribution Spectral features non-thermal spectrum, power law spectrum No cut at high energy up to now Temporal features diverse and spiky light curves. Duration : ~ ms ~ 1000s

Experimental motivation: EGRET(20 MeV~30 GeV) has detected GeV photons emission from few GRBs, such as GRB910503, , ; many bursts have very hard spectra  VHE GRB detection is feasible Theoretical : Many models predict the existence of high energy photons (e.g. SSC) Importance: put more constrains on models; provide information on the origin of GRBs (such as the distance and the emission mechanism) No positive >1GeV GRBs are observed by ground based experiment (even thousands of low energy GRBs are detected) Experiments searching for GRBs Space (satellite): HETE Ⅱ ( keV ), RXTE( keV ), Ulysses, SWIFT( keV plus UV/Optical telescope ( nm).) ---only efficient for low energy GRBs GLAST(20Mev ~ >300Gev) Ground-based: Milagro (~TeV), HEGRA(10~20TeV), Tibet AS γ(>1.5TeV), ARGO(>100GeV) Efficient for high energy GRBs Field of view, effective area and energy threshold are very important to explore high energy GRBs Motivation to search for GeV GRBs

Detector configuration ARGO-YBJ ARGO-YBJ experiment 1. Full cover : 6700m 2, High altitude : 4300m a.s.l. (Low energy threshold) 2. Large field of view (2.5sr) 3. High duty cycle （ >90%, continuously monitoring of the overhead sky ） 4. High trigger rate: ~4kHz Central carpet : 10×13 clusters Guard ring : 4×6 clusters The basic element: Pad (8 strips, digital readout) Data Acquisition modes: (1) shower (the lateral distribution and the arrival direction of primaries) (2) SPT ( single particle technique, no trigger, record the counting rate in fixed time interval, enable it to detect GRBs in the GeV region ) Longitude: 90.50, E Latitude: 30.10, N

SPT technique Low energy threshold of ~1GeV for photon Coincident time window ： 150ns Counting rates are not Poisson distribution over long duration (>1hour) due to the effects of pressure and temperature etc. ; but in short periods they are. Counting rate in four channels Channel 1 during 1 hour

SPT technique Effective area Effective area for photon with zenith angle θ = 20 deg for the four multiplicity channels (from top to bottom, 1, 2, 3, and ≥4) Sensitivity Typical upper limits in the 1~100GeV region reaching 5×10 -5 erg/cm 2 makes the ARGO-YBJ SPT one of the most sensitive detectors for the study of the high energy spectrum of GRBs.

2. Search for GRBs coincident with Swift 2.1 Check if the GRBs are in the field of view of ARGO. 2.2 Correction to the counting rates for long duration GRBs. 2.3 Estimate the flux over high energies ( 1GeV ~ 100GeV ) from the flux over low energies (15keV~150keV, Swift).

2.4 Data quality & stability Since a GRB is seen as an excess in counting rate ， the first step is to demonstrate the detector stability, i.e. it is working as a “statistical detector”, moreover, at a preliminary step, data must be checked to select only clusters working properly. Working in single particle mode requires very stable detectors, and a very careful and continuous monitoring of the experimental conditions. Stability Criteria There was no hardware changing over the data checking time; Ratios between 4 channels are reasonable; The normalized fluctuation parameter f from the following formula well modeled by Gaussian fit for detectors normally operated. S bb Here s is the signal of duration Δt (10s) and b is the background averaged in a duration 10 times Δt before and after the signal.

Distribution of fluctuation parameter f abnormal normal

Firstly ， calculate the GRBs’ significance f grb ; Secondly, calculate the significance f on several hours before and after the GRBs. Obviously the distribution of f for several hours follows Gaussian ( 0, σ); Finally, get the statistical significance of GRBs n σ. In this step, we use the parameter f offered by the previous step 2.5 Calculate the statistical significance of the GRBs

3. Preliminary results ( ~ ) There are 43 GRBs in the field of view of ARGO-YBJ among 267 GRBs detected by Swift ( ~ ) There are 134 clusters on operation normally up to now.

Photon index V.S. MJD T90 V.S. MJD

Zenith angle Statistical Significance

4. Conclusions ARGO-YBJ has advantage on GRB searching in the high energy region. It’s hopeful to detect high energy GRBs with ARGO. No significant excess has been found from 43 GRBs exploded within θ≤60° in the period August 15, 2005 ~ June 13, Next work : (1)applying wavelet and cross correlation on GRB analyzing; (2) the correction to the atmospheric and the hall temperature; (3) Vary time scale of search; (4) associate with data from ARGO-DAQ in shower mode.