Highlights of the ARGO-YBJ experiment P. Camarri University of Roma Tor Vergata and INFN on behalf of the ARGO-YBJ collaboration P. Camarri - ICATPP 2010 – Como, 7-8 October 2010

P. Camarri - ICATPP Como, 7-8 October 2010 Longitude 90° 31’ 50” East Latitude 30° 06’ 38” North 90 Km North from Lhasa (Tibet) An unconventional EAS-array exploiting the full coverage approach at very high altitude to detect small air showers at an energy threshold of a few hundreds of GeV. The ARGO-YBJ experiment Tibet ASγ ARGO The Yangbajing Cosmic Ray Laboratory 4300 m above the sea level ~ 600 g/cm 2

The ARGO-YBJ Collaboration INFN and Dpt. di Fisica Università, Lecce INFN and Dpt. di Fisica Universita’, Napoli INFN and Dpt. di Fisica Universita’, Pavia INFN and Dpt di Fisica Università “Roma Tre”, Roma INFN and Dpt. di Fisica Univesità “Tor Vergata”, Roma INAF/IFSI and INFN, Torino INAF/IASF, Palermo and INFN, Catania IHEP, Beijing Shandong University, Jinan South West Jiaotong University, Chengdu Tibet University, Lhasa Yunnan University, Kunming ZhengZhou University, ZhengZhou Hong Kong University, Hong Kong International Collaboration: Chinese Academy of Science (CAS) Istituto Nazionale di Fisica Nucleare (INFN)

Experimental Hall & Detector Layout RPC Single layer of Resistive Plate Chambers (RPCs) with full coverage (92% active surface) and large area (5600 m 2 ) + sampling guard ring (6700 m 2 in total) time resolution ~1-2 ns (pad) space resolution = strip 10 Pads (56 x 62 cm 2 ) for each RPC 8 Strips (6.5 x 62 cm 2 ) for each Pad 1 CLUSTER = 12 RPCs 78 m 111 m 99 m74 m (5.7  7.6 m 2 ) Gas Mixture: Ar/ Iso/TFE = 15/10/75 HV = 7200 V in streamer mode Central Carpet: 130 Clusters 1560 RPCs Strips

ARGO-YBJ: a multi-purpose experiment P. Camarri - ICATPP 2010 Como, 7-8 October 2010 by 2 operational modes:  scaler mode (counting rate, > 1 GeV)  shower mode (full reconstruction, > 300 GeV) Sky survey -10º  δ  70º above 300 GeV (  -sources, anisotropies) High exposure for flaring activity (  -sources, GRBs, solar flares) CR physics 1 TeV  10 4 TeV p/p ratio at TeV energies Solar and heliospheric physics p + He spectrum at low energies Knee region p-air and p-p cross sections Anisotropies Multicore events

P. Camarri - ICATPP 2010 Como, 7-8 October 2010 Operational Modes Object: flaring phenomena (high energy tail of GRBs, solar flares) detector and environment monitor Recording the counting rates (N hit ≥1, ≥2, ≥3, ≥4) for each cluster at fixed time intervals (every 0.5 s) lowers the energy threshold down to ≈ 1 GeV. No information on the arrival direction and spatial distribution of the detected particles. :  Scaler Mode: Detection of Extensive Air Showers (direction, size, core …) Coincidence of different detector units (pads) within 420 ns Trigger : ≥ 20 fired pads on the central carpet (rate ~3.6 kHz) Object: Cosmic Ray physics (above ~1 TeV) VHE γ-astronomy (above ~300 GeV)  Shower Mode: INDIPENDENT DAQ

High space/time granularity + Full coverage + High altitude detailed study on the EAS space/time structure with unique capabilities 3-D view of a detected shower Top view of the same shower EAS reconstruction Event Rate ~ 3.6 kHz for N hit >20

P. Camarri - ICATPP 2010 Como, 7-8 October 2010 Stable data taking since Nov with full detector Average duty cycle ~ 90% Trigger rate ~ pad threshold Dead time 4% 220 GB/day transferred to IHEP/CNAF data centers

P. Camarri - ICATPP 2010 Como, 7-8 October 2010 The Moon-Shadow technique Geomagnetic Field: positively charged particles are deflected towards the West. Ion spectrometer The observation of the Moon shadow may provide a direct check of the relation between size and primary energy Cosmic rays are blocked by the Moon Deficit of cosmic rays in the direction of the Moon Moon diameter ~0.5 deg Size of the deficit Position of the deficit Angular Resolution Pointing Error Energy calibration West displacement

P. Camarri - ICATPP 2010 Como, 7-8 October 2010 All data: 2006 → 2009 N > s.d.  9 standard deviations / month PSF of the detector 3200 hours on-source θ < 50° The deficit surface is the convolution of the PSF of the detector and the widespread Moon disc.

P. Camarri - ICATPP 2010 Como, 7-8 October 2010 Moon Shadow analysis Measured angular resolution Measured EW displacement Relation multiplicity – primary energy Δα = º · N -0.68

All-sky survey result (1 – 30 TeV) P. Camarri - ICATPP 2010 Como, 7-8 October sources with significance >5 σ in ~800 days (July 06 – Dec. 09) Crab 14 σ, Mrk σ, MGRO J σ Mean = -9.3 ± Sigma = ± Evidence for a signal at ≈4 s.d. from the MGRO J region. 12 CrabMrk421

P. Camarri - ICATPP 2010 Como, 7-8 October 2010 dN/dE = (3.62  0.29) · · (E/1 TeV) –2.55  0.10 cm –2 s –1 TeV –1 N PAD Events /dayE med (TeV) 40 –  –  >  Crab Nebula ~14 s.d. in ~800 days ≈ 50 % Crab/year NO event selection NO γ /h discrimination NO Pb on RPCs Absolute measurement of angular resolution capability.

MGRO J P. Camarri - ICATPP 2010 Como, 7-8 October 2010 Pulsar Wind Nebula discovered by Milagro (  8 s.d.) with a flux  80% Crab Confirmed by HESS and VERITAS. Detected by Tibet ASγ at 4.4 s.d. (ICRC05) LAT HESS Milagro HESS: intrinsic extension is 0.34 deg Milagro: extension is <2.6 deg dN/dE = 6.2· ·E -1.5 exp(-E/14.1) sec -1 cm -2 TeV -1 dN/dE = 4.14· ·E -2.1 sec -1 cm -2 TeV -1 by J.Smith arXiv: Discrepancy between HESS and Milagro

ARGO-YBJ preliminary analysis P. Camarri - ICATPP 2010 Como, 7-8 October 2010   Number of events  Integral angular distribution (N>100) Intrinsic extension:  MGRO J Energy Spectrum ARGO preliminary HESS Milagro

MGRO J1908: preliminary conclusions P. Camarri - ICATPP 2010 Como, 7-8 October 2010 Experimentindex1 TeV flux (10 -8 TeV -1 m -2 s -1 ) 10 TeV flux ( TeV -1 m -2 s -1 ) E cut (TeV)Source Extension HESS No0.34 Milagro < 2.6 ARGO-YBJ -2.2  No0.48 ± 0.28 dN/dE = (3.6 ± 0.8) · · (E/6 TeV) –2.2 ± 0.3 cm -2 sec -1 TeV -1 The observed extension is compatible with the HESS measurement ARGO data are not sensitive to the cutoff The ARGO and HESS measured fluxes are NOT consistent ! ARGO is consistent with MILAGRO in particular around 10 TeV

Mrk421 P. Camarri - ICATPP Como, 7-8 October 2010 Total significance: 12 s.d. in ~800 days Different flares observed by ARGO-YBJ in TeV range Full DAQ TEST data Mrk421 is characterized by a strong flaring activity both in X-rays and in TeV γ–rays. Swift (15-50 keV) The first source observed by ARGO (2006 July)

Mrk421 cumulative light curves P. Camarri - ICATPP 2010 Como, 7-8 October 2010 RXTE/ASM 2-12 keV Swift/BAT keV ARGO-YBJ TeV (N > 100) Active period The steepness of the curve stands for the flux. Feb,June 556 days with simultaneous data averaged over 5 days.

P. Camarri - ICATPP 2010 Como, 7-8 October 2010 GASP- WEBT SWIFT RXTE SWIFT SuperAGILE AGILE MAGIC VERITAS Mrk421 - June 2008 flare A set of simultaneous measurements covering 12 decades from optical to TeV energies was performed during the flaring activity in first half of June 2008 data from: GASP-WEBT (R-band; May 24 to June 23) SWIFT (UVOT & XRT; June 12-13) AGILE (E > 100 MeV; June 9-15) MAGIC and VERITAS (E> 400 GeV; May 27 - June 8) in Donnarumma et al., ApJ 691 L13 (2009) complemented by publicly-available data from RossiXTE/ASM (2-12 keV) and Swift/BAT (15-50 keV). No VHE data after June 8 the moonlight hampered the Cherenkov telescopes measurements ! 2 flaring episodes were reported: June 3-8 and June 9-15

Mrk421 - June 2008 flare P. Camarri - ICATPP 2010 Como, 7-8 October 2010 dN/dE = (3.2  1.0) · · (E/2.5) –2.1  0.7 e -  (E) cm –2 s –1 TeV –1 EBL from Raue, M. & Mazin, D., 2008, Int. J. Mod. Phys. D17, 1515 The spectrum slope is consistent with the one measured by Whipple in 2000/2001 observing a similar flare, suggesting that the relation spectral index – flux is a long-term property of the source. The integral flux above 1 TeV during June is ~6 Crab units: one of the most powerful flares from Mrk421 ever observed. G. Aielli et al. – ApJL 714 (2010) L208

Feb. 2010: strong Mrk421 flare P. Camarri - ICATPP 2010 Como, 7-8 October 2010  The flux exceeded 3 Crab units for the duration of the observations. The peak flux (16 Feb.) is >10 Crab units.  For the first time an EAS- array observed a TeV flare at 5σ on a daily basis.  VERITAS reported similar observations in Atel # Feb. 16 Feb. 17 Feb.18 Feb.  ARGO observed a strong TeV γ-ray flare from Mrk421 during Feb at 6σ. ~5 σ ~6 σ

P. Camarri - ICATPP 2010 Como, 7-8 October 2010 Why: Satellite detectors (EGRET, LAT -FERMI) confirmed E> 1 GeV Ground experiments (MILAGRITO, HEGRA, AIROBICC, Tibet AS-g) reported marginal detection on VHE tail Many models predicts the energy cut-off And: Absorbtion by Extragalatic Background Light (EBL) which modulates with redshift z The GeV-TeV range may reveal the spectral cutoff Constraints on the emission model Scaler mode (E>1 GeV) & Shower Mode (E>10 GeV) G. Aielli et al. – Search for GRBs with the ARGO-YBJ detector in scaler mode The Astrophysical Journal 699 (2009) G. Aielli et al. - ARGO-YBJ constraints on very high energy emission From GRBs Astroparticle Physics 32 (2009) Search for GRBs in the GeV-TeV range

P. Camarri - ICATPP 2010 Como, 7-8 October 2010 Long duration GRBs (>2s): 82 Short duration GRBs (≤2s): 11 No evidence of coincident signals over the GRB T90 duration In stacked analysis no evidence for any integral effect Number of GRBs analysed: 93 With known redshift: 16 GRB in the ARGO FOV since Dec 2004 to Apr 2010 Scaler-mode results Looking for excess in the counting rates in coincidence with satellite detections. No pointing capability and poor energy resolution.

P. Camarri - ICATPP 2010 Como, 7-8 October 2010 Fluence U.L. for known-redshift GRBs Only 14 over 16 analyzed are reported (up to January 2009)

Conclusions P. Camarri - ICATPP 2010 Como, 7-8 October 2010  The ARGO-YBJ detector has been taking data since November 2007 with excellent performance  First results on γ-ray astronomy Crab Nebula spectrum in agreement with other measurements All-sky survey: observation of MGRO J Long-term monitoring of Markarian 421 Limits on GeV fluence from GRBs  Studies to increase the sensitivity are in progress  Sky survey going on  More results in cosmic-ray physics will be presented in the next ARGO-YBJ talk by P. Montini

P. Camarri - ICATPP 2010 Como, 7-8 October 2010 Detector performance  Angular resolution  Pointing accuracy  Energy scale calibration  Long-term stability Moon Shadow analysis:

X-ray/TeV correlation P. Camarri - ICATPP 2010 Como, 7-8 October 2010 To quantify the correlation and the time lags between X-ray and TeV emission the Discrete Correlation Function has been used (Edelson & Krolik 1988), ± ± RXTE/ASM & ARGOSwift/BAT & ARGO No significant time lag is found (u.l. for the cross-band time lag 1 day). Correlation coefficient at zero ~0.8 Positive values means TeV lag behind X-rays

TeV spectral index – flux relation P. Camarri - ICATPP 2010 Como, 7-8 October 2010 To study the Mrk421 SED in different flux levels with 2 years data, both the X-ray and TeV data are grouped into 4 bands based on the RXTE/ASM daily counting rate: 0-2, 2-3, 3-5, >5 cm -2 s -1. EBL by Franceschini et al. (2008) The TeV flux measured by ARGO ranges from 0.9 to 7 Crab units. The TeV spectral index hardens with increasing flux in agreement with the relation between flux and spectral index calculated by Whipple in Krennrich et al. (2002)

X-ray/TeV flux correlation P. Camarri - ICATPP 2010 Como, 7-8 October 2010  A positive correlation is observed  The relation appears quadratic rather than linear. Similar result has been reported by Fossati et al. (2008). This result, based on a 2 years observation, extends similar studies carried out in much shorter periods. We investigated the correlation between ARGO-YBJ > 1 TeV and RXTE/ASM simultaneous integral fluxes.

SSC model P. Camarri - ICATPP 2010 Como, 7-8 October 2010 The ARGO data can be described by a one-zone SSC model (Mastichiadis & Kirk, 1997, Yang et al., 2008) Electron injection compactness Electron max Lorentz factor Very Preliminary