32 nd ICRC –Beijing – August 11-18, 2011 Silvia Vernetto IFSI-INAF Torino, ITALY On behalf of the ARGO-YBJ collaboration Observation of MGRO J with ARGO-YBJ

Tibet ASγ ARGO The Yangbajing Cosmic Ray Laboratory Longitude 90° 31’ 50” East Latitude 30° 06’ 38” North 4300 m a.s.l.

Resistive Plate Chambers carpet

The ARGO-YBJ detector  Central carpet 78 x 75 m 2 (95 % of active surface)  Sampling ring surrounds the carpet up to 111 x 99 m 2 (20 % of active surface) Single layer of RPCs detectors The PAD (56  62 cm 2 ) is the space-time “pixel” Time resolution  1-2 ns ARGO has PADs

The ARGO-YBJ detector  Central carpet 78 x 75 m 2 (95 % of active surface)  Sampling ring surrounds the carpet up to 111 x 99 m 2 (20 % of active surface) The PAD (56  62 cm 2 ) is the space-time “pixel” Time resolution  1 ns ARGO has PADs Single layer of RPCs detectors

Duty cycle 86%  First data in July 2006  Full detector in stable data taking since November 2007  Trigger: Number of fired pad N pad > 20  Rate ~3.5 kHz - Dead time 4%  220 GB/day transferred to IHEP/CNAF data centers

 Shower core reconstruction Maximum Likelihood Method applied to the lateral density profile of the shower  Fit of the shower front Primary direction EAS data Space and time coordinates of the fired PADs

The Moon shadow An important tool to check the detector performances Deficit: 71 standard deviations  9 standard deviations /month Physics: antiproton / proton ratio in cosmic rays

Displacement of the shadow due to the Geomagnetic field Bending  1.57° Z / E (TeV) The Moon Shadow Angular resolution Energy calibration Error of energy scale < 13%

HST Crab Nebula dN/dE =(3.0 ± 0.3) E –  ± 0.09 ph sec -1 cm -2 TeV -1 3 years data

Mrk421 TeV  rays ARGO X- rays 2-12 KeV RXTE/AMS X –rays KeV SWIFT/BAT Active periods Data sample: Nov 2007 – Feb 2011 Correlation with X rays Cumulative counting rate > 12 s.d.

MGRO J discovered by MILAGRO MILAGRO Galactic plane survey, Abdo et al data Median energy  20 TeV ARGO see poster by Chen S.Z. MGRO J Flux  80% Crab Extended source with extension < 2.6 deg Cygnus region

MGRO J confirmed by HESS Extension  0.34 ±0.04° HESS spectrum dN/dE = E -2.1 sec -1 cm -2 TeV -1 Aharonian et al., 2009

Fermi pulsar PSR J Abdo et al Pulsar period ms HESS Nebula flux FERMI upper limits to the nebula flux (pulsar off) FERMI pulsar

MGRO J spectrum Disagreement between HESS and MILAGRO spectra  8  HESS HESS spectrum: Aharonian et al., 2009 dN/dE = E -2.1 sec -1 cm -2 TeV -1 Milagro spectrum: Smith et al., 2009 dN/dE = E -1.5 exp(-E/14.1) sec -1 cm -2 TeV -1 MILAGRO

ARGO data and analysis  From November 2007 to February 2011 (5358 observation hours)  Events zenith angle < 45°  Source culmination zenith angle = 24°  Source observation time = 5.4 hours /day  Events with N pad > 40  No gamma/hadron discrimination  Background evaluation : time swapping method  4 N pad intervals: 40 – 100 – 300 – 1000

Assuming a bidimensional Gauss shape In agreement with HESS  MGRO J extension c Events Significance map Map centered on the HESS source Integral angular distribution Found extension:  = 0.50   0.35  N pad > 40

MGRO J spectrum … but it is  3 times larger than the HESS one dN/dE =(2.2 ± 0.4) (E/7 TeV) –  ± 0.3 ph sec -1 cm -2 TeV -1 The ARGO flux is in agreement with MILAGRO… Systematic errors < 30%

. Possible causes of disagreement 1) Statistical fluctuation ? 2) Complex morphology or contribution from other sources ? 3) Contribution by the diffuse galactic gamma ray flux (< 20%) 4) Variable flux ? No, because of the extension d > 40 pc variability time scale > 130 years The problem is still open…

Backup slides

Determination of the flux and extension The observed extension is determined by  Intrinsic source extension  Detector PSF (that depends on the energy) But to evaluate the spectrum one should know the extension Iterative procedure

Determination of the flux and extension Iterative procedure:  Assumed a power low spectrum with slope  = 2.5  MC: evaluation of the PSF  Data: determination of the intrinsic extension ( fitting the angular distribution of events)  MC: evaluation of the best opening angles  Data: determination of the spectrum slope ( fitting the event rates vs. nhit) After a few iterations the procedure converges