CRAB NEBULA: FIVE-YEAR OBSERVATION WITH ARGO-YBJ

The ARGO-YBJ air shower detector monitored the Crab Nebula gamma-ray emission from 2007 November to 2013 February. The integrated signal, consisting of ∼3.3 × 105 events, reached the statistical significance of 21.1 standard deviations. The obtained energy spectrum in the energy range 0.3–20 TeV can be described by a power law function dN/dE = I0 (E/2 TeV)−α, with a flux normalization I0 = (5.2 ± 0.2) × 10−12 photons cm−2 s−1 TeV−1 and α = 2.63 ± 0.05, corresponding to an integrated flux above 1 TeV of 1.97 × 10−11 photons cm−2 s−1. The systematic error is estimated to be less than 30% for the flux normalization and 0.06 for the spectral index. Assuming a power law spectrum with an exponential cutoff dN/dE = I0 (E/2 TeV)−α exp  (−E/Ecut), the lower limit of the cutoff energy Ecut is 12 TeV, at 90% confidence level. Our extended data set allows the study of the TeV emission over long timescales. Over five years, the light curve of the Crab Nebula in 200-day bins is compatible with a steady emission with a probability of 7.3 × 10−2. A correlated analysis with Fermi-LAT data over ∼4.5 yr using the light curves of the two experiments gives a Pearson correlation coefficient r = 0.56 ± 0.22. Concerning flux variations on timescales of days, a “blind” search for flares with a duration of 1–15 days gives no excess with a significance higher than four standard deviations. The average rate measured by ARGO-YBJ during the three most powerful flares detected by Fermi-LAT is 205 ± 91 photons day−1, consistent with the average value of 137 ± 10 day−1.

[1]  Danzengluobu,et al.  IDENTIFICATION OF THE TeV GAMMA-RAY SOURCE ARGO J2031+4157 WITH THE CYGNUS COCOON , 2014, 1406.6436.

[2]  A. Quirrenbach,et al.  H.E.S.S. observations of the Crab during its March 2013 GeV gamma-ray flare. , 2013, 1311.3187.

[3]  G. Richards A search for enhanced very-high-energy gamma-ray emission during the March 2013 Crab Nebula flare , 2013 .

[4]  Danzengluobu,et al.  TeV GAMMA-RAY SURVEY OF THE NORTHERN SKY USING THE ARGO-YBJ DETECTOR , 2013, 1311.3376.

[5]  G. Richards,et al.  A SEARCH FOR ENHANCED VERY HIGH ENERGY GAMMA-RAY EMISSION FROM THE 2013 MARCH CRAB NEBULA FLARE , 2013, 1309.5949.

[6]  S. Vernetto Gamma ray sources observed with ARGO-YBJ , 2013 .

[7]  Danzengluobu,et al.  OBSERVATION OF TeV GAMMA RAYS FROM THE UNIDENTIFIED SOURCE HESS J1841−055 WITH THE ARGO-YBJ EXPERIMENT , 2013, 1303.1258.

[8]  M. Trifoglio,et al.  VARIABLE GAMMA-RAY EMISSION FROM THE CRAB NEBULA: SHORT FLARES AND LONG “WAVES” , 2013, 1302.4342.

[9]  Danzengluobu,et al.  OBSERVATION OF THE TeV GAMMA-RAY SOURCE MGRO J1908+06 WITH ARGO-YBJ , 2012 .

[10]  Danzengluobu,et al.  LONG-TERM MONITORING OF MRK 501 FOR ITS VERY HIGH ENERGY γ EMISSION AND A FLARE IN 2011 OCTOBER , 2012, 1209.0534.

[11]  Danzengluobu,et al.  OBSERVATION OF TeV GAMMA RAYS FROM THE CYGNUS REGION WITH THE ARGO-YBJ EXPERIMENT , 2012, 1201.1973.

[12]  J. Galbraith-Frew,et al.  OBSERVATION AND SPECTRAL MEASUREMENTS OF THE CRAB NEBULA WITH MILAGRO , 2011, 1110.0409.

[13]  Danzengluobu,et al.  Calibration of the RPC charge readout in the ARGO-YBJ experiment , 2012 .

[14]  J. Chiang,et al.  GAMMA-RAY ACTIVITY IN THE CRAB NEBULA: THE EXCEPTIONAL FLARE OF 2011 APRIL , 2011, 1112.1979.

[15]  Danzengluobu,et al.  LONG-TERM MONITORING OF THE TeV EMISSION FROM Mrk 421 WITH THE ARGO-YBJ EXPERIMENT , 2011, 1106.0896.

[16]  F. Longo,et al.  Gamma-Ray Flares from the Crab Nebula , 2010, Science.

[17]  M. Trifoglio,et al.  Discovery of Powerful Gamma-Ray Flares from the Crab Nebula , 2011, Science.

[18]  Danzengluobu,et al.  GAMMA-RAY FLARES FROM Mrk421 IN 2008 OBSERVED WITH THE ARGO-YBJ DETECTOR , 2010 .

[19]  Danzengluobu,et al.  MULTI-TeV GAMMA-RAY OBSERVATION FROM THE CRAB NEBULA USING THE TIBET-III AIR SHOWER ARRAY FINELY TUNED BY THE COSMIC RAY MOON'S SHADOW , 2008, 0810.3757.

[20]  et al,et al.  VHE γ-Ray Observation of the Crab Nebula and its Pulsar with the MAGIC Telescope , 2007, 0705.3244.

[21]  Danzengluobu,et al.  Layout and performance of RPCs used in the Argo-YBJ experiment , 2006 .

[22]  Felix Aharonian,et al.  The Crab Nebula and Pulsar between 500 GeV and 80 TeV: Observations with the HEGRA Stereoscopic Air Cerenkov Telescopes , 2004 .

[23]  A. Mincer,et al.  Tests of Statistical Significance and Background Estimation in Gamma-Ray Air Shower Experiments , 2003, astro-ph/0306015.

[24]  S. Giani,et al.  GEANT Detector Description and Simulation Tool , 1994 .

[25]  D. Berley,et al.  Point source search techniques in ultra high energy gamma ray astronomy , 1993 .

[26]  Giuseppe Vacanti,et al.  Observation of TeV gamma rays from the Crab Nebula using the atmospheric Cerenkov imaging technique , 1989 .