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 ∼3.3×10^5 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 = I_0 (E / 2TeV)^{−α}, with a flux normalization I_0 = (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=I_0 (E/2TeV)^{−α} exp (−E/E_cut), the lower limit of the cutoff energy E_cut 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.