rab Nebula: five-year observation with ARGO-YBJ

he 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?1and?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 dataset 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 years 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.