The ARGO-YBJ air shower detector monitored the Crab Nebula gamma ray emission from 2007 November to 2013 February .
The integrated signal , consisting of \sim 3.3 \times 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 / 2 TeV ) ^ { - \alpha } , with a flux normalization I _ { 0 } = ( 5.2 \pm 0.2 ) \times 10 ^ { -12 } photons cm ^ { -2 } s ^ { -1 } TeV ^ { -1 } and \alpha = 2.63 \pm 0.05 , corresponding to an integrated flux above 1 TeV of 1.97 \times 10 ^ { -11 } photons cm ^ { -2 } s ^ { -1 } .
The systematic error is estimated to be less that 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 / 2 TeV ) ^ { - \alpha } \exp ( -E / E _ { cut } ) , the lower limit of the cutoff energy E _ { cut } 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 \times 10 ^ { -2 } .
A correlated analysis with Fermi-LAT data over \sim 4.5 years using the light curves of the two experiments gives a Pearson correlation coefficient r = 0.56 \pm 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 \pm 91 photons day ^ { -1 } , consistent with the average value of 137 \pm 10 day ^ { -1 } .