Crab Nebula: Five-year observation with ARGO-YBJ

Abstract

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 = I 0 (E/2 TeV)-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/2 TeV)-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.