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 $\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 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)${}^{-\alpha }$ $\exp$ (-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${}^{-1}$.

12 pages, 10 figures

References in corpus (8)

• Observations of the Crab Nebula with H.E.S.S
• Gamma-ray flares from the Crab Nebula
• Discovery of Powerful Gamma-Ray Flares from the Crab Nebula
• VHE Gamma-Ray Observation of the Crab Nebula and its Pulsar with the MAGIC telescope
• Identification of the TeV Gamma-ray Source ARGO J2031+4157 with the Cygnus Cocoon
• Variable gamma-ray emission from the Crab Nebula: short flares and long "waves"
• Long-term Monitoring on Mrk 501 for Its VHE gamma Emission and a Flare in October 2011
• Observation of TeV gamma-rays from the unidentified source HESS J1841-055 with the ARGO-YBJ experiment

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• Ultrahigh-energy Gamma-Ray Radiation from the Crab Pulsar Wind Nebula
• Modeling the broadest spectral band of the Crab nebula and constraining the ions acceleration efficiency
• EAS age determination from the study of the lateral distribution of charged particles near the shower axis with the ARGO-YBJ experiment
• Search for gamma-ray emission from the Sun during solar minimum with the ARGO-YBJ experiment
• VHE Gamma-ray Observation of Crab Nebula with HAGAR Telescope Array
• STACEX: RPC-based detector for a multi-messenger observatory in the Southern Hemisphere