High zenith angle observations of PKS 2155-304 with the MAGIC-I telescope

Abstract

The high frequency peaked BL Lac PKS 2155-304 with a redshift of z=0.116 was discovered in 1997 in the very high energy (VHE, E >100GeV) gamma-ray range by the University of Durham Mark VI gamma-ray Cherenkov telescope in Australia with a flux corresponding to 20% of the Crab Nebula flux. It was later observed and detected with high significance by the Southern Cherenkov observatory H.E.S.S. Detection from the Northern hemisphere is difficult due to challenging observation conditions under large zenith angles. In July 2006, the H.E.S.S. collaboration reported an extraordinary outburst of VHE gamma-emission. During the outburst, the VHE gamma-ray emission was found to be variable on the time scales of minutes and with a mean flux of ~7 times the flux observed from the Crab Nebula. Follow-up observations with the MAGIC-I standalone Cherenkov telescope were triggered by this extraordinary outburst and PKS 2155-304 was observed between 28 July to 2 August 2006 for 15 hours at large zenith angles. Here we present our studies on the behavior of the source after its extraordinary flare and an enhanced analysis method for data taken at high zenith angles. We developed improved methods for event selection that led to a better background suppression. The averaged energy spectrum we derived has a spectral index of -3.5 +/- 0.2 above 400GeV, which is in good agreement with the spectral shape measured by H.E.S.S. during the major flare on MJD 53944. Furthermore, we present the spectral energy distribution modeling of PKS 2155-304. With our observations we increased the duty cycle of the source extending the light curve derived by H.E.S.S. after the outburst. Finally, we find night-by-night variability with a maximal amplitude of a factor three to four and an intranight variability in one of the nights (MJD 53945) with a similar amplitude.

10 pages, 9 figures; Accepted for publication in A&A

References in corpus (6)

• Observations of the Crab Nebula with H.E.S.S
• Semi-analytic modeling of the EBL and consequences for extragalactic gamma-ray spectra
• VHE Gamma-Ray Observation of the Crab Nebula and its Pulsar with the MAGIC telescope
• Implementation of the Random Forest Method for the Imaging Atmospheric Cherenkov Telescope MAGIC
• Simultaneous observations of PKS 2155-304 with H.E.S.S., Fermi, RXTE and ATOM: spectral energy distributions and variability in a low state
• Mrk 421 active state in 2008: the MAGIC view, simultaneous multi-wavelength observations and SSC model constrained

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• Model-independent Redshift Estimation of BL Lac Objects through VHE Observations
• Infrared emission from dust and the spectral features of extragalactic gamma-ray sources
• Simultaneously unveiling the EBL and intrinsic spectral parameters of gamma-ray sources with Hamiltonian Monte Carlo
• Revisiting the evidences for spectral anomalies in distant blazars: new data on the photon-ALP mixing
• Spectral Hardening Revealed by Geometric De-boosting in the Masked Jet of PKS 2155$-$304