X-ray and very high energy gamma-ray observations of the galaxy Mrk 501 during May-June, 2006

Abstract

The galaxy Mrk 501 was observed with the GT-48 twinned atmospheric Cherenkov telescope over 9 moonless nights during May-June 2006. About 9 hours of observation time for the source, in good weather, were used to separate γ-ray showers from background showers. Excess γ-like events were detected with a significance of ~6σ. The flux was estimated assuming a Crab-like spectrum for Mrk 501 at energies E > 350 GeV using data from quasi-simultaneous observations with the Whipple telescope. The source was in a state of low activity in the TeV range, as confirmed by observations at other Cherenkov telescopes. Although the light curve of this object according to ground based observations manifested no significant variations on a diurnal scale, the soft and hard X-ray data indicate that the largest amplitudes of the variations occur at the lowest energies.