Very hard gamma-ray emission from a flare of Mrk 501

¹ ISDC Data Centre for Astrophysics, Ch. d’Ecogia 16, 1290 Versoix, Switzerland
e-mail: andrii.neronov@unige.ch
² APC, 10 rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France
³ Institute for Nuclear Research RAS, 60th October Anniversary prosp. 7a, Moscow 117312, Russia
e-mail: dmitri.semikoz@apc.univ-paris7.fr

Received: 14 April 2011
Accepted: 7 March 2012

Abstract

Aims. We investigate the peculiar properties of a large TeV γ-ray flare from Mrk 501 detected during the 2009 multiwavelength campaign.

Methods. We identified the counterpart of the flare in the Fermi/LAT telescope data and studied its spectral and timing characteristics.

Results. A strong order-of-magnitude increase of the very-high-energy γ-ray flux during the flare was not accompanied by an increase in the X-ray flux, so that the flare was one of the “orphan”-type TeV flares observed in BL Lacs. The flare lasted about one month at energies above 10 GeV. The flaring source spectrum in the 10–200 GeV range was very hard, with a photon index Γ = 1.1 ± 0.2, harder than that observed in any other blazar in the γ-ray band. No simultaneous flaring activity was detected below 10 GeV. Different variability properties of the emission below and above 10 GeV indicate the existence of two separate components in the spectrum. We investigate possible explanations of the very hard flaring component. We consider, among others, the possibility that the flare is produced by an electromagnetic cascade initiated by very-high-energy γ-rays in the intergalactic medium. Within such an interpretation, peculiar spectral and temporal characteristics of the flare could be explained if the magnetic field in the intergalactic medium is on the order of 10^-17 − 10^-16 G.