An active state of the BL Lacertae object Markarian 421 detected by INTEGRAL in April 2013^⋆

¹ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, via P. Gobetti 101, 40129 Bologna, Italy
e-mail: elena.pian@sns.it
² Scuola Normale Superiore, Piazza dei Cavalieri 7, 56122 Pisa, Italy
³ INFN, Sezione di Pisa, Largo Pontecorvo 3, 56127 Pisa, Italy
⁴ ISDC, Geneva Observatory, University of Geneva, Chemin d’Ecogia 16, 1290 Versoix, Switzerland
⁵ Istituto di Astrofisica e Planetologia Spaziali, via Fosso del Cavaliere 100, 00133 Roma, Italy
⁶ INAF – Osservatorio Astronomico di Brera, via Bianchi 46, 23207 Merate (LC), Italy
⁷ SISSA-ISAS, via Bonomea 265, 34136 Trieste, Italy
⁸ INAF – Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese (TO), Italy
⁹ François Arago Centre, APC, Université Paris Diderot, CNRS/IN2P3, 10 rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France
¹⁰ Dipartimento di Fisica, Università degli Studi di Perugia, via A. Pascoli, 06123 Perugia, Italy
¹¹ INFN, Sezione di Perugia, via A. Pascoli, 06123 Perugia, Italy
¹² INAF, Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy
¹³ Astronomical Institute, Academy of Sciences, Fricova 298, 25165 Ondrejov, Czech Republic
¹⁴ Czech Technical University in Prague, Faculty of Electrical Engineering, 16636 Praha 6, Czech Republic
¹⁵ INAF – Osservatorio Astronomico di Brera, via Brera 28, 20100 Milano, Italy
¹⁶ Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de La Palma, Spain
¹⁷ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, via U. La Malfa 153, 90146 Palermo, Italy
¹⁸ APC, Université Paris Diderot, CNRS/IN2P3, 10 rue Alice Domon et Léonie Duquet, 75025 Paris Cedex 13, France
¹⁹ Università degli Studi dell’Insubria, via Valleggio 11, 22100 Como, Italy

Received: 2 July 2013
Accepted: 9 September 2014

Abstract

Aims. Multiwavelength variability of blazars offers indirect, but very effective, insight into their powerful engines and on the mechanisms through which energy is propagated from the centre down the jet. The BL Lac object Mkn 421 is a TeV emitter, a bright blazar at all wavelengths, and therefore an excellent target for variability studies.

Methods. We activated INTEGRAL observations of Mkn 421 in an active state on 16–21 April 2013, and complemented them with Fermi-LAT data.

Results. We obtained well sampled optical, soft, and hard X-ray light curves that show the presence of two flares and time-resolved spectra in the 3.5–60 keV (JEM-X and IBIS/ISGRI) and 0.1–100 GeV (Fermi-LAT) ranges. The average flux in the 20–100 keV range is 9.1 × 10^-11 erg s^-1 cm^-2 (~4.5 mCrab) and the nuclear average apparent magnitude, corrected for Galactic extinction, is V ≃ 12.2. In the time-resolved X-ray spectra, which are described by broken power laws and, marginally better, by log-parabolic laws, we see a hardening that correlates with flux increase, as expected in refreshed energy injections in a population of electrons that later cool via synchrotron radiation. The hardness ratios between the JEM-X fluxes in two different bands and between the JEM-X and IBIS/ISGRI fluxes confirm this trend. During the observation, the variability level increases monotonically from the optical to the hard X-rays, while the large LAT errors do not allow a significant assessment of the MeV-GeV variability. The cross-correlation analysis during the onset of the most prominent flare suggests a monotonically increasing delay of the lower frequency emission with respect to that at higher frequency, with a maximum time-lag of about 70 min, that is however not well constrained. The spectral energy distributions from the optical to the TeV domain were compared to homogeneous models of blazar emission based on synchrotron radiation and synchrotron self-Compton scattering. They represent a satisfactory description, except in the state corresponding to the LAT softest spectrum and highest flux.

Conclusions. Multiwavelength variability of Mkn 421 can be very complex, with patterns changing from epoch to epoch down to intra-day timescales, depending on the emission state. This makes accurate monitoring of this source during bright hard X-ray states necessary and calls for the elaboration of multicomponent, multizone, time-dependent models.