Thursday, July 4, 2013

1307.0558 (E. Pian et al.)

An active state of the BL Lac Object Markarian 421 detected by INTEGRAL in April 2013    [PDF]

E. Pian, M. Tuerler, M. Fiocchi, A. Bazzano, L. Foschini, F. Tavecchio, V. Bianchin, R. Boissay, G. Castignani, C. Ferrigno, C. M. Raiteri, M. Villata, V. Beckmann, F. D'Ammando, R. Hudec, G. Malaguti, L. Maraschi, T. Pursimo, P. Romano, S. Soldi, A. Stamerra, A. Treves, P. Ubertini, S. Vercellone, R. Walter
(abridged) Multi-wavelength variability of blazars offers effective insight into the mechanisms through which energy is propagated from the center down the jet. In this context, we activated INTEGRAL observations of the blazar Markarian 421 in an active state on 16-21 April 2013, and complemented them with Fermi-LAT data. We obtained well sampled optical, soft and hard X-ray light curves that show the presence of two flares, the first one reaching a brighter maximum than the second one at X-rays. The average flux in the 20-100 keV range is 9.1e-11 cgs and the nuclear average apparent magnitude, corrected for Galactic extinction, is V ~ 12.2. In the time-resolved JEMX+IBIS spectra we see a change of spectral slope at an energy that correlates with flux, as expected in refreshed energy injections in a population of electrons that cool thereafter via synchrotron radiation. During the observation, the variability level increases monotonically from the optical to the hard X-rays, and the cross-correlation analysis during the onset of the most prominent flare shows an increasing delay of the lower frequencies emission with respect to the higher frequencies, with a maximum time-lag of about 70 minutes, that is however not very well constrained. The dataset of Mkn 421 presented here displays strong variability in the hard X-rays and GeV gamma-rays during a period of historically high optical state and extreme activity at TeV energies. The rapid spectral variation of the GeV emission requires fast changes of the electron characteristic energy and/or of the magnetic field, thus implying efficient acceleration mechanisms. The multi-wavelength variability in Mkn~421, in particular at X-rays, can be very complex and its patterns may change from epoch to epoch down to intra-day timescales, depending on the emission state, making accurate monitoring of this source necessary.
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