A Multi-wavelength Outburst of the Blazar Markarian 421
Blazars are the most luminous and variable active galaxies, with bright
spectra extending from the radio wavelengths to the X- and gamma-ray
energies. A fraction of them are detected up to the TeV gamma-rays from
ground-based Cherenkov telescopes; Mkn421 is the prototype of this class.
In 2013, Mkn421 underwent a phase of high activity, which culminated in
April with the detection of a huge TeV outburst. The source was also
exceptionally bright at hard X-rays, when a Target of Opportunity was
initiated by INTEGRAL. The observations (April 16-20, Revolutions 1283 & 1284)
lasted for a total of 400 ks and resulted in the detection, with IBIS/ISGRI,
JEM-X and OMC, of two fully resolved flares.
The source was observed also by the LAT instrument onboard NASA's Fermi
satellite, so that one was able to follow its behaviour simultaneously at
MeV to GeV energies. Strong variability was also detected, however,
remarkably, the flare maxima are not simultaneous with those occurring at
X-rays. This fact underlines the complexity of high energy variability in
blazars.
The analysis of the correlation of the IBIS/ISGRI and JEM-X light curves
during the rising part of the first flare indicates that the emission at
higher frequencies precedes that at lower frequencies by tens of minutes.
The correlation is chromatic, i.e., the lower the radiation frequency the
larger its delay with respect to the higher frequencies. This finding puts
constraints on the emission model for the multi-wavelength spectrum, which
is essentially of non-thermal nature, with the synchrotron mechanism
dominating at optical to X-ray energies, and self-Compton scattering being
responsible for the MeV-to-TeV gamma-rays.
In the image the multi-wavelength light curves of Mkn421 in April 2013
are shown: (a) OMC dereddened (using E(B-V) = 0.014) photometry, corrected
for the host galaxy and for a companion galaxy that is located in the OMC
field of view; (b) JEM-X count rates (resulting from the co-addition of the
signal measured by the 2 detectors), in the bands 3.04-5.52 keV (purple),
5.52-10.24 keV (pink), and 10.24-25.88 keV (light pink); (c) IBIS/ISGRI
count rates at 20-40 keV (red) and 40-100 keV (yellow); (d) Fermi-LAT fluxes
binned with 12-hr time-resolution. The background of the image is an artists
representation of a blazar.
Reference:
An active state of the BL Lac Object Markarian 421 detected by INTEGRAL in April 2013
E. Pian, M. Türler, M. Fiocchi et al.
http://arxiv.org/pdf/1307.0558v1.pdf, A&A, submitted, 2013
