INTEGRAL observations of the blazar Mrk 421 in outburst: results of a multi-wavelength campaign

Context. If one wants to understand the physics of blazars, better simultaneous observations are important at all wavelengths, so it was fortunate that a ToO observation of the TeV-emitting blazar Mrk 421 with INTEGRAL could be triggered in June 2006 by an increase in the RXTE count rate to more than 30 mCrab. The source was then observed with all INTEGRAL instruments, with the exception of the spectrometer SPI, for a total exposure of 829 ks. During this time several outbursts were observed by IBIS and JEM- X. Multiwavelength observations were immediately triggered, and the source was observed at radio, optical, and X-ray wavelengths up to TeV energies. Aims. The data obtained during these observations were analysed with respect to time variability, time lags, correlated vari ability, and spectral evolution and then compiled in aFspectrum. Methods. The observations of the different instruments/telescopes were analysed with the usual correlation and time-analysis methods. The spectral analysis of the X-ray data was performed with XSPEC. Results. Four strong flares at X-rays were observed that were not seen a t other wavelengths (partially because of missing data). From the fastest rise in the X-rays, an upper limit could be derived to the extension of the emission region. A time lag between high-energy and low-energy X-rays was observed, which allowed an estimation of the magnetic-field strength. The spectr al analysis of the X-rays revealed a slight spectral hardening of the low-energy (3-∼43 keV) spectral index. The hardness-ratio analysis of the Swift-XRT (0.2 - 10 keV) data indicated a small correlation with the intensity; i. e., a hard-to-soft evolution was obser ved. At the energies of IBIS/ISGRI (20 - 150 keV), such correlations are less obvious. A multiwavelength spectrum was composed and the X-ray and bolometric luminosities calculated. Conclusions. The observed flaring activity of Mrk 421 is mainly visible at X -rays. It is found that the spectral change with intensity is small. But at least one flare showed a completely di fferent spectral behaviour than the other flares, so one can con clude that each blob of relativistic-moving electrons has its own individu al physical environment that leads to different emission characteristics. From a fit of a leptonic emission model to the data, one finds that the observed variability may be due to a varying effi ciency of particle acceleration.