Mrk421 - The June 2008 flare observed from optical to TeV energies
Reference: I. Donnarumma et al. (The AGILE, Gasp-WEBT, MAGIC, and VERITAS Collaborations),The Astrophysical Journal, 691: L13-L19, 2009
ArXiv version: ArXiV:0812.1500
Contact person: Jeff Grube
Mrk 421 is a relatively bright and close blazer type AGN, with a redshift of z = 0.031, which exhibits large flaring states across the high-energy emission bands. One exceptionally strong flaring period in Mrk 421 was recorded during a two weeks in June 2008 at X-rays, high-energy (HE) gamma-rays (0.1 GeV < energy < 100 GeV) as detected by the AGILE GRID instrument, and in very-high-energy (VHE) gamma-rays (energies above 100 GeV) as detected by the imaging Cherenkov systems VERITAS and MAGIC.
Figure 1 shows for Mrk 421 the broadband light curve from May 24 to June 23, 2008. In the second and thirds panels from the top are shown two large flares in 2-10 keV and 15-60 keV X-rays, respectively. Each X-ray flare lasted approximately 7
days, with the second flare producing a dramatic increase in the 15-60 keV X-ray flux by an order of magnitude. During the first X-ray flare a large correlated VHE gamma-ray flare was recorded by VERITAS and MAGIC, as shown in the fourth panel from the top. Unfortunately no VHE data was taken during the second X-ray flare due to the bright moon period preventing VERITAS and MAGIC observations, however AGILE GRID observations during the second X-ray flare revealed an exceptionally bright HE gamma-ray flux. The two June 2008 flaring episodes are consistent with the highest ever recorded X-ray and HE gamma-ray fluxes for Mrk 421, allowing for the first detection of Mrk 421 by AGILE GRID.
Figure 2 shows the spectral energy distribution (SED) of Mrk 421 during the two large flaring periods in June 2008. The VHE gamma-ray spectrum measured by VERITAS on June 6, 2008, representing the peak in the first flare, is shown in red with the contemporaneous optical and X-ray data. The Swift XRT X-ray spectrum from June 12, 2008 is shown in black during the peak in the second flare, with the Swift UVOT, BAT, and AGILE GRID data shown for approximately the same time period. The SED shows a first broadband emission peak within the Swift XRT energy range, estimated at about 3 keV. The full SED is modeled with a synchrotron self-Compton (SSC) model, describing the beamed blazar jet emission. The measured short timescale variability seen from the light curve constrains the size of jet emitting region, described in the SSC model with a relativistic Doppler factor of 20 for the population of non-thermal electrons.
In summary, the exceptionally bright flaring measured from X-ray to VHE gamma-rays from Mrk 421 in June 2008 reveals remarkable rapid flaring structures, placing unique constraints to the synchrotron self-Compton (SSC) interpretation of this nearby blazar.
Figures from paper (click to get full size image):