Multiband variability studies and novel broadband SED modeling of Mrk 501 in 2009

Abstract

Aims. We present an extensive study of the BL Lac object Mrk 501 based on a data set collected during the multi-instrument campaign spanning from 2009 March 15 to 2009 August 1, which includes, among other instruments, MAGIC, VERITAS, Whipple 10 m, and Fermi-LAT to cover the γ-ray range from 0.1 GeV to 20 TeV; RXTE and Swift to cover wavelengths from UV to hard X-rays; and GASP-WEBT, which provides coverage of radio and optical wavelengths. Optical polarization measurements were provided for a fraction of the campaign by the Steward and St. Petersburg observatories. We evaluate the variability of the source and interband correlations, the γ-ray flaring activity occurring in May 2009, and interpret the results within two synchrotron self-Compton (SSC) scenarios. Methods. The multiband variability observed during the full campaign is addressed in terms of the fractional variability, and the possible correlations are studied by calculating the discrete correlation function for each pair of energy bands where the significance was evaluated with dedicated Monte Carlo simulations. The space of SSC model parameters is probed following a dedicated grid-scan strategy, allowing for a wide range of models to be tested and offering a study of the degeneracy of model-to-data agreement in the individual model parameters, hence providing a less biased interpretation than the "single-curve SSC model adjustment" typically reported in the literature. Results. We find an increase in the fractional variability with energy, while no significant interband correlations of flux changes are found on the basis of the acquired data set. The SSC model grid-scan shows that the flaring activity around May 22 cannot be modeled adequately with a one-zone SSC scenario (using an electron energy distribution with two breaks), while it can be suitably described within a two (independent) zone SSC scenario. Here, one zone is responsible for the quiescent emission from the averaged 4.5-month observing period, while the other one, which is spatially separated from the first, dominates the flaring emission occurring at X-rays and very-high-energy (>100 GeV, VHE) γ rays. The flaring

Statistical Analysis of the Microvariable AGN Source Mrk 501

“…The micro-variability we observed in the optical coincides with the intra-night variability reported at TeV energies in May 2009 [17]. They may be physically related, but unfortunately, the time resolution in the optical is minutes or better, while that in the TeV is much longer.…”

The VHE SED modelling of Markarian 501 in 2009

“…The VHE flare of May 1, 2009 was first observed by the Whipple 10m telescope when a high emission state was detected above 300 GeV [13,14]. A sudden increase in the flux in the first 0.5 h (MJD 54952.31 to MJD 54952.37) by about one order of magnitude compared to the average flux measured during the full campaign was recorded [13].…”

“…However, on May 22 (MJD 54973), the MAGIC telescope observed the blazar for 1.7 h and an elevated VHE flux (a VHE flare) ∼ 3 times the low flux level has been recorded [14].…”

“…Since its discovery, it has been the subject of extensive studies in multiwavelengths as it has undergone many major outbursts on long time scales and rapid flares on short time scales mostly in the X-rays and TeV energies [2][3][4][5][6][7][8][9][10][11][12]. As a part of multiwavelength campaign covering a period of 4.5 months from March 15 to August 1, 2009, Mrk 501 was observed by multi-instruments (∼ 30 different instruments) covering the entire electromagnetic spectrum [13,14]. In the optical and radio bands the flux was found to be almost constant and in the UV band it had some variation, however, during the VHE flare the flux was constant.…”

“…The VHE γray spectra observed during the 4.5 months period by different instruments are summarized in Table I and also the EBL corrected spectra are given in ref. [14]. In a recent paper Sahu et al, have explained the VHE flare of May 1 well using the photohadronic scenario by taking into account the EBL correction to the spectrum [15,16].…”