Multiwavelength observations of Mrk 501 in 2008

Abstract

Context. Blazars are variable sources on various timescales over a broad energy range spanning from radio to very high energy (>100 GeV, hereafter VHE). Mrk 501 is one of the brightest blazars at TeV energies and has been extensively studied since its first VHE detection in 1996. However, most of the γ-ray studies performed on Mrk 501 during the past years relate to flaring activity, when the source detection and characterization with the available γ-ray instrumentation was easier to perform. Aims. Our goal is to characterize the source γ-ray emission in detail, together with the radio-to-X-ray emission, during the non-flaring (low) activity, which is less often studied than the occasional flaring (high) activity. Methods. We organized a multiwavelength (MW) campaign on Mrk 501 between March and May 2008. This multi-instrument effort included the most sensitive VHE γ-ray instruments in the northern hemisphere, namely the imaging atmospheric Cherenkov telescopes MAGIC and VERITAS, as well as Swift, RXTE, the F-GAMMA, GASP-WEBT, and other collaborations and instruments. This provided extensive energy and temporal coverage of Mrk 501 throughout the entire campaign. Results. Mrk 501 was found to be in a low state of activity during the campaign, with a VHE flux in the range of 10%-20% of the Crab nebula flux. Nevertheless, significant flux variations were detected with various instruments, with a trend of increasing variability with energy and a tentative correlation between the X-ray and VHE fluxes. The broadband spectral energy distribution during the two different emission states of the campaign can be adequately described within the homogeneous one-zone synchrotron self-Compton model, with the (slightly) higher state described by an increase in the electron number density. Conclusions. The one-zone SSC model can adequately describe the broadband spectral energy distribution of the source during the two months covered by the MW campaign. This agrees with previous studies of the broadband emission of this source during flaring and non-flaring states. We report for the first time a tentative X-ray-to-VHE correlation during such a low VHE activity. Although marginally significant, this positive correlation between X-ray and VHE, which has been reported many times during flaring activity, suggests that the mechanisms that dominate the X-ray/VHE emission during non-flaring-activity are not substantially different from those that are responsible for the emission during flaring activity. Key words. astroparticle physics -BL

FIRSTNuSTAROBSERVATIONS OF MRK 501 WITHIN A RADIO TO TeV MULTI-INSTRUMENT CAMPAIGN

“…For the period of observations covered in this work, the fractional variability shows a double-peaked shape with the highest variability in the X-ray and VHE bands. A similar broadband variability pattern has recently been reported for Mrk 501 (Doert 2013;Aleksić et al 2015c), for Mrk 421 (Aleksić et al 2015b;M. Baloković et al 2015, in preparation), and for other high-synchrotronpeaked blazars in, for example, Aleksić et al (2014).…”

“…The relatively high fractional variability at 37 GHz is not produced by any single flaring event, but rather by a consistent flickering in the radio flux. Such flickering is not typically observed in blazars, but has been reported for Mrk 501 in Aleksić et al (2015c). At X-ray frequencies, F var gradually increases with energy, reaching the largest value (∼0.6) in the 7-30 keV band measured by NuSTAR.…”

Multicolor Optical Monitoring of the Blazar S5 0716+714 from 2017 to 2019

“…For the target source, the Compton dominance parameter q is taken to be 1, and the r band frequency is c/6231Å. The flux variability is measured by the fractional variability parameter F var (Vaughan et al 2003;Aleksic et al 2015):…”

“…where S is the standard deviation of F , < σ 2 err > is the mean value of squared error and < F > is the average flux. The uncertainty of F var (Poutanen et al 2008;Vaughan et al 2003;Aleksic et al 2015) is calculated by…”

Verification of the diffusive shock acceleration in Mrk 501

“…Despite the fact that multiwavelength SED of Mrk 501 have been extensively studied for quite some time (e.g., Bednarek & Protheroe 1999;Katarzynski et a. 2001;Tavecchio et al 2001;Kino et al 2002;Albert et al 2007;Acciari et al 2011;Lefa et al 2011;Zheng & Zhang 2011;Mankuzhiyil et al 2012;Neronov et al 2012;Peng et al 2014;Aleksic et al 2015;Shukla et al 2015;Aliu et al 2016;Ahnen et al 2017), the nature of these objects, such as the content of the jet, location and mechanism responsible for the γ-ray emission, are still far from being understood. Using the EED that is obtained in §2, we calculate the multi-wavelength SED of Mrk 501 in the homogeneous SSC scenario.…”