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 toperform.
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.
Bibliographical noteFunding Information:
We are grateful to the referee, who helped us to improve the quality of this manuscript. The authors acknowledge the valuable contribution from Daniel Kranich during the preparation of the multi-instrument observations, as well as in the first steps towards the data reduction and interpretation. We would like to thank the Instituto de Astrofísica de Canarias for the excellent working conditions at the Observatorio del Roque de los Muchachos in La Palma. The support of the German BMBF and MPG, the Italian INFN, the Swiss National Fund SNF, and the Spanish MICINN is gratefully acknowledged. This work was also supported by the CPAN CSD2007-00042 and MultiDark CSD2009-00064 projects of the Spanish Consolider-Ingenio 2010 programme, by grant DO02-353 of the Bulgarian NSF, by grant 127740 of the Academy of Finland, by Projekt 09/176 of the Croatian science Foundation, by the DFG Cluster of Excellence “Origin and Structure of the Universe”, by the DFG Collaborative Research Centers SFB823/C4 and SFB876/C3, and by the Polish MNiSzW grant 745/N-HESS-MAGIC/2010/0. This research is supported by grants from the US Department of Energy Office of Science, the US National Science Foundation and the Smithsonian Institution, by NSERC in Canada, by Science Foundation Ireland (SFI 10/RFP/AST2748) and by STFC in the UK. We acknowledge the excellent work of the technical support staff at the Fred Lawrence Whipple Observatory and at the collaborating institutions in the construction and operation of the instrument. The St. Petersburg University team acknowledges support from Russian RFBR foundation, grant 12-02-00452. The Abastumani Observatory team acknowledges financial support by the Georgian National Science Foundation through grant GNSF/ST07/180. The Metsähovi team acknowledges the support from the Academy of Finland to our observing projects (numbers 212656, 210338, 121148, and others). The RATAN-600 observations were carried out with financial support of the Ministry of Education and Science of the Russian Federation (contract 14.518.11.7054), partial support from the Russian Foundation for Basic Research (grant 13-02-12103) is also acknowledged. We also acknowledge the use of public data from the Swift and RXTE data archive.
- Astroparticle physics
- BL Lacertae objects: individual: Mrk 501
- Gamma rays: general