We report on TeV γ -ray observations of the blazar Mrk 421 (redshift of 0.031) with the VERITAS observatory and the Whipple 10 m Cherenkov telescope. The excellent sensitivity of VERITAS allowed us to sample the TeV γ -ray fluxes and energy spectra with unprecedented accuracy where Mrk 421 was detected in each of the pointings. A total of 47.3 hr of VERITAS and 96 hr of Whipple 10 m data were acquired between 2006 January and 2008 June. We present the results of a study of the TeV γ -ray energy spectra as a function of time and for different flux levels. On 2008 May 2 and 3, bright TeV γ -ray flares were detected with fluxes reaching the level of 10 Crab. The TeV γ -ray data were complemented with radio, optical, and X-ray observations, with flux variability found in all bands except for the radio wave band. The combination of the Rossi X-ray Timing Explorer and Swift X-ray data reveal spectral hardening with increasing flux levels, often correlated with an increase of the source activity in TeV 1 The Astrophysical Journal, 738:25 (19pp), 2011 September 1 Acciari et al.γ -rays. Contemporaneous spectral energy distributions were generated for 18 nights, each of which are reasonably described by a one-zone synchrotron self-Compton model.

We present 10 years of R-band monitoring data of 31 northern blazars which were either detected at very high energy (VHE) gamma rays or listed as potential VHE gamma-ray emitters. The data comprise 11820 photometric data points in the R-band obtained in 2002-2012. We analyze the light curves by determining their power spectral density (PSD) slopes assuming a power-law dependence with a single slope β and a Gaussian probability density function (PDF). We use the multiple fragments variance function (MFVF) combined with a forward-casting approach and likelihood analysis to determine the slopes and perform extensive simulations to estimate the uncertainties of the derived slopes. We also look for periodic variations via Fourier analysis and quantify the false alarm probability through a large number of simulations. Comparing the obtained PSD slopes to values in the literature, we find the slopes in the radio band to be steeper than those in the optical and gamma rays. Our periodicity search yielded one target, Mrk 421, with a significant (p < 5%) period. Finding one significant period among 31 targets is consistent with the expected false alarm rate, but the period found in Mrk 421 is very strong and deserves further consideration

Aims. In a previous study we suggested that the broad-band emission and variability properties of BL Lacertae can be accounted for by a double synchrotron emission component with related inverse-Compton emission from the jet, plus thermal radiation from the accretion disc. Here we investigate the matter with further data extending over a wider energy range. Methods. The GLAST-AGILE Support Program (GASP) of the whole earth blazar telescope (WEBT) monitored BL Lacertae in 2008-2009 at radio, near-IR, and optical frequencies to follow its flux behaviour. During this period, high-energy observations were performed by XMM-Newton, Swift, and Fermi. We analyse these data with particular attention to the calibration of Swift UV data, and apply a helical jet model to interpret the source broad-band variability.Results. The GASP-WEBT observations show an optical flare in 2008 February-March, and oscillations of several tenths of mag on a few-day time scale afterwards. The radio flux is only mildly variable. The UV data from both XMM-Newton and Swift seem to confirm a UV excess that is likely caused by thermal emission from the accretion disc. The X-ray data from XMM-Newton indicate a strongly concave spectrum, as well as moderate (∼4-7%) flux variability on an hour time scale. The Swift X-ray data reveal fast (interday) flux changes, not correlated with those observed at lower energies. We compare the spectral energy distribution (SED) corresponding to the 2008 low-brightness state, which was characterised by a synchrotron dominance, to the 1997 outburst state, where the inverse-Compton emission was prevailing. A fit with an inhomogeneous helical jet model suggests that two synchrotron components are at work with their self inverse-Compton emission. Most likely, they represent the radiation from two distinct emitting regions in the jet. We show that the difference between the source SEDs in 2008 and 1997 can be explained in terms of pure geometrical variations. The outburst state occurred when the jet-emitting regions were better aligned with the line of sight, producing an increase of the Doppler beaming factor. Conclusions. Our analysis demonstrates that the jet geometry can play an extremely important role in the BL Lacertae flux and spectral variability. Indeed, the emitting jet is probably a bent and dynamic structure, and hence changes in the emitting regions viewing angles are likely to happen, with strong consequences on the source multiwavelength behaviour.

The MAGIC Collaboration reports the detection of the blazar S5 0716+714 (z = 0.31 ± 0.08) in very high energy gamma rays. The observations were performed in 2007 November and in 2008 April, and were triggered by the Kungliga Vetenskapliga Akademi telescope due to the high optical state of the object. An overall significance of the signal accounts to S = 5.8σ for 13.1 hr of data. Most of the signal (S = 6.9σ) comes from the 2008 April data sample during a higher optical state of the object suggesting a possible correlation between the Very High Energy γ-ray and optical emissions. The differential energy spectrum of the 2008 data sample follows a power law with a photon index of Γ = 3.45 ± 0.54 stat ± 0.2 syst , and the integral flux above 400 GeV is at the level of (7.5 ± 2.2 stat ± 2.3 syst) × 10 −12 cm −2 s −1 , corresponding to a 9% Crab Nebula flux. Modeling of the broadband spectral energy distribution indicates that a structured jet model appears to be more promising in describing the available data than a simple one-zone synchrotron self-Compton model.

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