We present coordinated multiwavelength observations of the bright, nearby BL Lacertae object Mrk 421 taken in 2013 January-March, involving GASP-WEBT, Swift, NuSTAR, Fermi-LAT, MAGIC, VERITAS, and other collaborations and instruments, providing data from radio to very high energy(VHE) γ-ray bands. NuSTAR yielded previously unattainable sensitivity in the 3-79keV range, revealing that the spectrum softens when the source is dimmer until the X-ray spectral shape saturates into a steep 3 G » power law, with no evidence for an exponential cutoff or additional hard components up to ∼80keV. For the first time, we observed both the synchrotron and the inverse-Compton peaks of the spectral energy distribution (SED) simultaneously shifted to frequencies below the typical quiescent state by an order of magnitude. The fractional variability as a function of photon energy shows a double-bump structure that relates to the two bumps of the broadband SED. In each bump, the variability increases with energy, which, in the framework of the synchrotron self-Compton model, implies that the electrons with higher energies are more variable. The measured multi band variability, the significant X-ray-to-VHE correlation down to some of the lowest fluxes ever observed in both bands, the lack of correlation between optical/UV and X-ray flux, the low degree of polarization and its significant (random) variations, the short estimated electron cooling time, and the significantly longer variability timescale observed in the NuSTAR light curves point toward in situ electron acceleration and suggest that there are multiple compact regions contributing to the broadband emission of Mrk 421 during low-activity states.

Context. BL Lacertae is the prototype of the blazar subclass named after it. Yet, it has occasionally shown a peculiar behaviour that has questioned a simple interpretation of its broad-band emission in terms of synchrotron plus synchrotron self-Compton (SSC) radiation. Aims. In the 2007-2008 observing season we carried out a new multiwavelength campaign of the Whole Earth Blazar Telescope (WEBT) on BL Lacertae, involving three pointings by the XMM-Newton satellite in July and December 2007, and January 2008, to study its emission properties, particularly in the optical-X-ray energy range. Methods. The source was monitored in the optical-to-radio bands by 37 telescopes. The brightness level was relatively low. Some episodes of very fast variability were detected in the optical bands. Flux changes had larger amplitude at the higher radio frequencies than at longer wavelengths. Results. The X-ray spectra acquired by the EPIC instrument onboard XMM-Newton are well fitted by a power law with photon index Γ ∼ 2 and photoelectric absorption exceeding the Galactic value. However, when taking into account the presence of a molecular cloud on the line of sight, the EPIC data are best fitted by a double power law, implying a concave X-ray spectrum. The spectral energy distributions (SEDs) built with simultaneous radio-to-X-ray data at the epochs of the XMM-Newton observations suggest that the peak of the synchrotron emission lies in the near-IR band, and show a prominent UV excess, besides a slight soft-X-ray excess. A comparison with the SEDs corresponding to previous observations with X-ray satellites shows that the X-ray spectrum is very variable, since it can change from extremely steep to extremely hard, and can be more or less curved in intermediate states. We ascribe the UV excess to thermal emission from the accretion disc, and the other broad-band spectral features to the presence of two synchrotron components, with their related SSC emission. We fit the thermal emission with a black body law and the non-thermal components by means of a helical jet model. The fit indicates a disc temperature > ∼ 20 000 K and a luminosity > ∼ 6 × 10 44 erg s −1 .

Aims. The Whole Earth Blazar Telescope (WEBT) consortium has been monitoring the blazar 3C 454.3 from the radio to the optical bands since 2004 to study its emission variability properties. Methods. We present and analyse the multifrequency results of the 2007−2008 observing season, including XMM-Newton observations and near-IR spectroscopic monitoring, and compare the recent emission behaviour with the past one. The historical mm light curve is presented here for the first time.Results. In the optical band we observed a multi-peak outburst in July−August 2007, and other faster events in November 2007−February 2008. During these outburst phases, several episodes of intranight variability were detected. A mm outburst was observed starting from mid 2007, whose rising phase was contemporaneous to the optical brightening. A slower flux increase also affected the higher radio frequencies, the flux enhancement disappearing below 8 GHz. The analysis of the optical-radio correlation and time delays, as well as the behaviour of the mm light curve, confirm our previous predictions, suggesting that changes in the jet orientation likely occurred in the last few years. The historical multiwavelength behaviour indicates that a significant variation in the viewing angle may have happened around year 2000. Colour analysis confirms a general redder-when-brighter trend, which reaches a "saturation" at R ∼ 14 and possibly turns into a bluer-when-brighter trend in bright states. This behaviour is due to the interplay of different emission components, the synchrotron one possibly being characterised by an intrinsically variable spectrum. All the near-IR spectra show a prominent Hα emission line (EW obs = 50−120 Å), whose flux appears nearly constant, indicating that the broad line region is not affected by the jet emission. We show the broad-band SEDs corresponding to the epochs of the XMM-Newton pointings and compare them to those obtained at other epochs, when the source was in different brightness states. A double power-law fit to the EPIC spectra including extra absorption suggests that the soft-X-ray spectrum is concave, and that the curvature becomes more pronounced as the flux decreases. This connects fairly well with the UV excess, which becomes more prominent with decreasing flux. The most obvious interpretation implies that, as the beamed synchrotron radiation from the jet dims, we can see both the head and the tail of the big blue bump. The X-ray flux correlates with the optical flux, suggesting that in the inverse-Compton process either the seed photons are synchrotron photons at IR-optical frequencies or the relativistic electrons are those that produce the optical synchrotron emission. The X-ray radiation would thus be produced in the jet region from where the IR-optical emission comes.

We report the results of optical monitoring for a sample of 11 blazars including 10 BL Lacertae objects (BL Lacs) and one flat spectrum radio quasar (FSRQ). We have measured the multiband optical flux and colour variations in these blazars on intraday and short-term time-scales of months and have limited data for two more blazars. These photometric observations were made during 2009-2011, using six optical telescopes, four in Bulgaria, one in Greece and one in India. On short-term time-scales we found significant flux variations in nine of the sources and colour variations in three of them. Intraday variability was detected on six nights for two sources out of the 18 nights and four sources for which we collected such data. These new optical observations of these blazars plus data from our previous published papers (for three more blazars) were used to analyse their spectral flux distributions in the optical frequency range. Our full sample for this purpose includes six high-synchrotron-frequency-peaked BL Lacs (HSPs), three intermediate-synchrotron-frequency-peaked BL Lacs (ISPs) and six lowsynchrotron-frequency-peaked BL Lacs (LSPs; including both BL Lacs and FSRQs). We also investigated the spectral slope variability and found that the average spectral slopes of LSPs show a good accordance with the synchrotron self-Compton loss dominated model. Our analysis supports previous studies that found that the spectra of the HSPs and FSRQs have significant additional emission components. The spectra of all these HSPs and LSPs get flatter when they become brighter, while for FSRQs the opposite appears to hold. This supports the hypothesis that there is a significant thermal contribution to the optical spectrum for FSRQs.

We present recent optical photometric observations of the blazar OJ 287 taken during September 2015 -May 2016. Our intense observations of the blazar started in November 2015 and continued until May 2016 and included detection of the large optical outburst in December 2016 that was predicted using the binary black hole model for OJ 287. For our observing campaign, we used a total of 9 ground based optical telescopes of which one is in Japan, one is in India, three are in Bulgaria, one is in Serbia, one is in Georgia, and two are in the USA. These observations were carried out in 102 nights with a total of ∼ 1000 image frames in BVRI bands, though the majority were in the R band. We detected a second comparably strong flare in March 2016. In addition, we investigated multi-band flux variations, colour variations, and spectral changes in the blazar on diverse timescales as they are useful in understanding the emission mechanisms. We briefly discuss the possible physical mechanisms most likely responsible for the observed flux, colour and spectral variability.