Context.We present the results from a study of the long-term optical spectral variations of BL Lacertae, using the long and well-sampled B and R-band light curves of the Whole Earth Blazar Telescope ...(WEBT) collaboration, binned in time intervals of 1 day. Aims.We study the relation between the long-term spectral variations and the respective flux variations of the source. Methods.Using cross-correlation techniques to investigate whether there are any delays between the flux variations in different energy bands and between the flux and spectral variations. Results.The relation between spectral slope and flux (the spectrum gets bluer as the source flux increases) is well described by a power-law model, although there is significant scatter around the best-fitting model line. To some extent, this is due to the spectral evolution of the source (along well-defined loop-like structures) during low-amplitude events, which are superimposed on the major optical flares, and evolve on time scales of a few days. The B and R-band variations are well correlated, with no significant, measurable delays larger than a few days. On the other hand, we find that the spectral variations lead those in the flux light curves by ~4 days. During at least the largest amplitude flares, the B-band variations appear to evolve faster than those in the R band. Conclusions.We confirm the “bluer-when-brighter” mild chromatism of the long-term variations, and we show that it can be explained if the flux increases/decreases faster in the B than in the R band. We also report the discovery of the lag between spectral and flux changes. These two features can be explained in terms of Doppler factor variations due to changes in the viewing angle of a curved and inhomogeneous emitting jet.
Aims. Multiwavelength variability of blazars offers indirect, but very effective, insight into their powerful engines and on the mechanisms through which energy is propagated from the centre down the ...jet. The BL Lac object Mkn 421 is a TeV emitter, a bright blazar at all wavelengths, and therefore an excellent target for variability studies. Methods. We activated INTEGRAL observations of Mkn 421 in an active state on 16–21 April 2013, and complemented them with Fermi-LAT data. Results. We obtained well sampled optical, soft, and hard X-ray light curves that show the presence of two flares and time-resolved spectra in the 3.5–60 keV (JEM-X and IBIS/ISGRI) and 0.1–100 GeV (Fermi-LAT) ranges. The average flux in the 20–100 keV range is 9.1 × 10-11 erg s-1 cm-2 (~4.5 mCrab) and the nuclear average apparent magnitude, corrected for Galactic extinction, is V ≃ 12.2. In the time-resolved X-ray spectra, which are described by broken power laws and, marginally better, by log-parabolic laws, we see a hardening that correlates with flux increase, as expected in refreshed energy injections in a population of electrons that later cool via synchrotron radiation. The hardness ratios between the JEM-X fluxes in two different bands and between the JEM-X and IBIS/ISGRI fluxes confirm this trend. During the observation, the variability level increases monotonically from the optical to the hard X-rays, while the large LAT errors do not allow a significant assessment of the MeV-GeV variability. The cross-correlation analysis during the onset of the most prominent flare suggests a monotonically increasing delay of the lower frequency emission with respect to that at higher frequency, with a maximum time-lag of about 70 min, that is however not well constrained. The spectral energy distributions from the optical to the TeV domain were compared to homogeneous models of blazar emission based on synchrotron radiation and synchrotron self-Compton scattering. They represent a satisfactory description, except in the state corresponding to the LAT softest spectrum and highest flux. Conclusions. Multiwavelength variability of Mkn 421 can be very complex, with patterns changing from epoch to epoch down to intra-day timescales, depending on the emission state. This makes accurate monitoring of this source during bright hard X-ray states necessary and calls for the elaboration of multicomponent, multizone, time-dependent models.
The infrared properties of blazars can be studied from the statistical point of view with the help of sky surveys, like that provided by the Wide-field Infrared Survey Explorer and the Two Micron All ...Sky Survey. However, these sources are known for their strong and unpredictable variability, which can be monitored for a handful of objects only. In this paper, we consider the 28 blazars (14 BL Lac objects and 14 flat-spectrum radio quasars, FSRQs) that are regularly monitored by the GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope since 2007. They show a variety of infrared colours, redshifts, and infrared–optical spectral energy distributions (SEDs), and thus represent an interesting mini-sample of bright blazars that can be investigated in more detail. We present near-IR light curves and colours obtained by the GASP from 2007 to 2013, and discuss the infrared–optical SEDs. These are analysed with the aim of understanding the interplay among different emission components. BL Lac SEDs are accounted for by synchrotron emission plus an important contribution from the host galaxy in the closest objects, and dust signatures in 3C 66A and Mrk 421. FSRQ SEDs require synchrotron emission with the addition of a quasar-like contribution, which includes radiation from a generally bright accretion disc (νL
ν up to ∼4 × 1046 erg s−1), broad-line region, and a relatively weak dust torus.
We present all the publicly available data, from optical/UV wavelengths (UVOT) to X-rays (XRT, BAT), obtained from Swift observations of the blazar PKS 2155-304, performed in response to the rapid ...alert sent out after the strong TeV activity (up to 17 crab flux level at E > 200 GeV) at the end of 2006 July. The X-ray flux increased by a factor of 5 in the 0.3-10 keV energy band and by a factor of 1.5 at optical/UV wavelengths, with roughly 1 day of delay. The comparison of the spectral energy distribution built with data quasi-simultaneous to the TeV detections shows an increase of the overall normalization with respect to archival data but only a small shift of the frequency of the synchrotron peak that remains consistent with the values reported in past observations when the TeV activity was much weaker.
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 $\Gamma \sim 2$ 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 $ \ga$$ 20\,000 \rm \, K$ and a luminosity $ \ga$$ 6 \times 10^{44} ~\rm erg \, s^{-1}$.
Aims.Spectroscopic monitoring of BL Lac objects is a difficult task that nonetheless can provide important information on the different components of the active galactic nucleus. Methods.We performed ...optical spectroscopic monitoring of the BL Lac object AO 0235+164 ($z=0.94$) with the VLT and TNG telescopes from Aug. 2003 to Dec. 2004, during an extended WEBT campaign. The flux of this source is both contaminated and absorbed by a foreground galactic system at $z=0.524$, the stars of which can act as gravitational micro-lenses. Results.In this period the object was in an optically faint, though variable state, and a broad $\ion{Mg}{ii}$ emission line was visible at all epochs. The spectroscopic analysis reveals an overall variation in the $\ion{Mg}{ii}$ line flux of a factor 1.9, while the corresponding continuum flux density changed by a factor 4.3. Most likely, the photoionising radiation can be identified with the emission component that was earlier recognised to be present as a UV-soft-X-ray bump in the source spectral energy distribution and that is visible in the optical domain only in very faint optical states. We estimate an upper limit to the broad line region (BLR) size of a few light months from the historical minimum brightness level; from this we infer the maximum amplification of the $\ion{Mg}{ii}$ line predicted by the microlensing scenario. Conclusions.Unless we have strongly overestimated the size of the BLR, only very massive stars could significantly magnify the broad $\ion{Mg}{ii}$ emission line, but the time scale of variations due to these (rare) events would be of several years. In contrast, the continuum flux, coming from much smaller emission regions in the jet, could be affected by microlensing from the more plausible MACHO deflectors, with variability time scales of the order of some months.
Aims. New data and results on the optical behavior of the prominent blazar PKS 0735+178 (also known as OI 158, S3 0735+17, da 237, 1ES 0735+178, 3EG J0737+1721) are presented, through the most ...continuous BVRI data available in the period 1994-2004 (about 500 nights of observations). In addition, the whole historical light curve, and a new photometric calibration of comparison stars in the field of this source are reported. Methods. Several methods for time series analysis of sparse data sets are developed, adapted, and applied to the reconstructed historical light curve and to each observing season of our unpublished optical database on PKS 0735+178. Optical spectral indexes are calculated from the multi-band observations and studied on long-term (years) durations as well. For the first time in this source, variability modes, characteristic timescales, and the signal power spectrum are explored and identified over 3 decades in time with sufficient statistics. The novel investigation of mid-term optical scales (days, weeks), could be also applied and compared to blazar gamma-ray light curves that will be provided, on the same timescales, by the forthcoming GLAST observatory. Results. In the last 10 years the optical emission of PKS 0735+178 exhibited a rather achromatic behavior and a variability mode resembling the shot-noise. The source was at an intermediate or low brightness level, showing a mild flaring activity and a superimposition/succession of rapid and slower flares, without extraordinary and isolated outbursts, but, at any rate, characterized by one major active phase in 2001. Several mid-term scales of variability were found, the more common falling into duration intervals of about 27-28 days, 50-56 days and 76-79 days. Rapid variability in the historical light curve appears to be modulated by a general, slower, and rather oscillating temporal trend, where typical amplitudes of about 4.5, 8.5, and 11-13 years can be identified. This spectral and temporal analysis, accompanying our data publication, suggests the occurrence of distinctive signatures at mid-term durations that can likely be of transitory nature. On the other hand the possible pseudo-cyclical or multi-component modulations at long times could be more stable, recurrent and correlated to the bimodal radio flux behavior and the twisted radio structure observed over several years in this blazar.
Aims. Our goal is to understand the nature of blazars and the mechanisms for the generation of high-energy γ-rays, through the investigation of the prototypical blazar PKS $2155{-}304$, which shows ...complex behaviour. Methods. We analyze simultaneous infrared-to-X-ray observations obtained with XMM-Newton and REM on November 7, 2006, when the source was in a low X-ray state. We perform a comparative analysis of these results with those obtained from previous observations in different brightness states. Results. We found that the peak of the synchrotron emission moved from ultraviolet to optical wavelengths and the X-ray spectrum is best fit with a broken power law model with $\Gamma_2 \sim 2.4$ harder than $\Gamma_1 \sim 2.6$ and a break at about 3.5 keV. This suggests that the soft X-rays ($E < 3.5$ keV) are related to the high-energy tail of the synchrotron emission, while the hard X-rays ($E > 3.5$ keV) are from the energy region between the synchrotron and inverse-Compton humps. The different variability at energies below and above the break strengthens this hypothesis. Our results also stress the importance of monitoring this source at both low and high energies to better characterize its variability behaviour.
The long-term variability of the multiwavelength blazar emission can be interpreted in terms of orientation variations of a helical, inhomogeneous, non-thermally emitting jet, possibly caused by the ...orbital motion of the parent black hole in a binary system (Villata & Raiteri 1999). The helical-jet model is here applied to explain the quasi-periodic radio-optical light curves and the broad-band spectral energy distributions (SEDs) of the BL Lac object AO 0235+16. Through a suitable choice of the model parameters, the helix rotation can well account for the periodicity of the main radio and optical outbursts and for the corresponding SED variability, while the interspersed minor radio events could be interpreted as due either to some local distortions of the helical structure or to other phenomena contributing to the source emission. In particular, the probable existence of flow instabilities provides a viable interpretation for the non-periodic features.
PDF
