We present the results of extensive multiband intranight optical monitoring of BL Lacertae during 2010–2012. BL Lacertae was very active in this period and showed intense variability in almost all wavelengths. We extensively observed it for a total for 38 nights; on 26 of them, observations were done quasi-simultaneously in B, V, R and I bands (totalling 113 light curves), with an average sampling interval of around 8 min. BL Lacertae showed significant variations on hour-like time-scales in a total of 19 nights in different optical bands. We did not find any evidence for periodicities or characteristic variability time-scales in the light curves. The intranight variability amplitude is generally greater at higher frequencies and decreases as the source flux increases. We found spectral variations in BL Lacertae in the sense that the optical spectrum becomes flatter as the flux increases but in several flaring states, deviates from the linear trend suggesting different jet components contributing to the emission at different times.

Blazars can be divided into two sub-classes namely high energy and low energy peaked blazars. In spectral energy distribution, the first synchrotron hump of the former class peaks in UV/X-rays and in IR/optical bands for the latter class. The peak of the spectral energy distribution seems to be responsible for variability properties of these classes of blazars in X-ray and optical bands. Since, in low energy peaked blazars, the X-ray bands lies well below the synchrotron hump, one expects that the highest energy electrons available for the synchrotron emission would have slower effect of variability on X-ray intra-day time-scale. In this paper, by taking the advantage of a sample of 12 low energy peaked blazars with total 50 observations from XMM–Newton since its launch, we confirm that this class is less variable in X-ray bands. We found that out of 50 observational light curves, genuine intra-day variability is present in only two of light curves i.e 4 per cent. Similar results we obtained from our earlier optical intra-day variability studies of high energy peaked blazars where out of 144 light curves, only genuine intra-day variability was detected in 6 light curves i.e ∼4 per cent. Since, X-ray bands lie below the peak of the spectral energy distribution of LSPs where inverse Compton mechanism is dominating rather than synchrotron radiation at the peak of the optical band, leads to slower variability in the X-ray bands. Hence, reducing their intra-day variability in X-ray bands as compared to the variability in optical bands.

Blazars are classified into high-, intermediate- and low-energy-peaked sources based on the location of their synchrotron peak. This lies in infra-red/optical to ultra-violet bands for low- and intermediate-peaked blazars. The transition from synchrotron to inverse Compton emission falls in the X-ray bands for such sources. We present the spectral and timing analysis of 14 low- and intermediate-energy-peaked blazars observed with XMM–Newton spanning 31 epochs. Parametric fits to X-ray spectra help constrain the possible location of transition from the high-energy end of the synchrotron to the low-energy end of the inverse Compton emission. In seven sources in our sample, we infer such a transition and constrain the break energy in the range 0.6–10 keV. The Lomb–Scargle periodogram is used to estimate the power spectral density (PSD) shape. It is well described by a power law in a majority of light curves, the index being flatter compared to general expectation from active galactic nuclei, ranging here between 0.01 and 1.12, possibly due to short observation durations resulting in an absence of long-term trends. A toy model involving synchrotron self-Compton and external Compton (EC; disc, broad line region, torus) mechanisms are used to estimate magnetic field strength ≤0.03–0.88 G in sources displaying the energy break and infer a prominent EC contribution. The time-scale for variability being shorter than synchrotron cooling implies steeper PSD slopes which are inferred in these sources.

The synchrotron peak of the X-ray bright High Energy Peaked Blazar PKS 2155–304 occurs in the UV-EUV region and hence its X-ray emission (0.6–10 keV) lies mostly in the falling part of the synchrotron hump. We aim to study the X-ray emission of PKS 2155–304 during different intensity states in 2009–2014 using the XMM-Newton satellite. We studied the spectral curvature of all of the observations to provide crucial information on the energy distribution of the nonthermal particles. Most of the observations show curvature or deviation from a single power law and can be well modeled by a log parabola model. In some of the observations, we find spectral flattening after 6 keV. In order to find the possible origin of the X-ray excess, we built the Multiband Spectral Energy distribution. We find that the X-ray excess in PKS 2155–304 is difficult to fit in the one zone model but, could be easily reconciled in the spine/layer jet structure. The hard X-ray excess can be explained by the inverse Comptonization of the synchrotron photons (from the layer) by the spine electrons.

We present the results of extensive multi-band intra-night optical monitoring of BL Lacertae during 2010--2012. BL Lacertae was very active in this period and showed intense variability in almost all wavelengths. We extensively observed it for a total for 38 nights; on 26 of them observations were done quasi-simultaneously in B, V, R and I bands (totaling 113 light curves), with an average sampling interval of around 8 minutes. BL Lacertae showed significant variations on hour-like timescales in a total of 19 nights in different optical bands. We did not find any evidence for periodicities or characteristic variability time-scales in the light curves. The intranight variability amplitude is generally greater at higher frequencies and decreases as the source flux increases. We found spectral variations in BL Lacertae in the sense that the optical spectrum becomes flatter as the flux increases but in several flaring states deviates from the linear trend suggesting different jet components contributing to the emission at different times.

Blazars are classified into high, intermediate and low energy peaked sources based on the location of their synchrotron peak. This lies in infra-red/optical to ultra-violet bands for low and intermediate peaked blazars. The transition from synchrotron to inverse Compton emission falls in the X-ray bands for such sources. We present the spectral and timing analysis of 14 low and intermediate energy peaked blazars ob- served with XMMNewton spanning 31 epochs. Parametric fits to X-ray spectra helps constrain the possible location of transition from the high energy end of the syn- chrotron to the low energy end of the inverse Compton emission. In seven sources in our sample, we infer such a transition and constrain the break energy in the range 0.6 10 keV. The Lomb-Scargle periodogram is used to estimate the power spectral density (PSD) shape. It is well described by a power law in a majority of light curves, the index being flatter compared to general expectation from AGN, ranging here between 0.01 and 1.12, possibly due to short observation durations resulting in an absence of long term trends. A toy model involving synchrotron self-Compton (SSC) and exter- nal Compton (EC; disk, broad line region, torus) mechanisms are used to estimate magnetic field strength 6 0.03 - 0.88 G in sources displaying the energy break and infer a prominent EC contribution. The timescale for variability being shorter than synchrotron cooling implies steeper PSD slopes which are inferred in these sources.