PKS 2155-304 is one of the brightest blazars in the X-ray band. It was repeatedly monitored with BeppoSAX during three long campaigns of about 2 days each in November of 1996November of , 1997November of and 1999. The source underwent different states of intensity and was clearly variable with successive flares detected. This paper presents temporal and spectral analysis to study the X-ray variability trends for a blazar. The variability shows larger amplitude and shorter timescale at higher energies. The power spectral densities have steep power-law slopes of ∼ 2-3, indicating shot noise variability. Structure function analysis reveals the existence of "typical" timescales characteristic of the half duration of the flares. From the cross-correlation analysis we find that the values of soft lags, i.e., delays of soft (0.1-1.5 keV) photons with respect to hard (3.5-10 keV) ones, differ from flare to flare, ranging from a few hundred seconds to about one hour. There is a suggestion that the flares with shorter duration show smaller soft lags. The soft lags are also energy-dependent, with longer lags of lower energy emission with respect to the emission in the 4-10 keV. The time-resolved X-ray spectral fits with a curved model show that peak energies of the synchrotron component are located in the very soft X-ray range or even below the BeppoSAX lower energy limit, 0.1 keV. A correlation between peak energies and fluxes is marginal. Spectral evolution during some flares shows clockwise loops in the spectral index-flux plane, confirming the soft lags indicated

This paper reports the synthesis of tetragonal zirconia nanowires using template method. An as‐prepared sample was characterized by scanning and transmission electron microscopy. It was found that the as‐prepared materials were tetragonal zirconia nanowires with average diameters of ca. 80 nm and length of over 10 μm. The Raman spectrum showed peaks at 120, 461, and 629 cm–1, which are attributed to the Eg, Eg, and B1g phonon modes of the tetragonal zirconia structure, respectively. The UV‐vis absorption spectrum showed an absorption peak at 232.5 nm (5.33 eV in photon energy). Photoluminescence (PL) spectra of zirconia nanowires showed a strong emission peak at ca. 388 nm at room temperature, which is attributed to the ionized oxygen vacancy in the zirconia nanowires system.

Starting from XMM-Newton EPIC pn data, we present the X-ray variability characteristics of PKS 2155À304 using a simple analysis of the excess variance, 2 XS , and of the fractional rms variability amplitude, F var . The scatter in 2 XS and F var , calculated using 500 s long segments of the light curves, is smaller than the scatter expected for red-noise variability. This alone does not imply that the underlying process responsible for the variability of the source is stationary, since the real changes of the individual variance estimates are possibly smaller than the large scatters expected for a red-noise process. In fact, the averaged 2 XS and F var , which reduce the fluctuations of the individual variances, also change with time, indicating nonstationary variability. Moreover, both the averaged XS (absolute rms variability amplitude) and the averaged F var show linear correlation with source flux, but in an opposite sense: XS correlates with flux, but F var anticorrelates with flux. These correlations suggest that the variability process of the source is strongly nonstationary, as random scatters of variances should not yield any correlation. Spectra of F var were constructed to compare variability amplitudes in different energy bands. We found that the fractional rms variability amplitude of the source, when significant variability is observed, increases logarithmically with the photon energy, indicating significant spectral variability. The point-to-point variability amplitude may also track this trend, suggesting that the slopes of the power spectral density of the source are energy-independent. Using the normalized excess variance, the black hole mass of PKS 2155À304 was estimated to be about 1:45 ; 10 8 M . This is compared and contrasted with the estimates derived from measurements of the host galaxies.

Using the cross-spectral method, we confirm the existence of the X-ray hard lags discovered with cross-correlation function technique during a large flare of Mrk 421 observed with BeppoSAX. For the 0.1-2 versus 2-10 keV light curves, both methods suggest sub-hour hard lags. In the time domain, the degree of hard lag, i.e. the amplitude of the 3.2-10 keV photons lagging the lower energy ones, tends to increase with the decreasing energy. In the Fourier frequency domain, by investigating the cross-spectra of the 0.1-2/2-10 keV and the 2-3.2/3.2-10 keV pairs of light curves, the flare also shows hard lags at the lowest frequencies. However, with the present data, it is impossible to constrain the dependence of the lags on frequencies even though the detailed simulations demonstrate that the hard lags at the lowest frequencies probed by the flare are not an artefact of sparse sampling, Poisson and red noise. As a possible interpretation, the implication of the hard lags is discussed in the context of the interplay between the (diffusive) acceleration and synchrotron cooling of relativistic electrons responsible for the observed X-ray emission. The energy-dependent hard lags are in agreement with the expectation of an energy-dependent acceleration time-scale. The inferred magnetic field (B ∼ 0.11 G) is consistent with the value inferred from the spectral energy distributions of the source. Future investigations with higher quality data that show whether or not the time-lags are energy-/frequency-dependent will provide a new constraint on the current models of the TeV blazars.

With currently available XMM-Newton EPIC pn observations spanned over about 3 yr, we present a detailed spectral and temporal variability of the 0.2-10 keV X-ray emission from the X-ray-bright BL Lac object PKS 2155À304. The spectral variability is examined with a model-independent hardness ratio method. We find that the spectral evolution of the source follows the light curves well, indicating that the spectra harden when the fluxes increase. The plots of hardness ratios versus count rates show that the spectral changes are particularly significant during flares. The cross-correlation functions (CCFs) show that the light curves in the different energy bands are well correlated at different time lags. The CCF peaks (i.e., the maximum correlation coefficients) tend to become smaller with larger energy differences, and the variabilities in the different energy bands are more correlated for the flares than for the other cases. In most cases the higher energy band variations lead the lower energy band, but in two cases we observed the opposite behavior, that the lower energy variability possibly leads the higher energy variability. The time lags increase with the energy differences between the two cross-correlated light curves. The maximum lag is found to be up to about 1 hr, supporting the findings obtained with previous low Earth orbit X-ray missions. We discuss our results in the context of the particle acceleration,