20 years of monitoring: PKS 2155-304 and PKS 1510-089 in the eyes of Swift and Fermi. II. PKS 1510-089 and comparison

Abstract

We present a comprehensive, two-decade, multiwavelength variability study of the blazar PKS 1510-089, one of the most prominent and extensively monitored flat-spectrum radio quasars. Using Fermi-LAT $γ$-ray data together with Swift-XRT and UVOT observations spanning 2005-2024, we trace the long-term evolution of its flux, interband correlations, and spectral behaviour across the optical, X-ray, and $γ$-ray bands. We find that the HE $γ$-ray and X-ray flux distributions are log-normal, while the optical distributions are compatible with double-log-normal functions. The latter may be due to contributions from the accretion disk. The range of fluxes in a given band, as well as the fractional variability values are in-line with the expectations that high-energy parts of a given spectral component are more variable than low-energy parts. No obvious cross-correlations exist between the bands over the 20 years of observations. The X-ray and $γ$-ray spectra are variable, but do not show any trend with flux. These results are suggestive of different zones being active in the jet of PKS 1510-089 at any given time. In a previous paper, we used the same techniques to study the high-frequency-peaked BL Lac object PKS 2155-304. Both sources follow the aforementioned trend on the energy-dependent variability of the spectral components, as well as the lack of significant cross-correlations between the studied bands. While PKS 2155-304 exhibits a harder-when-brighter behaviour in its high-energy part of the synchrotron component, no such behaviour could be found in PKS 1510-089. Both sources show orphan flares, which can seemingly happen in any band. In summary, the long-term studies of these two sources reveal that the underlying physics is similar in these apparently different source classes, even though variability patterns keep changing and remain unpredictable.

Figures (6)

• Figure 1: Time-evolution of PKS 1510$-$089 from 2005 to 2024. (a) HE $\gamma$-ray light curve from Fermi-LAT combining both power-law and log-parabola spectral reconstructions. The black dashed line marks the weighted average flux of the data shown. (b) HE $\gamma$-ray spectral parameters with gray points for $\Gamma$, and black and red symbols for $\alpha$ and $\beta$, respectively, in case the log-parabola reconstruction is preferred. The black dashed line gives the weighted average of $\Gamma$ and $\alpha$ combined, while the red dashed line indicates the weighted average of $\beta$. The gray solid and dotted lines mark values of $0.0$ and $2.0$ respectively. (c) X-ray flux light curve from Swift-XRT with the black dashed line giving the weighted average. (d) X-ray spectral parameters, $\alpha$ and $\beta$, as indicated with the corresponding dashed lines giving the respective weighted averages. (e) B and V band light curves from Swift-UVOT with coloured dashed lines indicating the respective weighted average. (f) Year-wise $F_\text{var}$ values for the four bands shown above. The coloured dashed lines indicate the respective $F_\text{var}$ values considering all fluxes of a given band.
• Figure 2: (Top row) Correlation plots of flux vs flux for X-rays vs HE $\gamma$ rays (left), B band vs HE $\gamma$ rays (middle), and B band vs X-rays (right). (2nd row) HE $\gamma$-ray index vs flux (left), curvature vs flux (middle) and curvature vs index. (Bottom row) Same as second row, but for the X-ray data. In each panel, the Person R coefficient for the correlation is given, as well as the p-value of the probability that a similar or higher R coefficient could be obtained from an uncorrelated data set.
• Figure 3: DCCF with a time resolution of $\Delta\tau=5\,$d of PKS 1510$-$089 between the various bands as labelled. The gray bands mark the confidence intervals as indicated.
• Figure 4: Flux distributions of the HE $\gamma$-ray (top left), X-ray (top right), B band (bottom left) and V band (bottom right) data using logarithmic binning. The red lines mark Gaussian fits with its mean, standard deviation and p-value of the KS test indicated as red text. The solid blue line marks a double-Gaussian fit, while the dashed and dash-dotted lines represent the individual components. The corresponding parameter sets are given in blue text, where parameters 1 correspond to the dashed line and parameters 2 to the dash-dotted line.
• Figure 5: Similar to Fig. \ref{['fig:1510_flux_dist']}, but the blue shaded histogram is for data taken before MJD 59423, while the golden histograms are for data taken afterwards (as in Fig. \ref{['fig:1510_flux_dist']}. The blue and golden lines are Gaussian fits to the respective histograms with parameters given in the appropriate colour.