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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 all flux distributions prefer a double-log-normal fit. In case of the optical bands, this 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.

We present a comprehensive 20-year multiwavelength variability study of the blazar PKS 2155 - 304, one of the most luminous and extensively monitored high-frequency-peaked BL Lac objects in the southern hemisphere. 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. While the HE γ-ray and X-ray flux distributions are compatible with log-normality, the optical band prefers a double-log-normal fit. The latter matches well the baseline flux change that took place in the optical range around 2009, and which is clearly visible in the flux distribution. While interband flux-flux correlations are found, no stable temporal lags emerge. This implies varying correlation patterns between epochs. The X-ray emission displays a robust harder-when-brighter trend, however with epoch-dependent slopes, while the γ-ray spectra show only mild flux dependence. The fractional variability increases systematically with energy within a given radiation component. No direct correlation of the year-wise fractional variability with the corresponding average flux could be found. Interestingly, a pronounced X-ray spectral upturn, detected during a low state in 2012, points to an additional radiative component. As the connection from this upturn to the γ-ray spectrum is not smooth, it probably is not the onset of the inverse-Compton component, but more likely points either to a hadronic contribution or an additional spatially-separate emission zone. These findings reveal the complexity of variability patterns in PKS 2155 - 304 and the non-uniform nature of its particle acceleration and emission processes.

High-frequency gravitational waves, particularly in the range <inline-formula><mml:math><mml:mrow><mml:mi>f</mml:mi><mml:mo>≳</mml:mo><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mrow><mml:mn>10</mml:mn></mml:mrow></mml:msup><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>Hz</mml:mi></mml:mrow></mml:math></inline-formula>, represent a compelling probe of physics beyond the Standard Model. Due to the absence of direct detection methods in this frequency regime, alternative strategies may be pursued. One promising approach involves the conversion of gravitons into photons in the presence of magnetic fields, a process known as the inverse Gertsenshtein effect. In this study, we explore such graviton-to-photon conversions occurring within the magnetic field environment of the M87 galaxy, utilizing realistic models for the galactic magnetic field and plasma density structure. We use the broadband electromagnetic spectrum of M87, ranging from millimeter to TeV gamma rays, to search for hidden contributions from graviton-photon conversions. In the well-constrained frequency range <inline-formula><mml:math><mml:mrow><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mrow><mml:mn>10</mml:mn></mml:mrow></mml:msup><mml:mi>─</mml:mi><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mrow><mml:mn>27</mml:mn></mml:mrow></mml:msup><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>Hz</mml:mi></mml:mrow></mml:math></inline-formula>, the lack of excess emission allows us to place improved bounds on the gravitational wave strain amplitude <inline-formula><mml:math><mml:msub><mml:mi>h</mml:mi><mml:mi>c</mml:mi></mml:msub></mml:math></inline-formula> or on spectral energy density <inline-formula><mml:math><mml:msub><mml:mi>Ω</mml:mi><mml:mi>gw</mml:mi></mml:msub><mml:msup><mml:mi>h</mml:mi><mml:mn>2</mml:mn></mml:msup></mml:math></inline-formula>. We find that our results from M87 yield substantially stronger constraints compared to existing bounds derived from Milky Way magnetic field considerations, with improvements ranging from one to five orders of magnitude depending on the frequency band, thereby enhancing the prospects for probing high-frequency gravitational wave backgrounds through indirect electromagnetic signatures.

We investigate the gravitational wave spectrum from first-order QCD phase transitions using the parity doublet model at finite baryon chemical potential. The model incorporates the chiral invariant mass <inline-formula><mml:math><mml:msub><mml:mi>m</mml:mi><mml:mn>0</mml:mn></mml:msub></mml:math></inline-formula>, representing the portion of nucleon mass that persists even when chiral symmetry is restored. Within the model, we identify two first-order phase transition regions: the nuclear liquid-gas transition and the chiral phase transition. By solving the bounce equation and computing the Euclidean action <inline-formula><mml:math><mml:msub><mml:mi>S</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:mo>/</mml:mo><mml:mi>T</mml:mi></mml:math></inline-formula>, we obtain the gravitational wave spectra from both transitions. The liquid-gas transition yields transition strength <inline-formula><mml:math><mml:mi>α</mml:mi><mml:mo>∼</mml:mo><mml:mi>O</mml:mi><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>)</mml:mo></mml:math></inline-formula> and inverse duration of the phase transition <inline-formula><mml:math><mml:mi>β</mml:mi><mml:mo>/</mml:mo><mml:mi>H</mml:mi><mml:mo>∼</mml:mo><mml:mi>O</mml:mi><mml:mo>(</mml:mo><mml:mn>10</mml:mn><mml:mo>)</mml:mo><mml:mi>─</mml:mi><mml:mi>O</mml:mi><mml:mo>(</mml:mo><mml:mn>100</mml:mn><mml:mo>)</mml:mo></mml:math></inline-formula> near the end point of the first-order line, producing signals with peak frequencies from the millihertz to the nanohertz band that can fit the existing data. In contrast, the chiral transition produces signals suppressed by approximately five orders of magnitude, well below the sensitivity of all current and planned detectors. These results establish, within the parity doublet model, a quantitative link between first-order QCD phase transitions and their gravitational wave signatures, and provide a concrete framework for relating gravitational wave observations to the chiral invariant mass <inline-formula><mml:math><mml:msub><mml:mi>m</mml:mi><mml:mn>0</mml:mn></mml:msub></mml:math></inline-formula> and thus to the origin of nucleon mass.