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We present detailed spectral energy distribution (SED) modeling of 14 local ultraluminous infrared galaxies (ULIRGs) with outstanding photometric data from the literature covering the ultraviolet–infrared (FIR) and radio bands (∼50 MHz to ∼30 GHz). We employ the CIGALE SED fitting code to model the ultraviolet–FIR–radio SED. For the radio-only SED modeling, we use the UltraNest package, leveraging its nested sampling algorithm. Combining the results from our previous study on 11 luminous infrared galaxies (LIRGs), we discuss the global astrophysical properties of a sample of 25 starburst galaxies ( z < 0.5). Their radio spectra are frequently characterized by bends and turnovers, with no indication of ULIRGs exhibiting more complicated SEDs than LIRGs despite showing more signs of interactions. Including radio measurements in the CIGALE modeling constrained the dust luminosity and star formation rate (SFR) estimates by more than 1 order of magnitude better than previously reported for starburst galaxies. We show that total and nonthermal radio luminosity at 1.4 and 4.8 GHz frequencies can be good estimators of recent SFRs for all LIRGs and those ULIRGS with an insignificant influence of active galactic nuclei. A weaker but still significant correlation is observed between radio SFRs at 1.4 GHz and old (averaged over 100 Myr) SFRs based on SED modeling, indicative of multiple episodes of starburst activity during their lifetime. The thermal radio luminosity at 4.8 GHz is a better tracer of recent star formation than the thermal luminosity at 1.4 GHz. Statistically, our modeled nonthermal radio spectral indices do not significantly correlate with redshift, stellar mass, SFR, specific SFR, and dust mass.

We present the results of our Power Spectral Density (PSD) analysis for the BL Lac object PKS 2155−304, utilizing the nightly-binned long-term light curve from the decade-long monitoring, as well as the minute-binned intra-night light curve from the High Energy Stereoscopic Survey (H.E.S.S.; >200 GeV). The source is unique for exhibiting the shortest flux-doubling timescale at Very High Energy (VHE) among its class and thus provides a rare opportunity to study the particle acceleration on the smallest spatial scales in blazar jets. The light curves are modeled in terms of the Continuous-Time Auto-Regressive Moving Average (CARMA) process. The combined long-term and intra-night PSD extends up to ∼6 decades in the temporal frequency range; unprecedented at the TeV energies for a blazar source. Our systematic approach reveals that PKS 2155−304 shows, on average, a complex shape of variability power spectrum, with more variability power on longer timescales. The long-term variability is best modeled by the CARMA(2,1) process, while the intra-night variability is modeled by a CARMA(1,0) process. We note that the CARMA(1,0) process refers to an Ornstein−Uhlenbeck process where the power-law PSD slope (PSD varies as a function of variability frequency to the power of the negative slope) changes from two to zero, above a certain “characteristic/relaxation” timescale. Even though the derived power spectrum of the intra-night light curve did not reveal a flattening, we speculate such relaxation must occur on timescales longer than a few hours for the source.

Multiwavelength blazar variability is produced by noise-like processes with the power-law form of power spectral density (PSD). We present the results of our detailed investigation of multiwavelength ( γ -ray and optical) light curves covering decades to minutes timescales, of two BL Lac objects namely, PKS 0735+178 and OJ 287. The PSDs are derived using discrete Fourier transform (DFT) method. Our systematic approach reveals that OJ 287 is, on average, more variable than PKS 0735+178 at both optical and γ -ray energies on the corresponding time scales. On timescales shorter than ∼10 days, due to continuous and dense monitoring by the Kepler satellite, a steepening of power spectrum is observed for OJ 287. This indicates the necessity of an intermittent process generating variability on intra-night timescales for OJ 287.

We report the selection of blazar candidates behind the Large and Small Magellanic Clouds. Both flat spectrum radio quasar and BL Lacreate objects were selected based on the long-term, multi-colour Optical Gravitational Lensing Experiment photometric data. We cross-correlated the Magellanic Quasar Survey catalogue of spectroscopically confirmed quasars and quasar candidates located behind the Magellanic Clouds with the radio data at six frequencies from 0.8 to 20 GHz. Among the 1654 objects visible in optical range, we identified a sample of 44 newly selected blazar candidates, including 27 flat spectrum radio quasars and 17 BL Lacs. We examined selected objects with respect to their radio, optical, and mid-infrared properties.

The power-law shape of the power spectral density (PSD) of blazar light curves— P ( ν k ) ∝ ν k − β , where ν k is the temporal frequency—indicates that blazar variability is, in general, of the colored-type noise ( β ≃ 1 − 3 ). A precise characterisation of PSD slopes, normalizations, or characteristic timescales (if any) manifesting as distinct features in the power spectra of blazars is important for constraining the physics of the emission and energy dissipation processes in relativistic jets. Here we present the results of the PSD analysis for the BL Lac object PKS 0735+178 at GeV (Fermi-LAT), optical (R-band), and radio (GHz band from UMRAO and OVRO programmes) frequencies, covering a broad range in variability timescales. The novelty of our approach is that in the optical regime, by combining the long-term and densely sampled R-band intra-night light curves, we constructed the PSD for time periods ranging from 23 years down to minutes. Our analysis reveals that: (1) the nature of processes generating flux variability at optical and radio frequencies is different from those operating at GeV photon energies ( β ∼ 2 and 1, respectively); (2) the main driver behind the optical variability is the same on timescales of years, months, days, and hours (a single power-law with β ∼ 2). We discuss our findings in the framework of a model where the overall blazar variability is generated by an underlying single stochastic process (radio and optical frequencies), or a linear superposition of such processes (γ-ray regime).