http://hdl.handle.net/2248/2 2026-06-06T09:03:13Z http://hdl.handle.net/2248/8931 Detection of quasi-periodic oscillations in the 37 GHz radio light curve of the blazar Ton 599 during 1990─2020 Gupta, A. C; Volvach, A. E; Kishore, Shubham; Volvach, L. N; Wiita, P. J; Cui, Lang; Valtonen, Mauri J; Mondal, Sandeep K; Gaur, H Context. Blazars are a subclass of radio-loud active galactic nuclei (AGN) that display strong multi-wavelength variability on diverse timescales ranging from years down to minutes. In the last 1.5 decades, there have been occasional detections of quasi-periodic oscillations in several blazars in their time series data. Aims. We searched for quasi-periodic oscillations (QPOs) in the 37 GHz radio band light curve of the flat-spectrum radio quasar Ton 599 made at the RT-22 radio telescope in Simeiz, Crimea, from 1990 to 2020. We also searched for QPOs in the available gamma-ray and optical data during the time span of these radio observations. Methods. To identify and quantify the QPO nature of this radio light curve of Ton 599, we used the Lomb─Scargle periodogram (LSP), REDFIT, and weighted wavelet Z-transform (WWZ) analyses. We performed LSP analyses of the gamma-ray and optical data. Results. We report the detection of a likely QPO of about 2.4 years in a portion of the 37 GHz radio light curve of Ton 599. No QPO signatures of similar timescales were found in either the γ-ray or optical (R-band) wavebands. Conclusions. We briefly discuss possible emission models for radio-loud AGN that could explain such QPOs with periods of a few years. Open Access; Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 2026-03-01T00:00:00Z http://hdl.handle.net/2248/8930 Probing generalized emergent dark energy with DESI DR2 Sharma, Vipin Kumar; Chaudhary, Himanshu; Kolekar, Sanved As an update on the initial findings of DESI, the new results provide the first hint of potential deviations from a cosmological constant (𝜔 = −1), which, if confirmed with significance > (2 − 4)𝜎, would challenge the validity of Λ within the ΛCDM model. We explore the Generalized Emergent Dark Energy (GEDE) model using recent BAO measurements from DESI DR2, Type Ia supernova compilations, and CMB distance priors. Employing nested sampling, we constrain the parameter Δ, which characterizes deviations from ΛCDM. Our analysis shows that with CMB+DESI DR2 alone, GEDE tends to prefer positive values of Δ. However, when different SNe Ia calibrations are included, the model favors negative values of Δ, corresponding to an earlier injection of dark energy. The Marginalized constraints on 𝜔(𝑧) further shows that GEDE sharply emerges but then asymptotes to 𝜔 = −1 without crossing it. At 𝑧 ∼ 1 data, GEDE provides a better fit than ΛCDM, while at 𝑧 ≲ 0.5 the data favor 𝜔 > −1, bringing the model deviate from ΛCDM. Bayesian model comparison shows weak support for GEDE with CMB+DESI DR2 (ln𝐵𝐹 = 1.96), moderate with PP (ln𝐵𝐹 = 2.65), weak-to-moderate with Union3 (ln𝐵𝐹 = 2.34), and weak with DES-SN5Y (ln𝐵𝐹 = 1.44). Overall, GEDE is consistent with current data and mildly favored when SNe Ia are included, making it a viable extension of ΛCDM that merits further investigation with future high precision measurements. Restricted Access 2026-03-01T00:00:00Z http://hdl.handle.net/2248/8929 Variability study and searching for quasiperiodic oscillations with day-like periods in the blazar S5 0716+714 with TESS Kishore, Shubham; Gupta, A. C; Wiita, P. J Using an unprecedented cadence of 30 minutes provided by the Transiting Exoplanet Survey Satellite, we have examined the optical light curves (LCs) of the blazar S5 0716+714 obtained from its Sectors 40, 47, and 53 over a period of about 75 days. This source exhibited flux variability in each of those sectors, reaching a maximum variability amplitude of 5.6%. The power spectral density shapes were tested with a simple power law and two distinct bending power laws and were found to be better fit by bending power laws than simple power laws for all but one of the segments. To look for any periodicities in these LCs, we used weighted wavelet Z transform analysis and generalized Lomb–Scargle periodograms. We identified one possible quasiperiodic oscillation signature in a portion of sector 40 (period of ∼6.5 hr), having ∼95% global significance. A statistical approach to assess the LCs involving continuous autoregressive moving average was implemented, and the LCs were found to follow more complex processes than the simplest and typical damped random walk process. We briefly discuss the statistical properties of the LCs along with the general variability features and physical processes that could cause these types of fluctuations. Open Access; Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. 2026-02-20T00:00:00Z http://hdl.handle.net/2248/8928 Fluctuation dynamos in supersonic turbulence at Pm ≳ 1 Nagdeo, Ameya Uday; Sur, Sharanya; Vaidya, Bhargav Fluctuation dynamos provide a robust mechanism for amplifying weak seed magnetic fields in turbulent astrophysical plasmas. However, their behaviour in the highly compressible regimes characteristic of the interstellar medium remains incompletely understood. Using high-resolution 3D magnetohydrodynamic simulations of supersonic turbulence with rms Mach numberM 11rms , we explore fluctuation dynamos across magnetic Prandtl numbers Pm = 1–10. At Pm = 1, dynamo growth is slower and saturates at lower magnetic-to-kinetic energy ratios, with amplification in the kinematic phase dominated by compression rather than line stretching. In contrast, at Pm = 10, vortical stretching emerges as the dominant mechanism, yielding faster growth, higher saturation levels, and stronger suppression of density–magnetic field correlations by magnetic pressure. This transition is reflected in the correlation coefficient between density and magnetic field strength, which is strongly positive at Pm = 1 but decreases significantly at higher Pm. Across all runs, the ratio of velocity-to-magnetic integral scales is ∼3.4, in the saturated phase, independent of Pm, while the ratio of viscous to resistive dissipation scales increases with the increase in Pm. Synthetic Faraday rotation measures reveal coherence lengths of ∼one-fourth to one-third of the forcing scale across the range of Pm explored. Using these coherence scales, we discuss the potential contribution of fluctuation dynamos to Faraday rotation expected from turbulent, gas-rich young disk galaxies. Open Access; Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. 2026-02-20T00:00:00Z