http://hdl.handle.net/2248/1 Publications from IIAP are submitted here 2026-06-03T23:54:39Z http://hdl.handle.net/2248/8929 Title: Variability study and searching for quasiperiodic oscillations with day-like periods in the blazar S5 0716+714 with TESS Authors: Kishore, Shubham; Gupta, A. C; Wiita, P. J Abstract: 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. Description: 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 Title: Fluctuation dynamos in supersonic turbulence at Pm ≳ 1 Authors: Nagdeo, Ameya Uday; Sur, Sharanya; Vaidya, Bhargav Abstract: 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. Description: 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/2248/8927 Title: Unveiling the dual nature of V1180 cas: UXor-like dips and EXor-like bursts across a decade Authors: Chand, Tarak; Sharma, Saurabh; Singh, Koshvendra; Ninan, J. P; Ghosh, Arpan; Ojha, D. K; Baug, Tapas; Sahu, D. K; Bhatt, B. C; Pramod Kumar, S; Yadav, R. K; Panwar, Neelam; Verma, Aayushi; Kaur, Harmeen; Mamta; Chakraborty, Manojit; Gokhe, Kartik; Singh, Ajay Kumar Abstract: We present a detailed analysis of the long-term photometric and spectroscopic evolution of V1180 Cas over a decade, aiming to identify the dominant mechanisms behind its variability. We combine multiband light curves from 1999 to 2025 with over 30 epochs of optical to near-infrared (NIR) spectroscopy (0.5–2.5 μm), analyzing variability patterns, color behavior, and emission line diagnostics. We investigate the temporal evolution of accretion and outflow indicators and their correlation with photometric states. The light curve reveals a transition from sporadic early dimming events to a quasi-periodic pattern since 2018, with 11 major dips showing asymmetry and stochastic substructure. Color–magnitude diagrams show classic UXor-like blueing during deep minima, while NIR and mid-infrared color changes indicate thermal evolution of the disk. Spectroscopic analysis reveals persistent hydrogen, Ca II, He I, and forbidden line emission. Accretion diagnostics track photometric variability, and forbidden lines often intensify during dips, implying a physical link between extinction and outflows. Estimated accretion rates range from ∼10−8 to 10−7 M⊙ yr−1 ; the outflow rate and density diagnostics are consistent with disk winds and shock-excited jets. V1180 Cas demonstrates dual-mode variability driven by both variable circumstellar extinction and episodic accretion events. The hybrid UXor/EXor behavior, combined with evolving disk signatures and persistent outflows, suggests a young stellar object undergoing coupled accretion–extinction–outflow evolution. Continued monitoring will be essential to fully resolve the physical processes shaping its variability. Description: 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/2248/8926 Title: Probing accretion dynamics and spin evolution in the X-ray pulsar RX J0520.5–6932 during its 2024 outburst Authors: Rahul, Sharma; Beri, Aru; Paul, B; Sanna, Andrea; Maitra, Chandreyee; Yang, Haonan Abstract: Context. After nearly a decade of quiescence, the transient Be/X-ray binary pulsar RX J0520.5–6932 underwent an outburst in 2024. Weperformed X-ray monitoring of the source with NICER and AstroSat near the peak of the event. Aims. Our primary objective was to investigate the energy and luminosity dependence of the pulsed emission, characterize the spin evolution, and study the broadband X-ray spectral properties of RX J0520.5–6932 during the outburst. Methods. We extracted light curves and spectra from NICER and AstroSat observations carried out during the outburst. Pulsations weredetected using epoch-folding techniques, enabling a detailed study of pulse-profile evolution as a function of energy and intensity. Broadband spectral modeling was performed using simultaneous data from SXT, LAXPC, and NICER. The spectra from individual NICER observations were used to study spectral variability. Results. The AstroSat/LAXPC and NICER light curves reveal pronounced short-duration flaring activity lasting 400–700 s with enhancements in intensity by about a factor of two. The pulse profile exhibits a strong dependence on both energy and intensity, evolving from a simple single-peaked structure at low energies to complex multi-peaked shapes at intermediate energies and reverting to simpler morphologies at higher energies. Pulse profiles during the flares di er significantly from those in the persistent state, indicating changes in the pulsed beam pattern with a change in the intensity on a short timescale. Broadband spectral analysis revealed a soft excess and an emission feature at 1 keV likely arising from reprocessed emission in the accretion disk and fluorescence from Ne Kand Fe L ions. Continuous NICER monitoring over nearly one orbital cycle enabled us to track spin evolution with accretiondriven spin-up and spectral variability in the soft X-ray band. Additionally, we observed a declining spin-up rate during the outburst, likely due to a gradual reduction in mass accretion rate. Conclusions. Our results provide a comprehensive view of the complex accretion dynamics in RX J0520.5–6932 during its 2024 outburst. The strong variability in pulse shape and spin behavior highlights rapid changes in the accretion geometry and torque as a function of accretion rate. Description: 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. 2026-02-01T00:00:00Z