http://hdl.handle.net/2248/2 2026-04-15T12:36:46Z 2026-04-15T12:36:46Z Jha, Rishikesh G Raja, K. S Ramesh, R Kathiravan, C Monstein, C http://hdl.handle.net/2248/8897 2026-03-26T07:04:08Z 2025-12-01T00:00:00Z

Relative strengths of fundamental and harmonic emissions of solar radio type ii bursts Jha, Rishikesh G; Raja, K. S; Ramesh, R; Kathiravan, C; Monstein, C Solar radio type II bursts are slow-drifting bursts that exhibit various distinct features such as Fundamental (F) and Harmonic (H) emissions, band-splitting, and discrete fine structures in the dynamic spectra. Observationally, it has been found that in some cases the F emission is stronger than the H emission, and vice versa. The reason for such behavior has not been thoroughly investigated. To investigate this, we studied 58 meter wave (20 ─ 500 MHz) type II solar radio bursts showing both F and H emissions, observed during the period from 13 June 2010 to 25 December 2024, using data obtained with the Compound Astronomical Low frequency Low cost Instrument for Spectroscopy and Transportable Observatory (CALLISTO) spectrometers at different locations and Gauribidanur LOw-frequency Solar Spectrograph (GLOSS). We examined the intensity ratios of the H (IH) and F (IF) emissions and analyzed their variation with heliographic longitude. We found that 14 out of 19 bursts originating from heliographic longitudes beyond ±75∘ exhibited an IH/IF ratio greater than unity. In contrast, 32 out of 39 bursts originating from longitudes within ±75∘ showed a intensity ratio less than unity. From these results, we conclude that the relative strength of the F and H emissions can be influenced by refraction due to density gradient in the solar corona, directivity and viewing angle of the bursts. Restricted Access; The original publication is available at springerlink.com

2025-12-01T00:00:00Z Pawan Kumar, S Ravindra, B Chowdhury, Partha Karak, B. B http://hdl.handle.net/2248/8896 2026-03-26T07:01:54Z 2026-01-01T00:00:00Z

Quasi-biennial oscillations and rieger-type periodicities in a babcock–leighton solar dynamo Pawan Kumar, S; Ravindra, B; Chowdhury, Partha; Karak, B. B Context. The Sun’s magnetic field exhibits the 11 year solar cycle as well as shorter periodicities, popularly known as the quasibiennial oscillations (QBOs) and Rieger-type periods. Although several theories have been proposed to explain the origin of QBOs and Rieger-type periods, no single theory has had widespread acceptance. Aims. We explore whether the Babcock–Leighton dynamo can produce Rieger-type periodicity and QBOs and investigate their underlying physical mechanisms. Methods. We used the observationally guided 3D kinematic Babcock–Leighton dynamo model, which has emerged as a successful model for reproducing many characteristic features of the solar cycle. We used Morlet wavelet and global wavelet power spectrum techniques to analyze the data obtained from the model. Results. In our model, we report QBOs and Rieger-type periods for the first time. Further, we investigated the individual Babcock-Leighton parameters (fluctuations in flux, latitude, time delay, and tilt scatter) role in the occurrence of QBOs and Rieger-type periods. We find that while fluctuations in the individual parameters of the Babcock–Leighton process can produce QBOs and Rieger-type periodicity, their occurrence probability is enhanced when considering combined fluctuations of all parameters in the Babcock–Leighton process. Finally, we find that with the increase in dynamo supercriticality, the model tends to suppress the generation of Rieger-type periodicity. Thus, this result supports earlier studies that suggest the solar dynamo is not highly supercritical. Conclusions. The Babcock–Leighton dynamo model successfully reproduces QBOs and Rieger-type periodicities that are observed in various solar activity data. 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

2026-01-01T00:00:00Z Mohnani, S Banerjee, Biswajit Miceli, D Nava, L Oganesyan, G Tiwari, P Ierardi, A De Santis, A. L Macera, S Shukla, A Branchesi, M Chatterjee, Swarna Agarwal, Sushmita Datta, Abhirup Yadav, K. K Anupama, G. C http://hdl.handle.net/2248/8895 2026-03-26T06:59:15Z 2026-01-01T00:00:00Z

Broad-band modelling of the GRB 230812B afterglow: Implications for very-high-energy-ray detections with IACTs Mohnani, S; Banerjee, Biswajit; Miceli, D; Nava, L; Oganesyan, G; Tiwari, P; Ierardi, A; De Santis, A. L; Macera, S; Shukla, A; Branchesi, M; Chatterjee, Swarna; Agarwal, Sushmita; Datta, Abhirup; Yadav, K. K; Anupama, G. C A significant fraction of the energy from the gamma-ray burst (GRB) jets, after powering the keV─MeV emission, forms an ultra-relativistic shock propagating into the circum-burst medium. The particles in the medium accelerate through the shock and produce the afterglow emission. Recently, a number of GRB afterglows were observed in TeV γ-rays by Cherenkov Telescopes. This new observational window provides access to the broad-band spectra of GRB afterglows, which contain rich information on the microphysics of relativistic shocks and the profile of the circum-burst medium. Since the transition from synchrotron to inverse Compton regime in afterglow spectra occurs between hard X-rays and the very-high-energy (VHE) γ-rays, it is necessary to have a detection in one of these bands to identify the two spectral components. The early afterglow data in hard X-rays, along with the GeV emission, could help to accurately constrain the spectral shape and capture the spectral turnover to distinguish the two components. We present a multi-wavelength spectral and temporal study, focussed on the keV-VHE domain, of GRB 230812B, one of the brightest GRBs detected by Fermi Gamma-ray Burst Monitor (Fermi/GBM). We also include the detection of a 72 GeV photon by Large Area Telescope (Fermi/LAT) during the early afterglow phase. Through detailed modelling of the emission within the afterglow external forward shock in a wind-like scenario, we predict the multi-wavelength afterglow observations from optical (up to approximately one day) to high-energy band. We emphasise the importance of following up poorly localised GRBs by demonstrating that even in cases without prompt localisation, such as GRB 230812B, it is possible to recover the emission using imaging atmospheric Cherenkov telescopes (IACTs) thanks to their relatively wide field of view. The low energy threshold of the Large-Sized Telescope is essential in discovering the VHE component at the much higher redshifts typical for long GRBs. 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.

2026-01-01T00:00:00Z Sengupta, Sutirtha Sujit, Das Sarangi, Arkaprabha http://hdl.handle.net/2248/8894 2026-03-26T06:57:17Z 2026-01-01T00:00:00Z

Dance to demise—how massive stars may form dense circumstellar shells before explosion Sengupta, Sutirtha; Sujit, Das; Sarangi, Arkaprabha We investigate the evolution of red supergiant (RSG) progenitors of core-collapse supernovae (SNe) with initial masses between 12 and 20 M⊙, focusing on the effects of enhanced mass loss due to pulsation-driven instabilities in their envelopes and subsequent dynamical ejections during advanced stages of nuclear burning. Using time-dependent mass loss from detailed Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution models, including a parameterized prescription for pulsation-driven superwinds and time-averaged mass-loss rates attributed to resulting shock-induced ejections, we construct the circumstellar medium (CSM) before the SN explosion. We calculate resulting CSM density profiles and column densities considering the acceleration of the stellar wind. Our models produce episodes of enhanced mass loss (∼10−4─10−2 M⊙ yr−1) in the last centuries—decades before explosion forming dense CSM (≳10−15 g cm−3 at distances ≲1015 cm)—consistent with those inferred from multiwavelength observations of Type II SNe such as SN 2023ixf, SN 2020ywx, SN 2017hcc, SN 2005ip, and SN 1998S. The formation of such dense circumstellar shells, within the explored range of our single star RSG models, provides a natural explanation for observed flash-ionization signatures, X-ray and radio emission, and has important implications for dust formation around Type II SNe. 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

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