http://hdl.handle.net/2289/142 2026-05-29T22:58:13Z http://hdl.handle.net/2289/8719 Title: Site Quality Analysis for an Indian Submillimeter Telescope: A Reanalysis-based Approach Authors: Singh, Tanmay; Rao, Mayuri Sathyanarayana; Thakur, Ritoban Basu Abstract: The Himalayan plateau region of Ladakh, India, is a potential host for a science-class submillimeter observatory, building on existing astronomical infrastructure near Hanle and Merak. Using the fifth-generation European Centre for Medium-Range Weather Forecasts Reanalysis (ERA5) data, we analyze precipitable water vapor (PWV) at monthly resolution over 184 months from 2010 January to 2025 April, map PWV statistics across Ladakh, and identify candidate regions that reach PWV ≤1 mm. For promising locations, we compute atmospheric transmittance and the corresponding atmospheric photon-noise using the am (Atmospheric Model) radiative transfer code; we present transmittance and brightness temperature estimates over 10–1000 GHz and compare the inferred performance to sites hosting current or planned submillimeter facilities worldwide. We find Ladakh to be favorable for submillimeter observations, with multiple ERA5 grid cells reaching PWV ≤ 1 mm. Within ERA5’s spatial resolution, two regions emerge as particularly promising: Site A (≈34.25∘N, 78.75∘E) and Site B (≈32.50∘N, 79.00∘E), which satisfy PWV ≤1 mm for about 23% and 19% of the study duration, respectively, compared to about 5% and 8% for the Hanle and Merak grid cells. These results motivate targeted in situ radiometer measurements for final site selection. Description: Open Access. Also available at arXiv.org 2026-05-07T00:00:00Z http://hdl.handle.net/2289/8718 Title: State-dependent X-Ray Variability in Cygnus X-1: A 12 yr NuSTAR Timing Study of Accretion Flow Geometry Authors: Duraphe, Kshitij; Mandar, Kartik; Khanal, Chooda; Pareek, Abha; Kondhiya, Tejaswi; Suswara, V Sree; Dinesh, Deeksha; Bhat, Vidyasagar; Bhatta, Gopal Abstract: We present a comprehensive timing analysis of the black hole X-ray binary Cygnus X-1 using 26 NuSTAR observations spanning 2012–2024, providing the most detailed characterization to date of its accretion flow variability across spectral states. Our analysis reveals fundamental insights into the physics governing state transitions in stellar-mass black holes. We characterize the energy-dependent manifestation of the well-known bimodal state distribution, showing that raw count rate bimodality is intrinsic to the Comptonized spectral component above ∼10 keV, while thermal emission below 8 keV remains unimodal. Power spectral analysis uncovers state-dependent characteristic frequencies shifting from 0.050 Hz (hard) to 0.074 Hz (intermediate), with featureless red noise in soft states. These frequencies correspond to disk truncation radii evolving from ∼5.5 Rg to ∼2 Rg, providing direct observational evidence for the inward progression of the accretion disk during state transitions. Frequency-dependent time lags evolve systematically from ∼50 ms hard lags at 0.1 Hz in hard states to near-zero in soft states, quantifying the collapse of the Comptonizing corona. Linear root mean square–flux relations persist across all states with parameters that precisely track the relative contributions of thermal versus nonthermal emission components. Most remarkably, we identify a failed state transition (observation 30302019006) exhibiting anticorrelated band behavior, suppressed variability (Fvar < 1.38%), and apparent sub-ISCO truncation. This discovery challenges standard transition models and suggests new pathways for accretion flow evolution in wind-fed systems. Description: Open Access. 2026-05-11T00:00:00Z http://hdl.handle.net/2289/8716 Title: The Simons Observatory: forecasted constraints on primordial gravitational waves with the expanded array of Small Aperture Telescopes Authors: Abril-Cabezas, I; Adachi, S; Ade, P; Adler, A E; Agrawal, P; Aguirre, J; Aiola, S; Alford, T; Rao, M Sathyanarayana; + 246 Co-Authors Abstract: We present updated forecasts for the scientific performance of the degree-scale (0.5 deg FWHM at 93 GHz), deep-field survey to be conducted by the Simons Observatory (SO). By 2027, the SO Small Aperture Telescope (SAT) complement will be doubled from three to six telescopes, including a doubling of the detector count in the 93 GHz and 145 GHz channels to 48,160 detectors. Combined with a planned extension of the survey duration to 2035, this expansion will significantly enhance SO's search for a B-mode signal in the polarisation of the cosmic microwave background, a potential signature of gravitational waves produced in the very early Universe. Assuming a 1/f noise model with knee multipole ℓknee = 50 and a moderately complex model for Galactic foregrounds, we forecast a 1σ (or 68% confidence level) constraint on the tensor-to-scalar ratio r of σr = 1.2 × 10-3, assuming no primordial B-modes are present. This forecast assumes that 70% of the B-mode lensing signal can ultimately be removed using high resolution observations from the SO Large Aperture Telescope (LAT) and overlapping large-scale structure surveys. For more optimistic assumptions regarding foregrounds and noise, and assuming the same level of delensing, this forecast constraint improves to σr = 7 × 10-4. These forecasts represent a major improvement in SO's constraining power, being a factor of around 2.5 times better than what could be achieved with the originally planned campaign, which assumed the existing three SATs would conduct a five-year survey. Description: Open Access. Also available at arXiv.org (one of the alternative locations) 2026-04-16T00:00:00Z http://hdl.handle.net/2289/8714 Title: The Tracking Tapered Gridded Estimator for the 21-cm power spectrum from the Murchison Widefield Array (MWA) drift scan observations - III. Improved upper limits at z=8.2 from multiple pointings Authors: Sarkar, Shouvik; Elahi, Khandakar Md Asif; Choudhuri, Samir; Bharadwaj, Somnath; Chatterjee, Suman; Bhattacharya, Baijayanta; Sethi, Shiv; Patwa, Akash Kumar Abstract: We analyse zenith-pointing (δ = −26. 7◦) Murchison Widefield Array (MWA) νc = 154.2MHz drift scan observations covering 349.0◦ ≤ α ≤ 70. 0◦ with 163 pointing centres (PCs) spaced by 0. 5◦. We measure Dℓ, the mean-squared angular brightness temperature fluctuations, as a function of α. A broad peak at α ≈ 50. 0◦ corresponds to the bright extended source Fornax A in the main lobe of the primary beam. A smaller peak at α ≈ 5.0◦ possibly corresponds to Fornax A in the first sidelobe. For α ≤ 22. 0◦ and ℓ ≥ 200, we find Dℓ ∝ ℓ2, which we interpret as Poisson fluctuations from point sources. We present 2 (k), the mean-squared 21-cm brightness temperature fluctuations from the Epoch of Reionization, as a function of α. Fornax A causes strong contamination near α ≈ 50. 5◦, elsewhere several PCs are consistent with noise. The range 358. 5◦ ≤ α ≤ 11. 5◦ is relatively foreground-free and best suited for EoR science. The PC at α = 11. 0◦2 yields the best 2 σ upper limit 2 UL (k) = (173. 13)2 mK2 at k = 0. 161Mpc−1. We incoherently combine 23 PCs to obtain UL (k) = (98. 67)2 mK 2 at k = 0. 156Mpc −1. This is the tightest upper limit from the MWA, being ≈ 3 times lower than earlier MWA limits, but ≈ 2 and ≈ 21 times higher than the LOFAR and HERA limits, respectively, and ≈ 3 orders of magnitude above theoretical predictions. Description: Open Access. Also available at arXiv.org (one of the alternative locations) 2026-06-01T00:00:00Z