IIAP Publications

A unique application of the observation of stars in Indian astronomical texts

2026-06-25T05:28:56Z

A unique application of the observation of stars in Indian astronomical texts Shylaja, B. S; Shubha, B. S It is very well known that the observation of stars constituted an integral part of Indian astronomical texts. The twenty seven stars along the zodiac were used as references for longitudes. Here we discuss an application which is generally not highlighted. The method uses the meridian transit observation of stars to determine the ascendant (lagna), as cited in the seventeenth-century manuscript Brahmatulya Udāharaṇam by Viśvanātha. We assess the method of calculation and discuss the possibility of a table being prepared based on observations. Open Access

Variable stars in the field of the galactic globular cluster M71

2026-06-25T05:23:45Z

Variable stars in the field of the galactic globular cluster M71 Cortes, C. C; Deras, D; Ferro, A. A; Muneer, S; Fierro, Bustos M71 is a nearby, metal-rich globular cluster at low Galactic latitude, where field contamination and spatially variable extinction complicate colour─magnitude diagrams (CMDs) and the identification of cluster member variable stars. Our aims are (i) to construct a homogeneous census of variable stars in M71 by refining their periods and classifications and identifying new candidates, and (ii) to derive a decontaminated, differentially dereddened CMD to constrain its physical properties. We obtained Johnson─Kron─Cousins VI time-series CCD photometry and reduced it using difference image analysis. Cluster membership was established from Gaia DR3 proper motions, and a differential-reddening correction was applied across the field of view. The resulting CMD, cleaned of field stars, was compared with tailored isochrones to estimate age ( 12.9+0.9−0.8Gyr), metallicity ([Fe/H] = −0.88+0.13−0.15), mean reddening ( E(B−V) = 0.21±0.02 ), and distance modulus ( (m−M)0 = 13.01±0.06). Variable stars were identified using two complementary approaches: a periodogram-free string-length scan refined with phase dispersion minimization, and a robust inter-site screening based on median statistics combined with a generalised Lomb─Scargle significance criterion. We identified 21 variable stars not previously reported in the Catalog of Variable Stars in Globular Clusters and provided their periods, amplitudes, classifications, membership status, and light curves. This combined strategy yields a consistent picture of M71, expanding its known variable-star population and confirming parameters typical of metal-rich Galactic disc globular clusters. Open Access; This is an Open Access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.

Observations of circularly polarized radio emission from the "Quiet" sun at frequencies <100 MHz

2026-06-25T05:21:35Z

Observations of circularly polarized radio emission from the "Quiet" sun at frequencies <100 MHz Sayuf, Shaik; Kathiravan, C; Ramesh, R; Barve, Indrajit V; Gireesh, G. V. S; Bane Kshitij, Suhas Trupti The presence of the magnetic field makes the solar coronal medium birefringent. Research indicates that an appreciable degree of circular polarization (DCP) can be observed in the thermal radio emission at frequencies <100 MHz from the solar corona, due to the difference in the absorption coefficients of the ordinary ("o") and extraordinary ("e") modes of propagation in the magnetized coronal medium. Measurements of this DCP from observations are, however, rare. In this study, we report Stokes-I and Stokes-V observations of thermal radio emission from the "quiet" solar corona in the frequency range 50─80 MHz, using a one-dimensional radio interferometric polarimeter. The estimated DCP in the above frequency range is ≍2.5%─1.2%. The results indicate the potential in ground-based low-frequency radio observations to estimate the coronal magnetic field in the "quiet" corona in the heliocentric distance range (r < 2.0 R⊙), where solar radio emission in the above frequency range typically originates. 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.

The CatWISE2020 quasar dipole: A reassessment of the cosmic dipole anomaly

2026-06-25T05:19:10Z

The CatWISE2020 quasar dipole: A reassessment of the cosmic dipole anomaly Bashir, Masroor; Pravabati, C; Appleby, S The Ellis─Baldwin test probes the cosmological principle by comparing the kinematic cosmic microwave background dipole with the Doppler-driven dipole in the number counts of extragalactic radio sources. Recent analysis of the CatWISE2020 quasar catalog has reported a number-count dipole amplitude exceeding the kinematic expectation at 4.9σ significance. We present a comprehensive reassessment of this test using the same dataset, incorporating major sources of uncertainty in the statistical inference. We employ a simulation framework based on the FLASK package, using lognormal realizations of the large-scale structure, the quasar clustering bias, the survey's radial selection function, and its exact sky coverage. Our simulations account for the kinematic dipole, the intrinsic clustering dipole, the shot noise, and the survey geometry effects. The analysis yields a revised significance of 3.63σ in the absence of a clustering dipole and 3.44σ with a randomly oriented clustering dipole. When the clustering dipole is aligned with the kinematic dipole, the significance decreases further to 3.27σ. Although the anomaly is reduced, it cannot be explained solely by the clustering dipole or mode coupling from the survey mask. We further assess the dipole measurement robustness by fitting models with successively higher-order multipoles up to ℓ = 4. Partial sky coverage induces mode coupling, shifting the dipole estimate to higher values when the octopole is included and inflating its variance as additional modes are incorporated, reflected in the increasing condition number of the estimator. This behavior highlights a bias─variance tradeoff inherent in multipole fitting on partial-sky data. 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.