Indian Centre for Space Physics

To understand the nature of the NGC 146–King 14 cluster pair, we conducted a detailed photometric, astrometric, and dynamical study using multiwavelength data from Gaia DR3, Pan-STARRS1, WISE, and TESS. Using a probabilistic approach, we identified 770 and 690 high-probability members of NGC 146 and King 14, respectively. Both clusters exhibit well-defined radial density profiles consistent with King models. We estimate the cluster ages as 20 ± 5 Myr and 50 ± 10 Myr from isochrone fitting, and distances of 2.98 ± 0.33 kpc and 2.51 ± 0.23 kpc from parallaxes after applying the Bailer-Jones criteria. The clusters show consistent mean proper motions. The mass function slopes (1.51 ± 0.18 and 1.50 ± 0.15) are close to the Salpeter value, and the extinction follows a normal Galactic reddening law ( R _V ≈ 3.1). Three-dimensional mapping gives a projected separation of ∼9 pc. Orbit integration using the galpy MWPotential2014 model shows that NGC 146 and King 14 move in nearly circular, disk-like orbits with similar mean orbital radii ( R _m ∼ 9 kpc) and orbital periods of roughly 255 Myr. A dynamical separation of ∼32 pc indicates that both clusters share a common spatial and kinematic association, consistent with a comoving pair. However, their relative velocity exceeds the escape velocity set by their combined mass, indicating they are not gravitationally bound. TESS light curves reveal seven variable stars, including γ Doradus, SPB stars, and eclipsing binaries, though only one is a likely member. Overall, the clusters likely formed within the same giant molecular cloud and now exist as an unbound comoving pair.

We present a comprehensive long-term, multi-epoch spectral and timing study of the Seyfert 1 Active Galactic Nucleus Mrk 1040, utilizing X-ray observations spanning from 2009 to 2024 (similar to 15 yr). The source exhibits pronounced spectral and temporal variability, indicative of transitions between different accretion regimes in the vicinity of the central supermassive black hole. The earlier reported soft excess is re-examined within a uniform, physically motivated multi-epoch framework. We confirm the presence of this soft excess in the 2009 observation, where it is well described by a warm, extended Comptonizing corona with kT(e,warm) similar to 0.26 keV and a radial extent of R-warm similar to 30 r(g). In subsequent epochs, the soft excess is not statistically significant, possibly due to a combination of enhanced ionized absorption, intrinsic weakening of the warm Comptonizing region, or partial truncation of the inner disc. A strong correlation between the soft and hard X-ray fluxes suggests a common physical origin for both components, likely within a multilayered Comptonizing structure that evolved into a compact and thermally stable corona after 2013. The observed spectral variability, together with changes in the Fe K alpha line strength, reflects the evolving coronal geometry and accretion flow dynamics. Variations in the intrinsic column density (N-H) further indicate that Mrk 1040 is embedded within a clumpy, dynamically variable absorber responding to changes in the accretion rate. Using the TCAF model, we estimate the black hole mass as M-BH = (4 . 50 +/- 1 . 62) x 10(7)M(circle dot), consistent with previous estimates.

We present a comprehensive study of the spectrophotometric evolution of the classical nova QY Mus from eruption to quiescence. The light curve shows a notable dust dip, classifying it as a D (137)-type nova, with dust formation beginning at '123 d post-outburst and reaching a maximum optical depth of tau ' 3 . 2 . We classify QY Mus as a slow nova with t2 = 87 +/- 6 d, and derive an absolute magnitude of M-V = -6.55 +/- 0 . 54 using the magnitude versus rate of decline relation. The spectroscopic evolution, traced from 94 to 1348 d, shows prominent P-Cygni profiles in Balmer and Fe II lines during the early decline, consistent with an Fe II-type nova. The transition to the nebular phase occurs around '233 d, marked by the emergence of [O III ] emission. Photoionization modelling using CLOUDY of 41 emission lines on day 590 yields a central source temperature of (7 . 08 +/- 0 . 20) & times; 10(5) K, with enhanced nitrogen and oxygen abundances and moderate neon enrichment, suggesting that QY Mus is not a neon nova. Mid-infrared WISE observations at '502 d indicate the presence of cool dust at '400 K. Using a Gaia-based colour-magnitude diagram constructed in this work for 34 quiescent novae, we find that QY Mus occupies a region consistent with systems hosting main-sequence or subgiant secondaries; its orbital period further supports a subgiant companion. These results establish QY Mus as a slow, dust-forming nova with well-characterized evolution and a characterized evolution and a subgiant secondary.

We present an autonomous model to simulate the daytime variation of sub-ionospheric Very Low Frequency (VLF) signal amplitude, beginning with the computation of the D-region electron density by numerically solving the electron continuity equation (ECE). From the resulting altitude-dependent electron density profile ( N_e ), we extract Wait’s ionospheric parameters ( h^' and β ) using a log-linear fitting method. The study focuses on two VLF propagation paths (one short and one medium in length) in India, originating from the VTX / 18.2 kHz transmitter. The model effectively employs the Long Wave Propagation Capability (LWPC) framework to reproduce the daytime VLF signal amplitude profile. It accurately captures the daytime variations observed at the Bengaluru (BAN) station, where the ground wave component is dominant, as well as at Khukurdaha, WB (KHK). A quantitative comparison between the simulated ( A_sim ) and ( A_obs ) observed amplitudes shows justified agreement, validating the physical consistency and possible predictive capability of the proposed approach.

We present a homogeneous multiwavelength analysis of 35 blue straggler star (BSS) candidates in nine open clusters, combining Swift/UVOT near-ultraviolet data with Gaia DR3 astrometry and optical-to-infrared photometry. We construct spectral energy distributions (SEDs) to search for signatures of hot companions associated with past mass transfer. Among the sample, 15 BSSs (∼43%) show ultraviolet excesses that are better described by two-component SED fits. The inferred companions are consistent with hot white dwarfs and pre-extremely low-mass (pre-ELM) white dwarf candidates, suggesting systems observed at different stages following mass transfer. We examine the radial distribution of the BSSs and find evidence for mass segregation in dynamically evolved clusters, a result that is broadly consistent with the estimated half-mass relaxation timescales of the host systems. To place the clusters in a Galactic context, we compute their orbits using galpy , obtaining low eccentricities ( e ≤ 0.1) and disk-like trajectories. We also find a positive relation between the half-number radius of the BSS population ( r _50 ) and the total number of BSSs. Overall, our results are consistent with a scenario in which the BSS population in these clusters is dominated by binary evolution. The systems identified here provide observational constraints on post-mass-transfer evolutionary phases. While the number of robust detections is limited and intrinsic degeneracies remain in SED-based decomposition, these results provide a useful foundation for future spectroscopic confirmation.