arXiv subjects

Sarang Shah

Publications and source records attributed to Sarang Shah.

7 recordsLinked to original sources

arXiv2026

Large Magellanic Cloud Globular Clusters in the Near-infrared. I. RR Lyrae in Reticulum

Reticulum is an old, metal-poor, and sparsely populated globular cluster in the outer regions of the Large Magellanic Cloud (LMC) and hosts a rich population of RR Lyrae stars. Being as close as possible to a single stellar population with negligible metallicity spread and low reddening, Reticulum is an ideal laboratory for testing stellar pulsation models and calibrating population II distance indicators. We present homogeneous multi-epoch near-infrared (NIR, JHKs) observations of RR Lyrae variables in Reticulum obtained with the Flamingos-2 imager on the 8.1-m Gemini South Telescope. Using NIR light-curve templates, we derive accurate intensity-averaged magnitudes and peak-to-peak amplitudes for 32 RR Lyrae stars, including 22 fundamental-mode (RRab), 4 first-overtone (RRc), and 6 mixed-mode (RRd) pulsators. The empirical JHKs period-luminosity (PL) relations of Reticulum RR Lyrae are very tight, exhibiting dispersions (~0.05 mag) comparable to those observed in Galactic globular clusters. The derived PL slopes are shallower than those reported for Galactic cluster variables. Adopting recent empirical and theoretical period-luminosity-metallicity (PLZ) calibrations based on Galactic globular clusters and pulsation models, we derive a true distance modulus of $\mu_0 = 18.472 \pm 0.035$ mag to Reticulum. This cluster distance is in excellent agreement with the precise geometric distance to the LMC and places Reticulum close to the LMC barycentric distance. The well-characterized RR Lyrae population and a precise distance make Reticulum a potential anchor for calibrating Population II distance ladder.

astro-ph.SR↗

arXiv2026

A Pilot Study to Verify the RR Lyrae Candidates with Vera C. Rubin Observatory Early Alerts

We present a pilot study using the Vera C. Rubin Observatory early alerts to verify RR Lyrae candidates in the Pan-STARRS1, the Dark Energy Survey, and the Next-Generation Virgo Cluster Survey RR Lyrae catalogs. RR Lyrae candidates fainter than 16 mag in the $g$-band in these catalogs were crossmatched with the alerts observed in several deep drilling fields and the M49 field. After excluding alerts with a low number of detections, there are 40 alerts associated with the RR Lyrae candidates. The multiband Rubin-LSST light curves extracted from the alerts verify 32 variables as genuine RR Lyrae, although several were not classified as variable stars in the ALeRCE and Lasair community alert brokers. While ALeRCE and Lasair provide $\sim70\%$ and $\sim40\%$ true variable classification, respectively, we find that $20\%$ of the alert sample are non-RR Lyrae variables. The remaining eight candidate variables do not show typical RR Lyrae light curves and include two active galactic nuclei and two eclipsing binaries. Additionally, we have also found a small number of known variable candidates with no alerts, which would suggest that they are either not RR Lyrae variables or the template images are not yet available for their difference image analysis.

astro-ph.SR↗

arXiv2025

The nature of ASASSN-24fw's occultation: modelling the event as dimming by optically thick rings around a sub-stellar companion

ASASSN-24fw is a main-sequence F-type star that experienced a rapid and long-lasting dimming event beginning in late 2024 and continuing until mid 2025. Its pre-dimming spectral energy distribution shows a persistent infrared excess with a fractional luminosity of approximately 0.5 percent. We model this excess using a two-component blackbody fit and find dust components with temperatures of about 1070 K and 390 K. Archival light curves indicate that ASASSN-24fw was photometrically stable prior to the event, suggesting that the dimming is caused by an external occulting body rather than intrinsic stellar variability. The event lasted about 275 days and exhibits a distinctive flat-bottomed profile of nearly 200 days, unlike most long-duration occultation events reported in the last decade. We analyze the light curve and spectra obtained during dimming to study the properties of both the star and the occulting material. A parametric light-curve model reveals multiple ingress phases, consistent with variations in the density and structure of the obscuring material. A second transit model favors an occulting body consistent with a gas giant or brown dwarf with a minimum mass of about 3.4 Jupiter masses and surrounded by an extended circumplanetary disk or rings of radius roughly 0.17 au. Near-infrared spectra taken during dimming show enhanced infrared excess and spectral features consistent with a late-type companion, approximately M8. We also detect variable H-alpha emission, suggesting evolving gas and dust in the occulting structure. Imaging from LCOGT identifies a nearby object within 3 arcsec, likely a bound companion at a projected separation of about 3000 au. Systems like ASASSN-24fw appear rare, and continued follow-up will help constrain the nature of the occulting body and the circumstellar environment.

astro-ph.SR↗

arXiv2025

A Dynamic, Signals-Based Reinterpretation of Microeconomic Theory

Economics has long been a science of static equilibria, in which time is a second-order rather than first-order concern. Without time, economic modelers may neglect or obscure the role of time-dependent phenomena, e.g. path-dependency, and limit their ability to compare agnostically the model results with empirical observations. In this article, I outline a dynamic, signals-based recipe for building microeconomic models from traditional static models. I demonstrate this recipe using a classic "desert island" Robinson Crusoe (RC) model of consumption. Starting from a classic static derivation, I then move to a dynamic view, using the utility function as a generator of force on consumption. Finally, I show that the resulting dynamic model may be expressed in Lagrangian and Hamiltonian terms. I conclude by suggesting a recipe for scientific iteration using these alternate mechanical formulations, and the alternative explanations these dynamic models may suggest compared to employing a static approach to modeling.

econ.TH↗

arXiv2025

Moduli Space of SU(2) Singular Monopole

The $SU(2)$ monopole with a Dirac singularity was constructed in Durcan (2007) and Cherkis and Durcan (2007). We study its moduli space by identifying the tangent direction to the moduli space. The tangent vectors to the moduli space are composed of the monopole's phase and translational zero modes. We construct the phase and translational zero modes using the Nahm transform. These zero modes are then used to construct the metric components $g_{00}$ and $g_{0q}$ of the $SU(2)$ singular monopole moduli space, where $0$ denotes the gauge coordinate and q denotes the translational coordinates. We find the moduli space to be the Taub-NUT space. It has been shown by Nakajima (1990) and Maciocia (1991) that there is an isomorphism between the monopole moduli space of regular monopoles and the moduli space of the Nahm data used to construct them. We compute the moduli space of the Nahm data of a singular monopole using the hyperk\"ahler quotient construction as described in Gibbons and Rychenkova (1997). We find it to be isometric to the moduli space of the singular monopole. Finally we make an explicit connection between the moduli space of the Nahm data and the monopole moduli space using Corrigan's inner product formula (Osborn 1981) independently proving this isomorphism.

hep-th↗

arXiv2024

Casting Light on Degeneracies: A Comprehensive Study of Lightcurve Variations in Microlensing Events OGLE-2017-BLG-0103 and OGLE-2017-BLG-0192

This study investigates orbital parallax in gravitational microlensing events, focusing on OGLE-2017-BLG-0103 and OGLE-2017-BLG-0192. For events with timescales $\leq$ 60 days, a Jerk Parallax degeneracy arises due to high Jerk velocity ($\tilde{v_{j}}$), causing a four-fold continuous parallax degeneracy. OGLE-2017-BLG-0103, after incorporating orbital parallax, reveals four discrete degenerate parallax solutions, while OGLE-2017-BLG-0192 exhibits four discrete solutions without degeneracy. {The asymmetric light curve of OGLE-2017-BLG-0103 suggests a more probable model where xallarap is added to the parallax model, introducing tension. The galactic model analysis predicts a very low mass stellar lens for OGLE-2017-BLG-0192. For OGLE-2017-BLG-0103, degenerate solutions suggest a low-mass star or a darker lens in the disc, while the Xallarap+Parallax model also predicts a stellar lens in the bulge, with the source being a solar-type star orbited by a dwarf star.} This study presents five degenerate solutions for OGLE-2017-BLG-0103, emphasizing the potential for confirmation through high-resolution Adaptive Optics (AO) observations with Extremely Large Telescopes in the future. The complexities of degenerate scenarios in these microlensing events underscore the need to analyze special single-lens events in the Roman Telescope Era.

astro-ph.GA↗

arXiv2024

A Partial Near-infrared Guide Star Catalog for Thirty Meter Telescope Operations

At first light, the Thirty Meter Telescope (TMT) near-infrared (NIR) instruments will be fed by a multiconjugate adaptive optics instrument known as the Narrow Field Infrared Adaptive Optics System (NFIRAOS). NFIRAOS will use six laser guide stars to sense atmospheric turbulence in a volume corresponding to a field of view of 2', but natural guide stars (NGSs) will be required to sense tip/tilt and focus. To achieve high sky coverage (50% at the north Galactic pole), the NFIRAOS client instruments use NIR on-instrument wavefront sensors that take advantage of the sharpening of the stars by NFIRAOS. A catalog of guide stars with NIR magnitudes as faint as 22 mag in the J band (Vega system), covering the TMT-observable sky, will be a critical resource for the efficient operation of NFIRAOS, and no such catalog currently exists. Hence, it is essential to develop such a catalog by computing the expected NIR magnitudes of stellar sources identified in deep optical sky surveys using their optical magnitudes. This paper discusses the generation of a partial NIR Guide Star Catalog (IRGSC), similar to the final IRGSC for TMT operations. The partial catalog is generated by applying stellar atmospheric models to the optical data of stellar sources from the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) optical data and then computing their expected NIR magnitudes. We validated the computed NIR magnitudes of the sources in some fields by using the available NIR data for those fields. We identified the remaining challenges of this approach. We outlined the path for producing the final IRGSC using the Pan-STARRS data. We have named the Python code to generate the IRGSC as irgsctool, which generates a list of NGS for a field using optical data from the Pan-STARRS 3pi survey and also a list of NGSs having observed NIR data from the UKIRT Infrared Deep Sky Survey if they are available.

astro-ph.IM↗