arXiv subjects

Manika Singla

Publications and source records attributed to Manika Singla.

4 recordsLinked to original sources

arXiv2026

The Search for Technosignatures: a Review of Possibilities

This paper aims to review the diverse range of technosignatures that have been proposed in the literature. We organize the review by scales, starting carefully from Earth, then zooming out to Earth's orbit, the solar system, including the Moon, the Earth-Moon Lagrange points, the inner solar system, the asteroid belt, interstellar objects, the outer solar system, the Kuiper belt, the solar gravitational lens region, and the Oort cloud. We then introduce the Kardashev and Barrow scale before exploring exoplanetary technosignatures, from surface, atmospheric to orbital sources. We next consider stellar technosignatures that may involve massive energy utilization, stellar modification or stellar pollution, and end with a section about compact objects. We then review attempts to detect interstellar communication, and discuss many dimensions of the search space from first principles. Then we consider interstellar travel technosignatures, and end with galactic, extragalactic and universal signatures. We end with a discussion about synergies between biosignatures and technosignatures searches, anomaly detection, multimodal strategies, instruments for detecting technosignatures, how to evaluate and prioritize the search, as well as epistemological issues.

astro-ph.EP↗

arXiv2025

Effect of Thermal Emission in Isotropic Scattering Atmospheres: An Invariant-Embedding Extension of Chandrasekhar's $H(\mu)$-Function

Chandrasekhar's H(mu)-function forms the foundation of radiative transfer theory for semi-infinite, isotropically scattering atmospheres under external illumination. However, the classical formulation does not account for thermal emission from internal heat sources, which is essential in many astrophysical environments, including hot Jupiters, brown dwarfs, and strongly irradiated exoplanets, where re-radiated stellar energy significantly alters the emergent intensity. To address this limitation, we extend Chandrasekhar's diffuse reflection framework by incorporating intrinsic thermal emission within the invariant-embedding formalism. In this approach, thermal emission enters as an embedded invariant contribution to the source function, leading to a generalized angular redistribution function M(mu). We derive the governing non-linear integral equations for M(mu) and express them in terms of the direction cosine mu, the thermal emission coefficient U(T)=B(T)/F, and the single-scattering albedo omega_0. High-precision numerical values of M(mu,U,omega_0) are computed for mu in [0,1], U<0.7, and omega_0<1 using a stable iterative scheme based on Gaussian quadrature. In the limit of vanishing thermal emission, the formulation reduces to Chandrasekhar's classical H(mu)-function, validating the approach. As an application, we consider the ultra-short-period exoplanet K2-137b and identify the wavelength range 0.85--2.5 micron where the model is most applicable, corresponding to the capabilities of JWST, HST, and ARIEL.

astro-ph.EP↗

arXiv2023

New models of reflection spectra for terrestrial exoplanets: Present and prebiotic Earth orbiting around stars of different spectral types

In order to recognize a habitable exoplanet from future observed spectra, we present new model reflected spectra and geometric albedo for modern and prebiotic (3.9 Ga) Earth-like exoplanets orbiting within the habitable zone of stars of spectral types F, G, K and M. We compute this for various atmospheric and surface compositions of the planets. Molecules that are potential biosignatures and act as greenhouse agents are incorporated in our model atmosphere. Various combinations of solid and liquid materials such as ocean, coast, land consisting of trees, grass, sand or rocks determine the surface albedo of the planet. Geometric albedo and model reflected spectra for a set of nine potential habitable planets, including Proxima Centauri b, TRAPPIST-1d, Kepler-1649c and Teegarden's Star-b, are also presented. We employ the opacity data derived by using the open-source package Exo-Transmit and adopt different atmospheric Temperature-Pressure profiles depending on the properties of the terrestrial exoplanets. The model reflected spectra are constructed by numerically solving the multiple scattering radiative transfer equations. We verified our model reflected spectra for a few specific cases by comparing with those published by other researchers. We demonstrate that prebiotic Earth-like exoplanets and present Earth-like exoplanets with increased amount of greenhouse gases in their atmospheres scatter more starlight in the optical. We also present the transmission spectra for modern and prebiotic Earth-like exoplanets with cloudy and cloudless atmospheres.

astro-ph.EP↗

arXiv2023

Effect of multiple scattering on the Transmission spectra and the Polarization phase curves for Earth-like Exoplanets

It is the most appropriate time to characterize the Earth-like exoplanets in order to detect biosignature beyond the Earth because such exoplanets will be the prime targets of big-budget missions like JWST, Roman Space Telescope, HabEx, LUVOIR, TMT, ELT, etc. We provide models for the transmission spectra of the Earth-like exoplanets by incorporating effects of multiple scattering. For this purpose we numerically solve the full multiple-scattering radiative transfer equations instead of using Beer-Bouguer-Lambert's law that doesn't include the diffuse radiation due to scattering. Our models demonstrate that the effect of this diffuse transmission radiation can be observationally significant, especially in the presence of clouds. We also calculate the reflection spectra and polarization phase curves of Earth-like exoplanets by considering both cloud-free and cloudy atmospheres. We solve the 3D vector radiative transfer equations numerically and calculate the phase curves of albedo and disk-integrated polarization by using appropriate scattering phase matrices and integrating the local Stokes vectors over the illuminated part of the disks along the line of sight. We present the effects of the globally averaged surface albedo on the reflection spectra and phase curves as the surface features of such planets are known to significantly dictate the nature of these observational quantities. Synergic observations of the spectra and phase curves will certainly prove to be useful in extracting more information and reducing the degeneracy among the estimated parameters of terrestrial exoplanets. Thus, our models will play a pivotal role in driving future observations.

astro-ph.EP↗