arXiv subjects

Ishan Swamy

Publications and source records attributed to Ishan Swamy.

6 recordsLinked to original sources

arXiv2026

Impact on time delays due to milli-lensing by subhalos on lensed gravitational waves

Dark matter substructure properties such as their mass function and spatial distribution depend on the nature of dark matter and are strong tests of the cosmological model. Like luminous matter, these dark matter substructures cause gravitational lensing affecting observables such as time delays. Given the millisecond-level timing precision achievable with current and future gravitational-wave (GW) detectors, gravitationally lensed GWs provide a powerful probe of dark matter substructure, especially, at the lower end of the mass function. In contrast, the optical (or electromagnetic) observations can provide a precision of a few hours and are thus, not sensitive to deflections from the less massive subhalos. In this work, we investigate the impact of realistic population of DM subhalos ($10^6-10^9 M_\odot$) on the lensing time delays for two of the typical strong lens configurations, a fold and a cusp, seen in quadruply lensed sources for a galaxy-scale lens. We find that the subhalos with NFW density profiles cause perturbations of the order of few hours to the lensing time delays between the macro-lensed images produced by the main lensing galaxy. These time delay "anomalies", if not accounted for, may affect the results of strong lens searches conducted on the GW data by the LIGO--Virgo--Kagra collaboration. Lastly, for the 200 realisations analysed per fold and cusp lens systems, we find that the NFW subhalos produced no additional images of their own called milli-images. Statistical studies are needed to better determine the expected impact on the lensed GW time delays and (non-)detection of milli-images.

astro-ph.CO↗

arXiv2026

Black Holes as Catalysts for Cosmic String Detection and Axion Dark Matter Genesis

The global $U_{PQ}(1)$ Peccei-Quinn (PQ) symmetry, proposed to resolve the strong CP problem, predicts the existence of the axion, a pseudo Nambu-Goldstone boson and a leading dark matter candidate. The spontaneous breaking of this symmetry generates global strings that decay predominantly via the emission of massive axions and gravitational waves. In this work, we investigate the decay of cosmic axion strings in the vicinity of a Schwarzschild black hole and estimate the corresponding energy losses and decay timescales of the resulting string loops. For primordial black holes (PBHs) with masses as small as $10^{-16} M_\odot$, the total radiated energy by the string is found to be on the order of $10^{27}$ GeV, encompassing both axion emission and gravitational waves. A key finding is that the presence of a central black hole significantly accelerates the decay of cosmic string loops, substantially reducing their lifetimes. We present these results as an initial estimate of axion radiation from PBH-cosmic string systems along with the decay time as an important observational signature for axions strings.

hep-ph↗

arXiv2025

Investigating the interaction of a Cosmic String with an Accreting Black Hole

Rotating black holes when attached to a cosmic string have their rotational energy extracted leading to a change in its spin and mass. The spin of a black hole can be measured using various methods for an accreting black hole in an X-ray binary system. Accretion disks around black holes have an innermost stable circular orbit (ISCO) whose location is directly dependent on spin and mass of the black hole. The orbit's location changes as the black hole's spin changes and hence can be a method to detect the presence of cosmic strings. This study investigates this change and suggests the ejection of accretion material as black hole spin approaches maximum for prograde motion and material falling into the black hole for retrograde motion, regardless of the presence of cosmic string. However, in the presence of cosmic string, the spin-up process due to accretion is found out to be slower, even with high accretion rates and is detectable. There is a transition phase that occurs as the black hole approaches maximum spin, where even small changes in spin result in significant changes in the ISCO's position. Accreting black holes attached to a large string never reach this transition phase and this absence serves as potential evidence for the existence of a cosmic string.

hep-th↗

arXiv2025

Blandford-Znajek Jets and the Total Angular Momentum Evolution of a Black Hole Connected to a Cosmic String

Rotating black holes with strong magnetic fields lead to an outward energy flux in the form of jets governed by the Blandford-Znajek mechanism. These jets depend on factors such as accretion rate, magnetic flux and the spin of the black hole. When such rotating black holes get attached to a cosmic string, it leads to a further rotational energy extraction, leading to a reduced spin. We consider such a system and investigate the effect this reduced spin has on the jet power and its dependence on the cosmic string tension, $μ$. It is shown that for a constant magnetic flux and accretion rate, the jet energy flux is inversely proportional to $μ^2$. Interestingly, the rate of this energy flux varies with time and is again dependent on $μ$. We also study the total angular momentum evolution of the black hole by considering four major effects: accretion, jets, cosmic string energy extraction and the Bardeen-Petterson effect. Further, we attempt to analyse the condition for the spin-down of a black hole due to these effects and find out that it is possible for both small and large string tensions, with a higher possibility for larger string tensions. Another interesting phenomenon that has been proposed is the alignment of the jet with the cosmic string. Additionally, the Bardeen-Petterson effect also leads to alignment or misalignment of the inner and outer disks depending on the alignment of the string. In this manuscript we propose that these results might have an observable effect and hence could serve as a potential detection method for cosmic strings.

gr-qc↗

arXiv2025

Probing Cosmic Strings via Black Hole Quasinormal Modes in Gravitational Wave Astronomy

Black holes, the simplest solution to Einstein's field equations, do not emit light, making their observations a major challenge for researchers. However, discovery of binary black holes (BBHs) in 2015 by LIGO has transformed the study of compact objects, with over 300 BBHs recorded, providing a new avenue for probing new physics. GWs remain a prominent and precise method of observing not only BBHs, but also dark matter and cosmic strings. Cosmic strings -- hypothetical one dimensional topological defects formed in the early universe, are yet to be observed, with multiple detection methods such as particle radiation, gravitational waves and lensing being proposed. Here we present a novel framework to search for cosmic strings by modeling them as perturbations within non-rotating black hole spacetime, focusing on their imprint on the spectrum of quasinormal modes (QNMs). Our numerical simulations identify a lower limit on perturbation strength, $λ\sim 10^{-10}$ for uncharged string and $λ\sim 10^{-7}$ for charged string, below which cosmic string effects become unobservable in QNM signals. By analyzing eigenvalue splitting and centers, we show that cosmic string properties impart distinct and detectable features to GW signals. Our results establish QNM analysis as a powerful, alternative observational strategy for constraining or detecting cosmic strings, and offer an inverse approach to estimate string energy or charge if a signal is detected. With upgrades in LIGO technologies and advanced multimessenger astronomy under development, these findings highlight new potential for detecting cosmic strings.

gr-qc↗

arXiv2025

Impact on orbital period of X-ray Binary system attached to a cosmic string

Cosmic strings attached to rotating black holes extract its rotational energy, resulting in a mass loss and reduced spin. In this paper we discuss the proposed methods to detect these phenomena and present a novel methodology based on existing literature, by considering a Low Mass X-ray binary system. We investigate the impact of a cosmic string interacting with a black hole in an X-ray binary system and attempt to explain the observations of unexpected orbital period changes in such systems by proposing mass loss by cosmic strings to be a potential cause. For a period change of order $10^{-10}$, the string tension is $\sim 10^{-17}$, lying in the predicted range for cosmic string tension. An analysis of multiple low mass X-ray binary systems is carried out and it is shown that a significant and observable change occurs for a string tension $\sim 10^{-11}$.

gr-qc↗