arXiv subjects

Pawan Kumar Gupta

Publications and source records attributed to Pawan Kumar Gupta.

10 recordsLinked to original sources

arXiv2026

Binary Black Holes in Einstein-Maxwell-Dilaton Theory: Second Post-Newtonian Dynamics from Effective Field Theory

The detection of gravitational waves from compact binary coalescences provides a powerful opportunity to test general relativity in the strong-field regime and to search for signatures of alternative theories of gravity. In this work, we consider Einstein-Maxwell-Dilaton (EMd) theory, in which black holes can carry both electric and scalar (dilatonic) charges. We employ the effective field theory approach, together with a temporal Kaluza-Klein decomposition of the metric in terms of non-relativistic gravitational fields, to derive the conservative two-body Lagrangian for charged black-hole binaries in EMd theory through second post-Newtonian (PN) order. Our calculation extends the previously known conservative dynamics at 1PN order and includes the gravitational, electromagnetic, and dilaton interactions at 2PN order. We verify the result in the appropriate Einstein-Maxwell, scalar-tensor, and general relativistic limits, and perform an independent test-body-limit check of the static 2PN sector. These results provide the conservative dynamics needed for developing higher-accuracy waveform models and testing EMd theory with gravitational-wave observations.

gr-qc↗

arXiv2025

Full-scatter vector field analysis of an overmoded and periodically-loaded cylindrical structure for the transportation of THz radiation

Highly overmoded and periodically loaded structures, such as the iris-line waveguide, offer an attractive solution for the efficient transportation of diffraction-prone THz pulses over long distances (hundreds of meters). This paper presents the full-scatter field theory that allows us to analytically derive all the spectral (modal) coefficients on the discontuities of the iris line. The spectral analysis uses vector fields, superseding scalar field descriptions, to account for diffraction loss as well as polarization effects and ohmic loss on practical conductive surfaces. An advanced application of Lorentz's reciprocity theory, using a generalized guided-field configuration, is developed to reduce complexity of the mode-matching problem over nonuniform sections. The used technique is quite general and applies to a wide class of structures, as it only assumes a paraxial incidence (i.e. a parabolic wave equation) along the axis of the structure. It removes the traditional assumption of very thin screens, allowing for the study of thicker screens in the high-frequency limit, while formulating the problem efficiently by scattering matrices whose coefficients are found analytically. The theory agrees with and expands previously established techniques, including Vainstein's asymptotic limit and the forward-scatter approximation. The used formulation also facilitates accurate visualization of the transient regime at the entrance of the structure and how it evolves to reach steady state.

physics.acc-ph↗

arXiv2025

Superinsulating behavior in granular Pb film on gated few-layer MoS$_2$

We report a super-insulating behavior, in a device having granular Pb film on back-gated few-layer $\mathrm{MoS_2}$, below an onset temperature same as the critical temperature $T_{\rm C}\approx7$ K of bulk Pb. Below $T_{\rm C}$, the current-voltage characteristics exhibit a threshold voltage marking a crossover between the low-bias insulating and the high-bias normal-resistance states, consistent with the known super-insulating state behavior. A temperature dependent critical magnetic field is also found above which the insulating behavior is suppressed. The threshold voltage is found to vary with the gate-voltage but the critical field remains unchanged. With reducing temperature, the sample conductance saturates to a finite value, which depends on magnetic field and gate-voltage. This saturation behavior is found to be inconsistent with the charge-BKT and the thermal activation models but it can be fitted well to a combination of thermal activation and quantum fluctuations.

cond-mat.supr-con↗

arXiv2025

Characteristic transition of the dominant power loss from diffractive to ohmic in overmoded and periodically loaded waveguides at high frequency

The analysis of electromagnetic fields in cylindrical waveguiding structures that contain periodic ring loading, whether for applications in charged-particle accelerators or radiation transportation, has been traditionally conducted under simplifying limits, where the structure is either single-moded at the lower-frequency limit or overmoded at high-frequency limit. These limits have often allowed us to find spectral (modal) expansions for the fields under simpler analytical and computational conditions, with ohmic effects typically being the dominant power loss mechanism in the lower limit, while diffraction effects dominate the loss in the higher limit. In this Letter, we report the observation of a transition point in the character of the main loss mechanism, where ohmic loss becomes dominant in a structure typically presumed to be dominated by diffraction loss. The results follow a formal analysis for the scattered vector fields in a highly overmoded THz waveguide. The findings bridge between the traditional theoretical descriptions for the two limits and reveal key tradeoffs that inform experiments for the transportation of THz radiation over long distances.

physics.app-ph↗

arXiv2023

Science with the Einstein Telescope: a comparison of different designs

The Einstein Telescope (ET), the European project for a third-generation gravitational-wave detector, has a reference configuration based on a triangular shape consisting of three nested detectors with 10 km arms, where in each arm there is a `xylophone' configuration made of an interferometer tuned toward high frequencies, and an interferometer tuned toward low frequencies and working at cryogenic temperature. Here, we examine the scientific perspectives under possible variations of this reference design. We perform a detailed evaluation of the science case for a single triangular geometry observatory, and we compare it with the results obtained for a network of two L-shaped detectors (either parallel or misaligned) located in Europe, considering different choices of arm-length for both the triangle and the 2L geometries. We also study how the science output changes in the absence of the low-frequency instrument, both for the triangle and the 2L configurations. We examine a broad class of simple `metrics' that quantify the science output, related to compact binary coalescences, multi-messenger astronomy and stochastic backgrounds, and we then examine the impact of different detector designs on a more specific set of scientific objectives.

gr-qc↗

arXiv2023

Effect of dynamical gravitomagnetic tides on measurability of tidal parameters for binary neutron stars using gravitational waves

Gravitational waves (GWs) from binary neutron stars (NSs) have opened unique opportunities to constrain the nuclear equation of state by measuring tidal effects associated with the excitation of characteristic modes of the NSs. This includes gravitomagnetic modes associated with the Coriolis effect, whose frequencies are proportional to the NS's spin frequency, and for which the spin orientation determines the subclass of modes that are predominantly excited. We advance the GW models for these effects that are needed for data analysis by first developing a description for the adiabatic signatures from gravitomagnetic modes in slowly rotating NSs. We show that they can be encapsulated in an effective Love number which differs before and after a mode resonance. Combining this with a known generic model for abrupt changes in the GWs at the mode resonance and a point-mass baseline leads to an efficient description which we use to perform case studies of the impacts of gravitomagnetic effects for measurements with Cosmic Explorer, an envisioned next-generation GW detector. We quantify the extent to which neglecting (including) the effect of gravitomagnetic modes induces biases (significantly reduces statistical errors) in the measured tidal deformability parameters, which depend on the equation of state. Our results substantiate the importance of dynamical gravitomagnetic tidal effects for measurements with third generation detectors.

gr-qc↗

arXiv2022

Binary dynamics from Einstein-Maxwell theory at second post-Newtonian order using effective field theory

The detection of gravitational waves from binary black holes sources has opened the possibility to search for electric charges and "dark" charges on black holes, the latter being candidates for dark matter. This requires theoretical predictions about the effect of charges on the inspiral of binary black holes in order to place constraint on charges. The effects of these charges on the inspiral of binary black holes can be described using Einstein-Maxwell theory. They have previously been derived up to first post-Newtonian (1PN) order, and the results were recently used to place bounds on the charge-to-mass ratio on black holes. Here we use the effective field theory approach with a metric parameterization based on a temporal Kaluza-Klein decomposition with non-relativistic gravitational fields to arrive at the Lagrangian for binary motion under the influence of charges up to 2PN order.

gr-qc↗

arXiv2022

Determining the equation of state of neutron stars with Einstein Telescope using tidal effects and r-mode excitations from a population of binary inspirals

Third-generation gravitational wave (GW) observatories such as Einstein Telescope (ET) and Cosmic Explorer (CE) will be ideal instruments to probe the structure of neutron stars through the GWs they emit when undergoing binary coalescence. In this work we make predictions about how well ET in particular will enable us to reconstruct the neutron star equation of state through observations of tens of binary neutron star coalescences with signal-to-noise ratios in the hundreds. We restrict ourselves to information that can be extracted from the inspiral, which includes tidal effects and possibly r-mode resonances. In treating the latter we go beyond the Newtonian approximation, introducing and utilizing new universal relations. We find that the ability to observe resonant r-modes would have a noticeable impact on neutron star equation of state measurements with ET.

gr-qc↗

arXiv2021

Bounding dark charges on binary black holes using gravitational waves

In models of minicharged dark matter associated with a hidden $U(1)$ symmetry, astrophysical black holes may acquire a "dark" charge, in such a way that the inspiral dynamics of binary black holes can be formally described by an Einstein-Maxwell theory. Charges enter the gravitational wave signal predominantly through a dipole term, but their effect is known to effectively first post-Newtonian order in the phase, which enables measuring the size of the charge-to-mass ratios, $|q_i/m_i|$, $i = 1,2$, of the individual black holes in a binary. We set up a Bayesian analysis to discover, or constrain, dark charges on binary black holes. After testing our framework in simulations, we apply it to selected binary black hole signals from the second Gravitational Wave Transient Catalog (GWTC-2), namely those with low masses so that most of the signal-to-noise ratio is in the inspiral regime. We find no evidence for charges on the black holes, and place typical 1-$σ$ bounds on the charge-to-mass ratios of $|q_i/m_i| \lesssim 0.2 - 0.3$.

gr-qc↗

arXiv2021

Relativistic effective action of dynamical gravitomagnetic tides for slowly rotating neutron stars

Gravitomagnetic quasi-normal modes of neutron stars are resonantly excited by tidal effects during a binary inspiral, leading to a potentially measurable effect in the gravitational-wave signal. We take an important step towards incorporating these effects in waveform models by developing a relativistic effective action for the gravitomagnetic dynamics that clarifies a number of subtleties. Working in the slow-rotation limit, we first consider the post-Newtonian approximation and explicitly derive the effective action from the equations of motion. We demonstrate that this formulation opens a way to compute mode frequencies, yields insights into the relevant matter variables, and elucidates the role of a shift symmetry of the fluid properties under a displacement of the gravitomagnetic mode amplitudes. We then construct a fully relativistic action based on the symmetries and a power counting scheme. This action involves four coupling coefficients that depend on the internal structure of the neutron star and characterize the key matter parameters imprinted in the gravitational waves. We show that, after fixing one of the coefficients by normalization, the other three directly involve the two kinds of gravitomagnetic Love numbers (static and irrotational), and the mode frequencies. We discuss several interesting features and dynamical consequences of this action, and analyze the frequency-domain response function (the frequency-dependent ratio between the induced flux quadrupole and the external gravitomagnetic field), and a corresponding Love operator representing the time-domain response. Our results provide the foundation for deriving precision predictions of gravitomagnetic effects, and the nuclear physics they encode, for gravitational-wave astronomy.

gr-qc↗