arXiv subjects

Fereshteh Rajabi

Publications and source records attributed to Fereshteh Rajabi.

At least 19 recordsLinked to original sources

arXiv2026

Disentangling propagation effects from Fast Radio Burst spectra: An analysis on simulated data

We present a methodology to decouple propagation effects, specifically scattering and dispersion, from the intrinsic spectro-temporal properties of repeating Fast Radio Bursts. Utilizing the Triggered Relativistic Dynamical Model, and assuming superradiance as the emission mechanism, we generate simulated sub-bursts and inject controlled levels of scattering and residual dispersion. For each burst, we measure the sub-burst slope, defined as the trajectory of the centroids in the dynamic spectrum, and the characteristic duration of the burst profile. We then fit a modified sub-burst slope law to the resulting slope-duration measurements to recover the scattering timescale, residual dispersion measure, and other model parameters. Under the thin-screen approximation, the scattering timescale at $1~\mathrm{GHz}$ is precisely recovered, typically to within $\sim 1-2\%$ of the true value. In contrast, the residual dispersion is recovered with comparatively lower precision, with median absolute errors of $\sim 0.3-0.6\ \mathrm{pc \, cm^{-3}}$, reflecting its weaker constraint and degeneracy with an intrinsic parameter. Despite this, the modified sub-burst slope law successfully reproduces the spectro-temporal evolution and accurately constrains the scattering properties even for diverse intrinsic burst populations. These results demonstrate that our framework yields a tractable method for separating propagation-induced distortions from intrinsic emission characteristics to a meaningful degree, enabling more reliable inference of the physical properties of FRB sources.

astro-ph.HE↗

arXiv2026

Fast Population Leakage in Astronomical Masers: Maser Amplification and Transient Superradiance

We use a $\Lambda$-type three-level Maxwell--Bloch model to test whether an inverted molecular transition in an astronomical maser source can produce maser amplification or superradiance when its upper level also decays through a second radiative pathway with a much larger spontaneous decay rate. Such shared-upper-level configurations occur in multilevel, radiatively pumped molecules, including Class II methanol masers and several OH maser transitions. The model follows the coupled evolution of level populations, molecular coherences, radiation fields, and phenomenological relaxation and dephasing, separating population leakage from coherence loss. We focus on the mixed configuration in which the observed transition is inverted while the faster pathway is non-inverted and acts as a leakage channel. We find that rapid spontaneous decay through the competing pathway does not, by itself, suppress maser amplification or superradiant emission from the inverted transition. The response is controlled by the shared upper-level population reservoir, the available initial coherence, and the relaxation and dephasing timescales. For small effective coherence in the leakage pathway, its radiative output remains weak, while the inverted transition either amplifies a seed field in the quasi-steady maser regime or develops macroscopic coherence and produces a transient superradiant burst. A larger inversion does not necessarily produce a stronger burst if it is accompanied by a weaker initial coherence seed. As a benchmark, we apply the model to the 6.7 GHz methanol flare in S255IR-NIRS3, whose upper level also decays through the 239.7 GHz transition at a spontaneous rate more than four orders of magnitude larger. The calculated flare remains compatible with a transient-superradiance interpretation when this fast leakage pathway is included explicitly.

astro-ph.HE↗

arXiv2025

Machine-learning approaches to dispersion measure estimation for fast radio bursts

Fast radio bursts (FRBs) are bright, mostly millisecond-duration transients of extragalactic origin whose emission mechanisms remain unknown. As FRB signals propagate through ionized media, they experience frequency-dependent delays quantified by the dispersion measure (DM), a key parameter for inferring source distances and local plasma conditions. Accurate DM estimation is therefore essential for characterizing FRB sources and testing physical models, yet current dedispersion methods can be computationally intensive and prone to human bias. In this proof-of-concept study, we develop and benchmark three deep-learning architectures, a conventional convolutional neural network (CNN), a fine-tuned ResNet-50, and a hybrid CNN-LSTM model, for automated DM estimation. All models are trained and validated on a large set of synthetic FRB dynamic spectra generated using CHIME/FRB-like specifications. The hybrid CNN-LSTM achieves the highest accuracy and stability while maintaining low computational cost across the investigated DM range. Although trained on simulated data, these models can be fine-tuned on real CHIME/FRB observations and extended to future facilities, providing a pathway toward real-time, data-driven DM estimation in large FRB surveys with further development.

astro-ph.HE↗

arXiv2025

Relativistic Maxwell-Bloch Equations with Applications to Astrophysics

We derive relativistic Maxwell-Bloch equations for potential applications in astronomical environments, where various radiative processes are known to occur, including the maser action and Dicke's superradiance. We show that for both phenomena a radiating system's response is preserved at different relative velocities between the system's rest frame and the observer, while the relevant timescales and the radiation intensity transform as expected from relativistic considerations. We verify that the level of coherence between groups of emitters travelling at different speeds is unchanged in all reference frames. We also derive relativistic versions of the maser equations applicable in the steady-state regime.

quant-ph↗

arXiv2024

Quantum coherence and the invisible Universe: Subradiance as a dark matter mechanism

The origin of dark matter in galactic halos, one of the deepest unsolved problems in astrophysics, may find an unexpected contribution from the quantum mechanics of ordinary atomic hydrogen. We show that quantum entanglement and coherence among hydrogen atoms in a gas at thermal equilibrium can naturally lead to subradiance, a cooperative suppression of radiation that renders the gas simultaneously dark in emission, transparent to incident radiation, and effectively collision-less. These three properties, precisely those associated with dark matter, emerge from a single underlying physical mechanism: the entangled structure of Dicke states in the gas. Applying this framework to the 21 cm line of atomic hydrogen in galactic dark matter halos, we find that conditions there place the gas deep in the asymptotic subradiance regime, where the strongly suppressed spontaneous and stimulated intensities cancel exactly. The cold neutral cores of high-velocity clouds, with their observed temperatures near 100 K and inferred dark-to-visible mass ratios of ~100:1, are consistent with this picture. Our results suggest that a significant fraction of the non-luminous matter pervading galactic halos may be familiar atomic hydrogen whose quantum cooperative behavior hides it from view; a solution that may have been hiding in plain sight.

astro-ph.GA↗

arXiv2024

Spectro-temporal analysis of ultra-fast radio bursts using per-channel arrival times

Fast radio bursts (FRBs), especially those from repeating sources, exhibit a rich variety of morphologies in their dynamic spectra (or waterfalls). Characterizing these morphologies and spectro-temporal properties is a key strategy in investigating the underlying unknown emission mechanism of FRBs. This type of analysis has been typically accomplished using two-dimensional Gaussian techniques and the autocorrelation function (ACF) of the waterfall. These techniques are effective and precise at all duration scales, but can be limited in the presence of scattered tails, complex morphologies, or recently observed microshot forests. Here, we present a technique that involves the tagging of per-channel arrival times of an FRB to perform spectro-temporal measurements using a Gaussian profile model for each channel. While scattering and dispersion remain important and often dominating sources of uncertainty in measurements, this technique provides an adaptable and firm foundation for obtaining spectro-temporal properties from all types of FRB morphologies. We present measurements using this technique of several hundred bursts across 12 repeating sources, including over 400 bursts from the repeating sources FRB 20121102A, FRB 20220912A, and FRB 20200120E, all of which exhibit recently observed microsecond-long ultra-FRBs, as well as 143 multi-component drift rates. In addition to retrieving the known relationship between sub-burst slope and duration, we explore other correlations between burst properties. We find that the sub-burst slope law extends smoothly to ultra-FRBs, and that ultra-FRBs appear to form a distinct population in the duration-frequency relation.

astro-ph.HE↗

arXiv2024

Impact of propagation effects on the spectro-temporal properties of Fast Radio Bursts

We present a mathematical analysis of propagation-induced distortions in the spectro-temporal properties of Fast Radio Bursts (FRBs). Within the Triggered Relativistic Dynamical Model, we derive a centroid-based formulation of the sub-burst slope law, which is an inverse relation between frequency-drift rate and temporal width of sub-bursts. We extend our analysis to include two frequency-dependent propagation effects: (i) multipath scattering, characterized by a pulse-broadening timescale $\tau_\mathrm{sc} \propto \nu^{-4}$, and (ii) residual dispersion, parameterized by $\Delta \mathrm{DM}\propto \nu^{-2}$. Our analysis shows that scattering preserves the inverse relation between sub-burst slope and duration, but increases the scaling coefficient when $\tau_\mathrm{sc}$ exceeds the intrinsic width ($t_\mathrm{w}$) of sub-bursts. Residual DM errors act asymmetrically: under-dedispersion flattens the sub-burst slope, whereas over-dedispersion causes a non-linear increase and eventually a change of sign. When both effects are present, scattering counterbalances the steepening induced by over-dedispersion and augments the flattening produced by under-dedispersion, yielding characteristically distorted curves. We repeat measurements for ultra-short duration bursts (ultra-FRBs) with $t_\mathrm{w} = 50\ \mu\mathrm{s}$ at 1 GHz and found them to be far more sensitive to propagation errors. Deviations become measurable for $\left | \Delta \mathrm{DM} \right |\sim0.05$ pc cm$^{-3}$ and for $\tau_\mathrm{sc} \sim0.1$ ms at 1 GHz, levels that have negligible impact on the standard-width sub-bursts. Our analysis provides practical diagnostics to disentangle propagation effects from the observed spectro-temporal properties of FRBs, thereby recovering true correlations among their intrinsic parameters.

astro-ph.HE↗

arXiv2024

Validating the Sub-Burst Slope Law: A Comprehensive Multi-Source Spectro-Temporal Analysis of Repeating Fast Radio Bursts

We conduct a comprehensive spectro-temporal analysis of repeating Fast Radio Bursts (FRBs) utilizing nine distinct sources, the largest sample to date. Our data set includes 175 sub-bursts and 31 multi-component bursts from 11 data sets, with centre frequencies ranging from 149--7144 MHz and durations spanning from 73 $μ$s--13 ms. Our findings are consistent with the predictions of the Triggered Relativistic Dynamical Model (TRDM) of FRB emission. We affirm the predicted quadratic relationship between sub-burst slope and central frequency, as well as a linear dependence of the sub-burst bandwidth on central frequency that is consistent with mildly-relativistic Doppler broadening of narrow-band emission. Most importantly, we confirm the sub-burst slope law, a predicted inverse relationship between sub-burst slope and duration, to hold consistently across different sources. Remarkably, we also discover that the drift rates of multi-component bursts follow the same law as the sub-burst slopes, an unexplained result that warrants further investigation. These findings not only support the TRDM as a viable framework for explaining several aspects of FRB emission, but also provide new insights into the complex spectro-temporal properties of FRBs.

astro-ph.HE↗

arXiv2023

Investigations of 2D ion crystals in a hybrid optical cavity trap for quantum information processing

We numerically investigate a hybrid trapping architecture for 2D ion crystals using static electrode voltages and optical cavity fields for in-plane and out-of-plane confinements, respectively. By studying the stability of 2D crystals against 2D-3D structural phase transitions, we identify the necessary trapping parameters for ytterbium ions. Multiple equilibrium configurations for 2D crystals are possible, and we analyze their stability by estimating potential barriers between them. We find that scattering to anti-trapping states limits the trapping lifetime, which is consistent with recent experiments employing other optical trapping architectures. These 2D ion crystals offer an excellent platform for quantum simulation of frustrated spin systems, benefiting from their 2D triangular lattice structure and phonon-mediated spin-spin interactions. Quantum information processing with tens of ions is feasible in this scheme with current technologies.

quant-ph↗

arXiv2023

Variability, flaring and coherence -- the complementarity of the maser and superradiance regimes

We discuss the role that coherence phenomena can have on the intensity variability of spectral lines associated with maser radiation. We do so by introducing the fundamental cooperative radiation phenomenon of (Dicke's) superradiance and discuss its complementary nature to the maser action, as well as its role in the flaring behaviour of some maser sources. We will consider examples of observational diagnostics that can help discriminate between the two, and identify superradiance as the source of the latter. More precisely, we show how superradiance readily accounts for the different time-scales observed in the multi-wavelength monitoring of the periodic flaring in G9.62+0.20E.

astro-ph.GA↗

arXiv2023

A broad survey of spectro-temporal properties from FRB 20121102A

We survey the spectro-temporal properties of fast radio bursts from FRB 20121102A observed by earlier studies across a wide range of frequencies. We investigate 167 bursts from FRB 20121102A spanning frequencies 1--7.5GHz, durations of less than 1 ms to approximately 10 ms, with low and high energies, and with different wait-times. We find from this sample of bursts a strong agreement with the inverse relationship between sub-burst slope and duration and with other predictions made by the triggered relativistic dynamical model (TRDM). Earlier results found agreement with those predictions across three different repeating FRB sources. For this sample of bursts, we find that the sub-burst slope as well as the 'sad trombone' drift rate are consistent with being in a quadratic relationship with frequency and that both these quantities are inversely proportional to the duration. We also find that the duration decreases with increasing frequency as well as a statistically significant correlation between the sub-burst duration and bandwidth (proportional to $t^{-1/2}$) that is unexpected. No distinct group of bursts in this sample deviated from these relationships, however significant scatter can be seen in measurements. This study demonstrates the consistent existence of relationships between the spectro-temporal properties of bursts from a repeating FRB source. A simple explanation for the inverse relation between the sub-burst slope and duration is an inherently narrowband emission process. We make all measurements available as well as a graphical user interface called Frbgui developed and used to perform measurements of burst waterfalls.

astro-ph.HE↗

arXiv2022

Transient Structure in the Non-linear Superradiance Regime of Widely Doppler Broadened Media

We investigate transient radiation processes in the non-linear superradiance (SR) regime of the Doppler broadened Maxwell-Bloch equations when the velocity distribution is of total bandwidth greatly exceeding that of the transient process itself. We demonstrate the formation of global polarisation phase correlation and the quenching of temporal structure if a smooth distribution is inverted above the critical threshold required to enter the non-linear SR regime. We propose candidate stochastic velocity distributions capable of sustaining finite temporal structure in the non-linear emission process. We develop a novel algorithm for simulating the Doppler broadened Maxwell-Bloch equations which is $O(n)$ complex in the number of velocity channels $n$ whenever the emerging polarisation correlation is of moderate bandwidth, and we apply it to a stochastic velocity distribution in order to demonstrate sustained delay and duration of peak intensity in the widely Doppler broadened limit. We discuss the transverse inversion process and recognise an autoregulation mechanism on the number of molecules cooperatively participating in SR emission. This mechanism has the effect of limiting the temporal duration of the intensity pulse to a lower bound proportional to the length of the sample, which we confirm through simulation.

physics.optics↗

arXiv2022

Superradiance from a Relativistic Source

We construct a model of cooperative superradiant emission from a highly relativistic multi-particle source. We revise an existing model of the literature for a relativistic two-level particle, and construct from it a Hamiltonian describing relativistic velocity dependent multi-particle superradiance. We adapt the standard diagrammatic framework to compute time evolution and density operators from our Hamiltonian, and demonstrate during the process a departure from standard results and calculation methods. In particular, we demonstrate that the so-called vertical photon result of the literature is modified by the relativistic Lorentz factor of the sample; we also introduce a set of coupled differential equations describing certain propagators in the velocity-dependent small sample framework, which we solve numerically via a hybrid fourth order Runge-Kutta and convolution approach. We demonstrate our methods for the simple case of two highly relativistic particles travelling with slightly differing velocities simulated at varying relativistic mean sample $β$ factors, and evaluate velocity coherence requirements for a sample to demonstrate enhanced superradiant emission in the observer frame. We find these coherence requirements to become increasingly restrictive at higher $β$ factors, even in the context of standard results of relativistic velocity differential transformations.

quant-ph↗

arXiv2021

The generation and transformation of polarisation signals in molecular lines through collective anisotropic resonant scattering

We discuss the existence of elliptical polarisation in rotational spectral lines of CO and other molecules within the context of the Anisotropic Resonant Scattering (ARS) model. We show that the effect of ARS on the radiation field can lead to not only the previously predicted transformation of background linear polarisation into circular polarisation (i.e., Faraday conversion), but also the occurrence of Faraday rotation and the generation of elliptically polarised signals in an otherwise initially unpolarised radiation field. This is due to a collective behaviour between the large number of molecules acting as a diffraction ensemble that strongly favours forward scattering over any other mode. Our application to astronomical data demonstrates the dependency of the Stokes parameters on the strength and orientation of the ambient magnetic field, and suggests that ARS will manifest itself for a wide range of molecular species and transitions.

astro-ph.GA↗

arXiv2021

Evidence of a shared spectro-temporal law between sources of repeating fast radio bursts

We study the spectro-temporal characteristics of two repeating fast radio bursts (FRBs), namely, FRB 20180916B and FRB 20180814A, and combine the results with those from our earlier analysis on FRB 20121102A. The relationship between the frequency drift rate, or slope, of individual sub-bursts and their temporal duration is investigated. We consider a broad sample of possible dispersion measure (DM) values for each source to understand the range of valid sub-burst slope and duration measurements for all bursts and to constrain our results. We find good agreement with an inverse scaling law between the two parameters previously predicted using a simple dynamical relativistic model. The remarkably similar behaviour observed in all sources provides strong evidence that a single and common underlying physical phenomenon is responsible for the emission of signals from these three FRBs, despite their associations with different types of host galaxies at various redshifts. It also opens up the possibility that this sub-burst slope law may be a universal property among repeating FRBs, or indicates a distinct subclass among them.

astro-ph.HE↗

arXiv2021

Manipulating phonons of a trapped-ion system using optical tweezers

We propose an experimental architecture where an array of optical tweezers affords site-dependent control over the confining potential of a conventional radio-frequency ion trap. The site-dependent control enables programmable manipulation of phonon modes of ions, with many potential applications in quantum information processing (QIP) and thermodynamics. We describe protocols for programming the array of optical tweezers to attain a set of target phonon modes with high accuracy. We propose applications of such controls in simulating quantum thermodynamics of a particle of programmable effective mass via Jarzynski's equality and improving the efficiency of sympathetic cooling and quantum logic gates in a multi-species ion system of disparate masses. We discuss the required optical parameters in a realistic ion trap system and potential adverse effects of optical tweezers in QIP. Our scheme extends the utility of trapped-ions as a platform for quantum computation and simulation.

quant-ph↗

arXiv2020

A simple relationship for the spectro-temporal structure of bursts from FRB 121102

We consider a simple dynamical and relativistic model to explain the spectro-temporal structure often displayed by repeating fast radio bursts (FRBs). We show how this model can account for the downward frequency drift in a sequence of sub-bursts of increasing arrival time (the "sad trombone" effect) and their tendency for exhibiting a reduced pulse width with increasing frequency of observation. Most importantly, this model also predicts a systematic inverse relationship between the (steeper) slope of the frequency drift observed within a single sub-burst and its temporal duration. Using already published data for FRB 121102 we find and verify the relationship predicted by this model. We therefore argue that the overall behaviour observed for this object as a function of frequency is consistent with an underlying narrow-band emission process, where the wide-band nature of the measured FRB spectrum is due to relativistic motions. Although this scenario and the simple dynamics we consider could be applied to other theories, they are well-suited for a model based upon Dicke's superradiance as the physical process responsible for FRB radiation in this and similar sources.

astro-ph.HE↗

arXiv2020

Astronomical masers and Dicke's superradiance

We consider the radiation properties and processes of a gas with a population inversion using the formalism based on the Maxwell-Bloch equations. We focus on the maser action and Dicke's superradiance to establish their relationship in the overall radiation process during the temporal evolution of the system as a function of position. We show that the maser action and superradiance are not competing phenomena but are rather complementary, and define two distinct limits for the intensity of radiation. Masers characterise the quasi-steady state limit, when the population inversion density and the polarisation amplitude vary on time-scales longer than those of non-coherent processes affecting their evolution (e.g., collisions), while superradiance defines the fast transient regime taking place when these conditions are reversed. We show how a transition from a maser regime to superradiance will take place whenever a critical threshold for the column density of the population inversion is reached, at which point a strong level of coherence is established in the system and a powerful burst of radiation can ensue during the transient regime. This critical level also determines the spatial region where a transition from the unsaturated to the saturated maser regimes will take place; superradiance can thus be seen as the intermediary between the two. We also quantify the gain in radiation intensity attained during the superradiance phase relative to the two maser regimes, and show how the strong coherence level during superradiance is well suited to explain observations that reveal intense and fast radiation flares in maser-hosting regions.

astro-ph.GA↗