arXiv subjects

Prasenjit Saha

Publications and source records attributed to Prasenjit Saha.

At least 19 recordsLinked to original sources

arXiv2026

Beyond collective fluctuations: probing micro-image swarms in lensed quasars with intensity interferometry

Each strongly lensed image of a quasar behind a lensing galaxy (or galaxy cluster) is composed of a swarm of micro-images. This is a result of microlensing due to stellar-scale substructure in the lens. The presence of microlenses forms a network of micro-caustics, and a source transiting these micro-caustics gives rise to variation in observed strongly lensed images. These micro-image swarms are currently observable only through collective intensity fluctuations, which hide the underlying information on the stellar (and compact dark matter, if any) mass functions within the swarm. To unlock the information present in micro-image swarms, it is necessary to explore new techniques. In this work, we study the prospects of determining the micro-image swarm size in lensed quasar images using the intensity interferometry (i.e., the Hanbury Brown & Twiss effect). We consider QSO 2237+0305 and PS J0147+4630, two of the brightest lens quasars in the sky, and study micro-image swarm features in visibility space for both macro-minimum and macro-saddle-point images. At the end, we argue that, with ongoing and expected technical advances, observations of micro-image swarms are plausible, at least for the brightest lensed quasars.

astro-ph.CO↗

arXiv2026

ESO White Paper on Intensity Interferometry: Cosmology, Fundamental Physics, Quantum Optics

In this whitepaper, we outline how recent technological advances and ongoing developments open qualitatively new science opportunities in cosmology, fundamental physics, and quantum astrophysics. First, intensity interferometry can contribute to one of the most foundational observables in cosmology: the expansion rate of the Universe. Its angular resolution allows it to resolve the angular extent of extragalactic objects such as supernovae or quasars; combined with a physical scale local to the source, this yields an angular diameter distance and hence a 'Hubble diagram'. Second, the nature of dark matter can be probed via the astrometric lensing signatures of tiny dark matter halos. Third, intensity interferometry gives direct access to second-order coherence properties of astrophysical emission, opening a window onto genuinely quantum aspects of astrophysical light.

astro-ph.IM↗

arXiv2026

Observations of Binary Stars with the 1.3-m Devasthal Fast Optical Telescope Using Speckle Interferometry: An Attempt

We present a feasibility study exploring the implementation of optical interferometry and speckle techniques with the 1.3-m Devasthal Fast Optical Telescope (DFOT) at ARIES, which is currently dedicated to photometric observations. Using the sCMOS camera as the DFOT backend, we perform interferometric speckle observations of several binary stars. Standard Speckle Interferometry (SI) algorithms are applied to analyze the recorded data. While this study does not aim to achieve the diffraction limit of DFOT or address a full science-driven resolution case, it serves as a crucial testbed for instrumentation, data acquisition, and analysis of Speckles with DFOT. Notably, we successfully identify and correct tracking-related positional errors in the observed binary systems, demonstrating the viability of the approach. These results provide strong motivation for more systematic observations and future implementation of optical interferometry techniques at meter-class telescopes.

astro-ph.IM↗

arXiv2025

Towards measuring astrophysical third order correlation functions with the H.E.S.S. optical intensity interferometer

The closure phase, the sum of the three Fourier phases in a telescope triangle, is an important tool in astronomical interferometry, helping to reconstruct the geometries of the observed objects. While already established in amplitude interferometry, for the recently expanding field of intensity interferometers the closure phase enables recovering information of the interferometric phases that are otherwise inaccessible with this technique. To extract the (cosine of) the closure phase ($\cos \phi$) in intensity interferometry, third-order correlations between three simultaneously measuring telescopes have to be computed. As the signal-to-noise of such three-photon correlations is too small for current generation intensity interferometers, no third-order correlations of astrophysical targets have been recorded so far. In this paper we present the first measurements of third order correlation functions of two stellar systems, Nunki ($\sigma $ Sgr) and Dschubba ($\delta$ Sco), observed with the H.E.S.S. intensity interferometer in 2023. We show how to isolate the three-photon contribution term from the two-photon contributions, in order to access $\cos \phi$. For the observed stellar targets the sensitivity is not high enough to extract closure phase information. To demonstrate that the analysis works well we further extract $\cos \phi$ in a laboratory experiment, using the H.E.S.S. intensity interferometer and a pseudo-thermal light source.

astro-ph.IM↗

arXiv2025

Exploiting light coherence in astrophysics

The Hanbury Brown-Twiss (HBT) effect, discovered in the 1950s and further developed in the 1960s, was originally used to estimate stellar angular diameters through intensity correlations measured by spatially separated detectors. Further developments started from HBT experiments to exploit quantum bunching of photons in incoherent light sources played foundational role in the development of quantum optics. When the two detectors in an HBT experiment are co-located, typically implemented using a beam splitter, a zero-baseline intensity correlation is obtained, which after deconvolution of the detector response function, yields the temporal component of the second-order coherence function. Unlike spatial correlations, this function is independent of the source brightness distribution, or its size, giving direct insight into the properties of the source's emission process itself - photon statistics. Along with photometric and spectral information, the second order coherence function can be used to constrain the emission mechanisms and discriminate between thermal, synchrotron, bremsstrahlung and stimulated emission processes. Evolution of the emission processes would likewise drive changes in the second order coherence. Light coherence information along with multi-wavelength observations, can become a complementary "messenger", carrying internal information about the astronomical source.

astro-ph.GA↗

arXiv2025

Stellar Multiplicity via Speckle Interferometry with the 3.6 m Devasthal Optical Telescope

Conventional ground-based optical telescopes, even those with large apertures, primarily observe stars, close binaries, and multiple systems as unresolved point sources through photometric measurements. Spectroscopy can identify multiple stellar components within a system, but both techniques are fundamentally limited in resolving stellar surfaces and providing direct angular separations. Although photometric and spectroscopic observations yield critical information on magnitudes/flux, metallicities, and orbital properties, complementary high-angular-resolution methods are required to constrain additional system characteristics, including angular orbital parameters, model-independent distances, radii, and stellar masses. The limitations of these two methods arise due to the Diffraction Limit of the telescopes and atmospheric turbulence. Speckle Interferometry (SI) is a clever and affordable method for ground-based telescopes to work around atmospheric turbulence. In this work, we utilize the speckle images obtained by the 3.6 m DOT and demonstrate the capability of SI to resolve binary systems, measure their orbital separations, and determine their position angles. For systems with faint companions where conventional analysis fails, we employ Bayesian inference to model speckle patterns and estimate orbital parameters with high precision. These results establish the effective methodology for using a medium-sized, 4-m class telescope like the DOT as a high-resolution stellar interferometer and demonstrate the potential of speckle interferometry as a powerful technique to advance optical interferometric studies within Indian astronomy.

astro-ph.IM↗

arXiv2025

Probing Stellar Kinematics with the Time-Asymmetric Hanbury Brown and Twiss Effect

Intensity interferometry (II) offers a powerful means to observe stellar objects with a high resolution. In this work, we demonstrate that II can also probe internal stellar kinematics by revealing a time-asymmetric Hanbury Brown and Twiss (HBT) effect, causing a measurable shift in the temporal correlation peak away from zero delay. We develop numerical models to simulate this effect for two distinct astrophysical scenarios: an emission-line circumstellar disk and an absorption-line binary system. Our simulations reveal a clear sensitivity of this temporal asymmetry to the system's inclination angle, velocity symmetry, and internal dynamics. This suggests that, with sufficiently high time resolution, II can be used to extract quantitative information about internal kinematics, offering a new observational window on stellar dynamics.

astro-ph.IM↗

arXiv2025

Bridging the micro-Hz gravitational wave gap via Doppler tracking with the Uranus Orbiter and Probe Mission: Massive black hole binaries, early universe signals and ultra-light dark matter

With the recent announcement by NASA's \textit{Planetary Science and Astrobiology Decadal Survey 2023-2032}, a priority flagship mission to the planet Uranus is anticipated. Here, we explore the prospects of using the mission's radio Doppler tracking equipment to detect gravitational waves (GWs) and other analogous signals related to dark matter (DM) over the duration of its interplanetary cruise. We develop a methodology to stack tracking data and account for time varying detector geometry, thereby constructing the sensitivity of the mission to GWs over the wide frequency range of $3\times 10^{-9}$ Hz to $10^{-4}$ Hz. We find that the mission has the potential to fill the gap between pulsar timing and space-based-interferometry GW observatories. If improvements in reducing \textit{Cassini} era noise by a factor of $\sim$10 are implemented, we forecast the detection of $\mathcal{\mathcal{O}}(\rm{few})$ individual massive black hole binaries using two independent population models. Additionally, we determine the mission's sensitivity to both astrophysical and primordial stochastic gravitational wave backgrounds, as well as its capacity to test, or even confirm via detection, ultralight DM models. In all these cases, the tracking of the spacecraft over its interplanetary cruise would enable coverage of unexplored regions of parameter space, where signals from new phenomena in our Universe may be lurking.

astro-ph.HE↗

arXiv2025

Radial Pulsations in Polaris: A Secondary Science Application of Cherenkov Telescopes via Intensity Interferometry

Ground-based Cherenkov telescopes, although typically inoperative during moonlit nights for gamma-ray observations, offer a valuable opportunity for secondary scientific applications through Intensity Interferometry (II). Recent developments and observations suggest that implementing II instrumentation on existing Imaging Atmospheric Cherenkov Telescopes (IACTs) or the Cherenkov Telescope Array (CTA) can significantly advance optical stellar measurements. Motivated by the resurgence of II efforts over the past two decades, this work presents simulations demonstrating the estimation of stellar parameters for a radially pulsating star, such as Polaris, using either a single telescope or multiple telescopes. For single-telescope simulations, we assume that the photon pixels in the camera are mapped onto four distinct regions of the aperture, generating multiple baselines and enabling enhanced observational plane coverage. These results highlight the potential of Cherenkov telescopes in India for high-resolution optical astronomy during otherwise inoperative periods and offer promising insights into the characterization of bright stellar objects with unprecedented precision.

astro-ph.IM↗

arXiv2025

Generative AI for image reconstruction in Intensity Interferometry: a first attempt

In the last few years, Intensity Interferometry (II) has made significant strides in achieving high-precision resolution of stellar objects at optical wavelengths. Despite these advancements, phase retrieval remains a major challenge due to the nature of photon correlation. This paper explores the application of a conditional Generative Adversarial Network (cGAN) to tackle the problem of image reconstruction in II. This method successfully reconstructs the shape, size, and brightness distribution of simulated, fast-rotating stars based on sparsely sampled spatial power spectra obtained by using two different hypothetical ground-based II facilities composed of six and nine Imaging Atmospheric Cherenkov Telescopes (IACTs), respectively. Although this particular example could also be addressed using parameter fitting, our results suggest that with larger arrays of IACTs much more complex systems with varied surface features could be reconstructed by applying machine-learning techniques to II. Hence this approach merits closer examination.

astro-ph.IM↗

arXiv2025

Gravitational lensing of fast radio bursts: prospects for probing microlens populations in lensing galaxies

Gravitational lensing by a stellar microlens of mass $M$ forms two images separated by micro-arcseconds on the sky and has a time delay of $2\times10^{-5}(M/{\rm M_\odot})$ seconds. Although we cannot resolve such micro-images in the sky, they could be resolved in time if the source is a fast radio burst (FRB). In this work, we study the magnification ($|\mu|$) and time delay~($t_d$) distributions of micro-images led by different microlens populations. We find that, in microlensing of typical strongly lensed (macro-)images in galaxy lenses, micro-images stemmed from a population of stellar mass microlenses in the $[0.08, 1.5]\:{\rm M_\odot}$ range and a second (dark) microlens population in $[10^{-3} - 10^{-2}]\:{\rm M_\odot}$ range reside in different parts of $|\mu|-t_d$ plane. For the global minimum macro-image, due to low stellar mass density, we find that the stellar population leads to peaks in autocorrelation at ${>}10^{-6}$ seconds, whereas the secondary population leads to peaks at ${<}10^{-6}$ seconds, allowing us to differentiate different microlens populations. However, an increase in stellar density introduces new peaks at ${<}10^{-6}$ seconds, which can pollute the inference about the presence of multiple microlens populations. In addition, we also show that the number of micro-images, hence the number of peaks in the autocorrelation, is also sensitive to the underlying stellar mass function, allowing us to constrain the stellar initial mass function (IMF) with FRB microlesning in the future. This work is a first step towards using FRB lensing to probe the microlens population within strong lenses, and more detailed studies are required to assess the effect of various uncertainties that we only discussed qualitatively.

astro-ph.GA↗

arXiv2025

Dark Matter Particle Flux in a Dynamically Self-consistent Milky Way Model

We extend a recently developed dynamically self-consistent model of the Milky Way constrained by observations from the Gaia observatory to include a radially anisotropic component in the dark matter (DM) halo, which represents the debris from the accreted Gaia-Sausage-Enceladus (GSE) galaxy. In the new model, which we call a self-consistent Anisotropic Halo Model or scAHM, we derive distribution functions for DM velocity in heliocentric and geocentric reference frames. We compare them with the velocity distributions in the standard halo model (SHM) and another anisotropic model (SHM++). We compute predicted scattering rates in direct-detection experiments, for different target nuclei and DM particle masses. Seasonal dependencies of scattering rates are analyzed, revealing small but interesting variations in detection rates for different target nuclei and DM masses. Our findings show that the velocity distribution of the anisotropic GSE component significantly deviates from Gaussian, showing a modest impact on the detection rates. The peculiar kinematic signature of the radially anisotropic component would be most clearly observable by direction-sensitive detectors.

astro-ph.GA↗

arXiv2025

A possible wave-optical effect in lensed FRBs

Context: Fast Radio Bursts (FRBs) are enigmatic extragalactic bursts whose properties are still largely unknown, but based on their extremely small time duration, they are proposed to have a compact structure, making them candidates for wave-optical effects if gravitational lensed. If an FRB is lensed into multiple-images bursts at different times by a galaxy or cluster, a likely scenario is that only one image is detected, because the others fall outside the survey area and time frame. Aims: In this work we explore the FRB analog of quasar microlensing, namely the collective microlensing by stars in the lensing galaxy, now with wave optics included. The eikonal regime is applicable here. Methods. We study the voltage (rather than the intensity) in a simple simulation consisting of (a) microlensing stars, and (b) plasma scattering by a turbulent interstellar medium. Results: The auto-correlation of the voltage shows peaks (at order-microsecond separations) corresponding to wave-optical interference between lensed micro-images. The peaks are frequency dependent if plasma-scattering is significant. While qualitative and still in need of more realistic simulations, the results suggest that a strongly-lensed FRB could be identified from a single image. Conclusions: Microlensing could sniff out macro-lensed FRBs

astro-ph.HE↗

arXiv2024

Reconciling concentration to virial mass relations

The concentration-virial mass (c-M) relation is a fundamental scaling relation within the standard cold dark matter ($Λ$CDM) framework well established in numerical simulations. However, observational constraints of this relation are hampered by the difficulty of characterising the properties of dark matter haloes. Recent comparisons between simulations and observations have suggested a systematic difference of the c-M relation, with higher concentrations in the latter. In this work, we undertake detailed comparisons between simulated galaxies and observations of a sample of strong-lensing galaxies. We explore several factors of the comparison with strong gravitational lensing constraints, including the choice of the generic dark matter density profile, the effect of radial resolution, the reconstruction limits of observed versus simulated mass profiles, and the role of the initial mass function in the derivation of the dark matter parameters. Furthermore, we show the dependence of the c-M relation on reconstruction and model errors through a detailed comparison of real and simulated gravitational lensing systems. An effective reconciliation of simulated and observed c-M relations can be achieved if one considers less strict assumptions on the dark matter profile, for example, by changing the slope of a generic NFW profile or focusing on rather extreme combinations of stellar-to-dark matter distributions. A minor effect is inherent to the applied method: fits to the NFW profile on a less well-constrained inner mass profile yield slightly higher concentrations and lower virial masses.

astro-ph.GA↗

arXiv2024

Interference with (Pseudo) Thermal Light; The Hanbury Brown and Twiss Effect

The correlation of light from two sources leads to an interference pattern if they belong to a specific time interval known as the coherence time, denoted as $Δτ$. The relationship governing this phenomenon is $ΔτΔν\approx 1$, where $Δν$ represents the bandwidth of the light. This requirement is not satisfied, and hence, interference fringes are not observable in the case of ordinary (thermal) light. In the 1950s, Robert Hanbury Brown and Richard Q. Twiss explored interference phenomena using a narrow bandwidth of thermal light. This investigation led to the discovery of the Hanbury-Brown and Twiss effect (or the HBT effect in short), which has since found applications in various fields, particularly stellar observations and quantum optics. This article briefly traces the history of the HBT effect and its applications in various fields, including stellar observations. More importantly, it outlines the basic theoretical framework of this effect, followed by the design and results of the correlation in intensity fluctuation of a pseudo-thermal light in a college laboratory setting (Michelson interferometer).

astro-ph.IM↗

arXiv2024

Microlensing near macro-caustics

Microlensing near macro-caustics is a complex phenomenon in which swarms of micro-images produced by micro-caustics form on both sides of a macro-critical curve. Recent discoveries of highly magnified images of individual stars in massive galaxy cluster lenses, predicted to be formed by these micro-image swarms, have stimulated studies on this topic. In this Chapter, we explore microlensing near macro-caustics using both simulations and analytic calculations. We show that the mean total magnification of the micro-image swarms follows that of an extended source in the absence of microlensing. Micro-caustics join into a connected network in a region around the macro-critical line of a width proportional to the surface density of microlenses; within this region, the increase of the mean magnification toward the macro-caustic is driven by the increase of the number of micro-images rather than individual magnifications of micro-images. The maximum achievable magnification in micro-caustic crossings decreases with the mass fraction in microlenses. We conclude with a review of applications of this microlensing phenomenon, including limits to the fraction of dark matter in compact objects, and searches of Population III stars and dark matter subhalos. We argue that the discovered highly magnified stars at cosmological distances already imply that less than $\sim$ 10\% of the dark matter may be in the form of compact objects with mass above $\sim 10^{-6}\, M_{\odot}$.

astro-ph.GA↗

arXiv2024

Essentials of strong gravitational lensing

Of order one in 10^3 quasars and high-redshift galaxies appears in the sky as multiple images as a result of gravitational lensing by unrelated galaxies and clusters that happen to be in the foreground. While the basic phenomenon is a straightforward consequence of general relativity, there are many non-obvious consequences that make multiple-image lensing systems (aka strong gravitational lenses) remarkable astrophysical probes in several different ways. This article is an introduction to the essential concepts and terminology in this area, emphasizing physical insight. The key construct is the Fermat potential or arrival-time surface: from it the standard lens equation, and the notions of image parities, magnification, critical curves, caustics, and degeneracies all follow. The advantages and limitations of the usual simplifying assumptions (geometrical optics, small angles, weak fields, thin lenses) are noted, and to the extent possible briefly, it is explained how to go beyond these. Some less well-known ideas are discussed at length: arguments using wavefronts show that much of the theory carries over unchanged to the regime of strong gravitational fields; saddle-point contours explain how even the most complicated image configurations are made up of just two ingredients. Orders of magnitude, and the question of why strong lensing is most common for objects at cosmological distance, are also discussed. The challenges of lens modeling, and diverse strategies developed to overcome them, are discussed in general terms, without many technical details.

astro-ph.CO↗

arXiv2023

What are the parities of photon-ring images near a black hole?

Light that grazes a black-hole event horizon can loop around one or more times before escaping again, resulting for distance observers in an infinite sequence of ever fainter and more delayed images near the black hole shadow. In the case of the M87 and Sgr A$^*$ back holes, the first of these so-called photon-ring images have now been observed. A question then arises: are such images minima, maxima, or saddle-points in the sense of Fermat's principle in gravitational lensing? or more briefly, the title question above. In the theory of lensing by weak gravitational fields, image parities are readily found by considering the time-delay surface (also called the Fermat potential or the arrival-time surface). In this work, we extend the notion of the time delay surface to strong gravitational fields and compute the surface for a Schwarzschild black hole. The time-delay surface is the difference of two wavefronts, one travelling forward from the source and one travelling backwards from the observer. Image parities are read off from the topography of the surface, exactly as in the weak-field regime, but the surface itself is more complicated. Of the images, furthest from the black hole and similar to the weak-field limit, are a minimum and a saddle point. The strong field repeats the pattern, corresponding to light taking one or more loops around the back hole. In between, there are steeply-rising walls in the time-delay surface, which can be interpreted as maxima and saddle points that are infinitely delayed and not observable -- these correspond to light rays taking a U-turn around the black hole.

gr-qc↗