arXiv subjects

Saurabh

Publications and source records attributed to Saurabh.

18 recordsLinked to original sources

arXiv2026

GRMHD accretion beyond the black hole paradigm: Light from within the shadow

We present the first three-dimensional general relativistic magnetohydrodynamic simulation of sustained accretion onto a horizonless singularity in which matter reaches the central object rather than being accumulated outside of it or expelled in outflows. We consider a Joshi-Malafarina-Narayan (JMN-1) spacetime, a well-motivated black hole mimicker that arises from gravitational collapse with anisotropic pressure in general relativity, and adopt a compactness parameter for which the central singularity is null. We find that the system evolves into a sustained magnetically arrested disk state. For parameters appropriate to the low-luminosity active galactic nucleus M87*, we obtain an accretion rate of $\sim(3.0 \pm 0.5)\times 10^{-6} \dot{M}_{\rm Edd}$, in full agreement with estimates based on black hole models and, in particular, comparable to that of our reference Schwarzschild black hole simulation. Synthetic ray-traced images at $230\,{\rm GHz}$, computed using polarized general relativistic radiative transfer, are broadly consistent with the Event Horizon Telescope observations of M87*. We identify a key observational discriminant between a black hole and JMN-1: the presence of detectable brightness inside of the ``observable" shadow of JMN-1. This emission originates very close to the central singularity, in a region that would be hidden behind the event horizon in a black hole spacetime. Although this signature is beyond the reach of current observations, it falls within the projected imaging dynamic range of next-generation radio interferometric instruments, offering a robust test of the black hole paradigm.

astro-ph.HE↗

arXiv2026

Towards Spatio-Temporal World Scene Graph Generation from Monocular Videos

Spatio-temporal scene graphs provide a principled representation for modeling evolving object interactions, yet existing methods remain fundamentally frame-centric: they reason only about currently visible objects, discard entities upon occlusion, and operate in 2D. To address this, we first introduce ActionGenome4D, a dataset that upgrades Action Genome videos into 4D scenes via feed-forward 3D reconstruction, world-frame oriented bounding boxes for every object involved in actions, and dense relationship annotations including for objects that are temporarily unobserved due to occlusion or camera motion. Building on this data, we formalize World Scene Graph Generation (WSGG), the task of constructing a world scene graph at each timestamp that encompasses all interacting objects in the scene, both observed and unobserved. We then propose three complementary methods, each exploring a different inductive bias for reasoning about unobserved objects: PWG (Persistent World Graph), which implements object permanence via a zero-order feature buffer; MWAE (Masked World Auto-Encoder), which reframes unobserved-object reasoning as masked completion with cross-view associative retrieval; and 4DST (4D Scene Transformer), which replaces the static buffer with differentiable per-object temporal attention enriched by 3D motion and camera-pose features. We further design and evaluate the performance of strong open-source Vision-Language Models on the WSGG task via a suite of Graph RAG-based approaches, establishing baselines for unlocalized relationship prediction. WSGG thus advances video scene understanding toward world-centric, temporally persistent, and interpretable scene reasoning.

cs.CV↗

arXiv2025

Probing jet base emission of M87* with the 2021 Event Horizon Telescope observations

We investigate the presence and spatial characteristics of the jet base emission in M87* at 230 GHz, enabled by the enhanced uv coverage in the 2021 Event Horizon Telescope (EHT) observations. The addition of the 12-m Kitt Peak Telescope and NOEMA provides two key intermediate-length baselines to SMT and the IRAM 30-m, giving sensitivity to emission structures at scales of $\sim250~\mu$as and $\sim2500~\mu$as (0.02 pc and 0.2 pc). Without these baselines, earlier EHT observations lacked the capability to constrain emission on large scales, where a "missing flux" of order $\sim1$ Jy is expected. To probe these scales, we analyzed closure phases, robust against station-based gain errors, and modeled the jet base emission using a simple Gaussian offset from the compact ring emission at separations $>100~\mu$as. Our analysis reveals a Gaussian feature centered at ($\Delta$RA $\approx320~\mu$as, $\Delta$Dec $\approx60~\mu$as), a projected separation of $\approx5500$ AU, with a flux density of only $\sim60$ mJy, implying that most of the missing flux in previous studies must arise from larger scales. Brighter emission at these scales is ruled out, and the data do not favor more complex models. This component aligns with the inferred direction of the large-scale jet and is consistent with emission from the jet base. While our findings indicate detectable jet base emission at 230 GHz, coverage from only two intermediate baselines limits reconstruction of its morphology. We therefore treat the recovered Gaussian as an upper limit on the jet base flux density. Future EHT observations with expanded intermediate-baseline coverage will be essential to constrain the structure and nature of this component.

astro-ph.HE↗

arXiv2025

Shadow Formation Conditions Beyond the Kerr Black Hole Paradigm

A compact object illuminated by background radiation produces a dark silhouette. The edge of the silhouette or shadow (alternatively, the apparent boundary or the critical curve) is commonly determined by the presence of the photon sphere (or photon shell in the case of rotating spacetime), corresponding to the maximum of the effective potential for null geodesics. While this statement stands true for Kerr black holes, here we remark that the apparent boundary (as defined by Bardeen) forms under a more general condition. We demonstrate that a shadow forms if the effective potential of null geodesics has a positive finite upper bound and includes a region where photons are trapped or scattered. Our framework extends beyond conventional solutions, including but not limited to naked singularities. Furthermore, we clarify the difference between the apparent boundary of a dark shadow and the bright ring on the screen of a distant observer. These results provide a unified theoretical basis for interpreting observations from the Event Horizon Telescope (EHT) and guiding future efforts towards extreme-resolution observations of compact objects.

gr-qc↗

arXiv2025

Semi-analytic studies of accretion disk and magnetic field geometry in M87*

Context: Magnetic fields play a pivotal role in dynamics of black hole accretion flows and formation of relativistic jets. Observations by the Event Horizon Telescope (EHT) provided unprecedented insights into accretion structures near black holes. Interpreting these observations requires a theoretical framework linking polarized emission to underlying system properties and magnetic field geometries. Aims: We investigate how system properties, particularly magnetic field geometry in the event horizon scale region, influence the structure of the observable synchrotron emission in M87*. Specifically, we aim to quantify the sensitivity of observables used by the EHT to black hole spin, plasma dynamics, accretion disk thickness, and magnetic field geometry. Methods: We adopt a semi-analytic radiatively inefficient accretion flow model in Kerr spacetime. We vary magnetic field geometry, black hole spin, accretion disk dynamics, and geometric thickness of the disk. We perform general relativistic ray tracing with a full polarized radiative transfer to obtain synthetic images of M87*. We extract EHT observables, such as disk diameter, asymmetry, and polarimetric metrics from synthetic models. We also consider a number of general relativistic magnetohydrodynamics simulations and compare them with the semi-analytical models. Results: We see limited impact of the disk thickness on observables. On the other hand, toroidal field dominated and poloidal field dominated magnetic configurations can be distinguished reliably. The flow dynamics, in particular presence of radial inflow, also significantly impacts the EHT observables. Conclusions: The M87* system is most consistent with a poloidal magnetic field dominated flow with partially radial inflow. While the spin remain elusive, moderate or large positive values are preferred.

astro-ph.HE↗

arXiv2025

Time Delay of Pulsar Signals in Astrophysical Black Hole Spacetimes

In this paper, we investigate the fully relativistic time delay of pulsar signals propagating in the vicinity of a rotating black hole and its potential mimickers, including a deformed Kerr black hole and the Janis-Newman-Winicour naked singularity. We aim to compute and compare the pulsar time delays caused by different spacetime geometries to explore possible observational signatures that distinguish between black holes and their alternatives. We begin by solving the equations of motion for null geodesics in these background geometries. Subsequently, we address the emitter-observer problem to compute the time delay of pulsar signals in Kerr, deformed Kerr, and JNW spacetimes. A comparative analysis between Schwarzschild and Kerr black holes allows us to observe the effect of spin on propagation delay in pulsar timing. Further, we examine the impact of the deformation parameter in the deformed Kerr black hole and the influence of the scalar field on the rotating JNW spacetime. Our study considers both direct and indirect photons emitted by a source in the equatorial circular orbit. We find that the variations in the spin parameter show very small but detectable changes when we compare the time delay cases of a Kerr black hole with deformed Kerr and rotating JNW spacetimes. Our pulsar time delay results suggest a potential observable distinguishing feature of these astrophysical black hole geometries which could be useful for the forthcoming observational facilities such as Square Kilometer Array Observatory, Five-hundred-meter Aperture Spherical Telescope and Event Horizon Telescope.

gr-qc↗

arXiv2025

Novel Polarimetric Analysis of Near Horizon Flaring Episodes in M87* in Millimeter Wavelength

Recent multi-wavelength observations of M87* \citep{2024A&A...692A.140A} revealed a high-energy $\gamma$-ray flare without a corresponding millimeter counterpart. We present a theoretical polarimetric study to evaluate the presence and nature of a potential millimeter flare in M87*, using a suite of general relativistic magnetohydrodynamical simulations with varying black hole (BH) spins and magnetic field configurations. We find that the emergence of a millimeter flare is strongly influenced by both spin and magnetic structure, with limited sensitivity to the electron distribution (thermal vs. non-thermal). We model the intensity light curve with a damped random walk (DRW) and compare the characteristic timescale ($\tau$) with recent SMA observations, finding that the simulated $\tau$ exceeds observed values by over an order of magnitude. In a flaring case with BH spin a=+0.5, we identify a distinct millimeter flare followed by an order-of-magnitude flux drop. All Stokes parameters show variability near the flare, including a sign reversal in the electric vector position angle. While most $\beta_m$ modes remain stable, the $EB$-correlation phase is highly sensitive to both the flare peak and decay. We examine polarimetric signatures in photon sub-rings, focusing on modes ns=0 and ns=1. The ns=0 signal closely matches the full image, while ns=1 reveals distinct behaviors, highlighting the potential of space VLBI to isolate sub-ring features. Finally, we analyze the magnetic and velocity field evolution during the flare, finding that magnetic reconnection weakens during the flux decay, and the clockwise velocity flow transitions into an outflow-dominated regime. These results suggest that transient radio variability near flares encodes key information about black hole spin and magnetic field structure, offering a novel probe into the physics of active galactic nuclei.

astro-ph.HE↗

arXiv2025

Illuminating Youth: Decades of Mid-Infrared Variability and Color Evolution of Young Stellar Objects

The variability of Young Stellar Objects (YSOs) is a crucial tool for understanding the mechanisms driving flux changes. In this study, we present an infrared variability analysis of a large sample of over 20,000 candidate YSOs, using data from the ALLWISE and NEOWISE surveys, which span around a decade with a 6-month cadence. We applied Lomb-Scargle Periodogram (LSP) analysis and linear fitting to the light curves, classifying them into distinct categories: {\it Secular} ({\it Linear}, {\it Curved}, and {\it Periodic}) and {\it Stochastic} ({\it Burst}, {\it Drop}, and {\it Irregular}). Our findings show that 5,467 (26.2$\pm$0.3\%) of the sources exhibit variability, with most (19.7$\pm$0.3\%) showing {\it Irregular} variations, followed by {\it Curved} and {\it Periodic} variations. In addition, 235 sources of {\it Bursts} and 122 {\it Drop} sources were identified. Variability is more pronounced in Class I sources with a higher fraction of variables (36.3$\pm$0.6\%) compared to Class II (22.1$\pm$0.4\%) and Class III (22.5$\pm$1.0\%) sources. The color (W1 $-$ W2) versus magnitude analysis (W2) using linear fitting shows that the trend ``redder-when-brighter" (RWB) is more prevalent (85.4$\pm$0.5\%) among YSOs. In contrast, the trend ``bluer-when-brighter" (BWB) is more common in younger sources compared to more evolved ones, having a BWB fraction of 29.0$\pm$1.1\% for Class I to 4.0$\pm$0.9\% for Class III.

astro-ph.SR↗

arXiv2024

Towards Unbiased and Robust Spatio-Temporal Scene Graph Generation and Anticipation

Spatio-Temporal Scene Graphs (STSGs) provide a concise and expressive representation of dynamic scenes by modeling objects and their evolving relationships over time. However, real-world visual relationships often exhibit a long-tailed distribution, causing existing methods for tasks like Video Scene Graph Generation (VidSGG) and Scene Graph Anticipation (SGA) to produce biased scene graphs. To this end, we propose ImparTail, a novel training framework that leverages loss masking and curriculum learning to mitigate bias in the generation and anticipation of spatio-temporal scene graphs. Unlike prior methods that add extra architectural components to learn unbiased estimators, we propose an impartial training objective that reduces the dominance of head classes during learning and focuses on underrepresented tail relationships. Our curriculum-driven mask generation strategy further empowers the model to adaptively adjust its bias mitigation strategy over time, enabling more balanced and robust estimations. To thoroughly assess performance under various distribution shifts, we also introduce two new tasks Robust Spatio-Temporal Scene Graph Generation and Robust Scene Graph Anticipation offering a challenging benchmark for evaluating the resilience of STSG models. Extensive experiments on the Action Genome dataset demonstrate the superior unbiased performance and robustness of our method compared to existing baselines.

cs.CV↗

arXiv2024

Towards Scene Graph Anticipation

Spatio-temporal scene graphs represent interactions in a video by decomposing scenes into individual objects and their pair-wise temporal relationships. Long-term anticipation of the fine-grained pair-wise relationships between objects is a challenging problem. To this end, we introduce the task of Scene Graph Anticipation (SGA). We adapt state-of-the-art scene graph generation methods as baselines to anticipate future pair-wise relationships between objects and propose a novel approach SceneSayer. In SceneSayer, we leverage object-centric representations of relationships to reason about the observed video frames and model the evolution of relationships between objects. We take a continuous time perspective and model the latent dynamics of the evolution of object interactions using concepts of NeuralODE and NeuralSDE, respectively. We infer representations of future relationships by solving an Ordinary Differential Equation and a Stochastic Differential Equation, respectively. Extensive experimentation on the Action Genome dataset validates the efficacy of the proposed methods.

cs.CV↗

arXiv2023

Fundamental Physics Opportunities with the Next-Generation Event Horizon Telescope

The Event Horizon Telescope (EHT) Collaboration recently published the first images of the supermassive black holes in the cores of the Messier 87 and Milky Way galaxies. These observations have provided a new means to study supermassive black holes and probe physical processes occurring in the strong-field regime. We review the prospects of future observations and theoretical studies of supermassive black hole systems with the next-generation Event Horizon Telescope (ngEHT), which will greatly enhance the capabilities of the existing EHT array. These enhancements will open up several previously inaccessible avenues of investigation, thereby providing important new insights into the properties of supermassive black holes and their environments. This review describes the current state of knowledge for five key science cases, summarising the unique challenges and opportunities for fundamental physics investigations that the ngEHT will enable.

astro-ph.HE↗

arXiv2023

Imaging ultra-compact objects with radiatively inefficient accretion flows

Recent Event Horizon Telescope observations of M87* and Sgr A* strongly suggests the presence of supermassive black hole at their respective cores. In this work, we use the semi-analytic Radiatively Inefficient Accretion Flows (RIAF) model to investigate the resulting images of Joshi-Malafarina-Narayan (JMN-1) naked singularity and the Schwarzschild BH. We aim at choosing the JMN-1 naked singularity model and compare the synchrotron images with the Schwarzschild solution to search any distinct features which can distinguish the two objects and find alternative to the black hole solution. We perform general relativistic ray-tracing and radiative transfer simulations using Brahma code to generate synchrotron emission images utilising thermal distribution function for emissivity and absorptivity. We investigate effects in the images by varying inclination angle, disk width and frequency. The shadow images simulated by the JMN-1 model closely resemble those generated by the Schwarzschild black hole. When we compare these images, we find that the disparities between them are minimal. We conduct simulations using various plasma parameters, but the resulting images remain largely consistent for both scenarios. This similarity is evident in the horizontal cross-sectional brightness profiles of the two instances. Notably, the JMN-1 model exhibits slightly higher intensity in comparison to the Schwarzschild black hole. We conclude that JMN-1 presents itself as a viable substitute for the black hole scenario. This conclusion is not solely grounded in the fact that they are indistinguishable from their respective shadow observations, but also in the consideration that JMN-1 emerges as an end state of a continual gravitational collapse. This paradigm not only allows for constraints on spacetime but also provides a good probe for the nature of the central compact object.

astro-ph.HE↗

arXiv2022

GALLIFRAY -- A geometric modeling and parameter estimation framework for black hole images using bayesian techniques

Recent observations of the galactic centers of M87 and the Milky Way with the Event Horizon Telescope have ushered in a new era of black hole based tests of fundamental physics using very long baseline interferometry (VLBI). Being a nascent field, there are several different modeling and analysis approaches in vogue (e.g., geometric and physical models, visibility and closure amplitudes, agnostic and multimessenger priors). We present \texttt{GALLIFRAY}, an open-source Python-based framework for estimation/extraction of parameters using VLBI data. It is developed with modularity, efficiency, and adaptability as the primary objectives. This article outlines the design and usage of \texttt{GALLIFRAY}. As an illustration, we fit a geometric and a physical model to simulated datasets using markov chain monte carlo sampling and find good convergence of the posterior distribution. We conclude with an outline of further enhancements currently in development.

astro-ph.IM↗

arXiv2022

Probing the Shadow Image of the Sagittarius A* with Event Horizon Telescope

Recent observations of the Milky-way galactic center at various frequencies suggest a supermassive compact object. Generally, that supermassive compact object is assumed to be a `Black Hole', having more than four million solar masses. In this work, we study the observational appearance at $230$ GHz and probe the nature of Sagittarius-A* (Sgr A*) as the naked singularity. Here, we consider the first type of Joshi-Malafarina-Narayan (JMN-1) and Janis-Newman-Winicour (JNW) naked singularity spacetimes which are anisotropic fluid solutions of the Einstein field equations. Motivated by radiatively inefficient accretion flows (RIAF), we use an analytical model for emission and absorption coefficients to solve the general relativistic radiative transfer equation. The resulting emission is then utilized to generate images to predict the nature of the Sgr A* with synthetic Very Long Baseline Interferometry (VLBI) images from current and future Event Horizon Telescope (EHT) arrays. Three different EHT array configurations are being used to simulate the models of naked singularities and a black hole. This may have little effect on the baseline, but it would increase the u-v plane gridding, making it feasible to capture a better-resolved image. Therefore, it is quite interesting and useful for the upcoming shadow image of the Sgr A* to predict whether it is a supermassive black hole or a naked singularity.

gr-qc↗

arXiv2020

Black Hole Shadows in Verlinde's Emergent Gravity

We study the effect of baryonic matter and apparent dark matter on black hole shadow in Verlinde's Emergent Gravity. To do so, we consider different baryonic mass profiles and an optically-thin disk region described by a gas in a radial free fall around the black hole. Assuming that most of the baryonic matter in the galaxy is located near the galactic center surrounding a supermassive black hole, we use two models of power law mass profile for the baryonic matter to study the effect of apparent dark matter on the shadow and the corresponding intensity. We find that the effect of the surrounding matter on the shadow size using observational values is small, however, it becomes significant when the surrounding baryonic matter increases. To this end, we show that the effect of simple power law function in the limit of constant baryonic mass in Verlinde's theory implies an apparent dark matter effect which is similar to the standard gravity having an isothermal dark matter profile. We also find the intensity of the electromagnetic flux radiation depending on the surrounding mass.

gr-qc↗

arXiv2020

Deflection of Light and Shadow Cast by a Dual Charged Stringy Black Hole

Gravitational lensing and black hole shadows are one of the strongest observational evidences to prove the existence of black holes in the universe. The gravitational lensing arises due to the deflection of light by the gravitational field of a gravitating body such as a black hole. Investigation of the shadow cast by a compact object as well as deflection of light around it may provide the useful information about physical nature of the particular compact object and other related aspects. In this paper, we study the deflection of light by a dual charged stringy black hole spacetime derived in dilaton-Maxwell gravity. The variation of deflection angle with the impact parameter for different values of electric and magnetic charges is studied. We also study the shadow of this black hole spacetime to obtain the radius of shadow cast by it. We have considered an optically thin emission disk around it and observed that there are not significant changes in the shadow cast by this black hole compared to well-known Schwarzschild black hole spacetime in GR.

gr-qc↗

arXiv2020

Imprints of Dark Matter on Black Hole Shadows using Spherical Accretions

We study the possibility of identifying dark matter in the galactic center from the physical properties of the electromagnetic radiation emitted from a optical-thin disk region around a static and spherically symmetric black hole. In particular, we consider two specific models for the optical-thin disk region: a gas at rest and a gas in a radial free fall. Due to the effect of dark matter on the spacetime geometry, we find that the dark matter can increase or decrease the intensity of the electromagnetic flux radiation depending on the dark matter model. To this end, we analyze two simple dark matter models having different mass functions $\mathcal{M}(r)$, with a matter mass $M$, thickness $\Delta r_s$ along with a dark matter core radius surrounding the black hole. In addition to that, we explore the scenario of a perfect fluid dark matter surrounding the black hole. We show that in order to have significant effect of dark matter on the intensity of the electromagnetic flux radiation, a high energy density of dark matter near the black hole is needed. We also find that the surrounding dark matter distribution plays a key role on the shadow radius and the intensity of the electromagnetic flux radiation, respectively. Finally, we have used the relation between the shadow radius and the quasinormal modes (QNMs) to compute the real part of QNM frequencies.

gr-qc↗

arXiv2020

EinsteinPy: A Community Python Package for General Relativity

This paper presents EinsteinPy (version 0.3), a community-developed Python package for gravitational and relativistic astrophysics. Python is a free, easy to use a high-level programming language which has seen a huge expansion in the number of its users and developers in recent years. Specifically, a lot of recent studies show that the use of Python in Astrophysics and general physics has increased exponentially. We aim to provide a very high level of abstraction, an easy to use interface and pleasing user experience. EinsteinPy is developed keeping in mind the state of a theoretical gravitational physicist with little or no background in computer programming and trying to work in the field of numerical relativity or trying to use simulations in their research. Currently, EinsteinPy supports simulation of time-like and null geodesics and calculates trajectories in different background geometries some of which are Schwarzschild, Kerr, and KerrNewmann along with coordinate inter-conversion pipeline. It has a partially developed pipeline for plotting and visualization with dependencies on libraries like Plotly, matplotlib, etc. One of the unique features of EinsteinPy is a sufficiently developed symbolic tensor manipulation utilities which are a great tool in itself for teaching yourself tensor algebra which for many beginner students can be overwhelmingly tricky. EinsteinPy also provides few utility functions for hypersurface embedding of Schwarzschild spacetime which further will be extended to model gravitational lensing simulation.

gr-qc↗