arXiv subjects

Muhammad Zahid

Publications and source records attributed to Muhammad Zahid.

9 recordsLinked to original sources

arXiv2026

Testing charged Simpson-Visser-AdS spacetimes with twin-peak QPOs

In this manuscript, we study circular geodesics and twin-peak high-frequency quasi-periodic oscillations (HF-QPOs) in the charged Simpson--Visser--anti-de Sitter (AdS) spacetime, which depends on a charge parameter $Q$, a regularization scale $b$ and an AdS radius $l$. Written in terms of the areal radius $R=\sqrt{r^{2}+b^{2}}$, the specific energy, angular momentum, Keplerian frequency and innermost stable circular orbit (ISCO) of test particles coincide with those of the Reissner--Nordström--AdS black hole, so $b$ enters the epicyclic spectrum only through a factor $1-b^{2}/R^{2}$ in the radial frequency. We give this frequency and the ISCO condition in closed form. We then fit the relativistic-precession model to the HF-QPO pairs of GRO~J1655$-$40 and XTE~J1550$-$564, with their dynamical masses included as Gaussian priors. The Schwarzschild limit gives $χ^{2}=32.9$ for two degrees of freedom. One additional deformation removes this tension: $Q/M=0.814^{+0.059}_{-0.074}$ at fixed $l=100M$, or $l=40.6^{+6.2}_{-4.8}\,M$ at $Q=0$, both with $χ^{2}\simeq0.13$. These two solutions lie on a single degeneracy valley in the $(Q,l)$ plane, and, at $l=100M$, $b$ is left unconstrained up to $b\simeq3.9M$ (68\%), a limit set by the charge prior rather than by the data. A Kerr black hole with spin $a\simeq0.3$--$0.4$ reproduces the same pairs, so the inferred deformation most likely absorbs the spin neglected by the static metric. Our results therefore describe degeneracies among orbital-frequency deformations; they are not a measurement of an electric charge or of a cosmological constant.

gr-qc↗

arXiv2026

Rotating black holes surrounded by PFDM in STVG: Shadows and eikonal quasinormal modes

We investigate the optical and dynamical properties of a rotating black hole in scalar--tensor--vector gravity (STVG) surrounded by perfect fluid dark matter (PFDM). Starting from the static STVG--PFDM geometry, we consider a Kerr-like rotating extension characterized by the spin parameter $a$, the STVG coupling $α$, and the PFDM parameter $λ$. We first examine the horizon structure and a radial effective mass function, then analyze null geodesics and unstable spherical photon orbits. We use these results to construct the black-hole shadow and study its size, distortion, and dependence on model parameters and observer inclination. We then compare the predicted angular shadow diameter with the Event Horizon Telescope observations of M87$^*$ and Sgr~A$^*$ in the $(a,α)$ parameter space for representative values of the PFDM parameter. The comparison shows that the PFDM contribution can shift the parameter-space region compatible with the observed shadow diameter, while the shadow size is generally less sensitive to spin than to deformation. We further estimate the Hawking energy-emission spectrum in the high-frequency geometric-optics approximation, using the shadow radius as an effective absorption scale. Finally, we study the quasinormal-mode spectrum in the eikonal limit through the correspondence between unstable photon orbits and ringdown frequencies. The numerical results recover the expected Schwarzschild limit and show a clear separation between the prograde and retrograde modes as the spin increases. The combined analysis provides a consistent optical, energy emission, and ringdown characterization of the rotating STVG--PFDM geometry.

gr-qc↗

arXiv2026

Charged Simpson-Visser-AdS spacetime: QPOs and observational constraints

In this article, we examine particle dynamics and Quasi-periodic oscillations within a charged Simpson-Visser-Anti-de Sitter (AdS) spacetime. We first analyze the behavior of the metric and black hole horizons under the competing impact of electric charge $Q$, regularization parameter $b$, and AdS radius $l$. The roots of the metric function reveal transitions between three distinct topological classes: non-extremal configurations featuring distinct inner and outer horizons, extremal limits with a single degenerate horizon, and strictly positive, horizonless wormhole geometries. By evaluating the effective potential, we determine how this modified geometry shifts the Innermost Stable Circular Orbit (ISCO) and alters the angular momentum and energy of test particles. Moreover, we derive the Keplerian, radial, and vertical frequencies to model quasi-periodic oscillations (QPOs). We show that the interplay between the regularization parameter $b$ and the AdS radius $l$ significantly modifies twin-peak QPOs. In particular, $b$ drives fundamental QPO frequencies higher than those predicted by standard general relativistic backgrounds. Twin-peak high-frequency QPO data from two black-hole binaries disfavor the Schwarzschild limit at $\sim\!4σ$, but constrain only a degenerate combination of charge and AdS curvature, leaving the regularization parameter entirely unconstrained by the data.

gr-qc↗

arXiv2026

RoboLight: A Dataset with Linearly Composable Illumination for Robotic Manipulation

In this paper, we introduce RoboLight, the first real-world robotic manipulation dataset capturing synchronized episodes under systematically varied lighting conditions. RoboLight consists of two components. (a) RoboLight-Real contains 2,800 real-world episodes collected in our custom Light Cube setup, a calibrated system equipped with eight programmable RGB LED lights. It includes structured illumination variation along three independently controlled dimensions: color, direction, and intensity. Each dimension is paired with a dedicated task featuring objects of diverse geometries and materials to induce perceptual challenges. All image data are recorded in high-dynamic-range (HDR) format to preserve radiometric accuracy. Leveraging the linearity of light transport, we introduce (b) RoboLight-Synthetic, comprising 196,000 episodes synthesized through interpolation in the HDR image space of RoboLight-Real. In principle, RoboLight-Synthetic can be arbitrarily expanded by refining the interpolation granularity. We further verify the dataset quality through qualitative analysis and real-world policy roll-outs, analyzing task difficulty, distributional diversity, and the effectiveness of synthesized data. We additionally demonstrate three representative use cases of the proposed dataset. The full dataset, along with the system software and hardware design, will be released as open-source to support continued research.

cs.RO↗

arXiv2025

Facet Specific Electron Conduction in Pentavalent (W5+) WO3 Drives Superior Photocatalytic CO 2 Reduction in (002) Plane

This article reports a concept of heat-induced topological modifications of non-layered WO 3 followed by successful synthesis of oxygen-vacant more-porous nanosheets with exposed active (002) facet. Experimental measurements and Density Functional Theory (DFT) calculations have revealed that the photoexcited electrons are found to accumulate preferentially on (002) facet to yield enhanced electron conduction, and consequently, strengthen the reduction potential as active catalytic sites for photocatalytic CO2 reduction. Owing to these beneficial properties, the more-porous nanosheets of WO 3 with (002) facet have exhibited superior performance than that of less-porous nanosheets of WO3 with (220) facet and bulk WO3 with (205) facet. This study therefore provides a new understanding of regulating physical, optical, and electronic properties through intricate atomic structure modulation of WO3, and may find widespread application in optoelectronics, sensors, and energy conversion.

cond-mat.mtrl-sci↗

arXiv2025

R900: Understanding the Cost-Effectiveness of Random Exploration from 900 Hours of Robotic Data Collection

Data scarcity presents a key bottleneck for imitation learning in robotic manipulation. In this paper, we focus on random exploration data-actions and video sequences produced autonomously via motions to randomly sampled positions in the workspace-to investigate their potential as a cost-effective data source. Our investigation follows two paradigms: (a) random actions, where we assess their feasibility for autonomously bootstrapping data collection policies, and (b) random exploration video frames, where we evaluate their effectiveness in pre-training parameter-dense networks with self-supervised learning objectives. To minimize human supervision, we first develop a fully automated pipeline that handles episode labeling, termination, and resetting using cloud-based microservices for real-time monitoring. Building on this, we present a large-scale study on the cost-effectiveness of real-world random exploration in a non-trivial two-layer stacking task, drawing on statistical results from 807 hours of random actions, 71 hours of random exploration video (1.28M frames), and 1,260 times of policy evaluation. The dataset will be made publicly available and access to the robot environment with the automated pipeline is to be made accessible via cloud service for future research.

cs.RO↗

arXiv2024

How Physics and Background Attributes Impact Video Transformers in Robotic Manipulation: A Case Study on Planar Pushing

As model and dataset sizes continue to scale in robot learning, the need to understand how the composition and properties of a dataset affect model performance becomes increasingly urgent to ensure cost-effective data collection and model performance. In this work, we empirically investigate how physics attributes (color, friction coefficient, shape) and scene background characteristics, such as the complexity and dynamics of interactions with background objects, influence the performance of Video Transformers in predicting planar pushing trajectories. We investigate three primary questions: How do physics attributes and background scene characteristics influence model performance? What kind of changes in attributes are most detrimental to model generalization? What proportion of fine-tuning data is required to adapt models to novel scenarios? To facilitate this research, we present CloudGripper-Push-1K, a large real-world vision-based robot pushing dataset comprising 1278 hours and 460,000 videos of planar pushing interactions with objects with different physics and background attributes. We also propose Video Occlusion Transformer (VOT), a generic modular video-transformer-based trajectory prediction framework which features 3 choices of 2D-spatial encoders as the subject of our case study. The dataset and source code are available at https://cloudgripper.org.

cs.RO↗

arXiv2023

CloudGripper: An Open Source Cloud Robotics Testbed for Robotic Manipulation Research, Benchmarking and Data Collection at Scale

We present CloudGripper, an open source cloud robotics testbed, consisting of a scalable, space and cost-efficient design constructed as a rack of 32 small robot arm work cells. Each robot work cell is fully enclosed and features individual lighting, a low-cost custom 5 degree of freedom Cartesian robot arm with an attached parallel jaw gripper and a dual camera setup for experimentation. The system design is focused on continuous operation and features a 10 Gbit/s network connectivity allowing for high throughput remote-controlled experimentation and data collection for robotic manipulation. CloudGripper furthermore is intended to form a community testbed to study the challenges of large scale machine learning and cloud and edge-computing in the context of robotic manipulation. In this work, we describe the mechanical design of the system, its initial software stack and evaluate the repeatability of motions executed by the proposed robot arm design. A local network API throughput and latency analysis is also provided. CloudGripper-Rope-100, a dataset of more than a hundred hours of randomized rope pushing interactions and approximately 4 million camera images is collected and serves as a proof of concept demonstrating data collection capabilities. A project website with more information is available at https://cloudgripper.org.

cs.RO↗

arXiv2021

Shadow cast and center of mass energy in a charged Gauss-Bonnet-AdS black hole

This work is devoted to the exploration of shadow cast and center of mass energy in the background of a 4-dimensional charged Gauss-Bonnet AdS black hole. On investigating particle dynamics, we have examined BH's metric function. Whereas, with the help of null geodesics, we pursue to calculate the celestial coordinates and the shadow radius of the black hole. We have made use of the hawking temperature to study the energy emission rate. Moreover, we have explored the center of mass energy and discussed its characteristics under the influence of spacetime parameters. For a better understanding, we graphically represent all of our main findings. The acquired result shows that both charge and AdS radius ($l$) decrease the shadow radius, while the Gauss-Bonnet coupling parameter $α$ increases the shadow radius in AdS spacetime. On the other hand, both $Q$ and $α$ result in diminishing the shadow radius in asymptotically flat spacetime. Finally, we investigate the energy emission rate and center of mass energy under the influence of $Q$ and $α$.

gr-qc↗