arXiv subjects

Abhay Kumar

Publications and source records attributed to Abhay Kumar.

At least 19 recordsLinked to original sources

arXiv2026

In orbit background for hard X-ray CubeSat polarimeters: case study of the CUSP mission in low-earth orbit

The CUbesat Solar Polarimeter (CUSP) project aims to measure the linear polarization of solar flares in the 25 - 100 keV hard X-ray band using a Compton scattering polarimeter. CUSP is a project in the framework of the Alcor Program of the Italian Space Agency aimed to develop innovative CubeSat technologies and missions. As part of CUSPs Phase B study, initiated in December 2024 and closed on July 2nd, 2026, estimating the in orbit background to optimize the signal-to-background ratio was one of the key objectives. In low-Earth orbit, the instrument is exposed to cosmic and albedo X-ray backgrounds, charged particles, and secondary radiation from the spacecraft and atmosphere. Simulating these contributions enables optimization of detector geometry and shielding to maximize signal-to-noise performance. We present initial in orbit background estimates for CUSP using a Geant4-based simulator. A detailed mass model of the CUSP has been implemented to simulate background components and estimate the background count rate in the CUSP orbit.

astro-ph.IM↗

arXiv2026

Hard X-Ray Focal-Plane Compton Spectro-Polarimeter: Detector Development and Sensitivity Evaluation

The scientific potential of X-ray polarimetry has long been recognized, yet the challenges of measuring polarization have left it largely unexplored, mainly in the hard X-ray regime. With the advent of hard X-ray focusing optics, sensitive focal-plane Compton polarimeters are now feasible. An early example is CXPOL (Compton X-ray Polarimeter), developed at Physical Research Laboratory (PRL), India, which demonstrated 20 - 80 keV polarimetric capabilities using a plastic scatterer and a CsI(Tl) absorber array. The CXPOL prototype demonstrated polarimetric capabilities in the 20 - 80 keV range, establishing a foundation for further development. Building on this concept, we evaluate a hard X-ray spectro-polarimeter employing a position-sensitive plastic scatterer surrounded by position-sensitive absorber detectors. This geometry enables efficient reconstruction of Compton events and allows combined polarimetric and spectroscopic measurements via interaction positions and deposited energies in the detectors. We evaluate key performance parameters of the revised configuration of the second version of the CXPOL. Using Geant4 simulations, we assess key performance parameters, including modulation factor, polarimetric efficiency, and expected sensitivity with modern hard X-ray optics. We also present the characterization results of first prototype of a 100x20x5 mm3 NaI(Tl) absorber read out on both ends by silicon photomultiplier (SiPM) array operating in coincidence, evaluating energy and position resolution and light-output variation along the detector. The coincidence readout also reduces SiPM background by an order of magnitude. The results demonstrate the strong potential of a position-sensitive Compton- based focal-plane instrument for next-generation hard X-ray spectro-polarimetry.

astro-ph.IM↗

arXiv2026

The CUSP CubeSat mission for space weather multi-physics analysis, design, and testing

The CUbesat Solar Polarimeter (CUSP) mission aims to measure the linear polarization of solar flares in the hard X-ray band by means of a Compton scattering polarimeter. CUSP is a project in the framework of the Alcor Program of the Italian Space Agency aimed at developing new CubeSat missions. We present the outcomes of the CUSP's Phase B study, which is ended on 2 July 2026. The design solutions adopted for the mission's most critical multi-physics design drivers will be discussed, these solutions have been formulated and applied to demonstrate compliance with system requirements at both the spacecraft and platform levels. Moreover, we will discuss the validation of the Payload model based on the environmental testing campaign (e.g., vibration) carried out on a demonstrator.

astro-ph.IM↗

arXiv2026

Compton Polarimeter Prototype for the CUbesat Solar Polarimeter (CUSP) mission

The space-based CUbesat Solar Polarimeter (CUSP) mission aims to measure the linear polarization of solar flares in the hard X-ray band (25-100 keV) by means of a dual-phase Compton polarimeter. CUSP will allow to study the magnetic reconnection and particle acceleration in the flaring magnetic structures of our star with its unprecedented sensitivity to solar flare polarization. CUSP is a project under development as part of the Alcor Program of the Italian Space Agency aimed at developing new CubeSat missions. In the frame of CUSP's Phase B, which started in December 2024, a flight-representative prototype of the Compton polarimeter has been developed and characterized with hard X-ray sources in the laboratory. This prototype consists of a 4$\times$4 central matrix of plastic scintillator bars surrounded by 4 strips of 8 elongated GAGG scintillators, respectively coupled to a multi-anode photomultiplier tube and arrays of avalanche photodiodes. These sensors are read out by custom front-end electronics based on MAROC-3A and SKIROC-2A ASICs with a Xilinx Artix 7 FPGA. The plastic scintillators act as scatterers, while the GAGG bars fully absorb the scattered photons. Coincident plastic-GAGG events allow for reconstructing the Compton scattering direction, whose distribution allows for inferring the polarization parameters of the source. We report here the measured performance of the polarimeter prototype using well-known radioactive isotopes and X-ray tubes, allowing us to assess the performance of our polarimeter prototype over the full 25-100 keV energy range.

astro-ph.IM↗

arXiv2026

Train Smarter, Not Longer: Memorization-Guided Data Reuse for Efficient LLM Training

The training paradigm of large language models has shifted from traditional one-pass training to multi-epoch training, as reasonable reuse of limited high-quality data can improve both model performance and sample efficiency. Meanwhile, excessive repetition introduces the risk of overfitting and diminishing returns. Determining when and how to reuse data effectively thus emerges as a natural but under-explored question. Through a novel observation of model's "Memorization Window" signals derived from loss retention dynamics and downstream evaluation scores, we propose "Memorization-guided Data Reuse", a training paradigm that adaptively determines when and how data should be reused, enabling principled decisions on the number of training epochs and the scheduling of data replays. Our preliminary experiments reveal a consistent memorization-driven regime: performance continues to improve with repetition far beyond current practice (e.g., the commonly cited four-epoch limit). While a full scheduler remains future work, these insights provide a foundation for memorization-aware training schedules, helping to determine reuse budgets and move toward training LLMs smarter rather than longer with limited high-quality data.

cs.LG↗

arXiv2026

Grouped Query Experts: Mixture-of-Experts on GQA Self-Attention

Self-attention is central to Transformer performance and is often the most expensive part of the Transformer at long context lengths because its pairwise token interactions scale quadratically with sequence length. Standard dense attention also applies the same set of attention heads to every token regardless of token difficulty or information content. This uniform activation can waste compute, especially as sequences grow longer and attention cost increases rapidly. We propose Grouped Query Experts (GQE), a mixture-of-experts layer on top of grouped-query attention (GQA). Within each GQA group, a router selects k query-head experts per token while all key-value (KV) heads remain dense and unchanged. Thus, GQE keeps the KV cache benefits of GQA and reduces only the active query-head computation. On a fixed 30B token budget at the 250M parameter scale, GQE matches the all-active GQA baseline in downstream accuracy while activating half the query heads per token.

cs.LG↗

arXiv2026

Development of a one-dimensional position sensitive detector for Compton X-ray polarimeters

The scientific potential of X-ray polarimetry has long been recognized, but the challenges in measuring polarization have left it largely unexplored, particularly in the hard X-ray regime. While tremendous advancement has been made in soft X-ray polarimetery, the lack of sensitive hard X-ray polarimeters and polarisation measurements continues to limit our understanding of high-energy astrophysical processes. With the development of hard X-ray mirrors, it is now possible to develop a sensitive focal plane hard X-ray polarimeter. One such effort is CXPOL, a prototype developed at PRL, India, which consists of a plastic scintillator as active scatterer readout by PMT surrounded by CsI(Tl) scintillators in cylindrical array with SiPM readout from one side. First results of the prototype have been demonstrated in 20 to 80 keV energy range. The sensitivity of the instrument can be significantly enhanced using faster and better light yield scintillator like NaI as absorbers. Further, the use of a position-sensitive scatterer and absorbers, can also provide spectroscopic information by measuring the interaction position along the length and from the known energy depositions in the detectors. Position sensitive detectors are also helpful in mitigating the systematic effects introduced by the off-axis events in the polarisation measurements. Here, we demonstrate the detection sensitivity in the 100x20x5 mm^3 NaI(Tl) scintillator absorber readout on both ends by SiPM arrays operating in co-incidence. In this work, we characterize the first prototype of this detector system and investigate the variation in energy and position resolution, and light output with irradiation position along the length of the detector. The two end readout in co-incidence also reduces the overall SiPM background per absorber by an order of magnitude, further enhancing the polarimetric sensitivity of the instrument.

astro-ph.IM↗

arXiv2026

Spectral performance of single-channel plastic and GAGG scintillator bars of the CUbesat Solar Polarimeter (CUSP)

Our Sun is the closest X-ray astrophysical source to Earth. As such, it makes a formidable case study to better understand astrophysical processes. Solar flares are in particular very interesting as they are linked to coronal mass ejections as well as magnetic field reconnection sites in the solar atmosphere. Flares can therefore provide insightful information on the physical processes at play on their production sites, but also on the emission and acceleration of energetic charged particles towards our planet, making it a formidable forecasting tool for space weather. While solar flares are critical to understanding magnetic reconnection and particle acceleration, their hard X-ray polarization -- key to distinguishing between competing theoretical models -- remains poorly constrained by existing observations. To address this, we present the CUbesat Solar Polarimeter (CUSP), a mission under development to perform solar flare polarimetry in the 25-100 keV energy range. CUSP consists of a 6U-XL platform hosting a dual-phase Compton polarimeter. The polarimeter is made of a central assembly of four 4x4 arrays of plastic scintillators, each coupled to multi-anode photomultiplier tubes, surrounded by four strips of eight elongated GAGG scintillator bars coupled to avalanche photodiodes. Both types of sensors from Hamamatsu are respectively read out by the MAROC-3A and SKIROC-2A ASICs from Weeroc. In this manuscript, we present the preliminary spectral performances of single plastic and GAGG channels measured in the laboratory using development boards of the ASICs foreseen for the flight model.

astro-ph.IM↗

arXiv2025

Solar Flare Hard X-ray Polarimetry with the CUbesat Solar Polarimeter (CUSP) mission

The CUbesat Solar Polarimeter (CUSP) project is a CubeSat mission planned for a launch in low-Earth orbit and aimed to measure the linear polarization of solar flares in the hard X-ray band by means of a Compton scattering polarimeter. CUSP will allow us to study the magnetic reconnection and particle acceleration in the flaring magnetic structures of our star. CUSP is a project in the framework of the Alcor Program of the Italian Space Agency aimed at developing new CubeSat missions. It is undergoing a 12-month Phase B that started in December 2024. The Compton polarimeter on board CUSP is composed of two acquisition chains based on plastic scintillators read out by Multi-Anode PhotoMultiplier Tubes for the scatterer part and GAGG crystals coupled to Avalanche PhotoDiodes for the absorbers. An event coincident between the two readout schemes will lead to a measurement of the incoming X-ray's azimuthal scattering angle, linked to the polarization of the solar flare in a statistical manner. The current status of the CUSP mission design, mission analysis, and payload scientific performance will be reported. The latter will be discussed based on preliminary laboratory results obtained in parallel with Geant4 simulations.

astro-ph.SR↗

arXiv2025

Personalized Product Search Ranking: A Multi-Task Learning Approach with Tabular and Non-Tabular Data

In this paper, we present a novel model architecture for optimizing personalized product search ranking using a multi-task learning (MTL) framework. Our approach uniquely integrates tabular and non-tabular data, leveraging a pre-trained TinyBERT model for semantic embeddings and a novel sampling technique to capture diverse customer behaviors. We evaluate our model against several baselines, including XGBoost, TabNet, FT-Transformer, DCN-V2, and MMoE, focusing on their ability to handle mixed data types and optimize personalized ranking. Additionally, we propose a scalable relevance labeling mechanism based on click-through rates, click positions, and semantic similarity, offering an alternative to traditional human-annotated labels. Experimental results show that combining non-tabular data with advanced embedding techniques in multi-task learning paradigm significantly enhances model performance. Ablation studies further underscore the benefits of incorporating relevance labels, fine-tuning TinyBERT layers, and TinyBERT query-product embedding interactions. These results demonstrate the effectiveness of our approach in achieving improved personalized product search ranking.

cs.IR↗

arXiv2025

The multi-physics analysis, design and testing of CUSP, a CubeSat mission for space weather and solar flares x-ray polarimetry

The space-based CUbesat Solar Polarimeter (CUSP) mission aims to measure the linear polarization of solar flares in the hard X-ray band by means of a Compton scattering polarimeter. CUSP is a project in the framework of the Alcor Program of the Italian Space Agency aimed at developing new CubeSat missions. As part of CUSP's Phase B study, which began in December 2024 and will last one year, we present the current development status of the design solutions adopted for the mission's most critical multi-physics design drivers. These solutions have been formulated and applied to demonstrate compliance with system requirements at both the spacecraft and platform levels. In particular, we describe the mechanical design of each structural component, the results of static, dynamic finite element analyses, and a proposal for topological optimization of the interface between the platform and payload and some fixture for test, and the preliminary environmental testing campaign (e.g., vibration, shock) that will be carried out on a mechanical demonstrator.

astro-ph.IM↗

arXiv2025

The CUbesat Solar Polarimeter (CUSP): mission overview II

The CUbesat Solar Polarimeter (CUSP) project is an Earth-orbiting CubeSat mission designed to measure the linear polarization of solar flares in the hard X-ray band using a Compton scattering polarimeter. CUSP will enable the study of magnetic reconnection and particle acceleration within the Sun's flaring magnetic structures. This project is being developed within the framework of the Italian Space Agency's Alcor Program, which aims to foster new CubeSat missions. CUSP entered its Phase B in December 2024, a phase scheduled to last 12 months. This paper reports on the current status of the CUSP mission design, mission analysis, and payload scientific performance.

astro-ph.SR↗

arXiv2025

CUbesat Solar Polarimeter (CUSP) Sensitivity Estimation and Performance Optimization using Geant4

The CUbesat Solar Polarimeter (CUSP) aims to measure the linear polarization of solar flares in the 25-100 keV X-ray band using a Compton scattering polarimeter. CUSP will allow us to study the magnetic reconnection and particle acceleration in the flaring magnetic structures of our star by providing high-sensitivity polarization measurements. CUSP is a project in the framework of the Alcor Program of the Italian Space Agency aimed to develop innovative CubeSat technologies and missions. As part of CUSPs Phase B study, which began in December 2024 and will continue for one year, we present the development status of the Geant4 based simulator to accurately simulate the detectors response and initial results on the sensitivity of the instrument. Geant4 Monte Carlo simulation is used to assess the physical interactions of the source photons with the detector and the passive materials. We implemented a detailed CUSP Mass Model within Geant4 to simulate and estimate the instruments sensitivity, correcting the geometric effects of the instrument. We also evaluated the effect of backscattering shielding on the sensitivity to optimize the mass model of the instrument.

astro-ph.SR↗

arXiv2025

Prototype Development and Calibration of the CUbesat Solar Polarimeter (CUSP)

The space-based CUbesat Solar Polarimeter (CUSP) mission aims to measure the linear polarization of solar flares in the hard X-ray band by means of a Compton scattering polarimeter. CUSP will allow to study the magnetic reconnection and particle acceleration in the flaring magnetic structures of our star with its unprecedented sensitivity to solar flare polarization. CUSP is a project in the framework of the Alcor Program of the Italian Space Agency aimed to develop new CubeSat missions. It has been proposed as a constellation of a two Cubesat mission to monitor the Sun for Space Weather, and will proceed with a single-satellite asset in its baseline implementation. In the frame of CUSP's Phase B study, that started in December 2024 for a 1-year period, we present the development status of this dual-phase polarimeter. Preliminary laboratory results using two chains of acquisition will be discussed. The first chain of acquisition, based on the Hamamatsu R7600 multi-anode photomultiplier tubes coupled to plastic scintillator bars and read out by the MAROC-3A ASIC, is used to detect the Compton scattering of incoming photons. On the other hand, GAGG crystals coupled to avalanche photo-diodes with a readout based on the SKIROC-2A ASIC are used to absorb the scattered photons. By reconstructing the azimuthal scattering direction for many incoming photons, one can infer the linear polarization degree and angle of the source. We will discuss the calibration results obtained with our prototype detector by using well-known radioactive isotopes, allowing us to assess the performances of our detector over the full 25-100 keV energy range.

astro-ph.IM↗

arXiv2025

Study of the HV power supply modules for the CUbesat Solar Polarimeter (CUSP)

The CUbesat Solar Polarimeter (CUSP) project is a CubeSat mission orbiting the Earth aimed to measure the linear polarization of solar flares in the hard X-ray band by means of a Compton scattering polarimeter. CUSP will allow to study the magnetic reconnection and particle acceleration in the flaring magnetic structures of our star. CUSP is a project in the framework of the Alcor Program of the Italian Space Agency aimed to develop new CubeSat missions. CUSP undergoing the Phase B started in December 2024 that will last for 12 month. The Compton polarimeter of the CUSP payload performs coincidence measurements between plastic scintilaltors and GaGG(Ce) crystals to derive the polarization of X-rays. These sensors are readout by Multi Anode Photomultiplier Tubes (MAPMTs) and Avalanche Photodiodes (APDs) respectively. Both sensors need an HV power supply up to -1~kV (for the MAPMT) and +500~V (for the APD). We tested precision regulated High Voltage DC/DC Converters by HVM Technology Inc. with Sub-Miniature Case Size ($0.85''\times0.85''\times0.60''$) of the SMHV series. These modules are compact and suited for CubeSat missions.

astro-ph.IM↗

arXiv2025

A Hidden Pulse: Uncovering a New Timing Signal in Cygnus X-1 with AstroSat

The study of fast variability properties in X-ray binaries advances our understanding of the physical processes and geometric properties of the accretion flow around the compact object. In this work, we study the evolution of the timing properties of Cygnus X-1 with AstroSat/LAXPC, during the transition of the source from the hard to soft state in 2017. We use a novel frequency-segmented technique to fit simultaneously the cross spectra and parts of the power spectra and coherence function with a multi-Lorentzian model and predict the phase-lags and the complementary parts of the power spectra and coherence function. We study the evolution of the frequency and power of the main variability components that are present throughout all the states. In particular, we identify previously undetected variability components, one of which manifests as a narrow dip in the coherence function and a broad drop in the phase-lag spectrum at the same frequency. This dip in coherence, which we detected for the first time in Cygnus X-1 at energies above 3 keV, appears in a state in which the source shows high-amplitude radio variability and significant hard X-ray polarization. While the contribution of the compact jet in X-rays is debated in the literature, this study provides a new avenue for investigating jet properties as well as the geometry of the Comptonizing medium.

astro-ph.HE↗

arXiv2025

IndieFake Dataset: A Benchmark Dataset for Audio Deepfake Detection

Advancements in audio deepfake technology offers benefits like AI assistants, better accessibility for speech impairments, and enhanced entertainment. However, it also poses significant risks to security, privacy, and trust in digital communications. Detecting and mitigating these threats requires comprehensive datasets. Existing datasets lack diverse ethnic accents, making them inadequate for many real-world scenarios. Consequently, models trained on these datasets struggle to detect audio deepfakes in diverse linguistic and cultural contexts such as in South-Asian countries. Ironically, there is a stark lack of South-Asian speaker samples in the existing datasets despite constituting a quarter of the worlds population. This work introduces the IndieFake Dataset (IFD), featuring 27.17 hours of bonafide and deepfake audio from 50 English speaking Indian speakers. IFD offers balanced data distribution and includes speaker-level characterization, absent in datasets like ASVspoof21 (DF). We evaluated various baselines on IFD against existing ASVspoof21 (DF) and In-The-Wild (ITW) datasets. IFD outperforms ASVspoof21 (DF) and proves to be more challenging compared to benchmark ITW dataset. The complete dataset, along with documentation and sample reference clips, is publicly accessible for research use on project website.

cs.SD↗

arXiv2025

ZClip: Adaptive Spike Mitigation for LLM Pre-Training

Training large language models (LLMs) presents numerous challenges, including gradient instability and loss spikes. These phenomena can lead to catastrophic divergence, requiring costly checkpoint restoration and data batch skipping. Traditional gradient clipping techniques, such as constant or norm-based methods, fail to address these issues effectively due to their reliance on fixed thresholds or heuristics, leading to inefficient learning and requiring frequent manual intervention. In this work, we propose ZClip, an adaptive gradient clipping algorithm that dynamically adjusts the clipping threshold based on statistical properties of gradient norms over time. Unlike prior reactive strategies, ZClip proactively adapts to training dynamics without making any prior assumptions on the scale and the temporal evolution of gradient norms. At its core, it leverages z-score-based anomaly detection to identify and mitigate large gradient spikes, preventing malignant loss spikes while not interfering with convergence otherwise. Our code is available at: https://github.com/bluorion-com/ZClip.

cs.LG↗