arXiv subjects

Amit Kumar

Publications and source records attributed to Amit Kumar.

At least 181 records · Page 10Linked to original sources

arXiv2022

HIME: Efficient Headshot Image Super-Resolution with Multiple Exemplars

A promising direction for recovering the lost information in low-resolution headshot images is utilizing a set of high-resolution exemplars from the same identity. Complementary images in the reference set can improve the generated headshot quality across many different views and poses. However, it is challenging to make the best use of multiple exemplars: the quality and alignment of each exemplar cannot be guaranteed. Using low-quality and mismatched images as references will impair the output results. To overcome these issues, we propose an efficient Headshot Image Super-Resolution with Multiple Exemplars network (HIME) method. Compared with previous methods, our network can effectively handle the misalignment between the input and the reference without requiring facial priors and learn the aggregated reference set representation in an end-to-end manner. Furthermore, to reconstruct more detailed facial features, we propose a correlation loss that provides a rich representation of the local texture in a controllable spatial range. Experimental results demonstrate that the proposed framework not only has significantly fewer computation cost than recent exemplar-guided methods but also achieves better qualitative and quantitative performance.

cs.CV↗

arXiv2022

GRB 210217A: A short or a long GRB?

Gamma-ray bursts are traditionally classified as short and long bursts based on their $T_{\rm 90}$ value (the time interval during which an instrument observes $5\%$ to $95\%$ of gamma-ray/hard X-ray fluence). However, $T_{\rm 90}$ is dependent on the detector sensitivity and the energy range in which the instrument operates. As a result, different instruments provide different values of $T_{\rm 90}$ for a burst. GRB 210217A is detected with different duration by {\it Swift} and {\it Fermi}. It is classified as a long/soft GRB by {\it Swift}-BAT with a $T_{\rm 90}$ value of 3.76 sec. On the other hand, the sub-threshold detection by {\it Fermi}-GBM classified GRB 210217A as a short/hard burst with a duration of 1.024 sec. We present the multi-wavelength analysis of GRB 210217A (lying in the overlapping regime of long and short GRBs) to identify its actual class using multi-wavelength data. We utilized the $T_{\rm 90}$-hardness ratio, $T_{\rm 90}$-\Ep, and $T_{\rm 90}$-$t_{\rm mvts}$ distributions of the GRBs to find the probability of GRB 210217A being a short GRB. Further, we estimated the photometric redshift of the burst by fitting the joint XRT/UVOT SED and place the burst in the Amati plane. We found that GRB 210217A is an ambiguous burst showing properties of both short and long class of GRBs.

astro-ph.HE↗

arXiv2022

FARSI: Facebook AR System Investigator for Agile Domain-Specific System-on-Chip Exploration

Domain-specific SoCs (DSSoCs) are attractive solutions for domains with stringent power/performance/area constraints; however, they suffer from two fundamental complexities. On the one hand, their many specialized hardware blocks result in complex systems and thus high development effort. On the other, their many system knobs expand the complexity of design space, making the search for the optimal design difficult. Thus to reach prevalence, taming such complexities is necessary. This work identifies necessary features of an early-stage design space exploration (DSE) framework that targets the complex design space of DSSoCs and further provides an instance of one called FARSI, (F)acebook (AR) (S)ystem (I)nvestigator. Concretely, FARSI provides an agile system-level simulator with speed up and accuracy of 8,400X and 98.5% comparing to Synopsys Platform Architect. FARSI also provides an efficient exploration heuristic and achieves up to 16X improvementin convergence time comparing to naive simulated annealing (SA). This is done by augmenting SA with architectural reasoning such as locality exploitation and bottleneck relaxation. Furthermore, we embed various co-design capabilities and show that on average, they have a 32% impact on the convergence rate. Finally, we demonstrate that using simple development-cost-aware policies can lower the system complexity, both in terms of the component count and variation by as much as 150% and 118% (e,g., for Network-on-a-Chip subsystem)

cs.AR↗

arXiv2022

Data Pipeline Architecture and Development for VELC onboard Space Solar Mission AdityaL1

ADITYA L-1 is India's first dedicated mission to study Sun and its atmosphere with Visible Emission Line Coronagraph (VELC), a major payload on ADITYA-L1. VELC has provision to make imaging and spectroscopic observations of the corona, simultaneously. Imaging with the Field of View (FOV) from 1.05Ro to 3Ro will be done in continuum at 500nm. The spectroscopic observations of solar corona in three emission lines, namely 5303 Å [Fe XIV], 7892 Å [Fe XI], 10747 Å [Fe XIII], and Spectro-polarimetry at 10747 Å [Fe XIII] will be performed with FOV of 1.05-1.5Ro. In this work, the end-to-end data pipeline architecture and development of the VELC payload are presented. The VELC proposal submission form, satellite observation parameters, data products, level definitions, data pipeline and analysis software to process the big raw data sets obtained using VELC instruments onboard satellite to science-ready data are discussed.

astro-ph.IM↗

arXiv2022

Imaging Ferroelectrics: Charge Gradient Microscopy (CGM) versus Potential Gradient Microscopy (PGM)

In 2014, Charge Gradient Microscopy (CGM) was first reported as a new scanning probe imaging mode, particularly well-suited for the characterisation of ferroelectrics. The implementation of the technique is straightforward; it involves monitoring currents that spontaneously develop between a passive conducting atomic force microscopy tip and Earth, as the tip is scanned across the specimen surface. However, details on the fundamental origin of contrast and what images mean, in terms of associated ferroelectric microstructures, are not yet fully understood. Here, by comparing information from CGM and Kelvin Probe Force Microscopy (KPFM), obtained from the same sets of ferroelectric domains (in both lithium niobate and barium titanate), we show that CGM reasonably reflects the spatial derivative of the measured surface potential. This is conceptually different from measuring local gradients in the surface bound-charge density or in any associated screening charges: after all, we see clear CGM signals, even when polarisation is entirely in-plane. We therefore suggest that CGM in ferroelectrics might be more accurately called Potential Gradient Microscopy (PGM). Intriguingly, in all cases examined, the measured surface potential (determined both through KPFM and by integrating the CGM signal) is of the opposite sign to that intuitively expected for a completely clean ferroelectric in vacuum. This is commonly observed and presumed due to a charge accumulation on the ferroelectric surface which is not easily removed.

cond-mat.mtrl-sci↗

arXiv2022

Probing into emission mechanisms of GRB 190530A using time-resolved spectra and polarization studies: Synchrotron Origin?

Multi-pulsed GRB 190530A, detected by the GBM and LAT onboard \fermi, is the sixth most fluent GBM burst detected so far. This paper presents the timing, spectral, and polarimetric analysis of the prompt emission observed using \AstroSat and \fermi to provide insight into the prompt emission radiation mechanisms. The time-integrated spectrum shows conclusive proof of two breaks due to peak energy and a second lower energy break. Time-integrated (55.43 $\pm$ 21.30 \%) as well as time-resolved polarization measurements, made by the Cadmium Zinc Telluride Imager (CZTI) onboard \AstroSat, show a hint of high degree of polarization. The presence of a hint of high degree of polarization and the values of low energy spectral index ($α_{\rm pt}$) do not run over the synchrotron limit for the first two pulses, supporting the synchrotron origin in an ordered magnetic field. However, during the third pulse, $α_{\rm pt}$ exceeds the synchrotron line of death in few bins, and a thermal signature along with the synchrotron component in the time-resolved spectra is observed. Furthermore, we also report the earliest optical observations constraining afterglow polarization using the MASTER (P $<$ 1.3 \%) and the redshift measurement ($z$= 0.9386) obtained with the 10.4m GTC telescopes. The broadband afterglow can be described with a forward shock model for an ISM-like medium with a wide jet opening angle. We determine a circumburst density of $n_{0} \sim$ 7.41, kinetic energy $E_{\rm K} \sim$ 7.24 $\times 10^{54}$ erg, and radiated $γ$-ray energy $E_{\rm γ, iso} \sim$ 6.05 $\times 10^{54}$ erg, respectively.

astro-ph.HE↗

arXiv2021

EyePAD++: A Distillation-based approach for joint Eye Authentication and Presentation Attack Detection using Periocular Images

A practical eye authentication (EA) system targeted for edge devices needs to perform authentication and be robust to presentation attacks, all while remaining compute and latency efficient. However, existing eye-based frameworks a) perform authentication and Presentation Attack Detection (PAD) independently and b) involve significant pre-processing steps to extract the iris region. Here, we introduce a joint framework for EA and PAD using periocular images. While a deep Multitask Learning (MTL) network can perform both the tasks, MTL suffers from the forgetting effect since the training datasets for EA and PAD are disjoint. To overcome this, we propose Eye Authentication with PAD (EyePAD), a distillation-based method that trains a single network for EA and PAD while reducing the effect of forgetting. To further improve the EA performance, we introduce a novel approach called EyePAD++ that includes training an MTL network on both EA and PAD data, while distilling the `versatility' of the EyePAD network through an additional distillation step. Our proposed methods outperform the SOTA in PAD and obtain near-SOTA performance in eye-to-eye verification, without any pre-processing. We also demonstrate the efficacy of EyePAD and EyePAD++ in user-to-user verification with PAD across network backbones and image quality.

cs.CV↗

arXiv2021

Application of Hierarchical Temporal Memory Theory for Document Categorization

The current work intends to study the performance of the Hierarchical Temporal Memory(HTM) theory for automated classification of text as well as documents. HTM is a biologically inspired theory based on the working principles of the human neocortex. The current study intends to provide an alternative framework for document categorization using the Spatial Pooler learning algorithm in the HTM Theory. As HTM accepts only a stream of binary data as input, Latent Semantic Indexing(LSI) technique is used for extracting the top features from the input and converting them into binary format. The Spatial Pooler algorithm converts the binary input into sparse patterns with similar input text having overlapping spatial patterns making it easy for classifying the patterns into categories. The results obtained prove that HTM theory, although is in its nascent stages, performs at par with most of the popular machine learning based classifiers.

cs.CL↗

arXiv2021

Deep V and I CCD photometry of young star cluster NGC 1893 with the 3.6m DOT

Young star clusters consisting of massive stars are the ideal sites to study the star formation processes and influence of massive stars on the subsequent star formation. NGC 1893 is a young star cluster associated with the HII region Sh2-236. It contains about five `O'-type stars and several early `B'-type stars. It is located at a distance of $\sim$3.25 kpc and has a reddening, E(B-V)$\sim$0.4 mag. To characterize the young low-mass stellar population in the central portion of the cluster, we carried out deep VI band observations of the region using the 4K$\times$4K CCD IMAGER mounted on the 3.6-m DOT. Our analysis shows that the present data are deep enough to detect stars below $\sim$24 mag. We found optical counterparts of $\sim$220 candidate members, including young stars and unclassified cluster members from Caramazza et al. (2008). We estimated the membership probabilities of the Gaia sources (mostly bright stars with G$<$19 mag) located within the cluster radius using the Gaia EDR3. Toward the fainter end, we used the optical color-magnitude diagram (CMD) to select the cluster members from a sample of young stars. The unclassified member candidates and X-ray sources from Caramazza et al. (2012) are also found to be young low-mass stars. In total, we identified $\sim$425 young stars with age$<$10 Myr, and 110 of these are new. Most of these stars appear kinematic members of the cluster. By examining the CMD for the stars in the cluster region, we suggest that the cluster has insignificant contamination due to field stars in the pre-main sequence zone of the CMD. The slope of the mass function in the mass range 0.2$\le$M/M$\odot$$\le$2.5 is found to be $Γ$ = -1.43 $\pm$ 0.15, consistent with those of other star-forming complexes. The spatial distribution of the young stars as a function of mass suggests that toward the cluster center, most of the stars are massive.

astro-ph.SR↗

arXiv2021

Revealing nature of GRB 210205A, ZTF21aaeyldq (AT2021any), and follow-up observations with the 4K$\times$4K CCD Imager+3.6m DOT

Optical follow-up observations of optical afterglows of gamma-ray bursts are crucial to probe the geometry of outflows, emission mechanisms, energetics, and burst environments. We performed the follow-up observations of GRB 210205A and ZTF21aaeyldq (AT2021any) using the 3.6m Devasthal Optical Telescope (DOT) around one day after the burst to deeper limits due to the longitudinal advantage of the place. This paper presents our analysis of the two objects using data from other collaborative facilities, i.e., 2.2m Calar Alto Astronomical Observatory (CAHA) and other archival data. Our analysis suggests that GRB 210205A is a potential dark burst once compared with the X-ray afterglow data. Also, comparing results with other known and well-studied dark GRBs samples indicate that the reason for the optical darkness of GRB 210205A could either be intrinsic faintness or a high redshift event. Based on our analysis, we also found that ZTF21aaeyldq is the third known orphan afterglow with a measured redshift except for ZTF20aajnksq (AT2020blt) and ZTF19abvizsw (AT2019pim). The multiwavelength afterglow modelling of ZTF21aaeyldq using the afterglowpy package demands a forward shock model for an ISM-like ambient medium with a rather wider jet opening angle. We determine circumburst density of $n_{0}$ = 0.87 cm$^{-3}$, kinetic energy $E_{k}$ = 3.80 $\times 10^{52}$ erg and the afterglow modelling also indicates that ZTF21aaeyldq is observed on-axis ($θ_{obs} < θ_{core}$) and a gamma-ray counterpart was missed by GRBs satellites. Our results emphasize that the 3.6m DOT has a unique capability for deep follow-up observations of similar and other new transients for deeper observations as a part of time-domain astronomy in the future.

astro-ph.HE↗

arXiv2021

A Hitting Set Relaxation for $k$-Server and an Extension to Time-Windows

We study the $k$-server problem with time-windows. In this problem, each request $i$ arrives at some point $v_i$ of an $n$-point metric space at time $b_i$ and comes with a deadline $e_i$. One of the $k$ servers must be moved to $v_i$ at some time in the interval $[b_i, e_i]$ to satisfy this request. We give an online algorithm for this problem with a competitive ratio of ${\rm polylog} (n,Δ)$, where $Δ$ is the aspect ratio of the metric space. Prior to our work, the best competitive ratio known for this problem was $O(k \cdot {\rm polylog}(n))$ given by Azar et al. (STOC 2017). Our algorithm is based on a new covering linear program relaxation for $k$-server on HSTs. This LP naturally corresponds to the min-cost flow formulation of $k$-server, and easily extends to the case of time-windows. We give an online algorithm for obtaining a feasible fractional solution for this LP, and a primal dual analysis framework for accounting the cost of the solution. Together, they yield a new $k$-server algorithm with poly-logarithmic competitive ratio, and extend to the time-windows case as well. Our principal technical contribution lies in thinking of the covering LP as yielding a {\em truncated} covering LP at each internal node of the tree, which allows us to keep account of server movements across subtrees. We hope that this LP relaxation and the algorithm/analysis will be a useful tool for addressing $k$-server and related problems.

cs.DS↗

arXiv2021

Blast waves in two and three dimensions: Euler versus Navier Stokes equations

The exact solution of the Euler equation, which describes the time evolution of a blast wave created by an intense explosion, is a classic problem in gas dynamics. However, it has been found that the analytical results do not match with results from molecular dynamics simulation of hard spheres in two and three dimensions. In this paper, we show that the mismatch between theory and simulations can be resolved by considering the Navier Stokes equation. From the direct numerical simulation of the Navier Stokes equation in two and three dimensions, we show that the inclusion of heat conduction and viscosity terms is essential to capture the results from molecular dynamics simulations.

cond-mat.stat-mech↗

arXiv2021

Online Discrepancy with Recourse for Vectors and Graphs

The vector-balancing problem is a fundamental problem in discrepancy theory: given T vectors in $[-1,1]^n$, find a signing $σ(a) \in \{\pm 1\}$ of each vector $a$ to minimize the discrepancy $\| \sum_{a} σ(a) \cdot a \|_{\infty}$. This problem has been extensively studied in the static/offline setting. In this paper we initiate its study in the fully-dynamic setting with recourse: the algorithm sees a stream of T insertions and deletions of vectors, and at each time must maintain a low-discrepancy signing, while also minimizing the amortized recourse (the number of times any vector changes its sign) per update. For general vectors, we show algorithms which almost match Spencer's $O(\sqrt{n})$ offline discrepancy bound, with ${O}(n\cdot poly\!\log T)$ amortized recourse per update. The crucial idea is to compute a basic feasible solution to the linear relaxation in a distributed and recursive manner, which helps find a low-discrepancy signing. To bound recourse we argue that only a small part of the instance needs to be re-computed at each update. Since vector balancing has also been greatly studied for sparse vectors, we then give algorithms for low-discrepancy edge orientation, where we dynamically maintain signings for 2-sparse vectors. Alternatively, this can be seen as orienting a dynamic set of edges of an n-vertex graph to minimize the absolute difference between in- and out-degrees at any vertex. We present a deterministic algorithm with $O(poly\!\log n)$ discrepancy and $O(poly\!\log n)$ amortized recourse. The core ideas are to dynamically maintain an expander-decomposition with low recourse and then to show that, as the expanders change over time, a natural local-search algorithm converges quickly (i.e., with low recourse) to a low-discrepancy solution. We also give strong lower bounds for local-search discrepancy minimization algorithms.

cs.DS↗

arXiv2021

Spectropolarimeter on-board the Aditya-L1: Polarization Modulation and Demodulation

One of the major science goals of the Visible Emission Line Coronagraph (VELC) payload aboard the Aditya-L1 mission is to map the coronal magnetic field topology and the quantitative estimation of longitudinal magnetic field on routine basis. The infrared (IR) channel of VELC is equipped with a polarimeter to carry out full Stokes spectropolarimetric observations in the Fe XIII line at 1074.7~nm. The polarimeter is in dual-beam setup with continuously rotating waveplate as the polarization modulator. Detection of circular polarization due to Zeeman effect and depolarization of linear polarization in the presence of magnetic field due to saturated Hanle effect in the Fe~{\sc xiii} line require high signal-to-noise ratio (SNR). Due to limited number of photons, long integration times are expected to build the required SNR. In other words signal from a large number of modulation cycles are to be averaged to achieve the required SNR. This poses several difficulties. One of them is the increase in data volume and the other one is the change in modulation matrix in successive modulation cycles. The latter effect arises due to a mismatch between the retarder's rotation period and the length of the signal detection time in the case of VELC spectropolarimeter (VELC/SP). It is shown in this paper that by appropriately choosing the number of samples per half rotation the data volume can be optimized. A potential solution is suggested to account for modulation matrix variation from one cycle to the other.

astro-ph.IM↗

arXiv2021

A Search for Blazar Activity in Broad-Absorption-Line Quasars

Our recently reported lack of Intra-Night Optical Variability (INOV) among Broad-Absorption-Line (BAL) quasars exhibiting some blazar-like radio properties, either questions polar ejection of BAL clouds, and/or hints at a physical state of the relativistic jet modified due to interaction with the thermal BAL wind on the innermost sub-parsec scale. As a robust check on this unexpected finding for the BAL_blazar candidates, we report here the INOV study of a new and much more rigorously defined comparison sample consisting of 9 normal (non-BAL) blazars, matched in both magnitude and redshift to the aforementioned sample of BAL_blazar candidates. The present campaign spanning 27 sessions yields an INOV duty cycle of ~23% for the comparison sample of normal blazars, employing the enhanced F-test. However, even this more sensitive test does not detect INOV for the sample of BAL_blazar candidates. This stark INOV contrast found here between the BAL_blazar candidates and normal blazars can probably be traced to a physical interaction of the relativistic jet with the thermal wind, within sub-parsec range from the nucleus. The consequent enfeebling of the jet would additionally explain the striking deficiency among BAL quasars of powerful FR II radio lobes on the much larger scale of 10-100 kpc.

astro-ph.GA↗

arXiv2021

Photometric, polarimetric, and spectroscopic studies of the luminous, slow-decaying Type Ib SN 2012au

Optical, near-infrared (NIR) photometric and spectroscopic studies, along with the optical imaging polarimetric results for SN 2012au, are presented in this article to constrain the nature of the progenitor and other properties. Well-calibrated multiband optical photometric data (from $-$0.2 to +413 d since $B$-band maximum) were used to compute the bolometric light curve and to perform semi-analytical light-curve modelling using the $\texttt{MINIM}$ code. A spin-down millisecond magnetar-powered model explains the observed photometric evolution of SN 2012au reasonably. Early-time imaging polarimetric follow-up observations ($-$2 to +31 d) and comparison with other similar cases indicate signatures of asphericity in the ejecta. Good spectral coverage of SN 2012au (from $-$5 to +391 d) allows us to trace the evolution of layers of SN ejecta in detail. SN 2012au exhibits higher line velocities in comparison with other SNe Ib. Late nebular phase spectra of SN 2012au indicate a Wolf$-$Rayet star as the possible progenitor for SN 2012au, with oxygen, He-core, and main-sequence masses of $\sim$1.62 $\pm$ 0.15 M$_\odot$, $\sim$4$-$8 M$_\odot$, and $\sim$17$-$25 M$_\odot$, respectively. There is a clear absence of a first overtone of carbon monoxide (CO) features up to +319 d in the $K$-band region of the NIR spectra. Overall analysis suggests that SN 2012au is one of the most luminous slow-decaying Type Ib SNe, having comparatively higher ejecta mass ($\sim$4.7$-$8.3 M$_\odot$) and kinetic energy ($\sim$[4.8 $-$ 5.4] $\times$ 10$^{51}$ erg). Detailed modelling using $\texttt{MESA}$ and the results obtained through $\texttt{STELLA}$ and $\texttt{SNEC}$ explosions also strongly support spin-down of a magnetar with mass of around 20 M$_\odot$ and metallicity Z = 0.04 as a possible powering source of SN 2012au.

astro-ph.HE↗

arXiv2021

Bag-of-Tasks Scheduling on Related Machines

We consider online scheduling to minimize weighted completion time on related machines, where each job consists of several tasks that can be concurrently executed. A job gets completed when all its component tasks finish. We obtain an $O(K^3 \log^2 K)$-competitive algorithm in the non-clairvoyant setting, where $K$ denotes the number of distinct machine speeds. The analysis is based on dual-fitting on a precedence-constrained LP relaxation that may be of independent interest.

cs.DS↗

arXiv2021

Stochastic Makespan Minimization in Structured Set Systems

We study stochastic combinatorial optimization problems where the objective is to minimize the expected maximum load (a.k.a.\ the makespan). In this framework, we have a set of $n$ tasks and $m$ resources, where each task $j$ uses some subset of the resources. Tasks have random sizes $X_j$, and our goal is to non-adaptively select $t$ tasks to minimize the expected maximum load over all resources, where the load on any resource $i$ is the total size of all selected tasks that use $i$. For example, when resources are points and tasks are intervals in a line, we obtain an $O(\log\log m)$-approximation algorithm. Our technique is also applicable to other problems with some geometric structure in the relation between tasks and resources; e.g., packing paths, rectangles, and "fat" objects. Our approach uses a strong LP relaxation using the cumulant generating functions of the random variables. We also show that this LP has an $Ω(\log^* m)$ integrality gap, even for the problem of selecting intervals on a line; here $\log^* m$ is the iterated logarithm function.

cs.DS↗