arXiv subjects

P. Patel

Publications and source records attributed to P. Patel.

16 recordsLinked to original sources

arXiv2026

Localization and Confidence Region Estimation of Short GRBs with the COSI BGO Shield Using a HEALPix-Based Deep Learning Approach

The Compton Spectrometer and Imager is a NASA satellite mission under development that will survey the entire sky in the 0.2-5 MeV range using a wide-field germanium detector array, surrounded on the sides and bottom by active shields (the Anticoincidence Subsystem, ACS). The ACS aims to suppress and monitor background events, as well as detect transient sources, such as Gamma-Ray Bursts (GRBs), through its onboard triggering algorithm. The data related to GRBs are sent to the ground and analyzed by an automated pipeline to localize the GRBs and share their positions with the community. In this work, we present a brief GRB localization method based on ACS data, utilizing deep learning (DL) techniques, which can estimate the 90\% confidence region, including cases where it is split into multiple areas. To address this, we developed a neural network classifier that predicts the GRB location as a probability distribution across the sky map following the HEALPix framework. The distribution can be used to compute the 90\% confidence regions. Future work will compare this DL-based localization approach with classical methods such as $\chi^2$ fitting and Maximum Likelihood Estimation.

astro-ph.HE↗

arXiv2024

Probing the rest-frame wavelength dependence of quasar variability : Insights from the Zwicky Transient Facility Survey

Context: Quasar variability can potentially unlock crucial insights into the accretion process. Understanding how this variability is influenced by wavelength is crucial for validating and refining quasar variability models. Aims: This paper aims to enhance the understanding of the dependence of variability on the rest-frame wavelength ($\lambda_{RF}$) by isolating the variance in different timescales in well-defined wavelength bins and examining the corona-heated accretion-disk (CHAR) model. Methods: We investigated the relation between variance and rest-frame wavelength ($\lambda_{RF}$) using optical g- and r-band light curves from the Zwicky Transient Facility (ZTF) Data Release 15 for $\sim 5000$ quasars within narrow ranges of black hole mass ($M_{BH}$) and Eddington ratio ($R_{Edd}$). A spectral model taking into account disk continuum emission, Balmer transitions, Fe II pseudo-continuum emission, and other emission lines is necessary to best interpret the variance spectrum. Results: Our analysis indicates a strong anticorrelation between median variance and $\lambda_{RF}$ for quasars with $M_{BH} = 10^{8}$ and $R_{Edd} = 10^{-1}$ at different timescales. This anti-correlation is more pronounced at shorter timescales. The results align well with a bending power-law power spectrum density (PSD) model with both the damping timescale and the high-frequency slope of the PSD depending on the wavelength. The predictions provided by the CHAR model on the variance spectrum across most timescales studied showcase its potential in constraining temperature variations within the accretion disk. Key words. accretion, accretion discs; galaxies:active; quasars: supermassive black holes

astro-ph.GA↗

arXiv2023

The universal power spectrum of Quasars in optical wavelengths: Break timescale scales directly with both black hole mass and accretion rate

Aims: Establish the dependence of variability properties, such as characteristic timescales and variability amplitude, on basic quasar parameters such as black hole mass and accretion rate, controlling for the rest-frame wavelength of emission. Methods: Using large catalogs of quasars, we selected the g-band light curves for 4770 objects from the Zwicky Transient Facility archive. All selected objects fall into a narrow redshift bin, $0.6<z<0.7$, but cover a wide range of accretion rates in Eddington units (REdd) and black hole masses ($M$). We grouped these objects into 26 independent bins according to these parameters, calculated low-resolution $g$-band variability power spectra for each of these bins, and approximated the power spectra with a simple analytic model that features a break at a timescale $t_b$. Results: We found a clear dependence of the break timescale $t_b$ on REdd, on top of the known dependence of $t_b$ on the black hole mass $M$. In our fits, $t_b\propto M^{0.65 - 0.55}$ REdd $^{0.35 - 0.3}$, where the ranges in the exponents correspond to the best-fitting parameters of different power spectrum models. Scaling $t_b$ to the orbital timescale of the innermost stable circular orbit (ISCO), $t_{\rm ISCO}$, results approximately in $t_{b}/t_{\rm ISCO} \propto ($REdd$/M)^{0.35}$. The observed values of $t_b$ are $\sim 10$ longer than the orbital timescale at the light-weighted average radius of the disc region emitting in the (observer frame) $g$-band. The different scaling of the break frequency with $M$ and REdd shows that the shape of the variability power spectrum cannot be solely a function of the quasar luminosity, even for a single rest-frame wavelength. Finally, the best-fitting models have slopes above the break in the range -2.5 and -3. A slope of -2, as in the damped random walk models, fits the data significantly worse.

astro-ph.GA↗

arXiv2022

Benchtop magnetic shielding for benchmarking atomic magnetometers

Here, a benchtop hybrid magnetic shield containing four mumetal cylinders and nine internal flexible printed circuit boards is designed, constructed, tested, and operated. The shield is designed specifically as a test-bed for building and operating ultra-sensitive quantum magnetometers. The geometry and spacing of the mumetal cylinders are optimized to maximize shielding efficiency while maintaining Johnson noise $<15$ fT/$\sqrt{}$Hz. Experimental measurements at the shield's center show passive shielding efficiency of $\left(1.0\pm0.1\right){\times}10^6$ for a $0.2$ Hz oscillating field applied along the shield's axis. The nine flexible printed circuit boards generate three uniform fields, which all deviate from perfect uniformity by ${\leq}0.5$% along $50$% of the inner shield axis, and five linear field gradients and one second-order gradient, which all deviate by ${\leq}4$% from perfect linearity and curvature, respectively, over measured target regions. Together, the target field amplitudes are adjusted to minimize the remnant static field along $40$% of the inner shield axis, as mapped using an atomic magnetometer. In this region, the active null reduces the norm of the magnitudes of the three uniform fields and six gradients by factors of $19.5$ and $19.8$, respectively, thereby reducing the total static field from $1.68$ nT to $0.23$ nT.

physics.ins-det↗

arXiv2021

Super-resolution linear optical imaging in the far field

The resolution of optical imaging devices is ultimately limited by the diffraction of light. To circumvent this limit, modern super-resolution microscopy techniques employ active interaction with the object by exploiting its optical nonlinearities, nonclassical properties of the illumination beam, or near-field probing. Thus, they are not applicable whenever such interaction is not possible, for example, in astronomy or non-invasive biological imaging. Far-field, linear-optical super-resolution techniques based on passive analysis of light coming from the object would cover these gaps. In this paper, we present the first proof-of-principle demonstration of such a technique. It works by accessing information about spatial correlations of the image optical field and, hence, about the object itself via measuring projections onto Hermite-Gaussian transverse spatial modes. With a basis of 21 spatial modes in both transverse dimensions, we perform two-dimensional imaging with twofold resolution enhancement beyond the diffraction limit.

physics.optics↗

arXiv2020

A Blockchain-based Approach for Assessing Compliance with SLA-guaranteed IoT Services

Within cloud-based internet of things (IoT) applications, typically cloud providers employ Service Level Agreements (SLAs) to ensure the quality of their provisioned services. Similar to any other contractual method, an SLA is not immune to breaches. Ideally, an SLA stipulates consequences (e.g. penalties) imposed on cloud providers when they fail to conform to SLA terms. The current practice assumes trust in service providers to acknowledge SLA breach incidents and executing associated consequences. Recently, the Blockchain paradigm has introduced compelling capabilities that may enable us to address SLA enforcement more elegantly. This paper proposes and implements a blockchain-based approach for assessing SLA compliance and enforcing consequences. It employs a diagnostic accuracy method for validating the dependability of the proposed solution. The paper also benchmarks Hyperledger Fabric to investigate its feasibility as an underlying blockchain infrastructure concerning latency and transaction success/fail rates.

cs.DC↗

arXiv2018

Collisionless shock acceleration of narrow energy spread ion beams from mixed species plasmas using 1 $\mu$m lasers

Collisionless shock acceleration of protons and C$^{6+}$ ions has been achieved by the interaction of a 10$^{20}$ W/cm$^2$, 1 $\mu$m laser with a near-critical density plasma. Ablation of the initially solid density target by a secondary laser allowed for systematic control of the plasma profile. This enabled the production of beams with peaked spectra with energies of 10-18 MeV/a.m.u. and energy spreads of 10-20$\%$ with up to 3x10$^9$ particles within these narrow spectral features. The narrow energy spread and similar velocity of ion species with different charge-to-mass ratio are consistent with acceleration by the moving potential of a shock wave. Particle-in-cell simulations show shock accelerated beams of protons and C$^{6+}$ ions with energy distributions consistent with the experiments. Simulations further indicate the plasma profile determines the trade-off between the beam charge and energy and that with additional target optimization narrow energy spread beams exceeding 100 MeV/a.m.u. can be produced using the same laser conditions.

physics.plasm-ph↗

arXiv2017

Photoionization Models for the Inner Gaseous Disks of Herbig Be Stars: Evidence Against Magnetospheric Accretion?

We investigate the physical properties of the inner gaseous disks of the three, hot, Herbig B2e stars, HD 76534, HD 114981 and HD 216629, by modelling CFHT-ESPaDOns spectra using non-LTE radiative transfer codes. We assume that the emission lines are produced in a circumstellar disk heated solely by the photospheric radiation from the central star in order to test if the optical and near-IR emission lines can be reproduced without invoking magnetospheric accretion. The inner gaseous disk density was assumed to follow a simple power-law in the equatorial plane, and we searched for models that could reproduce observed lines of HI (H$\alpha$ and H$\beta$), HeI, CaII and FeII. For the three stars, good matches were found for all emission line profiles individually; however, no density model based on a single power-law was able to reproduce all of the observed emission lines. Among the single power-law models, the one with the gas density varying as $\sim 10^{-10}\,(R_{*}/R)^3\, \rm g\,cm^{-3}$ in the equatorial plane of a 25 R$_{*}$ ($0.78$ AU) disk did the best overall job of representing the optical emission lines of the three stars. This model implies a mass for the H$\alpha$-emitting portion of the inner gaseous disk of $\sim 10^{-9} M_*$. We conclude that the optical emission line spectra of these HBe stars can be qualitatively reproduced by a $\approx\,1$ AU, geometrically thin, circumstellar disk of negligible mass compared to the central star in Keplerian rotation and radiative equilibrium.

astro-ph.SR↗

arXiv2015

Photoionization Models of the Inner Gaseous Disk of the Herbig Be Star BD+65 1637

We attempt to constrain the physical properties of the inner, gaseous disk of the Herbig Be star BD+65 1637 using non-LTE, circumstellar disk codes and observed spectra (3700 to 10,500 \r{A}) from the ESPaDOnS instrument on CFHT. The photoionizing radiation of the central star is assumed to be the sole source of input energy for the disk. We model optical and near-infrared emission lines that are thought to form in this region using standard techniques that have been successful in modeling the spectra of Classical Be stars. By comparing synthetic line profiles of hydrogen, helium, iron and calcium with the observed line profiles, we try to constrain the geometry, density structure, and kinematics of the gaseous disk. Reasonable matches have been found for all line profiles individually; however, no disk density model based on a single power-law for the equatorial density was able to simultaneously fit all of the observed emission lines. Amongst the emission lines, the metal lines, especially the Ca II IR triplet, seem to require higher disk densities than the other lines. Excluding the Ca II lines, a model in which the equatorial disk density falls as $10^{-10} (R_{*}/R)^3 g\,cm^{-3}$ seen at an inclination of 45{\deg} for a $50\,R_{*}$ disk provides reasonable matches to the overall line shapes and strengths. The Ca II lines seem to require a shallower drop off as $10^{-10} (R_{*}/R)^2 g\,cm^{-3}$ to match their strength. More complex disk density models are likely required to refine the match to the BD+65 1637 spectrum.

astro-ph.SR↗

arXiv2015

Weak gravitational lensing with the Square Kilometre Array

We investigate the capabilities of various stages of the SKA to perform world-leading weak gravitational lensing surveys. We outline a way forward to develop the tools needed for pursuing weak lensing in the radio band. We identify the key analysis challenges and the key pathfinder experiments that will allow us to address them in the run up to the SKA. We identify and summarize the unique and potentially very powerful aspects of radio weak lensing surveys, facilitated by the SKA, that can solve major challenges in the field of weak lensing. These include the use of polarization and rotational velocity information to control intrinsic alignments, and the new area of weak lensing using intensity mapping experiments. We show how the SKA lensing surveys will both complement and enhance corresponding efforts in the optical wavebands through cross-correlation techniques and by way of extending the reach of weak lensing to high redshift.

astro-ph.CO↗

arXiv2013

Probing the accelerating Universe with radio weak lensing in the JVLA Sky Survey

We outline the prospects for performing pioneering radio weak gravitational lensing analyses using observations from a potential forthcoming JVLA Sky Survey program. A large-scale survey with the JVLA can offer interesting and unique opportunities for performing weak lensing studies in the radio band, a field which has until now been the preserve of optical telescopes. In particular, the JVLA has the capacity for large, deep radio surveys with relatively high angular resolution, which are the key characteristics required for a successful weak lensing study. We highlight the potential advantages and unique aspects of performing weak lensing in the radio band. In particular, the inclusion of continuum polarisation information can greatly reduce noise in weak lensing reconstructions and can also remove the effects of intrinsic galaxy alignments, the key astrophysical systematic effect that limits weak lensing at all wavelengths. We identify a VLASS "deep fields" program (total area ~10-20 square degs), to be conducted at L-band and with high-resolution (A-array configuration), as the optimal survey strategy from the point of view of weak lensing science. Such a survey will build on the unique strengths of the JVLA and will remain unsurpassed in terms of its combination of resolution and sensitivity until the advent of the Square Kilometre Array. We identify the best fields on the JVLA-accessible sky from the point of view of overlapping with existing deep optical and near infra-red data which will provide crucial redshift information and facilitate a host of additional compelling multi-wavelength science.

astro-ph.CO↗

arXiv2013

A Solution to the Edge-Balanced Index Set Problem for Complete Odd Bipartite Graphs

In 2009, Kong, Wang, and Lee began work on the problem of finding the edge-balanced index sets (EBI) of complete bipartite graphs K_{m,n} by solving the cases where n = 1, 2, 3, 4, and 5, and also the case where m = n. In 2011, Krop and Sikes expanded upon that work by finding EBI(K_{m,m-2a}) for odd m > 5 and 1 <= a <= (m-3)/4. In this paper, we provide a general solution to the edge-balanced index set problem for all complete odd bipartite graphs, thereby concluding the problem for this case.

math.CO↗

arXiv2013

The Correlation Between H\alpha\ Emission and Visual Magnitude During Long-Term Variations in Classical Be Stars

H\alpha\ equivalent widths and UBV magnitudes are calculated for Be star disk models that grow in size and/or density with time. We show that these simple models are consistent with the known Be star classes of positive and inverse correlations between long-term variations in H\alpha\ and V magnitude as identified by Harmanec. We support the conclusion of Harmanec that the distinction is controlled by the inclination of the disk to the line of sight. We demonstrate that the strength of these correlations, particularly those of an inverse correlation where the system becomes fainter as the H\alpha\ emission strength grows, is strongly influenced by the scale height of the inner Be star disk and the extent of the gravitational darkening of the central B star due to rapid rotation. This dependence may allow coordinated spectroscopic and photometric observations to better constrain these poorly known Be star parameters.

astro-ph.SR↗

arXiv2010

Implementation of barycentric resampling for continuous wave searches in gravitational wave data

We describe an efficient implementation of a coherent statistic for continuous gravitational wave searches from neutron stars. The algorithm works by transforming the data taken by a gravitational wave detector from a moving Earth bound frame to one that sits at the Solar System barycenter. Many practical difficulties arise in the implementation of this algorithm, some of which have not been discussed previously. These difficulties include constraints of small computer memory, discreteness of the data, losses due to interpolation and gaps in real data. This implementation is considerably more efficient than previous implementations of these kinds of searches on Laser Interferometer Gravitational Wave (LIGO) detector data.

gr-qc↗

arXiv2009

Radio Weak Gravitational Lensing with VLA and MERLIN

We carry out an exploratory weak gravitational lensing analysis on a combined VLA and MERLIN radio data set: a deep (3.3 micro-Jy beam^-1 rms noise) 1.4 GHz image of the Hubble Deep Field North. We measure the shear estimator distribution at this radio sensitivity for the first time, finding a similar distribution to that of optical shear estimators for HST ACS data in this field. We examine the residual systematics in shear estimation for the radio data, and give cosmological constraints from radio-optical shear cross-correlation functions. We emphasize the utility of cross-correlating shear estimators from radio and optical data in order to reduce the impact of systematics. Unexpectedly we find no evidence of correlation between optical and radio intrinsic ellipticities of matched objects; this result improves the properties of optical-radio lensing cross-correlations. We explore the ellipticity distribution of the radio counterparts to optical sources statistically, confirming the lack of correlation; as a result we suggest a connected statistical approach to radio shear measurements.

astro-ph.CO↗

arXiv2008

Searching for gravitational waves from Cassiopeia A with LIGO

We describe a search underway for periodic gravitational waves from the central compact object in the supernova remnant Cassiopeia A. The object is the youngest likely neutron star in the Galaxy. Its position is well known, but the object does not pulse in any electromagnetic radiation band and thus presents a challenge in searching the parameter space of frequency and frequency derivatives. We estimate that a fully coherent search can, with a reasonable amount of time on a computing cluster, achieve a sensitivity at which it is theoretically possible (though not likely) to observe a signal even with the initial LIGO noise spectrum. Cassiopeia A is only the second object after the Crab pulsar for which this is true. The search method described here can also obtain interesting results for similar objects with current LIGO sensitivity.

gr-qc↗