arXiv subjects

Pankaj Jain

Publications and source records attributed to Pankaj Jain.

At least 37 records · Page 2Linked to original sources

arXiv2024

Detection of simultaneous QPO triplets in 4U 1728-34 and constraining the neutron star mass and moment of inertia

We report simultaneous detection of twin kHz and $\sim 40$ Hz quasi-periodic oscillations (QPOs) in the time-resolved analysis of the AstroSat/LAXPC observation of the neutron star low mass X-ray binary, 4U 1728-34. The frequencies of the multiple sets of triplets are correlated with each other and are consistent with their identification as the orbital, periastron and twice the nodal precessions frequencies. The observed relations, along with the known spin of the neutron star, put constraints on the mass and the ratio of moment of inertia to the mass of the neutron star to be $M^*_\odot = 1.92\pm 0.01$ and $I_{45}/M^*_\odot = 1.07\pm 0.01$ under the simplistic assumption that the metric is a Kerr one. We crudely estimate that the mass and moment of inertia values obtained may differ by about 1 % and 5 %, respectively, if a self-consistent metric is invoked. Using the TOV equations for computing the moment of inertia of a neutron star in slow rotation approximation, having different equations of state, we find that the predicted values of neutron star parameters favor stiffer equations of state. We expect more stringent constraints would be obtained using a more detailed treatment, where the EOS-dependent metric is used to compute the expected frequencies rather than the Kerr metric used here. The results provide insight into both the nature of these QPOs and the neutron star interior.

astro-ph.HE↗

arXiv2024

Medium Assisted Low Energy Nuclear Fusion

We study the process of nuclear fusion at low energies in a medium using the second order time dependent perturbation theory. We consider a specific process which involves fusion of a low energy proton with a Nickel nucleus. The reaction proceeds in two steps or interactions. We refer to the amplitudes corresponding to these two interactions as the the molecular and the nuclear matrix elements. The first amplitude involves Coulomb interaction with another nucleus in the medium while the second corresponds to the nuclear fusion process. It has been shown in earlier papers that such a second order process has negligible amplitude unless it is assisted by special medium effects. In the present paper we show the presence of a special configuration of atoms which greatly enhances the process. We find that if the spacings among the atoms can be tuned, the rate can be sufficiently enhanced so that easily observable. The spacings do not require acute fine tuning, however, if they are significantly off the rate falls sharply to negligible values. This might also explain both the successes and failures experienced by many experimentalists studying this phenomenon. We study only a particular final state which involves emission of one photon. However we show that many final states are possible which need not involve photon emission.

nucl-th↗

arXiv2023

Probing Cosmological Principle using the spectral index of quasar flux distribution

We study the dipole signal in the spectral index (x) of the differential number counts using quasars in the CatWISE2020 catalog of infrared sources. The index is extracted by using the log-likelihood method. We obtain the value $x=1.579 \pm 0.001$ for a quasar sample of 1355352 sources. We extract the dipole signal in this parameter by employing $χ^{2}$ minimization, assuming a sky model of x up to the quadrupole term. We find that the dipole amplitude |D| is 0.005 \pm 0.002 and dipole direction (l, b) in Galactic coordinate system equal to $(201.50^{\circ} \pm 27.87^{\circ}, -29.37^{\circ} \pm 19.86^{\circ})$. The direction of dipole anisotropy is found to be very close to the hemispherical power asymmetry $(l,b)=(221^\circ,-27^{\circ})$ in the Cosmic Microwave Background (CMB). We also obtain a signal of quadrupole anisotropy which is correlated with the ecliptic poles and can be attributed to ecliptic bias.}

astro-ph.CO↗

arXiv2023

Low energy nuclear reactions in a crystal lattice

We extend the recently proposed mechanism for inducing low energy nuclear reactions (LENRs) to a crystal lattice. The process gets a dominant contribution at second order in quantum perturbation theory. The tunnelling barrier is evaded due to the presence of high energy intermediate states. However, the process is found to have a negligible rate in free space due to a delicate cancellation between amplitudes corresponding to different states. The delicate cancellation can be evaded in medium in special circumstances leading to an observable rate. Here we consider this process in a one dimensional crystal lattice and find that the rate depends strongly on the form of interaction. We find that for some allowed choices of interactions, the rate is substantial, while other interactions produce a very small rate.

nucl-th↗

arXiv2023

Correlations between QPO frequencies and spectral parameters of GRS 1915+105 using AstroSat observations

In this work, we study the correlation between Quasi-periodic Oscillation (QPO) frequency and the spectral parameters during various X-ray states in the black hole binary GRS 1915+105 which matches well with the predicted relativistic dynamic frequency (i.e. the inverse of the sound crossing time) at the truncated radii. We have used broadband data of LAXPC and SXT instruments onboard AstroSat. Spectral fitting shows that the accretion rate varies from $\sim 0.1$ to $\sim 5.0 \times 10^{18}$ gm/s and the truncated radius changing from the last stable orbit of an almost maximally spinning black hole, $\sim$ 1.2 to $\sim$ 19 Gravitational radii. For this wide range, the frequencies of the C-type QPO (2 - 6 Hz) follow the trend predicted by the relativistic dynamical frequency model and interestingly, the high-frequency QPO at $\sim$ 70 Hz also follows the same trend, suggesting they originate from the innermost stable circular orbit with the same mechanism as the more commonly observed C-type QPO. While the qualitative trend is as predicted, there are quantitative deviations between the data and the theory, and the possible reasons for these deviations are discussed.

astro-ph.HE↗

arXiv2023

Probing Cosmology beyond $Λ$CDM using the SKA

The cosmological principle states that the Universe is statistically homogeneous and isotropic at large distance scales. There currently exist many observations which indicate a departure from this principle. It has been shown that many of these observations can be explained by invoking superhorizon cosmological perturbations and may be consistent with the Big Bang paradigm. Remarkably, these modes simultaneously explain the observed Hubble tension, i.e., the discrepancy between the direct and indirect measurements of the Hubble parameter. We propose several tests of the cosmological principle using SKA. In particular, we can reliably extract the signal of dipole anisotropy in the distribution of radio galaxies. The superhorizon perturbations also predict a significant redshift dependence of the dipole signal which can be nicely tested by the study of signals of reionization and the dark ages using SKA. We also propose to study the alignment of radio galaxy axes as well as their integrated polarization vectors over distance scales ranging from a few Mpc to Gpc. We discuss data analysis techniques that can reliably extract these signals from data.

astro-ph.CO↗

arXiv2023

Fully Elman Neural Network: A Novel Deep Recurrent Neural Network Optimized by an Improved Harris Hawks Algorithm for Classification of Pulmonary Arterial Wedge Pressure

Heart failure (HF) is one of the most prevalent life-threatening cardiovascular diseases in which 6.5 million people are suffering in the USA and more than 23 million worldwide. Mechanical circulatory support of HF patients can be achieved by implanting a left ventricular assist device (LVAD) into HF patients as a bridge to transplant, recovery or destination therapy and can be controlled by measurement of normal and abnormal pulmonary arterial wedge pressure (PAWP). While there are no commercial long-term implantable pressure sensors to measure PAWP, real-time non-invasive estimation of abnormal and normal PAWP becomes vital. In this work, first an improved Harris Hawks optimizer algorithm called HHO+ is presented and tested on 24 unimodal and multimodal benchmark functions. Second, a novel fully Elman neural network (FENN) is proposed to improve the classification performance. Finally, four novel 18-layer deep learning methods of convolutional neural networks (CNNs) with multi-layer perceptron (CNN-MLP), CNN with Elman neural networks (CNN-ENN), CNN with fully Elman neural networks (CNN-FENN), and CNN with fully Elman neural networks optimized by HHO+ algorithm (CNN-FENN-HHO+) for classification of abnormal and normal PAWP using estimated HVAD pump flow were developed and compared. The estimated pump flow was derived by a non-invasive method embedded into the commercial HVAD controller. The proposed methods are evaluated on an imbalanced clinical dataset using 5-fold cross-validation. The proposed CNN-FENN-HHO+ method outperforms the proposed CNN-MLP, CNN-ENN and CNN-FENN methods and improved the classification performance metrics across 5-fold cross-validation. The proposed methods can reduce the likelihood of hazardous events like pulmonary congestion and ventricular suction for HF patients and notify identified abnormal cases to the hospital, clinician and cardiologist.

cs.LG↗

arXiv2023

The comptonizing medium of the black-hole X-ray binary MAXI~J1535$-$571 through type-C quasi-periodic oscillations

We present a detailed spectral and temporal analysis of the black-hole candidate MAXI~J1535$-$571 using NICER observations in September and October 2017. We focus specifically on observations in the hard-intermediate state when the source shows type-C quasi-periodic oscillations (QPOs). We fitted the time-averaged spectrum of the source and the rms and phase-lag spectra of the QPO with a one-component time-dependent Comptonization model. We found that the corona contracts from $\sim 10^4$ to $\sim 3 \times 10^3$ km as the QPO frequency increases from $\sim 1.8$ Hz to $\sim 9.0$ Hz. The fits suggest that the system would consists of two coronas, a small one that dominates the time-averaged spectrum and a larger one, possibly the jet, that dominates the rms and lag spectra of the QPO. We found a significant break in the relation of the spectral parameters of the source and the properties of the QPO, including its lag spectra, with QPO frequency. The change in the relations happens when the QPO frequency crosses a critical frequency $ν_c \approx 3.0$ Hz. Interestingly, the QPO reaches this critical frequency simultaneously as the radio emission from the jet in this source is quenched.

astro-ph.HE↗

arXiv2022

Dipole Anisotropy in Gravitational Wave Source Distribution

Our local motion with respect to the cosmic frame of rest is believed to be dominantly responsible for the observed dipole anisotropy in the Cosmic Microwave Background Radiation (CMBR). We study the effect of this motion on the sky distribution of gravitational wave (GW) sources. We determine the resulting dipole anisotropy in GW source number counts, mass weighted number counts, which we refer to as mass intensity, and mean mass per source. The mass M dependence of the number density n(M) distribution of BBH is taken directly from the data. We also test the anisotropy in the observable mean mass per source along the direction of the CMB dipole. The current data sample is relatively small and consistent with isotropy. The number of sources required for this test is likely to become available in the near future.

astro-ph.CO↗

arXiv2022

A Toy Model for Low Energy Nuclear Fusion

We study the fusion of a proton with a nucleus with the emission of two photons at low incident energy of the order of eV or smaller. We use a step model for the repulsive potential between proton and the nuclei. We consider the reaction both in free space and inside a medium. We make a simple model for the medium by assuming a hard wall potential beyond a certain length scale. This essentially leads to discretization of the energy spectrum which is expected inside a medium and is seen both for a crystalline lattice structure and for amorphous materials. We use second order perturbation theory to compute the transition rate. We find that the rate in free space is very small. However in medium, the rate may be substantial. Hence, we conclude that nuclear fusion reactions may take place at low energies at observable rates.

nucl-th↗

arXiv2022

A study of Dipolar Signal in distant Quasars with various observables

We study the signal of anisotropy in AGNs/quasars of CatWISE2020 catalogue using different observables. It has been reported earlier that this data shows a strong signal of dipole anisotropy in the source number counts. We test this claim using two independent data analysis procedures and find our number count dipole consistent with the earlier results. In addition to number counts, we test for the anisotropy signal in two other observables -- mean spectral index $\barα$ and mean flux density $\bar{B}$. We find a dipole signal of considerable strength both in the mean spectral index and the mean flux density. The dipole in mean flux density points towards the galactic center and becomes very weak after imposing a flux cut to remove sources with flux greater than 1 mJy. This can be attributed to the presence of some bright sources. The signal in mean spectral index, however, is relatively stable as a function of both flux and galactic cuts. The dipole in this observable points roughly opposite to the galactic center and hence most likely arises due to galactic bias. Hence, the signal in both the mean spectral index and mean flux density appears to be consistent with isotropy.

astro-ph.CO↗

arXiv2022

A mechanism to explain Galaxy alignment over a range of scales

The observed large-scale alignment of polarization angles and galaxy axis have been challenging the fundamental assumption of homogeneity and isotropy in standard cosmology since more than two decades. The intergalactic magnetic field, and its correlations in real space, potentially seems as a viable candidate for explaining this phenomenon. It has been shown earlier that the large-scale intergalactic magnetic field correlations can explain the alignment signal of quasars over Gpc scale, interestingly they can also explain the radio polarization alignment observed in JVAS/CLASS data over 100 Mpc. Motivated with recent observations of galaxy axis alignment over several tens of Mpc, and Mpc scale, i.e., the cluster scale, we further explore the correlations of background magnetic field to explain these relatively small scale alignment observations. In particular, we explore two recently claimed signals of alignment in the radio sources in the FIRST catalog and in the ACO clusters. We find that the FIRST alignment signal is well explained in terms of the intergalactic magnetic field with a spectral index of $-2.62\pm 0.03$. Furthermore, the model also partially explains the very small scale alignment (alignment within clusters). Though the elementary model proposed in this work seems to have its limitations at very small scales, the large-scale magnetic field correlations potentially seem to explain the polarization and galaxy axis alignment from Gpc to Mpc scales.

astro-ph.CO↗

arXiv2022

Rubio de Francia Extrapolation Theorems for Quasi-Monotone Functions

We prove Rubio de Francia extrapolation results in Lebesgue and grand Lebesgue spaces for quasi monotone functions with $QB_{β,p}$ weights. The extrapolation in Lebesgue spaces with the weight class $QB_{β,\infty}$ has also been investigated. As an application, we characterize the boundedness of the Hardy averaging operator for quasi monotone functions in the grand Lebesgue spaces.

math.FA↗

arXiv2022

Time-resolved spectroscopy on the heartbeat state of GRS 1915+105 using AstroSat

AstroSat spectra of the black hole system GRS 1915+105 during the heartbeat state (with a varying oscillation period from 150 to 100 secs) were analysed using a truncated relativistic disc model along with a Comptonization component. Spectra were fitted for segments of length ~ 24 secs. The oscillation can be described as coordinated variations of the accretion rate, Comptonised flux, and the inner disc radius, with the latter ranging from 1.235-5 gravitational radii. Comparison with results from the $χ$ and Intermediate states shows that while the accretion rate and the high energy photon index were similar, the inner disc radius and the fraction of Comptonised photons were larger for these states than for the heartbeat one. The coronal efficiency $η\equiv L_{ac}/\dot M c^2$, where $L_{ac}$ is the radiative luminosity generated in the corona is found to be approximately $\propto \dot M^{-2/3}$ for all the observations. The efficiency decreases with inner radii for the heartbeat state but has similar values for the $χ$ and Intermediate states where the inner radii is larger. The implications of these results are discussed.

astro-ph.HE↗

arXiv2021

Superhorizon perturbations: A possible explanation of the Hubble--Lemaître Tension and the Large Scale Anisotropy of the Universe

Current cosmological observations point to a serious discrepancy between the observed Hubble parameter obtained using direct and cosmic microwave background radiation (CMBR) measurements. Besides this, the so called Hubble--Lemaître tension, we also find considerable evidence in diverse cosmological observables that indicate violation of the cosmological principle. In this paper, we suggest that both these discrepancies are related and can be explained by invoking superhorizon perturbations in the Universe. We implement this by considering a single superhorizon mode and showing that it leads to both a dipole in large scale structures and a shift in the Hubble--Lemaître parameter. Furthermore, the shift is found to be independent of redshift up to a certain distance. This is nicely consistent with the data.

astro-ph.CO↗

arXiv2021

Explaining Excess Dipole in NVSS Data Using Superhorizon Perturbation

Many observations in recent times have shown evidence against the standard assumption of isotropy in the Big Bang model. Introducing a superhorizon scalar metric perturbation has been able to explain some of these anomalies. In this work, we probe the net velocity arising due to the perturbation. We find that this extra component does not contribute to the CMB dipole amplitude while it does contribute to the dipole in large scale structures. Thus, within this model's framework, our velocity with respect to the large scale structure is not the same as that extracted from the CMB dipole, assuming it to be of purely kinematic origin. Taking this extra velocity component into account, we study the superhorizon mode's implications for the excess dipole observed in the NRAO VLA Sky Survey (NVSS). We find that the mode can consistently explain both the CMB and NVSS observations. We also find that the model leads to small contributions to the local bulk flow and the dipole in Hubble parameter, which are consistent with observations. The model leads to several predictions which can be tested in future surveys. In particular, it implies that the observed dipole in large scale structure should be redshift dependent and should show an increase in amplitude with redshift. We also find that the Hubble parameter should show a dipole anisotropy whose amplitude must increase with redshift in the CMB frame. Similar anisotropic behaviour is expected for the observed redshift as a function of the luminosity distance.

astro-ph.CO↗

arXiv2021

Testing evolution of LFQPOs with mass accretion rate in GRS 1915+105 with Insight-HXMT

Using the Insight-HXMT observations of GRS 1915+105 when it exhibits low frequency quasi-periodic oscillations (QPOs), we measure the evolution of the QPO frequency along with disk inner radius and mass accretion rate. We find a tight positive correlation between the QPO frequency and mass accretion rate. Our results extend the finding of previous work with AstroSat to a larger range of accretion rate with independent instruments and observations. Treating the QPO frequency of GRS 1915+105 as the relativistic dynamic frequency of a truncated disk, we are able to confirm the high spin nature of the black hole in GRS 1915+105. We also address the potential of our finding to test general relativity in the future.

astro-ph.HE↗

arXiv2020

Alignment of Radio Galaxy Axes using FIRST Catalogue

We study the alignment of radio galaxies axes using the FIRST catalogue. we impose several cuts in order to select the candidates which are most likely to be free of systematic bias. In our study we primarily focus on testing for alignment among sources within a certain angular separation from one another since for most sources redshift information is not available. We find a very significant effect for angular distances less than 1 degrees. The distance scale of alignment is found to be roughly 28 Mpc, in agreement with earlier estimates, assuming that these sources are dominantly at redshift of 0.8. However we are not able to entirely rule out the possibility of systematic bias in data. We also perform a full three dimensional analysis using a smaller data sample for which redshift information is available. In this case we only find a very weak signal at much larger distances.

astro-ph.CO↗