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Sushovan Mondal

Publications and source records attributed to Sushovan Mondal.

9 recordsLinked to original sources

Profile Reconstruction from Temporally Stable Emission Components for Timing PSR J1713+0747

The assumption of long-term pulse-profile stability underpins high-precision pulsar timing and forms the basis of pulsar timing array experiments. However, several millisecond pulsars exhibit temporal profile variability that can introduce systematic biases in pulse time of arrival measurements and compromise timing precision. We present a profile-domain analysis of PSR J1713+0747 at low radio frequencies, in the 300-500 MHz band, using upgraded GMRT observations for the Indian Pulsar Timing Array experiment. We model frequency-resolved pulse profiles using a Bayesian Gaussian decomposition framework in which individual Gaussian components are associated with persistent emission regions through informative phase priors that permit modest temporal variations. By tracking the evolution of the decomposed components across observing epochs and frequency sub-bands, we identify central Gaussian components that remain precisely localized despite changes in the integrated pulse morphology. We then reconstruct pulse profiles with realistic noise using these central components and perform timing analysis. Our approach provides a physically motivated framework for mitigating pulse-profile variability and offers a generic methodology for recovering robust timing information from pulsars exhibiting profile evolution.

astro-ph.HE

The Indian Pulsar Timing Array Data Release 2: III. Search for a Stochastic Gravitational Wave Background

We present the first independent search for an isotropic stochastic gravitational wave background in the second data release of the Indian Pulsar Timing Array, comprising of 27 millisecond pulsars monitored simultaneously in two frequency bands with the upgraded Giant Metrewave Radio Telescope over a maximum 7.2 year baseline. Building on a comprehensive single pulsar noise analysis, we search for a common uncorrelated red noise process within a Bayesian inference framework and with the noise-marginalized optimal statistics, and we test the robustness of the result through per-pulsar dropout analyses and solar-wind exclusion cuts. Leaving the spectral index free, we recover a broad amplitude posterior, $\log_{10} A_{\rm CURN} = -13.71^{+1.06}_{-3.28}$, with an unconstrained spectral index $γ_{\rm CURN} = 2.98^{+3.62}_{-2.70}$ and a Savage-Dickey Bayes factor of $2.5$ for a common red process over the no signal model. The optimal-statistic signal to noise ratios for the monopole, dipole, and Hellings-Downs correlations are all consistent with zero. Fixing the spectral index to $γ= 13/3$, the value predicted by an idealized toy model in which the background is sourced by a population of supermassive black hole binaries in circular orbits evolving purely under leading-order gravitational radiation reaction, we place a $95\%$ upper limit on the common-process amplitude of $A_{\rm GWB} < 3.4\times10^{-14}$, stable across solar elongation cuts of $10^\circ$, $20^\circ$, and $30^\circ$. This limit lies approximately an order of magnitude above the amplitudes reported by other, longer-running pulsar timing array experiments. We also demonstrate through simulated datasets with the addition of simple chromatic and achromatic noise components that it will take at least a 10 year baseline to start recovering the common red noise signal.

astro-ph.HE

Axial $w$-modes of anisotropic neutron stars

We investigate the axial $w$-mode oscillations of anisotropic neutron stars. Stellar configurations are constructed using two realistic equations of state, BSk21 and SLy4, with two prescriptions for pressure anisotropy, the Horvat ansatz and the Bowers-Liang ansatz. The axial $w$-mode frequencies are computed by solving the linearized perturbation equations using a continued-fraction method. For each fixed anisotropy strength, the axial $w$-mode frequency decreases monotonically with increasing stellar mass along the stable branch, with its magnitude depending on both the equation of state and the nature of the anisotropy. At low stellar masses, configurations with dominant radial pressure ($p_r>p_t$) exhibit higher frequencies than those with dominant tangential pressure, whereas toward the upper end of the stable branch this ordering is reversed, and configurations with $p_t>p_r$ attain higher frequencies at the same mass. The axial $w$-mode frequency displays an approximately linear dependence on compactness, with anisotropy modifying both the slope and the intercept. The Bowers-Liang ansatz produces a wider spread in the frequency values compared to the Horvat ansatz. We also analyze the damping times associated with the axial $w$-modes and find that they increase with stellar mass, with a rapid rise toward the upper end of the stable branch. At a fixed mass, increasing the tangential pressure relative to the radial pressure leads to shorter damping times, while configurations with dominant radial pressure exhibit longer damping times. The sensitivity of the damping time to anisotropy is more pronounced for more compact stars, and the Bowers-Liang ansatz yields systematically larger damping times than the Horvat ansatz. Finally, we provide empirical expressions for the axial $w$-mode frequency and damping time as functions of stellar compactness and anisotropy strength.

gr-qc

Revisiting wideband pulsar timing measurements

In the wideband paradigm of pulsar timing, the time of arrival of a pulsar pulse is measured simultaneously with the corresponding dispersion measure from a frequency-resolved integrated pulse profile. We present a new method for performing wideband measurements that rigorously accounts for measurement noise. We demonstrate this method using observations of PSR J2124$-$3358 made as part of the Indian Pulsar Timing Array experiment using the upgraded Giant Metre-wave Radio Telescope, and show that our method produces more realistic measurement uncertainty estimates compared to the existing wideband measurement method.

astro-ph.IM

Effects of Coronal Mass Ejection on PSR J1022+1001 and Possible Mode Change of PSR J2145-0750 in the InPTA DR2

The Indian Pulsar Timing Array (InPTA) has recently published its second data release (DR2), comprising the timing analysis of seven years of data on 27 millisecond pulsars (MSPs), observed simultaneously in the 300-500 MHz (band 3) and 1260-1460 MHz (band 5), using the upgraded Giant Metrewave Radio Telescope (uGMRT). The low-frequency data, particularly in band 3, is highly sensitive to propagation effects such as dispersion measure (DM) fluctuations, which can be imprints of some astrophysical phenomena (scientific outliers). Here, we analyze the two outliers of possible astrophysical origin coming from the band 3 DM time series of two pulsars: PSR J1022+1001, with an ecliptic latitude of -0.06 degree, and PSR J2145-0750, one of the brightest MSPs, with multi-component profile morphology. Our study reveals compelling evidence for a coronal mass ejection (CME) event traced in the data of PSR J1022+1001, and reports evidence for a potential mode-changing event in PSR J2145-0750. By contrasting these two cases, we show that DM fluctuations due to CME interacions and intrinsic mode-changing events produce distinct observational signatures, enabling a physically informed classification of scientific outliers in PTA datasets. Extending the analyses presented here to the full sample of InPTA-DR2 pulsars is expected to reveal additional CME events, and possible mode-changing events. Such detections will not only improve our understanding of solar and pulsar magnetospheric plasma interactions but will also enable more accurate modelling of DM variations, leading to improved pulsar timing solutions, which are crucial for high-precision Pulsar Timing Array (PTA) science.

astro-ph.HE

The Indian Pulsar Timing Array Data Release 2: I. Dataset and Timing Analysis

The Indian Pulsar Timing Array (InPTA) employs unique features of the upgraded Giant Metrewave Radio Telescope (uGMRT) to monitor dozens of the International Pulsar Timing Array (IPTA) millisecond pulsars (MSPs), simultaneously in the 300-500 MHz and the 1260-1460 MHz bands. This dual-band approach ensures that any frequency-dependent delays are accurately characterized, significantly improving the timing precision for pulsar observations, which is crucial for pulsar timing arrays. We present details of InPTA's second data release that involves 7 yrs of data on 27 IPTA MSPs. This includes sub-banded Times of Arrival (ToAs), Dispersion Measures (DM), and initial timing ephemerides for our MSPs. A part of this dataset, originally released in InPTA's first data release, is being incorporated into IPTA's third data release which is expected to detect and characterize nanohertz gravitational waves in the coming years. The entire dataset is reprocessed in this second data release providing some of the highest precision DM estimates so far and interesting solar wind related DM variations in some pulsars. This is likely to characterize the noise introduced by the dynamic inter-stellar ionised medium much better than the previous release thereby increasing sensitivity to any future gravitational wave search.

astro-ph.IM

Quasi-normal f-modes of anisotropic quark stars in full general relativity

We investigate f-mode oscillations of anisotropic quark stars within the framework of full general relativity. We consider two different equations of state (EOSs), one is the MIT bag model EOS and the other is EOS-A. Our study examines the impact of the pressure anisotropy on the equilibrium structure, as well as on the frequencies and damping times of f-mode oscillations. Our results confirm that the f-mode frequency scales linearly with the square root of the average density, with anisotropy influencing both the slope and intercept of this relation. The dependence of the f-mode frequency on total mass reveals distinct trends based on the relative dominance of tangential and radial pressure. When the tangential pressure exceeds the radial pressure, the frequency increases with mass, exhibiting rapid growth for massive quark stars. When the radial pressure dominates, the frequency increases with mass; however, in cases where the radial pressure is significantly greater than the tangential pressure, the frequency decreases as mass increases. For low-mass quark stars, stronger tangential pressure leads to an increase in frequency, while beyond a threshold mass, a further increase in tangential pressure results in a decrease in frequency. For the chosen range of anisotropic strengths, the frequency varies between 1.3 kHz and 2.3 kHz for the MIT bag EOS and between 1.8 kHz and 3.4 kHz for EOS-A. We find that the normalized damping time follows a linear trend with compactness. For a fixed stellar mass, an increase in tangential pressure relative to radial pressure reduces the damping time, whereas a decrease in tangential pressure significantly increases it. The damping time ranges from 83 ms to 900 ms for the MIT bag EOS and from 60 ms to 761 ms for EOS-A. We present semi-empirical expressions for both the frequency and damping time as functions of mass, radius, and anisotropic strength.

gr-qc

f-mode oscillations of anisotropic neutron stars in full general relativity

We investigate f-mode oscillations of static anisotropic stable neutron stars within the framework of full general relativity. We present equations governing unperturbed stellar structures and oscillations with an ansatz to account for the anisotropy. We solve those equations for two different equations of states. We see that, moderately anisotropic neutron stars with the tangential pressure larger than the radial pressure can give more massive neutron stars than the isotropic or very anisotropic ones. We find that the frequency of the f-mode exhibits a linear relationship with the square root of the average density of the stars and the slope of the fit depends on the anisotropic strength. For any given value of the anisotropic strength, the frequency increases with the increase of the mass of the neutron star, linearly for lower masses, and rapidly at higher masses. However, this non-linear rise in the frequency with the mass is not prominent when the radial pressure is larger than the tangential pressure. For a fixed value of a small mass, higher anisotropy leads to a larger value of the frequency, but when the fixed mass is above a threshold value, higher anisotropy leads to a smaller value of the frequency. The nature of the variation in the frequency with the change in the anisotropic strength is similar for the two equations of state, but for a fixed mass and the same amount of the anisotropy, the softer equations of state gives higher frequency. We also find that the damping time of the f-mode oscillation decreases as the mass of the neutron star increases for all values of the anisotropic strength. For a fixed mass of the neutron star and for the same amount of the anisotropy, the value of the damping time is lower for the softer equation of state, but the nature of the variation in the damping time with the change in the anisotropic strength is similar.

gr-qc

Propagation of gravitational waves in various cosmological backgrounds

The present work investigates some exact solutions of the gravitational wave equation in some widely used cosmological spacetimes. The examples are taken from spatially flat and closed isotropic models as well as Kasner metric which is anisotropic. Various matter distributions are considered, from the standard dust or radiation distribution to exotic matters like that with an equation of state $P = - \frac{1}{3} ρ$. In almost all cases the frequency and amplitude of the wave are found to be decaying with evolution, except for a closed radiation universe where there is a resurgence of the waveform, consistent with the recollapse of the universe into the big crunch singularity.

gr-qc