arXiv subjects

Dhruv Pathak

Publications and source records attributed to Dhruv Pathak.

15 recordsLinked to original sources

arXiv2025

A White Paper on The Multi-Messenger Science Landscape in India

The multi-messenger science using different observational windows to the Universe such as Gravitational Waves (GWs), Electromagnetic Waves (EMs), Cosmic Rays (CRs), and Neutrinos offer an opportunity to study from the scale of a neutron star to cosmological scales over a large cosmic time. At the smallest scales, we can explore the structure of the neutron star and the different energetics involved in the transition of a pre-merger neutron star to a post-merger neutron star. This will open up a window to study the properties of matter in extreme conditions and a guaranteed discovery space. On the other hand, at the largest cosmological scales, multi-messenger observations allow us to study the long-standing problems in physical cosmology related to the Hubble constant, dark matter, and dark energy by mapping the expansion history of the Universe using GW sources. Moreover, the multi-messenger studies of astrophysical systems such as white dwarfs, neutron stars, and black holes of different masses, all the way up to a high redshift Universe, will bring insightful understanding into the physical processes associated with them that are inaccessible otherwise. This white paper discusses the key cases in the domain of multi-messenger astronomy and the role of observatories in India which can explore uncharted territories and open discovery spaces in different branches of physics ranging from nuclear physics to astrophysics.

astro-ph.HE↗

arXiv2024

Can the second time-derivative of the orbital frequency of binary pulsars be used for testing general relativity?

With precision pulsar timing, measured values of a large set of pulsar parameters are obtainable. For some of those parameters, such as the time-derivatives of spin or orbital periods (in the case of binary pulsars), the measured values are not the intrinsic values of the parameters as they contain contributions from the dynamical effects. In the case of orbital period derivatives, the intrinsic values are essentially the general relativistic results. Pulsar timing solution also provides measurement of higher time-derivatives of orbital frequency for some pulsars. We specifically focus on the second time-derivative of the orbital frequency to explore its application in testing general relativity. In this work, we have provided a formalism to estimate the general relativistic contribution to the second derivative of the orbital frequency. We have calculated the dynamical effect contributions as well as the general relativistic contributions to the second time-derivative of the orbital period for real as well as synthetic pulsars. We find that the general relativistic contribution to the second time-derivative of the orbital period is negligibly small compared to the observed values of the real pulsars.

gr-qc↗

arXiv2023

Cost of inferred nuclear parameters towards the f-mode dynamical tide in binary neutron stars

Gravitational Wave (GW) observations from Neutron Stars (NS) in a binary system provide an excellent scenario to constrain the nuclear parameters. The investigation of Pratten et al. (2022) has shown that the ignorance of f-mode dynamical tidal correction in the GW waveform model of the binary neutron star (BNS) system can lead to substantial bias in the measurement of NS properties and NS equations of state (EOS). In this work, we investigate the bias in the nuclear parameters resulting from the ignorance of dynamical tidal correction. In addition, this work demonstrates the sensitivity of the nuclear parameters and the estimated constraints on them from future GW observations. We infer the nuclear parameters from GW observations by describing the NS matter within the relativistic mean field model. For a population of GW events, we notice that the ignorance of dynamical tide predicts a lower median for nucleon effective mass ($m^*$) by $\sim6\%$ compared to the scenario when dynamical tidal correction is considered. Whereas at a 90\% credible interval(CI), $m^*$ gets constrained up to $\sim 5\%$ and $\sim 3\%$ in A+ (the LIGO-Virgo detectors with a sensitivity of 5th observing run) and Cosmic Explorer (CE) respectively. We also discuss the resulting constraints on all other nuclear parameters, including compressibility, symmetry energy, and slope of symmetry energy, considering an ensemble of GW events. We do not notice any significant impact in analyzing nuclear parameters other than $m^*$ due to the ignorance of f-mode dynamical tides.

gr-qc↗

arXiv2023

Constraining nuclear parameters using Gravitational waves from f-mode Oscillations in Neutron Stars

Gravitational waves (GW) emanating from unstable quasi-normal modes in Neutron Stars (NS) could be accessible with the improved sensitivity of the current GW detectors or with the next-generation GW detectors and, therefore, can be employed to study the NS interior. Assuming f-mode excitation in isolated pulsars with typical energy of pulsar glitches and considering potential f-mode GW candidates for A+ (upgraded LIGO detectors operating at 5th observation run design sensitivity) and Einstein Telescope (ET), we demonstrate the inverse problem of NS asteroseismology within a Bayesian formalism to constrain the nuclear parameters and NS Equation of State (EOS). We describe the NS interior within relativistic mean field formalism. Taking the example of glitching pulsars, we find that for a single event in A+ and ET, among the nuclear parameters, the nucleon effective mass ($m^*$) within 90\% credible interval (CI) can be restricted within $10\%$ and $5\%$, respectively. At the same time, the incompressibility ($K$) and the slope of the symmetry energy ($L$) are only loosely constrained. Considering multiple (10) events in A+ and ET, all the nuclear parameters are well constrained, especially $m^*$, which can be constrained to 3\% and 2\% in A+ and ET, respectively. Uncertainty in the observables of a $1.4M_{\odot}$ NS such as radius ($R_{1.4M_{\odot}}$), f-mode frequency ($f_{1.4M_{\odot}}$), damping time ($\tau_{1.4M_{\odot}}$) and a few EOS properties including squared speed of sound ($c_s^2$) are also estimated.

astro-ph.HE↗

arXiv2022

Improving the spin-down limits of the continuous gravitational waves emitted from rotating triaxial pulsars

The spin-down limit of the continuous gravitational wave strain from pulsars assumed to be triaxial stars rotating about a principal moment of inertia axis depends upon the value of the intrinsic spin frequency derivative of the pulsar, among other parameters. In order to get more accurate intrinsic spin frequency derivative values, dynamical effects contributing to the measured spin frequency derivative values must be estimated via more realistic approaches. In this work, we calculated improved values for the spin-down limit of the continuous gravitational wave strain (assuming that pulsars are triaxial stars rotating about a principal moment of inertia axis) for a set of 237 pulsars for which a targeted search for continuous gravitational waves was recently carried out by the LIGO-Virgo-KAGRA (LVK) Collaboration. We used `GalDynPsr', a Python-based public package, to calculate more realistic values for the intrinsic spin frequency derivatives and, consequently, we get more realistic values of the spin-down limit. The realistic values that we obtain for the intrinsic spin frequency derivatives can also provide a valuable contribution to improving the sensitivity of searches for continuous gravitational waves from known pulsars.

astro-ph.HE↗

arXiv2022

Relativistic correction to the r-mode frequency in light of multi-messenger constraints

R-mode oscillations of rotating neutron stars are promising candidates for continuous gravitational wave (GW) observations. The r-mode frequencies for slowly rotating Newtonian stars are well-known and independent of the equation of state (EOS) but for neutron stars, several mechanisms can alter the r-mode frequency of which the relativistic correction is dominant and relevant for most of the neutron stars. The most sensitive searches for continuous GWs are those for known pulsars for which GW frequencies are in targeted narrow frequency bands of few Hz. In this study, we investigate the effect of several state-of-the-art multi-messenger constraints on the r-mode frequency for relativistic, slowly rotating, barotropic stars. Imposing these recent constraints on the EOS, we find that the r-mode frequency range is slightly higher from the previous study and the narrow band frequency range can increase upto 8-25% for the most promising candidate PSR J0537-6910 depending on the range of compactness. We also derive universal relations between r-mode frequency and dimensionless tidal deformability which can be used to estimate the dynamical tide of the r-mode resonant excitation during the inspiral signal. These results can be used to construct the parameter space for r-mode searches in gravitational wave data and also constrain the nuclear equation of state following a successful r-mode detection.

gr-qc↗

arXiv2022

Nanohertz Gravitational Wave Astronomy during the SKA Era: An InPTA perspective

Decades long monitoring of millisecond pulsars, which exhibit highly stable rotational periods, in pulsar timing array experiments is on the threshold of discovering nanohertz stochastic gravitational wave background. This paper describes the Indian Pulsar timing array (InPTA) experiment, which employs the upgraded Giant Metrewave Radio Telescope (uGMRT) for timing an ensemble of millisecond pulsars for this purpose. We highlight InPTA's observation strategies and analysis methods, which are relevant for a future PTA experiment with the more sensitive Square Kilometer Array (SKA) telescope. We show that the unique multi-sub-array multi-band wide-bandwidth frequency coverage of the InPTA provides Dispersion Measure estimates with unprecedented precision for PTA pulsars, e.g., ~ 2 x 10{-5} pc-cm{-3} for PSR J1909-3744. Configuring the SKA-low and SKA-mid as two and four sub-arrays respectively, it is shown that comparable precision is achievable, using observation strategies similar to those pursued by the InPTA, for a larger sample of 62 pulsars requiring about 26 and 7 hours per epoch for the SKA-mid and the SKA-low telescopes respectively. We also review the ongoing efforts to develop PTA-relevant general relativistic constructs that will be required to search for nanohertz gravitational waves from isolated super-massive black hole binary systems like blazar OJ 287. These efforts should be relevant to pursue persistent multi-messenger gravitational wave astronomy during the forthcoming era of the SKA telescope, the Thirty Meter Telescope, and the next-generation Event Horizon Telescope.

astro-ph.HE↗

arXiv2022

The Indian Pulsar Timing Array: First data release

We present the pulse arrival times and high-precision dispersion measure estimates for 14 millisecond pulsars observed simultaneously in the 300-500 MHz and 1260-1460 MHz frequency bands using the upgraded Giant Metrewave Radio Telescope (uGMRT). The data spans over a baseline of 3.5 years (2018-2021), and is the first official data release made available by the Indian Pulsar Timing Array collaboration. This data release presents a unique opportunity for investigating the interstellar medium effects at low radio frequencies and their impact on the timing precision of pulsar timing array experiments. In addition to the dispersion measure time series and pulse arrival times obtained using both narrowband and wideband timing techniques, we also present the dispersion measure structure function analysis for selected pulsars. Our ongoing investigations regarding the frequency dependence of dispersion measures have been discussed. Based on the preliminary analysis for five millisecond pulsars, we do not find any conclusive evidence of chromaticity in dispersion measures. Data from regular simultaneous two-frequency observations are presented for the first time in this work. This distinctive feature leads us to the highest precision dispersion measure estimates obtained so far for a subset of our sample. Simultaneous multi-band uGMRT observations in Band 3 and Band 5 are crucial for high-precision dispersion measure estimation and for the prospect of expanding the overall frequency coverage upon the combination of data from the various Pulsar Timing Array consortia in the near future. Parts of the data presented in this work are expected to be incorporated into the upcoming third data release of the International Pulsar Timing Array.

astro-ph.IM↗

arXiv2021

Low-frequency wideband timing of InPTA pulsars observed with the uGMRT

High-precision measurements of the pulsar dispersion measure (DM) are possible using telescopes with low-frequency wideband receivers. We present an initial study of the application of the wideband timing technique, which can simultaneously measure the pulsar times of arrival (ToAs) and DMs, for a set of five pulsars observed with the upgraded Giant Metrewave Radio Telescope (uGMRT) as part of the Indian Pulsar Timing Array (InPTA) campaign. We have used the observations with the 300-500 MHz band of the uGMRT for this purpose. We obtain high precision in DM measurements with precisions of the order 10^{-6}cm^{-3}pc. The ToAs obtained have sub-{\mu}s precision and the root-mean-square of the post-fit ToA residuals are in the sub-{\mu}s range. We find that the uncertainties in the DMs and ToAs obtained with this wideband technique, applied to low-frequency data, are consistent with the results obtained with traditional pulsar timing techniques and comparable to high-frequency results from other PTAs. This work opens up an interesting possibility of using low-frequency wideband observations for precision pulsar timing and gravitational wave detection with similar precision as high-frequency observations used conventionally.

astro-ph.IM↗

arXiv2021

Evidence for profile changes in PSR J1713+0747 using the uGMRT

PSR J1713+0747 is one of the most precisely timed pulsars in the international pulsar timing array experiment. This pulsar showed an abrupt profile shape change between April 16, 2021 (MJD 59320) and April 17, 2021 (MJD 59321). In this paper, we report the results from multi-frequency observations of this pulsar carried out with the upgraded Giant Metrewave Radio Telescope (uGMRT) before and after the event. We demonstrate the profile change seen in Band 5 (1260 MHz - 1460 MHz) and Band 3 (300 MHz - 500 MHz). The timing analysis of this pulsar shows a disturbance accompanying this profile change followed by a recovery with a timescale of $\sim 159$ days. Our data suggest that a model with chromatic index as a free parameter is preferred over models with combinations of achromaticity with DM bump or scattering bump. We determine the frequency dependence to be $\sim\nu^{+1.34}$.

astro-ph.HE↗

arXiv2021

High Precision Measurements of Interstellar Dispersion Measure with the upgraded GMRT

Pulsar radio emission undergoes dispersion due to the presence of free electrons in the interstellar medium (ISM). The dispersive delay in the arrival time of pulsar signal changes over time due to the varying ISM electron column density along the line of sight. Correcting for this delay accurately is crucial for the detection of nanohertz gravitational waves using Pulsar Timing Arrays. In this work, we present in-band and inter-band DM estimates of four pulsars observed with uGMRT over the timescale of a year using two different template alignment methods. The DMs obtained using both these methods show only subtle differences for PSR 1713+0747 and J1909$-$3744. A considerable offset is seen in the DM of PSR J1939+2134 and J2145$-$0750 between the two methods. This could be due to the presence of scattering in the former and profile evolution in the latter. We find that both methods are useful but could have a systematic offset between the DMs obtained. Irrespective of the template alignment methods followed, the precision on the DMs obtained is about $10^{-3}$ pc cm$^{-3}$ using only BAND3 and $10^{-4}$ pc cm$^{-3}$ after combining data from BAND3 and BAND5 of the uGMRT. In a particular result, we have detected a DM excess of about $5\times10^{-3}$ pc cm$^{-3}$ on 24 February 2019 for PSR J2145$-$0750. This excess appears to be due to the interaction region created by fast solar wind from a coronal hole and a coronal mass ejection (CME) observed from the Sun on that epoch. A detailed analysis of this interesting event is presented.

astro-ph.HE↗

arXiv2020

pinta: The uGMRT Data Processing Pipeline for the Indian Pulsar Timing Array

We introduce pinta, a pipeline for reducing the upgraded Giant Metre-wave Radio Telescope (uGMRT) raw pulsar timing data, developed for the Indian Pulsar Timing Array experiment. We provide a detailed description of the workflow and usage of pinta, as well as its computational performance and RFI mitigation characteristics. We also discuss a novel and independent determination of the relative time offsets between the different back-end modes of uGMRT and the interpretation of the uGMRT observation frequency settings, and their agreement with results obtained from engineering tests. Further, we demonstrate the capability of pinta to generate data products which can produce high-precision TOAs using PSR J1909-3744 as an example. These results are crucial for performing precision pulsar timing with the uGMRT.

astro-ph.IM↗

arXiv2019

A study of dynamical effects in the observed second time-derivative of the spin or orbital frequencies of pulsars

The observed values of the time-derivatives of the spin or orbital frequency of pulsars are affected by their dynamical properties. We derive thorough analytical expressions for such dynamical contributions in terms of the Galactic coordinates, the proper motion, the pulsar distance, and the radial velocity. We find that the effects of the dynamical terms in the second-derivative of frequencies or parameters based on such second derivatives, e.g., braking index, are usually negligible. However, unique pulsars for which the effects of the dynamical terms are significant can exist. In particular, dynamical effects can make the magnitude of the observed value of the braking index to be in the order of thousand while the true value of it is close to the theoretically expected value three, especially if the pulsars lie close to the Galactic centre. Dynamics can also affect the value of the second derivative of the orbital frequency of a binary pulsar at the first decimal place. We also emphasize the fact that our expressions provide more accurate results than pre-existing approximate ones that exclude some of the terms. Comparison with a set of pulsars showed that the median value of the difference between the results obtained by our method and a pre-existing method is about 50 percent.

astro-ph.HE↗

arXiv2019

Correcting the measured values of the rate of change of the spin and orbital periods of rotation powered pulsars

For rotation powered pulsars, the rate of change of the spin period is expected to be positive. On the other hand, for a clean binary system (where gravity is the only force acting on the pulsar) the rate of change of the orbital period is expected to be negative. There are, however, some pulsars with the measurements of the rate of change of the spin period as negative quantities. Likewise, there are some pulsars with positive measurements of the rate of change of the orbital period. We investigate these cases by eliminating the external dynamical effects. We also look at those cases where at first, the measured values possess the correct sign, but on subtracting the dynamical effects, the sign changes. We use `GalDynPsr' package to perform these tasks. Moreover, we investigate possible reasons for such anomalies.

astro-ph.HE↗

arXiv2017

Dynamical effects in the observed rate of change of the orbital and the spin periods of radio pulsars: Improvement in the method of estimation and its implications

The observed values of the rate of change of the orbital and the spin periods of pulsars are affected by different dynamical effects, for example, the line-of-sight acceleration and the proper motion of the pulsar relative to the sun. We explore these dynamical effects thoroughly and point out the drawbacks of popular methods. We introduce a package, `GalDynPsr', that evaluates different dynamical effects following traditional as well as improved methods based on the model of the Galactic potential provided in a publicly available package called `galpy'. We argue that the improved methods introduced in this paper should be used for pulsars located 1 kpc or farther away from the solar system, especially when precise values of the rate of change of the periods are required, e.g., while placing limits on alternative theories of gravity, calculating the spin-down limit of the continuous gravitational waves emitted from a rotationally deformed neutron star, understanding pulsar `death-line', etc. GalDynPsr is available online and open for contributions.

astro-ph.IM↗