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In this work, we explore the impact of higher dimensional spacetime on the stellar structure and thermodynamic properties of neutron stars. Utilizing the density-dependent relativistic hadron field theory, we introduce modifications to incorporate the influence of higher dimensionality, a novel approach not explored in existing literature to our best knowledge. Our methodology involves solving the essential stellar structure equations in D-dimensional spacetime ($D \geq 4$), starting with the modification of the Einstein-Hilbert action, derivation of the Einstein field equation in D dimensions, and application of the resulting exterior Schwarzschild spacetime metric for D-dimension. Our findings reveal that with incremental dimensions, the central density $\rho_{c} G_D$ and central pressure $p_c G_D$ gradually increase, leading to progressively stiffer neutron matter. Incremental dimensionality also results in a gradual increase in the maximum mass attained, limited to our study between $D=4$ and $D=6$, as no maximum mass value is obtained for $D>6$. We consistently observe the criteria $dM/d\rho_c>0$ fulfilled up to the maximum mass point, supported by stability analysis against infinitesimal radial pulsations. The validity of our solution is confirmed through causality conditions, ensuring that the matter sound speed remains within the speed of light for all cases. Additionally, our examination indicates that the total mass-to-radius ratio for all discussed D-dimensional cases comfortably resides within the modified Buchdahl limit, which exhibits the physical validity of achieved results., Comment: 12 pages, 7 figures, 3 tables

High-precision pulsar timing observations are limited in their accuracy by the jitter noise that appears in the arrival time of pulses. Therefore, it is important to systematically characterise the amplitude of the jitter noise and its variation with frequency. In this paper, we provide jitter measurements from low-frequency wideband observations of PSR J0437$-$4715 using data obtained as part of the Indian Pulsar Timing Array experiment. We were able to detect jitter in both the 300 - 500 MHz and 1260 - 1460 MHz observations of the upgraded Giant Metrewave Radio Telescope (uGMRT). The former is the first jitter measurement for this pulsar below 700 MHz, and the latter is in good agreement with results from previous studies. In addition, at 300 - 500 MHz, we investigated the frequency dependence of the jitter by calculating the jitter for each sub-banded arrival time of pulses. We found that the jitter amplitude increases with frequency. This trend is opposite as compared to previous studies, indicating that there is a turnover at intermediate frequencies. It will be possible to investigate this in more detail with uGMRT observations at 550 - 750 MHz and future high sensitive wideband observations from next generation telescopes, such as the Square Kilometre Array. We also explored the effect of jitter on the high precision dispersion measure (DM) measurements derived from short duration observations. We find that even though the DM precision will be better at lower frequencies due to the smaller amplitude of jitter noise, it will limit the DM precision for high signal-to-noise observations, which are of short durations. This limitation can be overcome by integrating for a long enough duration optimised for a given pulsar., Comment: 13 pages, 12 figures, 3 tables, accepted for Publication of the Astronomical Society of Australia

A pulsar's pulse profile gets broadened at low frequencies due to dispersion along the line of sight or due to multi-path propagation. The dynamic nature of the interstellar medium makes both of these effects time-dependent and introduces slowly varying time delays in the measured times-of-arrival similar to those introduced by passing gravitational waves. In this article, we present an improved method to correct for such delays by obtaining unbiased dispersion measure (DM) measurements by using low-frequency estimates of the scattering parameters. We evaluate this method by comparing the obtained DM estimates with those, where scatter-broadening is ignored using simulated data. A bias is seen in the estimated DMs for simulated data with pulse-broadening with a larger variability for a data set with a variable frequency scaling index, $\alpha$, as compared to that assuming a Kolmogorov turbulence. Application of the proposed method removes this bias robustly for data with band averaged signal-to-noise ratio larger than 100. We report the measurements of the scatter-broadening time and $\alpha$ from analysis of PSR J1643$-$1224, observed with upgraded Giant Metrewave Radio Telescope as part of the Indian Pulsar Timing Array experiment. These scattering parameters were found to vary with epoch and $\alpha$ was different from that expected for Kolmogorov turbulence. Finally, we present the DM time-series after application of this technique to PSR J1643$-$1224., Comment: 10 pages, 8 figures, Accepted for publication in MNRAS

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., Comment: 19 pages, 14 figures, 1 table; abstract shortened as per arXiv requirement (see pdf version). Accepted for publication in PRD

The values of the bending delays in the signal of a radio pulsar in a binary with a stellar mass black hole as a companion have been calculated accurately within a full general relativistic framework considering the Schwarzchid spacetime near the companion. The results match with the pre-existing approximate analytical expressions unless both of the orbital inclination angle and the orbital phase are close to $90^{\circ}$. For such a case, the approximate analytical expressions underestimate the value of the bending delay. On the other hand, for systems like the double pulsar, those expressions are valid throughout the orbital phase, unless its inclination angle is very close to 90 degrees. For a pulsar-black hole binary, the bending phenomenon also increases the strength of the pulse profile and sometimes can lead to a small low intensity tail., Comment: 21 pages, 16 figures, 1 table. Accepted for publication in MNRAS

We present the discovery and timing solutions of four millisecond pulsars (MSPs) discovered in the Arecibo 327 MHz Drift-Scan Pulsar Survey. Three of these pulsars are in binary systems, consisting of a redback (PSR J2055+1545), a black widow (PSR J1630+3550), and a neutron star-white dwarf binary (PSR J2116+1345). The fourth MSP, PSR J2212+2450, is isolated. We present the multiyear timing solutions as well as polarization properties across a range of radio frequencies for each pulsar. We perform a multiwavelength search for emission from these systems and find an optical counterpart for PSR J2055+1545 in Gaia DR3, as well as a gamma-ray counterpart for PSR J2116+1345 with the Fermi-LAT telescope. Despite the close colocation of PSR J2055+1545 with a Fermi source, we are unable to detect gamma-ray pulsations, likely due to the large orbital variability of the system. This work presents the first two binaries found by this survey with orbital periods shorter than a day; we expect to find more in the 40% of the survey data that have yet to be searched., Comment: 22 pages, 11 figures, 4 tables. Published in ApJ

The wideband timing technique enables the high-precision simultaneous estimation of pulsar Times of Arrival (ToAs) and Dispersion Measures (DMs) while effectively modeling frequency-dependent profile evolution. We present two novel independent methods that extend the standard wideband technique to handle simultaneous multi-band pulsar data incorporating profile evolution over a larger frequency span to estimate DMs and ToAs with enhanced precision. We implement the wideband likelihood using the libstempo python interface to perform wideband timing in the tempo2 framework. We present the application of these techniques to the dataset of fourteen millisecond pulsars observed simultaneously in Band 3 (300 - 500 MHz) and Band 5 (1260 - 1460 MHz) of the upgraded Giant Metrewave Radio Telescope (uGMRT) with a large band gap of 760 MHz as a part of the Indian Pulsar Timing Array (InPTA) campaign. We achieve increased ToA and DM precision and sub-microsecond root mean square post-fit timing residuals by combining simultaneous multi-band pulsar observations done in non-contiguous bands for the first time using our novel techniques., Comment: Published in MNRAS

The Indian Pulsar Timing Array (InPTA) collaboration has recently made its first official data release (DR1) for a sample of 14 pulsars using 3.5 years of uGMRT observations. We present the results of single-pulsar noise analysis for each of these 14 pulsars using the InPTA DR1. For this purpose, we consider white noise, achromatic red noise, dispersion measure (DM) variations, and scattering variations in our analysis. We apply Bayesian model selection to obtain the preferred noise models among these for each pulsar. For PSR J1600$-$3053, we find no evidence of DM and scattering variations, while for PSR J1909$-$3744, we find no significant scattering variations. Properties vary dramatically among pulsars. For example, we find a strong chromatic noise with chromatic index $\sim$ 2.9 for PSR J1939+2134, indicating the possibility of a scattering index that doesn't agree with that expected for a Kolmogorov scattering medium consistent with similar results for millisecond pulsars in past studies. Despite the relatively short time baseline, the noise models broadly agree with the other PTAs and provide, at the same time, well-constrained DM and scattering variations., Comment: Accepted for publication in PRD, 30 pages, 17 figures, 4 tables

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., Comment: Accepted for publication in Journal of Astronomy and Astrophysics for Special Issue on Indian Participation in the SKA (Editors : Abhirup Datta, Nirupam Roy, Preeti Kharb and Tirthankar Roy Choudhury)

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., Comment: 23 pages, 21 figures, 3 tables. Published in PASA

We present the long-term spectro-temporal evolution of the average radio emission properties of the magnetar XTE J1810-197 (PSR J1809-1943) following its most recent outburst in late 2018. We report the results from two and a half years of monitoring campaign with the upgraded Giant Metrewave Radio Telescope carried out over the frequency range of 300 - 1450 MHz. Our observations show intriguing time variability in the average profile width, flux density, spectral index and the broadband spectral shape. While the average profile width appears to gradually decrease at later epochs, the flux density shows multiple episodes of radio re-brightening over the course of our monitoring. Our systematic monitoring observations reveal that the radio spectrum has steepened over time, resulting in evolution from a magnetar-like to a more pulsar-like spectrum. A more detailed analysis reveals that the radio spectrum has a turnover, and this turnover shifts towards lower frequencies with time. We present the details of our analysis leading to these results, and discuss our findings in the context of magnetar radio emission mechanisms as well as potential manifestations of the intervening medium. We also briefly discuss whether an evolving spectral turnover could be an ubiquitous property of radio magnetars., Comment: 12 pages, 6 figures, accepted for publication in ApJ

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., Comment: Accepted for publication in MNRAS

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 $\sim\nu^{+1.34}$., Comment: Accepted for publication in MNRAS-Letters

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., Comment: 11 pages, 6 figures, 2 tables. Accepted by A&A

We revisit the population of normal pulsars in the Galactic field in an `evolutionary' approach. By comparing the distributions of various parameters of synthetic pulsars detectable by the Parkes Multibeam Pulsar Survey, the Pulsar Arecibo L-band Feed Array Survey, and two Swinburne Multibeam surveys with those of the real pulsars detected by the same surveys, we find that a good and physically realistic model can be obtained by using a uniform distribution of the braking index in the range of 2.5 to 3.0, a uniform distribution of the cosine of the angle between the spin and the magnetic axis in the range of 0 to 1, a log-normal birth distribution of the surface magnetic field with the mean and the standard deviation as 12.85 and 0.55 respectively while keeping the distributions of other parameters unchanged from the ones most commonly used in the literature. We have also replaced the universal 'death-line' by a `death-condition' specific to each individual pulsar. We find that our model is better than the most popular model. With our improved model, we predict that an all-sky pulsar survey with phase-I SKA-MID will detect about nine thousand normal pulsars in the Galactic field. Among these pulsars, a considerable number will produce continuous gravitational waves in the operational range of the future ground-based gravitational waves detectors like LIGO A$+$, and in certain cases, the spin-down limit of the gravitational wave strains will be well below the detection sensitivity limit. We also provide a fit for the present-day distributions of the spin periods and 1400 MHz luminosities of the whole normal pulsar population in the Galactic field (which are potentially observable) and those can be used in future population studies under the snapshot approach., Comment: 13 pages, 10 figures, 5 tables

The relativistic equations of hydrostatic equilibrium for a spherically symmetric star, or the Tolman-Oppenheimer-Volkoff equations are known in higher dimensions. In this paper, these equations have been expressed in terms of parameters of 4 dimensional spacetime and solved numerically for 4, 5, 6, and 7 dimensions using a standard equation of state for the neutron star matter derived for the 4 dimensional spacetime. It has been shown that with the increase of the dimensionality, the maximum value of the mass of the neutron star decreases and the stars become less compact. Thus, although the compactness limit decreases with increased dimensionality, neutron stars never violate this limit. Simultaneous measurements of the mass, radius, and gravitational redshift for a neutron star might enable us to conclude about the central density, equation of state and the dimensionality of the spacetime in and around the star.

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.

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., Comment: This version, without appendix D, has been accepted for publication in New Astronomy

XTE J1810-197 (PSR J1809-1943) was the first ever magnetar which was found to emit transient radio emission. It has recently undergone another radio and high-energy outburst. This is only the second radio outburst that has been observed from this source. We observed J1810-197 soon after its recent radio outburst at low radio frequencies using the Giant Metrewave Radio Telescope. We present the 650 MHz flux density evolution of the source in the early phases of the outburst, and its radio spectrum down to frequencies as low as 300 MHz. The magnetar also exhibits radio emission in the form of strong, narrow bursts. We show that the bursts have a characteristic intrinsic width of the order of 0.5-0.7 ms, and discuss their properties in the context of giant pulses and giant micropulses from other pulsars. We also show that the bursts exhibit spectral structures which cannot be explained by interstellar propagation effects. These structures might indicate a phenomenological link with the repeating fast radio bursts which also show interesting, more detailed frequency structures. While the spectral structures are particularly noticeable in the early phases of the outburst, these seem to be less prominent as well as less frequent in the later phases, suggesting an evolution of the underlying cause of these spectral structures., Comment: A few minor changes in the previous version; consistent with the published article

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., Comment: 12 pages

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., Comment: Accepted for publication in ApJ

The Lense-Thirring effect from spinning neutron stars in double neutron star binaries contribute to the periastron advance of the orbit. This extra term involves the moment of inertia of the neutron stars. Moment of inertia, on the other hand, depends on the mass and spin of the neutron star as well as the equation of state of the matter. If at least one member of the double neutron star binary (better the faster one) is a radio pulsar, then accurate timing analysis might lead to the estimation of the contribution of the Lense-Thirring effect to the periastron advance, which will lead to the measurement of the moment of inertia of the pulsar. Combination of the knowledge on the values of the moment of inertia, the mass, and the spin of the pulsar, will give a new constraint on the equation of state. Pulsars in double neutron star binaries are the best for this purpose as short orbits and moderately high eccentricities make the Lense-Thirring effect substantial, whereas tidal effects are negligible (unlike pulsars with main sequence or white-dwarf binaries). The most promising pulsars are PSR J0737-3039A and PSR J1757-1854. The spin-precession of pulsars due to the misalignment between the spin and the orbital angular momentum vectors affect the contribution of the Lense-Thirring effect to the periastron advance. This effect has been explored for both PSR J0737-3039A and PSR J1757-1854, and as the misalignment angles for both of these pulsars are small, the variation in the Lense-Thirring term is not much. However, to extract the Lense-Thirring effect from the observed rate of the periastron advance, more accurate timing solutions including precise proper motion and distance measurements are essential., Comment: To appear in the special issue of `Universe' dedicated to the conference: "Compact Stars in the QCD Phase Diagram VI" held at The Joint Institute for Nuclear Research (JINR), Dubna, Russia; during 26 - 29 September, 2017

The model that fast radio bursts are caused by plunges of asteroids onto neutron stars can explain both repetitive and non-repetitive bursts. If a neutron star passes through an asteroid belt around another star, there would be a series of bursts caused by series of asteroid impacts. Moreover, the neutron star would cross the same belt repetitively if it is in a binary with the star hosting the asteroid belt, leading to a repeated series of bursts. I explore the properties of neutron star binaries which could lead to the only known repetitive fast radio burst so far (FRB121102). In this model, the next two epochs of bursts are expected around 27-February-2017 and 18-December-2017. On the other hand, if the asteroid belt is located around the neutron star itself, then a chance fall of an asteroid from that belt onto the neutron star would lead to a non-repetitive burst. Even a neutron star grazing an asteroid belt can lead to a non-repetitive burst caused by just one asteroid plunge during the grazing. This is possible even when the neutron star is in a binary with the asteroid hosting star, if the belt and the neutron star orbit are non-coplanar., Comment: 6 pages

It is an exceptionally opportune time for Astrophysics when a number of next-generation mega-instruments are poised to observe the universe across the entire electromagnetic spectrum with unprecedented data quality. The Square Kilometre Array (SKA) is undoubtedly one of the major components of this scenario. In particular, the SKA is expected to discover tens of thousands of new neutron stars giving a major fillip to a wide range of scientific investigations. India has a sizeable community of scientists working on different aspects of neutron star physics with immediate access to both the uGMRT (an SKA pathfinder) and the recently launched X-ray observatory Astrosat. The current interests of the community largely centre around studies of - a) the generation of neutron stars and the SNe connection}, b) the neutron star population and evolutionary pathways}, c) the evolution of neutron stars in binaries and the magnetic fields}, d) the neutron star equation of state}, e) the radio pulsar emission mechanism}, and, f) the radio pulsars as probes of gravitational physics}. Most of these studies are the main goals of the SKA first phase, which is likely to be operational in the next four years. This article summarises the science goals of the Indian neutron star community in the SKA era, with significant focus on coordinated efforts among the SKA and other existing/upcoming instruments., Comment: To appear in Journal of Astrophysics and Astronomy (JOAA) special issue on "Science with the SKA: an Indian perspective"

The Square Kilometre Array (SKA), when it becomes functional, is expected to enrich neutron star (NS) catalogues by at least an order of magnitude over their current state. This includes the discovery of new NS objects leading to better sampling of under-represented NS categories, precision measurements of intrinsic properties such as spin period and magnetic field, as also data on related phenomena such as microstructure, nulling, glitching, etc. This will present a unique opportunity to seek answers to interesting and fundamental questions about the extreme physics underlying these exotic objects in the universe. In this paper, we first present a meta-analysis (from a methodological viewpoint) of statistical analyses performed using existing NS data, with a two-fold goal: First, this should bring out how statistical models and methods are shaped and dictated by the science problem being addressed. Second, it is hoped that these analyses will provide useful starting points for deeper analyses involving richer data from SKA whenever it becomes available. We also describe a few other areas of NS science which we believe will benefit from SKA which are of interest to the Indian NS community., Comment: To appear in Journal of Astrophysics and Astronomy (JOAA) special issue on "Science with the SKA: an Indian perspective"

The Thirty Meter Telescope (TMT) International Observatory (TIO) will be a revolutionary leap forward in astronomical observing capabilities, enabling us to address some of the most profound questions about the universe. From unraveling the mysteries of dark matter and dark energy to exploring the origins of stars and planets, TMT will transform our understanding of the cosmos. The TIO Detailed Science Case (DSC) presents science goals that inform the top-level requirements for the observatory's design and operations, including the telescope, enclosure, instruments, and adaptive optics system., Comment: 275 pages. 2024 version. Updated from 2015 and 2022

Pulsars in binary systems have been very successful to test the validity of general relativity in the strong field regime. So far, such binaries include neutron star-white dwarf (NS-WD) and neutron star-neutron star (NS-NS) systems. It is commonly believed that a neutron star-black hole (NS-BH) binary will be much superior for this purpose. But in what sense is this true? Does it apply to all possible deviations?, Comment: Accepted for publication in JCAP. A shorter version received Honorable Mention in the 2014 Awards for Essays on Gravitation by the Gravity Research Foundation

Studies of binary pulsars provide insight into various theories of physics. Detection of such systems is challenging due to the Doppler modulation of the pulsed signal caused by the orbital motion of the pulsar. We investigated the loss of sensitivity in eccentric binary systems for different types of companions. This reduction of sensitivity should be considered in future population synthesis models for binary pulsars. This loss can be recovered partially by employing the `acceleration search' algorithm and even better by using the `acceleration-jerk search' algorithm., Comment: 2 pages. To appear in the proceeding of "The Metrewavelength Sky Conference" held at NCRA-TIFR, Pune, from December 9-13 2013, to mark 50 years of radio astronomy research at the Tata Institute of Fundamental Research (TIFR), as well as 10 years of operation of the Giant Metrewave Radio Telescope (GMRT) as an international observatory

Luminosity is an intrinsic property of radio pulsars related to the properties of the magnetospheric plasma and the beam geometry, and inversely proportional to the observing frequency. In traditional models, luminosity has been considered as a function of the spin parameters of pulsars. On the other hand, parameter independent models like power law and lognormal have been also used to fit the observed luminosities. Some of the older studies on pulsar luminosities neglected observational biases, but all of the recent studies tried to model observational effects as accurately as possible. Luminosities of pulsars in globular clusters and in the Galactic disk have been studied separately. Older studies concluded that these two categories of pulsars have different luminosity distributions, but the most recent study concluded that those are the same. This article reviews all significant works on pulsar luminosities and discusses open questions., Comment: Invited review in International Journal of Modern Physics D. 21 pages, 8 tables, 3 figures

By generalizing earlier work of Johnston & Kulkarni, we present a detailed description of the reduction in the signal-to-noise ratio for observations of binary pulsars. We present analytical expressions, and provide software, to calculate the sensitivity reduction for orbits of arbitrary eccentricity. We find that this reduction can be quite significant, especially in the case of a massive companion like another neutron star or a black hole. On the other hand, the reduction is less for highly eccentric orbits. We also demonstrate that this loss of sensitivity can be recovered by employing "acceleration search" or "acceleration-jerk search" algorithms., Comment: 18 pages, 17 figures, 6 tables; slightly shorter version accepted for publication in MNRAS

The luminosity distribution of Galactic radio pulsars is believed to be log-normal in form. Applying this functional form to populations of pulsars in globular clusters, we employ Bayesian methods to explore constraints on the mean and standard deviation of the function, as well as the total number of pulsars in the cluster. Our analysis is based on an observed number of pulsars down to some limiting flux density, measurements of flux densities of individual pulsars, as well as diffuse emission from the direction of the cluster. We apply our analysis to Terzan 5 and demonstrate, under reasonable assumptions, that the number of potentially observable pulsars is in a 95.45% credible interval of 133$^{+101}_{-58}$. Beaming considerations would increase the true population size by approximately a factor of two., Comment: Proceedings of IAUS 291 "Neutron Stars and Pulsars: Challenges and Opportunities after 80 years", J. van Leeuwen (ed.); 4 pages, 1 figure

As neutron star - black hole binaries are expected to be discovered through future pulsar surveys using upcoming facilities, it is necessary to understand various observable properties of such systems. In the present work, we study the advance of the periastron of such binaries under the post-Newtonian formalism over a wide range of parameters. We find that the first and second order post-Newtonian effects and the leading order spin-orbit coupling effects are significant for such binaries but higher order effects can be neglected., Comment: Accepted for publication in MNRAS; 7 pages, 5 figures, no table

A number of different classes of potentially extra-terrestrial bursts of radio emission have been observed in surveys with the Parkes 64m radio telescope, including "Rotating Radio Transients", the "Lorimer burst" and "perytons". Rotating Radio Transients are radio pulsars which are best detectable in single-pulse searches. The Lorimer burst is a highly dispersed isolated radio burst with properties suggestive of extragalactic origin. Perytons share the frequency-swept nature of the Rotating Radio Transients and Lorimer burst, but unlike these events appear in all thirteen beams of the Parkes Multibeam receiver and are probably a form of peculiar radio frequency interference. In order to constrain these and other radio source populations further, we searched the archival Parkes Multibeam Pulsar Survey data for events similar to any of these. We did not find any new Rotating Radio Transients or bursts like the Lorimer burst. We did, however, discover four peryton-like events. Similar to the perytons, these four bursts are highly dispersed, detected in all thirteen beams of the Parkes multibeam receiver, and have pulse widths between 20--30 ms. Unlike perytons, these bursts are not associated with atmospheric events like rain or lightning. These facts may indicate that lightning was not responsible for the peryton phenomenon. Moreover, the lack of highly dispersed celestial signals is the evidence that the Lorimer burst is unlikely to belong to a cosmological source population., Comment: Accepted for publication in MNRAS; 7 pages, 3 figures, 1 table

Using Monte Carlo simulations, we model the luminosity distribution of recycled pulsars in globular clusters as the brighter, observable part of an intrinsic distribution and find that the observed luminosities can be reproduced using either log-normal or power-law distributions as the underlying luminosity function. For both distributions, a wide range of model parameters provide an acceptable match to the observed sample, with the log-normal function providing statistically better agreement in general than the power-law models. Moreover, the power-law models predict a parent population size that is a factor of between two and ten times higher than for the log-normal models. We note that the log-normal luminosity distribution found for the normal pulsar population by Faucher-Gigu\`ere and Kaspi is consistent with the observed luminosities of globular cluster pulsars. For Terzan~5, our simulations show that the sample of detectable radio pulsars, and the diffuse radio flux measurement, can be explained using the log-normal luminosity law with a parent population of $\sim 150$ pulsars. Measurements of diffuse gamma-ray fluxes for several clusters can be explained by both power-law and log-normal models, with the log-normal distributions again providing a better match in general. In contrast to previous studies, we do not find any strong evidence for a correlation between the number of pulsars inferred in globular clusters and globular cluster parameters including metallicity and stellar encounter rate., Comment: Accepted for publication in MNRAS; 14 pages, 10 figures and 5 tables

Millisecond pulsars are believed to descend from low mass x-ray binaries. Observable parameters of binary millisecond pulsars e.g. mass of the pulsar, mass of the companion, spin period of the pulsar, orbital period, orbital eccentricity etc are used to probe the past accretion history of the millisecond pulsars. But unfortunately in these studies, binding energy of the neutron star is not considered commonly. We show that the effect of the binding energy is significant in the estimation of the amount of mass accretion and thus should be incorporated in models for binary evolutions. Moreover, different Equations of State for dense matter give different values for the accreted mass for the same amount of increase in the gravitational mass of the neutron star implying the need of constraining dense matter Equations of State even to understand the spin-up procedure properly., Comment: Accepted for publication in MNRAS Letters

We study luminosities of millisecond pulsars in globular clusters by fitting the observed luminosity distribution with single and double power laws. We use simulations to model the observed distribution as the brighter part of some parent distribution for Terzan 5 and try to find a model which simultaneously agrees with the observed diffuse radio flux, total predicted number of pulsars and observed luminosity distribution. We find that wide ranges of parameters for log-normal and power-law distributions give such good models. No clear difference between the luminosity distributions of millisecond pulsars in globular clusters and normal disk pulsars was seen., Comment: 4 pages, 2 figures, to appear in AIP Conference Proceedings of Pulsar Conference 2010 "Radio Pulsars: a key to unlock the secrets of the Universe", Sardinia, October 2010

Orbital parameters of binary radio pulsars reveal the history of the pulsars' formation and evolution including dynamic interactions with other objects. Advanced technology has enabled us to determine these orbital parameters accurately in most of the cases. Determination of post-Keplerian parameters of double neutron star binaries (especially of the double pulsar) provide clean tests of GTR and in the future may lead us to constrain the dense matter EoS. For binary pulsars with MS or WD companions, knowledge about the values of the orbital parameters as well as of the spin periods and the masses of the pulsars and the companions might be useful to understand the evolutionary history of the systems. As accreting neutron star binaries lead to orbit circularization due to the tidal coupling during accretion, their descendants i.e. binary MSPs are expected to be in circular orbits. On the other hand, dense stellar environments inside globular clusters (GCs) cause different types of interactions of single stars with pulsar binaries. These interactions can impart high eccentricities to the pulsar binaries. So it is quite common to get eccentric millisecond pulsar binaries in GCs and we find that "fly-by" causes intermediate values of eccentricities while "exchange" or "merger" causes high values of eccentricities. We also show that "ionization" is not much effective in the present stage of GCs. Even in the absence of such kinds of stellar interactions, a millisecond pulsar can have an eccentric orbit as a result of Kozai resonance if the pulsar binary is a member of a hierarchical triple system. PSR J1903+0327 is the only one eccentric millisecond pulsar binary in the galactic disk where stellar interactions are negligible. The possibility of this system to be a member of a hierarchical triple system or past association of a GC have been studied and found to be less likely., Comment: Invited chapter in "Pulsars: Theory, Categories and Applications", Editor: Alexander D. Morozov, 2010, Nova Science Publishers, ISBN: 978-1-61668-919-3; Typos corrected

The observed distribution of globular cluster binary radio pulsars in the eccentricity versus orbital period plane can be explained as a result of binary-single star interactions. Our numerical and analytical study hints that the highest eccentricity binaries in clusters are likely to be from exchange and/or merger of a single star with a binary component, while the intermediate eccentricity systems are probably results of fly-by interactions., Comment: Published in the Low-Frequency Radio Universe ASP Conference Series, Vol. 407, proceedings of the conference held 8-12 December 2008, at National Centre for Radio Astrophysics (NCRA), TIFR, Pune, India. Edited by D. J. Saikia, D. A. Green, Y. Gupta, and T. Venturi. San Francisco: Astronomical Society of the Pacific

We investigate the observational signatures associated with one of the proposed formation scenario for the recently discovered highly eccentric binary millisecond pulsar (MSP) PSR J1903+0327 in the galactic plane. The scenario requires that the MSP to be part of a hierarchical triple (HT), consisting of inner and outer binaries, experiencing the Kozai resonance. Numerical modeling of a bound point mass HT, while incorporating the effects due to the quadrupolar interactions between the binary orbits and dominant contributions to the general relativistic periastron precession in the inner binary, reveals that, at the present epoch, the orbital eccentricity of the binary MSP should decrease for reasonable ranges in the HT parameters. The estimated decrements in the orbital eccentricity of the inner binary are few parts in $10^{5}$, substantially higher than the reported accuracies in the estimation of the orbital eccentricity of the binary MSP, while employing various general relativistic timing models for isolated binary pulsars. For wide ranges in the allowed orbital parameters, the estimated rate of change in the eccentricity of the inner binary is orders of magnitude higher than the value recently measured by the pulsar timing analysis. Therefore, we rule out the scenario that the MSP is part of a HT undergoing the Kozai oscillations. The origin of this system in a typical globular cluster is also shown to be less likely than inferred in the discovery paper., Comment: Accepted for publication in MNRAS Letters. Minor typos corrected in the second version

High sensitivity searches of globular clusters (GC) for radio pulsars by improved pulsar search algorithms and sustained pulsar timing observations have so far yielded some 140 pulsars in more than two dozen GCs. The observed distribution of orbital eccentricity and period of binary radio pulsars in GCs have imprints of the past interaction between single pulsars and binary systems or of binary pulsars and single passing non-compact stars. It is seen that GCs have different groups of pulsars. These may have arisen out of exchange or merger of a component of the binary with the incoming star or a "fly-by" in which the original binary remains intact but undergoes a change of eccentricity and orbital period. We consider the genesis of the distribution of pulsars using analytical and computational tools such as STARLAB, which performs numerical scattering experiments with direct N-body integration. Cluster pulsars with intermediate eccentricities can mostly be accounted for by fly-bys whereas those with high eccentricities are likely to be the result of exchanges and/or mergers of single stars with the binary companion of the pulsar, although there are a few objects which do not easily fit into this description. The corresponding distribution for galactic field pulsars shows notable differences from the GC pulsar orbital period and eccentricity distribution. The long orbital period pulsars in the galactic field with frozen out low eccentricities are largely missing from the globular clusters, and we show that ionization of these systems in GCs cannot alone account for the peculiarities., Comment: Accepted for publication in ApJ

We analyze the observed distribution of the orbital eccentricity and period of binary radio pulsars in globular clusters using computational tools to simulate binary-single star interactions. Globular clusters have different groups of pulsars arising from separate interaction scenarios. Intermediate eccentricities of cluster pulsars can be mostly accounted by fly-bys although locally lower stellar densities at pulsar positions may alter the situation. Very high eccentricities are likely to be results of exchanges and/or mergers of single stars with the binary companion of the pulsar., Comment: Accepted for publication in ApJ Letters; version modified after referee's comments

I study stellar structures $i.e.$ the mass, the radius, the moment of inertia and the oblateness parameter at different spin frequencies for strange stars and neutron stars in a comparative manner. I also calculate the values of the radii of the marginally stable orbits and Keplerian orbital frequencies. By equating kHz QPO frequencies to Keplerian orbital frequencies, I find corresponding orbital radii. Knowledge about these parameters might be useful in further modeling of the observed features from LMXBs with advanced and improved future techniques for observations and data analysis., Comment: Accepted for publication in New Astronomy

We have studied strange star properties both at zero temperature and at finite temperatures and searched signatures of strange stars in gamma-ray, x-ray and radio astronomy. We have a set of Equations of State (EoS) for strange quark matter (SQM) and solving the TOV equations, we get the structure of strange stars. The maximum mass for a strange star decreases with the increase of temperature, because at high temperatures, the EoS become softer. One important aspect of strange star is that, surface tension depends on the size and structure of the star and is significantly larger than the conventional values. Moment of inertia is another important parameter for compact stars as by comparing theoretical values with observed estimate, it is possible to constrain the dense matter Equation of State. We hope that this approach will help us to decide whether the members of the double pulsar system PSR J0737-3039 are neutron stars or strange stars., Comment: Published in the proceedings of "Astrophysics Of Compact Objects: " International Conference on Astrophysics of Compact Objects, July 1-7, 2007 Huangshan, China

We study the dependence of the surface magnetic fields of radio pulsars on the choice of Equations of State, pulsar masses and the values of the angle between the magnetic axis and the spin axis of the pulsars within simple dipole model. We show that the values of the surface magnetic field can be even order of magnitude different from its canonical values. This difference will effect any magnetosphere related model to explain observational features of radio pulsars and magnetars. We find a significant difference of the value of the surface magnetic field from the commonly quoted value for the faster member of the double pulsar system, i.e. PSR J0737-3039A as here both the mass of the pulsar and the angle between the magnetic axis and the spin axis are known. Our study reveals the importance of constraining the dense matter Equations of State in pulsar astrophysics as well as hints an alternative way to constrain these by independent determination of the pulsar magnetic field., Comment: 4 pages, 3 figures, 1 table

We examine neutron star properties based on a model of dense matter composed of B=1 skyrmions immersed in a mesonic mean field background. The model realizes spontaneous chiral symmetry breaking non-linearly and incorporates scale-breaking of QCD through a dilaton VEV that also affects the mean fields. Quartic self-interactions among the vector mesons are introduced on grounds of naturalness in the corresponding effective field theory. Within a plausible range of the quartic couplings, the model generates neutron star masses and radii that are consistent with a preponderance of observational constraints, including recent ones that point to the existence of relatively massive neutron stars with mass M 1.7 Msun and radius R (12-14) km. If the existence of neutron stars with such dimensions is confirmed, matter at supra-nuclear density is stiffer than extrapolations of most microscopic models suggest., Comment: 27 pages, 5 figures, AASTeX style; to be published in The Astrophysical Journal

High energy density ($\eps$) and temperature (T) links general relativity and hydrodynamics leading to a lower bound for the ratio of shear viscosity ($\eta$) and entropy density ($s$). We get the interesting result that the bound is saturated in the simple model for quark matter that we use for strange stars at the surface for $T \sim 80 MeV$. At this $T$ we have the possibility of cosmic separation of phases. At the surface of the star where the pressure is zero - the density $\eps$ has a fixed value for all stars of various masses with correspondingly varying central energy density $\eps_c$. Inside the star where this density is higher, the ratio of $\eta/s$ is larger and are like the known results found for perturbative QCD. This serves as a check of our calculation. The deconfined quarks at the surface of the strange star at $T = 80 MeV$ seem to constitute the most perfect interacting fluid permitted by nature., Comment: 10 pages, 2 figures, 1 table; Accepted for publication in Phys. Lett. B