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Sushan Konar

Publications and source records attributed to Sushan Konar.

At least 19 recordsLinked to original sources
arXiv2025

Astrophysics with Compact Objects: An Indian Perspective, Present Status and Future Vision

Astrophysical compact objects, viz., white dwarfs, neutron stars, and black holes, are the remnants of stellar deaths at the end of their life cycles. They are ideal testbeds for various fundamental physical processes under extreme conditions that are unique in nature. Observational radio astronomy with uGMRT and OORT facilities has led to several important breakthroughs in studies of different kinds of pulsars and their emission mechanisms. On the other hand, accretion processes around compact objects are at the core of Indian astronomy research. In this context, AstroSat mission revolutionized spectro-temporal observations and measurements of accretion phenomena, quasi-periodic oscillations, and jet behaviour in binary systems hosting compact objects. Moreover, recently launched XPoSat mission is set to provide an impetus to these high-energy phenomena around compact objects by enabling us to conduct polarization measurements in the X-ray band. Further, during the past decade, numerous gravitational wave signals have been observed from coalescing black holes and neutron stars in binary systems. Recent simultaneous observation of the GW170817 event in both gravitational waves and electromagnetic channels has ushered in the era of multi-messenger astronomy. In the future, synergistic efforts among several world-class observational facilities, e.g., LIGO-India, SKA, TMT, etc., within the Indian astrophysics community will provide a significant boost to achieve several key science goals that have been delineated here. In general, this article plans to highlight scientific projects being pursued across Indian institutions in this field, the scientific challenges that this community would be focusing on, and the opportunities in the coming decade. Finally, we have also mentioned the required resources, both in the form of infrastructural and human resources.

astro-ph.HE↗
arXiv2023

Defining Millisecond Pulsars

Millisecond pulsars (MSP) are an important subclass of rotation powered pulsars (RPP), traditionally defined as those with $P_s < 20-30$~ms and $B_s \lesssim 10^{10}$~G. We re-examine this definition by applying Gaussian mixture model (GMM) analysis to identify distinct clusters within the RPP population and find that the MSPs appear to be better demarcated by the condition $\mathbf{P_s \lesssim 16}$~ms.

astro-ph.HE↗
arXiv2022

The Enigma of GLEAM-X~J162759.5-523504.3

It is proposed that GLEAM-X~J162759.5-523504.3, the newly discovered radio transient with an unusually long spin-period (P$_s$ = 1091.1690s), can be identified to be a Radio Magnetar which has a dipolar surface magnetic field of $2.5 \times 10^{16}$~G. It is shown that - a) it is possible to anchor such a strong field at the core-crust boundary of a neutron star, and b) the energy of field dissipation can explain the observed luminosity (radio & X-ray) of this source.

astro-ph.HE↗
arXiv2022

Radio Pulsar Sub-Populations (II) : The Mysterious RRATs

Several conjectures have been put forward to explain the RRATs, the newest subclass of neutron stars, and their connections to other radio pulsars. This work discusses these conjectures in the context of the characteristic properties of the RRAT population. Contrary to expectations, it is seen that - a) the RRAT population is statistically un-correlated with the nulling pulsars, and b) the RRAT phenomenon is unlikely to be related to old age or death-line proximity. It is perhaps more likely that the special emission property of RRATs is a signature of them being later evolutionary phases of other types of neutron stars which may have resulted in restructuring of the magnetic fields.

astro-ph.HE↗
arXiv2022

The Magnetar Connection

We investigate the combined evolution of the dipolar surface magnetic field (B$_{s}$) and the spin-period (P$_s$) of known magnetars and high magnetic field (B$_s$ $ \gtrsim 10^{13}$~G) radio pulsars. We study the long term behaviour of these objects assuming a simple ohmic dissipation of the magnetic field. Identifying the regions (in the P$_s$-B$_s$ plane) in which these neutron stars would likely move into, before crossing the death-line to enter the pulsar graveyard, we comment upon the possible connection between the magnetars and other classes of neutron stars.

astro-ph.HE↗
arXiv2019

Radio Pulsar Sub-Populations (I) : The Curious Case of Nulling Pulsars

About 200 radio pulsars have been observed to exhibit nulling episodes - short and long. We find that the nulling fraction of a pulsar does not have any obvious correlation with any of the intrinsic pulsar parameters. It also appears that the phenomenon of nulling may be preferentially experienced by pulsars with emission coming predominantly from the polar cap region, and also having extremely curved magnetic fields.

astro-ph.HE↗
arXiv2019

The Sounds of Music : Science of Musical Scales III -- Indian Classical

In the previous articles of this series, we have discussed the development of musical scales particularly that of the heptatonic scale which forms the basis of Western classical music today. In this last article, we take a look at the basic structure of scales used in Indian classical music and how different `raga's are generated through the simple process of scale shifting.

cs.SD↗
arXiv2019

The Sounds of Music : Science of Musical Scales II -- Western Classical

A set of basic notes, or `scale', forms the basis of music. Scales are specific to specific genre of music. In this second article of the series we explore the development of various scales associated with western classical music, arguably the most influential genre of music of the present time.

physics.pop-ph↗
arXiv2019

The Sounds of Music : Science of Musical Scales I -- Human Perception of Sound

Both, human appreciation of music and musical genres, transcend time and space. The universality of musical genres and associated musical scales is intimately linked to the physics of sound and the special characteristics of human acoustic sensitivity. In this series of articles, we examine the science underlying the development of the heptatonic scale, one of the most prevalent scales of the modern musical genres, both western and Indian.

physics.pop-ph↗
arXiv2017

Magnetic Fields of Neutron Stars

This article briefly reviews our current understanding of the evolution of magnetic fields in neutron stars, which basically defines the evolutionary pathways between different observational classes of neutron stars. The emphasis here is on the evolution in binary systems and the newly emergent classes of millisecond pulsars.

astro-ph.HE↗
arXiv2017

Gravity Defied (from potato asteroids to magnetised neutron stars) IV. Neutron Stars (dead stars of the second kind)

A star burns its nuclear fuel and balances gravitation by the pressure of the heated gas, during its active lifetime. After the exhaustion of the nuclear fuel, a low mass star finds peace as a {\em white dwarf}, where the pressure support against gravitation is provided by Fermi-degenerate electrons. However, for massive stars the gravitational squeeze becomes so severe that in the final phase of evolution, the average density approximately equals the nuclear density. At such densities most of the protons combine with electrons to convert themselves into neutrons. A {\em Neutron star}, composed of such neutron-rich material, is host to some fascinating physics arising out of its amazingly compact state of matter (where a solar mass is packed inside a sphere of radius $\sim$ 10Km).

physics.pop-ph↗
arXiv2017

Gravity Defied (from potato asteroids to magnetised neutron stars) III. White Dwarfs (dead stars of the first kind)

During its active lifetime a star burns its nuclear fuel and gravitation is held off by the pressure of the heated gas. Gravity should take over once this fuel is exhausted unless some other agency saves the star from such a fate. Low mass stars find peace as {\bf \em white dwarfs} when the electrons settle into a Fermi degenerate phase where the pressure of degenerate electrons balance the gravitational pressure.

physics.pop-ph↗
arXiv2017

Gravity Defied (from potato asteroids to magnetised neutron stars) I : The self-gravitating objects

Gravitation, the universal attractive force, acts upon all matter (and radiation) relentlessly. Left to itself, gravity would pull everything together and the Universe would be nothing but a gigantic black hole. Nature throws almost every bit of physics - rotation, magnetic field, heat, quantum effects and so on, at gravity to escape such a fate. In this series of articles we shall explore systems where the eternal pull of gravity has been held off by one or another such means.

physics.pop-ph↗
arXiv2016

Neutron Star Physics in the Square Kilometer Array Era : An Indian Perspective

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.

astro-ph.HE↗
arXiv2016

Of Mountains and Molehills : Gravitational Waves from Neutron Stars

Surface asymmetries of accreting neutron stars are investigated for their mass quadrupole moment content. Though the amplitude of the gravitational waves from such asymmetries seem to be beyond the limit of detectability of the present generation of detectors, it appears that rapidly rotating neutron stars with strong magnetic fields residing in HMXBs would be worth considering for targeted search for continuous gravitational waves with the next generation of instruments.

astro-ph.HE↗
arXiv2016

Under-Graduate Research in Physics : An Indian Perspective

It is now widely believed that research should be an essential and integral part of under-graduate studies. In recent years there has been a conscious effort to bring research opportunities to the physics under-graduates in India. We argue that the need for the hour is a methodical evaluation of the existing under-graduate research programs for their effectiveness in preparing the students for a career in physics.

physics.ed-ph↗
arXiv2015

Strong constraints on magnetized white dwarfs surpassing the Chandrasekhar mass limit

We show that recently proposed white dwarf models with masses well in excess of the Chandrasekhar limit, based on modifying the equation of state by a super-strong magnetic field in the centre, are very far from equilibrium because of the neglect of Lorentz forces. An upper bound on the central magnetic fields, from a spherically averaged hydrostatic equation, is much smaller than the values assumed. Robust estimates of the Lorentz forces are also made without assuming spherical averaging. These again bear out the results obtained from a spherically averaged model. In our assessment, these estimates rule out the possibility that magnetic tension could change the situation in favor of larger magnetic fields. We conclude that such super-Chandrasekhar models are unphysical and exploration of their astrophysical consequences is premature. Erratum : We correct certain numerical estimates made in our paper. This correction however does not alter the conclusion that when Lorentz forces are taken into account the super-massive white dwarf models, obtained simply by modifying the equation of state in presence of a super-strong magnetic field in the centre, fail to be in equilibrium.

astro-ph.SR↗