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Jishnu Dey

Publications and source records attributed to Jishnu Dey.

At least 37 records · Page 2Linked to original sources
arXiv2005

Strange stars and superbursts at near-Eddington mass accretion rates

Careful assessment of four good superburst candidates for GX 17+2 reveals that superburst is possible at near Eddington mass accretion rates. For the other seven stars, where superburst is found, there is the standard model of burning accumulated carbon from normal type I bursts of the accreting stars. However, there is the need for carbon, nitrogen and oxygen mass fraction (Z_{CNO}) which must be larger than Z_{CNO,\odot}, where the latter refers to the standard value found in the sun. Also it is very difficult to incorporate GX 17+2 into the standard picture of superbursts. In case of superbursts from strange stars, arising from broken quark pairs going over to diquarks at the surface of the star, these problems do not arise. Furthermore there is a natural explanation for the large value of ~ 1000 for αwhich is defined in the literature as the ratio of energy released between normal bursts to the energy released during the normal burst. In the scenario for superbursts in strange stars it may be argued that the relatively smaller value of αof ~ 440 indicates frequent recurrence of superbursts which is reflected in 4U 1636-53.

astro-ph↗
arXiv2005

Newtonian and general relativistic contribution of gravity to surface tension of strange stars

Surface tension (S) is due to the inward force experienced by particles at the surface and usually gravitation does not play an important role in this force. But in compact stars the gravitational force on the particles is very large and S is found to depend not only on the interactions in the strange quark matter, but also on the structure of the star, i.e. on its mass and radius. Indeed, it has been claimed recently that 511 keV photons observed by the space probe INTEGRAL from the galactic bulge may be due to electron-positron annihilation, and their source may be the positron cloud outside of an antiquark star. Such stars, if they exist, may also go a long way towards explaining away the antibaryon deficit of the universe. For that to happen S must be high enough to allow for survival of quark/antiquark stars born in early stages of the formation of the universe. High value of S may also assist explanation of delayed gamma-ray burst after a supernova explosion, as conversion from normal matter to strange matter takes place. The possibility of some implications from formation of surface waves are also discussed.

astro-ph↗
arXiv2005

Stability of strange stars (SS) under radial oscillation

A realistic Equation of State (EOS) leads to strange stars (ReSS) which are compact in the mass radius plot, close to the Schwarzchild limiting line (Dey et al. 1998). We carry out a stability analysis under radial oscillations and compare with the EOS of other SS models. We find that the ReSS is stable and an M-R region can be identified to that effect.

astro-ph↗
arXiv2004

General relativistic effects of strong magnetic fields on the gravitational force: a driving engine for bursts of gamma rays in SGRs?

In general relativity all forms of energy contribute to gravity and not only just ordinary matter as in Newtonian Physics. This fact can be seen in the modified hydrostatic equilibrium equation for relativistic stars pervaded by magnetic (B) fields. It has an additional term coupled to the matter part as well as an anisotropic term which is purely of magnetic origin. That additional term coming from the pressure changed by the radial component of the diagonal electromagnetic field tensor, weakens the gravitational force when B is strong enough and can even produce an unexpected change in the attractive nature of the force by reversing its sign. In an extreme case, this new general relativistic (GR) effect can even trigger an instability in the star as a consequence of the sudden reversal of the hydrostatic pressure gradient. We suggest here that this GR effect may be the possible central engine driving the transient giant outbursts observed in Soft Gamma-ray Repeaters (SGRs). In small regions of the neutron star (NS), strong magnetic condensation can take place. Beyond a critical limit, these highly magnetised bubbles may explode releasing the trapped energy as a burst of gamma-rays of ~ 10^{36-40} erg.

astro-ph↗
arXiv2004

Possible evidence of surface vibration of strange stars from stellar observations

Emission lines in the eV and keV range by certain stellar candidates from their recent analysis invoke the question of their possible origin. These stars under consideration, are the 4U 0614+091 (0.65, 0.86, and 1.31 keV), 2S 0918-549 (0.8 keV with width 55 eV), 4U 1543-624 (0.7 keV), 4U 1850-087 (0.7 keV) and 4U 1820-30 (0.6 and 0.9 keV) and also the 0.6 keV excess emission in RX J170930.2-263927. Recently, it has been suggested that the resonance absorption at ~ in 0.7, 1.4, 2.1 and 2.8 keV 1E1207-5209 and 0.35, 0.7 and 1.4 keV RX J1856.5-3754 are due to harmonic surface vibrations in strange stars. We propose that these harmonic vibrations may also responsible for emission lines in the above mentioned compact stellar candidates.

astro-ph↗
arXiv2004

A model finding a new Richardson potential with different scales for confinement and asymptotic freedom, by fitting the properties of ${\D}^{++}$ and ${\Om}^{-}$

Phenomenological Richardson potential has built in asymptotic freedom (AF in short) and confinement, with only one parameter $\La$ in the potential. But it is known that the scales of AF and confinement are not the same. In the present work a relativistic mean field calculation for baryons is tried out with two parameters $\La$ and $\La^\prime$ for AF and confinement respectively .To test the two parameter potential we calculate the energies and the magnetic moments, of the triple u - quark system (${\D}^{++}$) and the triple s - quark system (${\Om}^{-}$) and found good values for $\La=100 MeV$ and $\La^\prime ~= 350 MeV$. So we believe that the modified Richardson potential should have AF scale $\La ~= 100 MeV$ and the confinement scale $\La^\prime ~= 350 MeV$.

hep-ph↗
arXiv2004

Two new sum rules for octet-baryon magnetic moments (μ) and constraints on QCD sum rules from new experimental determination of μ-s for the decuplet

Recently the μ_{Δ^{++}} was found from a fit to (π^+)p scattering. This enable us to pinpoint condensate parameters more precisely in the context of QCD sum rules (QCDSR). In the octet sector, the Coleman-Glashow sum rule (CGSR) is violated by the experimental μ-s. QCDSR allows us to write down two sum rules similar to the CGSR, which are obeyed by the experimental magnetic moments, whereas they rule out a specific model using the Wilson loop approach and a particular chiral quark model. It is amusing to note that the QCDSR allows us to write down the quark and gluon condensates in terms of measurables like the μ-s of the nucleons and the Σ^{+/-}.

hep-ph↗
arXiv2004

Incompressibility of strange matter

Strange stars calculated from a realistic equation of state (EOS), that incorporate chiral symmetry restoration as well as deconfinement at high density show compact objects in the mass radius curve. We compare our calculations of incompressibility for this EOS with that of nuclear matter. One of the nuclear matter EOS has a continuous transition to ud-matter at about five times normal density. Another nuclear matter EOS incorporates density dependent coupling constants. From a look at the consequent velocity of sound, it is found that the transition to ud-matter seems necessary.

hep-ph↗
arXiv2004

Theoretical Description of kHz QPOs in accreting LMXB systems

We describe kHz QPOs from the hydrodynamical model of accretion disks for LMXB systems. Out of the pair, the higher frequency originates due to the viscous effects of an accretion disk involving the formation of shock, while the lower one is due to the Keplerian motion of accreting matter. Comparing our results with observations for two fast rotating compact stellar candidates, namely, 4U 1636-53 and KS 1731-260, we find that they match to a very good approximation.

astro-ph↗
arXiv2003

Origin and interpretation of kilohertz QPOs from strange stars in X-ray binary system: theoretical hydrodynamical description

We model and interpret the Kilohertz QPOs from the hydrodynamical description of accretion disk around a rapidly rotating compact strange star. The higher QPO frequency is described by the viscous effects of accretion disk leading to shocks, while the lower one is taken to be the Keplerian motion of the accreting matter. Comparing our results with the observations for two of the fastest rotating compact stellar candidates namely, 4U~1636-53 and KS 1731-260, we find that they match to a very good approximation, thus interpreting them as strange stars.

astro-ph↗
arXiv2002

Have we observed the skin vibration of realistic strange stars (ReSS) ?

Skin vibration of ReSS and consequent resonance absorption can account for the absorption lines in the spectrum of X-ray emission from many compact stellar objects and in particular, the stars J1210$-$5226 and RXJ1856$-$3754. Observations of the X-ray spectrum of these stars is difficult to explain, if they are neutron stars.

astro-ph↗
arXiv2002

Superbursts and long bursts as surface phenomenon of compact objects

X-ray bursts from compact stars is believed to be due to type I thermonuclear processes which are short lived, typically ~ 10 to 100 s. There are some low mass X-ray binaries (LMXB) like 4U 1820-30, 4U 1636-53, KS 1731-260 and Serpens X-1, known as Super Bursters (SB) which emit X-rays close to the Eddington luminosity limit for long periods of several hours. Recently there are reports of some long bursters (LB), which have bursts lasting 6-25 minutes, whereas the 4U 1735-44 has a burst period of 86 minutes. We suggest that these bursts from SB and LB may be due to breaking and re-formation of diquark pairs, on the surface of realistic strange quark stars. We use the beta equilibrated u, d and s quark model of Dey et al. (1998) and Li et al. (1999a and 1999b) and allow for spin dependent hyperfine interaction between quarks. The interaction produces pairing of specific colour-spin diquarks, leading to further lowering of energy by several MeV-s for each pair, on the average. Diquarks are expected to break up due to the explosion and shock of the TN process. The subsequent production of copious diquark pairing may produce sufficient energy to produce the very long bursts seen in SB or LB. The estimated total energy liberated, 10^{42} ergs, can be explained in our model with the calculated pair density ~ 0.275/fm^3 and a surface thickness of only half a micron, if the entire surface is involved. The depth of the surface involved in the process may be only few microns if the process is restricted to small part of the surface near the equator as suggested by Bildsten.

astro-ph↗
arXiv2002

Stability of strange stars (SS) derived from a realistic equation of state

A realistic equation of state (EOS) leads to realistic strange stars (ReSS) which are compact in the mass radius plot, close to the Schwarzchild limiting line (Dey et al 1998). Many of the observed stars fit in with this kind of compactness, irrespective of whether they are X-ray pulsars, bursters or soft $γ$ repeaters or even radio pulsars. We point out that a change in the radius of a star can be small or large, when its mass is increasing and this depends on the position of a particular star on the mass radius curve. We carry out a stability analysis against radial oscillations and compare with the EOS of other strange star (SS) models. We find that the ReSS is stable and an M-R region can be identified to that effect.

astro-ph↗
arXiv2001

Nucleons and Isobars at finite density ($ρ$) and temperature (T)

The importance of studying matter at high $ρ$ increases as more astrophysical data becomes available from recently launched spacecrafts. The importance of high T studies derives from heavy ion data. In this paper we set up a formalism to study the nucleons and isobars with long and short range potentials non-pertubatively, bosonizing and expanding semi-classically the Feyman integrals up to one loop. We address the low density, finite T problem=A0 first, the case relevant to heavy ion collisions, hoping to adresss the high density case later. Interactions change the nucleon and isobar numbers at different $ρ$ and T non-trivially.

nucl-th↗
arXiv2000

Density dependent strong coupling constant of QCD derived from compact star data

The present work is an endeavour to connect the properties of tiny nearly massless objects with those of some of the most massive ones, the compact stars. Since 1996 there is major influx of X-ray and $γ$ ray data from binary stars, one or both of which are compact objects that are difficult to explain as neutron stars since they contain a mass M in too small a radius R . The suggestion has been put forward that these are strange quark stars (SS) explainable in a simple model with chiral symmetry restoration (CSR) for the quarks and the M, R and other properties like QPOs (quasi periodic oscillations) in their X-ray power spectrum. It would be nice if this astrophysical data could shed some light on fundamental properties of quarks obeying QCD. One can relate the strong coupling constant of QCD, $α_s$ to the quark mass through the Dyson-Schwinger gap equation using the real time formalism of Dolan and Jackiw. This enables us to obtain the density dependence of $α_s$ from the simple CSR referred to above. This way fundamental physics, difficult to extract from other models like for example lattice QCD, can be constrained from present-day compact star data and may be put back to modelling the dense quark phase of early universe.

astro-ph↗
arXiv2000

Glimpses of a strange star

There are about 2000 gamma ray burst (GRB) events known to us with data pouring in at the rate of one per day. While the afterglows of GRBs in radio, optical and X-ray bands are successfully explained by the fireball model, a significant difficulty with the proposed mechanisms for GRBs is that a small amount ($\le 10^{-6} M_{\odot}$) of baryons in the ejecta can be involved. There are very few models that fulfill this criteria together with other observational features, among which are the differentially rotating collapsed object model and the "supernova" model. These models generally invoke rapidly rotating neutron stars, and may be subject to uncertainties in the formation mechanisms and the equations of state of neutron stars. According to Spruit, the problem of making a GRB from an X-ray binary is reduced to finding a plausible way to make the star rotate differentially. We suggest that a model of strange star (SS) can naturally explain many of these bursts with not only their low baryon content, but the differential rotation which leads to an enhanced magnetic field that surfaces up and is responsible for GRBs.

astro-ph↗