Searcharxiv⌕ Search

arXiv subjects

Mira Dey

Publications and source records attributed to Mira Dey.

At least 19 recordsLinked to original sources
arXiv2015

A Strange Star Scenario for the Formation of Eccentric Millisecond Pulsar/Helium White Dwarf Binaries

According to the recycling scenario, millisecond pulsars (MSPs) have evolved from low-mass X-ray binaries (LMXBs). Their orbits are expected to be circular due to tidal interactions during the binary evolution, as observed in most of the binary MSPs. There are some peculiar systems that do not fit this picture. Three recent examples are PSRs J2234$+$06, J1946$+$3417 and J1950$+$2414, all of which are MSPs in eccentric orbits but with mass functions compatible with expected He white dwarf companions. It has been suggested these MSPs may have formed from delayed accretion-induced collapse of massive white dwarfs, or the eccentricity may be induced by dynamical interaction between the binary and a circumbinary disk. Assuming that the core density of accreting neutron stars in LMXBs may reach the density of quark deconfinement, which can lead to phase transition from neutron stars to strange quark stars, we show that the resultant MSPs are likely to have an eccentric orbit, due to the sudden loss of the gravitational mass of the neutron star during the transition. The eccentricities can be reproduced with a reasonable estimate of the mass loss. This scenario might also account for the formation of the youngest known X-ray binary Cir X$-$1, which also possesses a low-field compact star in an eccentric orbit.

astro-ph.HE↗
arXiv2013

Strange star equation of state fits the refined mass measurement of 12 pulsars and predicts their radii

There are three categories of stars whose masses have been found accurately in recent times: (1) two for which Shapiro delay is used which is possible due to GR light bending as the partner is heavy : PSR J1614-2230 and PSR J1903+0327 (2) six eclipsing stars for which numerical Roche Lobe geometry is used and (3) 3 stars for which spectroscopic methods are used and in fact for these three the mass and radii both are estimated. Motivated by large color (N_c) expansion using a modified Richardson potential, along with density dependent quark masses thereby allowing chiral symmetry restoration, we get compact strange stars fitting all the observed masses.

astro-ph.HE↗
arXiv2011

KHz QPOs in LMXBs, relations between different frequencies and compactness of stars

We suggest that the mass of four compact stars SAX J1808.4$-$3658, KS 1731$-$260, SAX J1750.8$-$2900 and IGR J17191$-$2821 can be determined from the difference in the observed kiloHertz quasi periodic oscillations (kHz QPO-s) of these stars. The stellar radius is very close to the marginally stable orbit $R_{ms}$ as predicted by Einstein's general relativity. It may be noted that the first of these stars was suggested to be a strange star more than a decade back by Li \emph{et al.} (1999) from the unique millisecond X-ray pulsations with an accurate determination of its rotation period. It showed kHz QPO-s eight years back and so far it is the only set that has been observed. This is the first time we give an estimate of the mass of the star and of three other compact stars in Low-Mass X-ray Binaries using their observed kHz QPO-s.

astro-ph.HE↗
arXiv2009

The micro-glitch in PSR B1821-24 : A case for a strange pulsar?

The single glitch observed in PSR B1821-24, a millisecond pulsar in M28, is unusual on two counts. First, the magnitude of this glitch is at least an order of magnitude smaller ($Δν/ ν\sim 10^{-11}$) than the smallest glitch observed to date. Secondly, all other glitching pulsars have strong magnetic fields with $B \gsim 10^{11} G$ and are young, whereas PSR B1821-24 is an old recycled pulsar with a field strength of $2.25\times10^9 G$. We have suggested earlier that some of the recycled pulsars could actually be strange quark stars. In this work we argue that the crustal properties of such a {\em strange} pulsar are just right to give rise to a glitch of this magnitude, explaining the scarcity of larger glitches in millisecond pulsars.

astro-ph.SR↗
arXiv2008

Bound for entropy and viscosity ratio for strange quark matter

High energy density ($\eps$) and temperature (T) links general relativity and hydrodynamics leading to a lower bound for the ratio of shear viscosity ($η$) 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 $η/s$ 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.

astro-ph↗
arXiv2008

Members of the double pulsar system PSR J0737-3039 : neutron stars or strange stars ?

One interesting method of constraining the dense matter Equations of State is to measure the advancement of the periastron of the orbit of a binary radio pulsar (when it belongs to a double neutron star system). There is a great deal of interest on applicability of this procedure to the double pulsar system PSR J0737-3039 (A/B). Although the above method can be applied to PSR A in future within some limitations, for PSR B this method can not be applied. On the other hand, the study of genesis of PSR B might be useful in this connection and its low mass might be an indication that it could be a strange star.

astro-ph↗
arXiv2008

Strange Stars : An interesting member of the compact object family

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.

astro-ph↗
arXiv2008

Chromo-thermal oscillations and collapse of strange stars to black holes : Astrophysical Implications

The effects of temperature on strange stars are studied and it is found that the maximum mass of the star decreases with the increase of temperature since at high temperatures the equations of state become softer. Moreover, if the temperature of a strange star increases, keeping its baryon number fixed, its gravitational mass increases and radius decreases. This leads to a limiting temperature where it turns into a black hole. These features are due to a combined effect of the change of gluon mass and the quark distribution with temperature. We report a new kind of radial oscillations of strange stars driven by what we call chromo-thermal instability. We also discuss the relevance of our findings in the astrophysics of core collapse supernovae and gamma ray bursts.

astro-ph↗
arXiv2007

High Density Strange Star Matter and Observed Parity Doubling of Excited Hadrons

Parity doubling is observed in hadron states around 1.5 GeV - and the estimated energy density is found to be high. When a large excitation energy is available and the pions decouple from the quarks, the QCD interaction is still not perturbative. And signature of such a system can exist in the form of small ratio of the shear viscosity to entropy density. This is true for an equation of state which is applicable to the surface of strange star. We indicate the correspondence of parity doubling with the apparently disconnected model calculation of compactness of some pulsars.

hep-ph↗
arXiv2007

Entropy & viscosity bound of strange stars

At finite temperature (T) there is a link with general relativity and hydrodynamics that leads to a lower bound for the ratio of shear viscosity and entropy density (η/s). We find that the bound is saturated in the simple model for quark matter that we use for strange stars at T = 80 MeV, at the surface of a strange star. At this T we have the possibility of cosmic separation of phases. We find that, although strongly correlated, the quark matter at the surface of strange stars constitute the most perfect interacting fluid permitted by nature. At the centre of the star, however, the density is higher and conditions are more like the results found for perturbative QCD.

hep-ph↗
arXiv2006

Mean field baryon magnetic moments and sumrules

New developments have spurred interest in magnetic moments ($μ$-s) of baryons. The measurement of some of the decuplet $μ$-s and the findings of new sumrules from various methods are partly responsible for this renewed interest. Our model, inspired by large colour approximation, is a relativistic self consistent mean field description with a modified Richardson potential and is used to describe the $μ$-s and masses of all baryons with up (u), down (d) and strange (s) quarks. We have also checked the validity of the Franklin sumrule (referred to as CGSR in the literature) and sumrules of Luty, March-Russell and White. We found that our result for sumrules matches better with experiment than the non-relativistic quark model prediction. We have also seen that quark magnetic moments depend on the baryon in which they belong while the naive quark model expects them to be constant.

hep-ph↗
arXiv2006

Strange Star Equation of State With a Modified Richardson Potential

Richardson potential is an phenomenological interquark interaction taking care of two aspects of QCD, namely the asymptotic freedom and the confinement. The original potential has a scale parameter having value around 400 MeV and is well tested in hadronic property calculations. This potential was then used in strange star calculation. Strange stars are very compact stars composed of strange quark matter i.e. a very high density strange quark phase consisting of deconfined u, d and s quarks. Here the value of the scale parameter was taken as 100 MeV. The argument was that for a deconfined quark system like a strange star, the scale parameter may have a value quite different from that used in hadronic sector. To remove this discrepancy we introduced two scale parameters in the potential, one for the asymptotic freedom part and the other for the confining part. With suitable values of the parameters, this modified potential has been successfully used in both baryonic property and strange star calculations. The Equation of States obtained with the modified potential are also used to obtain mass-radius relations for the strange stars.

astro-ph↗
arXiv2006

Evidence for strange stars from joint observation of harmonic absorption bands and of redshift

From recent reports on terrestrial heavy ion collision experiments it appears that one may not obtain information about the existence of asymptotic freedom (AF) and chiral symmetry restoration (CSR) for quarks of QCD at high density. This information may still be obtained from compact stars - if they are made up of strange quark matter. Very high gravitational redshift lines (GRL), seen from some compact stars, seem to suggest high ratios of mass and radius (M/R) for them. This is suggestive of strange stars (SS) and can in fact be fitted very well with SQM equation of state deduced with built in AF and CSR. In some other stars broad absorption bands appear at about ~ 0.3 keV and multiples thereof, that may fit in very well with resonance with harmonic compressional breathing mode frequencies of these SS. Emission at these frequencies are also observed in six stars. If these two features of large GRL and BAB were observed together in a single star, it would strengthen the possibility for the existence of SS in nature and would vindicate the current dogma of AF and CSR that we believe in QCD. Recently, in 4U 1700-24, both features appear to be detected, which may well be interpreted as observation of SS - although the group that analyzed the data did not observe this possibility. We predict that if the shifted lines, that has been observed, are from neon with GRL shift z = 0.4 - then the compact object emitting it is a SS of mass 1.2 M_sun and radius 7 km. In addition the fit to the spectrum leaves a residual with broad dips at 0.35 keV and multiples thereof, as in 1E1207-5209 which is again suggestive of SS.

astro-ph↗
arXiv2006

Strange stars at finite temperature

We calculate strange star properties, using large N_c approximation with built-in chiral symmetry restoration (CSM). We used a relativistic Hartree Fock mean field approximation method, using a modified Richardson potential with two scale parameters Λand Λ^\prime, to find a new set of equation of states for strange quark matter. We take the effect of temperature (T) on gluon mass, in addition to the usual density dependence, and find that the transition T from hadronic matter to strange matter is 80 MeV. Therefore formation of strange stars may be the only signal for formation of QGP with asymptotic freedom and CSM.

astro-ph↗
arXiv2006

Compact strange stars with a medium dependence in gluons at finite temperature

The possible existence of strange stars in the universe will help in the understanding of various properties of quantum chromodynamics, like asymptotic freedom and chiral symmetry restoration, which is otherwise very difficult to prove in laboratory experiments. Strange star properties were calculated using large $N_c$ approximation with built-in chiral symmetry restoration. A relativistic Hartree Fock calculation was performed using the Richardson potential as an interquark interaction. This potential has the asymptotic freedom and a confinement-deconfinement mechanism built into it and the present calculation employs an application of this potential with modified two scale parameters $Λ$ and $Λ^{\prime}$, to find a new set of equations of state for strange quark matter. The linear confinement string tension from lattice calculations is 350 $MeV$ and the Coulomb -like part has the parameter 100 $MeV$ from deep inelastic scattering experiments. We also consider the effect of temperature, $T$, on gluon mass in a simple way, in addition to the usual density dependence, and find that the transition $T$ from hadronic matter to strange matter is at 80 MeV, close to the 100 MeV estimated in litarature. Therefore formation of strange stars may be the only signal for formation of quark-gluon plasma with asymptotic freedom and chiral symmetry restoration and this may be observable through many processes -such as for example through delayed $γ$ ray afterglow.

astro-ph↗
arXiv2005

Strange Pulsar Hypothesis

It appears that there is a genuine shortage of radio pulsars with surface magnetic fields significantly smaller than $\sim 10^8$ Gauss. We propose that the pulsars with very low magnetic fields are actually strange stars locked in a state of minimum free energy and therefore at a limiting value of the magnetic field which can not be lowered by the system spontaneously.

astro-ph↗
arXiv2005

Superburst: surface phenomenon of compact objects

We suggest that superbursts from some low mass X-ray binaries may be due to breaking and re-formation of diquark pairs, on the surface of realistic strange stars. Diquarks are expected to break up due to the explosion and shock of the thermonuclear process. The subsequent production of copious diquark pairing may produce sufficient energy to produce the superbursts.

astro-ph↗