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Monika Sinha

Publications and source records attributed to Monika Sinha.

54 records · Page 3Linked to original sources
arXiv2011

Possibility of conversion of neutron star to quark star in presence of high magnetic field

Recent results and data suggests that high magnetic field in neutron stars (NS) strongly affects the characteristic (radius, mass) of the star. They are even separated as a class known as magnetars, for whom the surface magnetic field are greater than $10^{14}$ G. In this work we discuss the effect of such high magnetic field on the phase transition of NS to quark star (QS). We study the effect of magnetic field on the transition from NS to QS including the magnetic field effect in equation of state (EoS). The inclusion of the magnetic field increases the range of baryon number density, for which the flow velocities of the matter in the respective phase are finite. The magnetic field helps in initiation of the conversion process. The velocity of the conversion front however decreases due to the presence of magnetic field, as the presence of magnetic field reduces the effective pressure (P). The magnetic field of the star gets decreased by the conversion process, and the resultant QS has lower magnetic field than that of the initial NS.

astro-ph.HE↗
arXiv2009

Transition from radiatively inefficient to cooling dominated phase in two temperature accretion discs around black holes

We investigate the transition of a radiatively inefficient phase of a viscous two temperature accreting flow to a cooling dominated phase and vice versa around black holes. Based on a global sub-Keplerian accretion disc model in steady state, including explicit cooling processes self-consistently, we show that general advective accretion flow passes through various phases during its infall towards a black hole. Bremsstrahlung, synchrotron and inverse Comptonization of soft photons are considered as possible cooling mechanisms. Hence the flow governs a much lower electron temperature ~10^8 - 10^{9.5}K compared to the hot protons of temperature ~10^{10.2} - 10^{11.8}K in the range of the accretion rate in Eddington units 0.01 - 100. Therefore, the solutions may potentially explain the hard X-rays and the gamma-rays emitted from AGNs and X-ray binaries. We finally compare the solutions for two different regimes of viscosity and conclude that a weakly viscous flow is expected to be cooling dominated compared to its highly viscous counterpart which is radiatively inefficient. The flow is successfully able to reproduce the observed luminosities of the under-fed AGNs and quasars (e.g. Sgr A*), ultra-luminous X-ray sources (e.g. SS433), as well as the highly luminous AGNs and ultra-luminous quasars (e.g. PKS 0743-67) at different combinations of the mass accretion rate and ratio of specific heats.

astro-ph.HE↗
arXiv2009

Hyperon bulk viscosity in strong magnetic fields

We study the bulk viscosity of neutron star matter including $Λ$ hyperons in the presence of quantizing magnetic fields. Relaxation time and bulk viscosity due to both the non-leptonic weak process involving $Λ$ hyperons and direct Urca processes are calculated here. In the presence of a strong magnetic field of $10^{17}$ G, the hyperon bulk viscosity coefficient is reduced whereas bulk viscosity coefficients due to direct Urca processes are enhanced compared with their field free cases when many Landau levels are populated by protons, electrons and muons.

astro-ph↗
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↗
arXiv2007

CPT and lepton number violation in neutrino sector: Modified mass matrix and oscillation due to gravity

We study the consequences of CPT and lepton number violation in neutrino sector. For CPT violation we take gravity with which neutrino and antineutrino couple differently. Gravity mixes neutrino and antineutrino in an unequal ratio to give two mass eigenstates. Lepton number violation interaction together with CPT violation gives rise to neutrino-antineutrino oscillation. Subsequently, we study the neutrino flavor mixing and oscillation under the influence of gravity. It is found that gravity changes flavor oscillation significantly which influences the relative abundance of different flavors in present universe. We show that the neutrinoless double beta decay rate is modified due to presence of gravity- the origin of CPT violation, as the mass of the flavor state is modified.

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↗
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↗
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

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↗
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↗