19 research outputs found

### Possible signatures for strange stars in stellar X-ray binaries

Kilohertz quasi-periodic brightness oscillations (kHz QPOs) observed in
certain X-ray burst sources may represent Keplerian frequencies in the inner
regions of the accretion disk in such systems. If this assumption is strictly
adhered to, we show here that if the central accretor in stellar X-ray burst
sources is a strange star (made up of u, d and s quarks in beta equilibrium,
referred to as strange matter) then the calculated QPO frequencies are
reconcilable with the observed QPO frequencies (corresponding to the highest
frequency of 1.22 kHz, observed so far from the source 4U 1636-53) only for
particular values of the QCD-related parameters which describe the equation of
state of strange matter. We demonstrate that QPO frequencies in the very high
range (1.9-3.1) kHz can be understood in terms of a (non- magnetized) strange
star X-ray binary (SSXB) rather than a neutron star X-ray binary (NSXB). Future
discovery of such high frequency QPOs from X-ray burst sources will constitute
a new astrophysical di- agnostic for identifying solar mass range stable
strange stars in our galaxy.Comment: 4 pages, 2 figs., uses psbox.tex, submitted to A&

### A general relativistic calculation of boundary layer and disc luminosity for accreting non-magnetic neutron stars in rapid rotation

We calculate the disc and boundary layer luminosities for accreting rapidly rotating neutron stars with low magnetic fields in a fully general relativistic manner. Rotation increases the disc luminosity and decreases the boundary layer luminosity. A rapid rotation of the neutron star substantially modifies these quantities as compared with the static limit. For a neutron star rotating close to the centrifugal mass shed limit, the total luminosity has contribution only from the extended disc. For such maximal rotation rates, we find that well before the maximum stable gravitational mass configuration is reached, there exists a limiting central density, for which particles in the innermost stable orbit will be more tightly bound than those at the surface of the neutron star. We also calculate the angular velocity profiles of particles in Keplerian orbits around the rapidly rotating neutron star. The results are illustrated for a representative set of equation of state models of neutron star matter

### General Relativistic Spectra from Accretion Disks around Rapidly Rotating Neutron Stars

We compute spectra from accretion disks around rapidly rotating neutron
stars. The full effect of general relativity is considered for the structure
calculation of the stars. We take into account the Doppler shift, gravitational
redshift and light-bending effects in order to compute the observed spectra. To
facilitate direct comparison with observations, a simple empirical function is
presented which describes the numerically computed spectra well. This function
can in principle be used to distinguish between the Newtonian spectra and the
relativistic spectra. We also discuss the possibility of constraining neutron
star's equation of state using our spectral models.Comment: 2 pages, to appear in the proceedings of 'The Physics of Cataclysmic
Variables and Related Objects', Goettingen, August 5-10, 200

### Two coexisting families of compact stars: observational implications for millisecond pulsars

It is usually thought that a single equation of state (EoS) model "correctly"
represents cores of all compact stars. Here we emphasize that two families of
compact stars, viz., neutron stars and strange stars, can coexist in nature,
and that neutron stars can get converted to strange stars through the
nucleation process of quark matter in the stellar center. From our fully
general relativistic numerical computations of the structures of fast-spinning
compact stars, known as millisecond pulsars, we find that such a stellar
conversion causes a simultaneous spin-up and decrease in gravitational mass of
these stars. This is a new type of millisecond pulsar evolution through a new
mechanism, which gives rise to relatively lower mass compact stars with higher
spin rates. This could have implication for the observed mass and spin
distributions of millisecond pulsars. Such a stellar conversion can also rescue
some massive, spin-supported millisecond pulsars from collapsing into black
holes. Besides, we extend the concept of critical mass $M_{\rm cr}$ for the
neutron star sequence (Berezhiani et al. 2003; Bombaci et al. 2004) to the case
of fast-spinning neutron stars, and point out that neutron star EoS models
cannot be ruled out by the stellar mass measurement alone. Finally, we
emphasize the additional complexity for constraining EoS models, for example,
by stellar radius measurements using X-ray observations, if two families of
compact stars coexist.Comment: 10 pages, 5 figures, accepted for publication in The Astrophysical
Journa

### Fast spinning strange stars: possible ways to constrain interacting quark matter parameters

For a set of equation of state (EoS) models involving interacting strange
quark matter, characterized by an effective bag constant (B_eff) and a
perturbative QCD corrections term (a_4), we construct fully general
relativistic equilibrium sequences of rapidly spinning strange stars for the
first time. Computation of such sequences is important to study millisecond
pulsars and other fast spinning compact stars. Our EoS models can support a
gravitational mass (M_G) and a spin frequency at least up to approximately 3.0
solar mass and approximately 1250 Hz respectively, and hence are fully
consistent with measured M_G and spin frequency values. This paper reports the
effects of B_eff and a_4 on measurable compact star properties, which could be
useful to find possible ways to constrain these fundamental quark matter
parameters, within the ambit of our EoS models. We confirm that a lower B_eff
allows a higher mass. Besides, for known M_G and spin frequency, measurable
parameters, such as stellar radius, radius-to-mass ratio and moment of inertia,
increase with the decrease of B_eff. Our calculations also show that a_4
significantly affects the stellar rest mass and the total stellar binding
energy. As a result, a_4 can have signatures in evolutions of both accreting
and non-accreting compact stars, and the observed distribution of stellar mass
and spin and other source parameters. Finally, we compute the parameter values
of two important pulsars, PSR J1614-2230 and PSR J1748-2446ad, which may have
implications to probe their evolutionary histories, and for constraining EoS
models.Comment: 17 pages, 11 figures, 7 tables, accepted for publication in Monthly
Notices of the Royal Astronomical Societ

### Millisecond radio pulsars with known masses: Parameter values and equation of state models

The recent fast growth of a population of millisecond pulsars with precisely measured mass provides an excellent opportunity to characterize these compact stars at an unprecedented level. This is because the stellar parameter values can be accurately computed for known mass and spin rate and an assumed equation of state (EoS) model. For each of the 16 such pulsars and for a set of EoS models from nucleonic, hyperonic, strange quark matter and hybrid classes, we numerically compute fast spinning stable stellar parameter values considering the full effect of general relativity. This first detailed catalogue of the computed parameter values of observed millisecond pulsars provides a testbed to probe the physics of compact stars, including their formation, evolution and EoS. We estimate uncertainties on these computed values from the uncertainty of the measured mass, which could be useful to quantitatively constrain EoS models. We note that the largest value of the central density ρc in our catalogue is ∼5.8 times the nuclear saturation density ρsat, which is much less than the expected maximum value 13ρsat. We argue that the ρc-values of at most a small fraction of compact stars could be much larger than 5.8ρsat. Besides, we find that the constraints on EoS models from accurate radius measurements could be significantly biased for some of our pulsars, if stellar spinning configurations are not used to compute the theoretical radius values

### Rapidly rotating strange stars for a new equation of state of strange quark matter

For a new equation of state of strange quark matter, we construct equilibrium
sequences of rapidly rotating strange stars in general relativity. The
sequences are the normal and supramassive evolutionary sequences of constant
rest mass. We also calculate equilibrium sequences for a constant value of
$\Omega$ corresponding to the most rapidly rotating pulsar PSR 1937 + 21. In
addition to this, we calculate the radius of the marginally stable orbit and
its dependence on $\Omega$, relevant for modeling of kilo-Hertz quasi-periodic
oscillations in X-ray binaries.Comment: Two figures, uses psbox.tex and emulateapj5.st

### Implications of kHz Quasi-Periodic Brightness Oscillations in X--Ray Binaries for Neutron Star Structure

Kilohertz quasi-periodic oscillations (QPOs) in low-mass X-ray binaries
(LMXBs) may represent the orbital frequencies of the innermost Keplerian orbits
around accreting neutron stars. Attempts have recently been made to derive
constraints on the mass and the equation of state of the neutron star, by
identifying the highest observed QPO frequency with the Keplerian frequency at
the marginally stable orbit given by the general theory of relativity. These
estimates have either neglected the effect of neutron star rotation or used an
approximate treatment of rotation in general relativity. We rederive these
constraints using a fully general relativistic formalism including the effect
of rapid rotation. We also present constraints corresponding to the case where
the innermost stable orbit touches the stellar surface.Comment: Replaced with new extensively revised version. To appear MNRAS, 302,
L6