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&

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