Implementation of an F-statistic all-sky search for continuous gravitational waves in Virgo VSR1 data

We present an implementation of the $mathcal{F}$-statistic to carry out the first search in data from the Virgo laser interferometric gravitational wave detector for periodic gravitational waves from a priori unknown, isolated rotating neutron stars. We searched a frequency $f_0$ range from 100 Hz to 1 kHz and the frequency dependent spindown $f_1$ range from $-1.6,(f_0/100,{rm Hz}) times 10^{-9},$ Hz/s to zero. A large part of this frequency - spindown space was unexplored by any of the all-sky searches published so far. Our method consisted of a coherent search over two-day periods using the $mathcal{F}$-statistic, followed by a search for coincidences among the candidates from the two-day segments. We have introduced a number of novel techniques and algorithms that allow the use of the Fast Fourier Transform (FFT) algorithm in the coherent part of the search resulting in a fifty-fold speed-up in computation of the $mathcal{F}$-statistic with respect to the algorithm used in the other pipelines. No significant gravitational wave signal was found. The sensitivity of the search was estimated by injecting signals into the data. In the most sensitive parts of the detector band more than 90% of signals would have been detected with dimensionless gravitational-wave amplitude greater than $5 times 10^{-24}$.

Searching for gravitational waves from known pulsars at once and twice the spin frequency

The existence of a superfluid core in the interior of a rotating neutron star may have an influence on its gravitational wave emission. In addition to the usually-assumed pure quadrupole radiation with the gravitational wave frequency at twice the spin frequency, a frequency of rotation itself may also be present in the gravitational wave spectrum. We study the parameters of a general model for such emission, compare it with previously proposed, simpler models, discuss the feasibility of the recovery of the stellar parameters and carry out the Monte Carlo simulations to test the performance of our estimation method.

Parameters of rotating neutron stars with and without hyperons

The discovery of a 2 Msun neutron star provided a robust constraint for the theory of exotic dense matter, bringing into question the existence of strange baryons in the interiors of neutron stars. Although many theories fail to reproduce this observational result, several equations of state containing hyperons are consistent with it. We study global properties of stars using equations of state containing hyperons, and compare them to those without hyperons to find similarities, differences, and limits that can be compared with the astrophysical observations. Rotating, axisymmetric, and stationary stellar configurations in general relativity are obtained, and their global parameters are studied. Approximate formulae describing the behavior of the maximum and minimum stellar mass, compactness, surface redshifts, and moments of inertia as functions of spin frequency are provided. We also study the thin disk accretion and compare the spin-up evolution of stars with different moments of inertia.

Formation of millisecond pulsars - NS initial mass and EOS constraints

Recent measurement of a high millisecond pulsar mass (PSR J1614-2230, 1.97+-0.04 Msun) compared with the low mass of PSR J0751+1807 (1.26+-0.14 Msun) indicates a large span of masses of recycled pulsars and suggests a broad range of neutron stars masses at birth. We aim at reconstructing the pre-accretion masses for these pulsars while taking into account interaction of the magnetic field with a thin accretion disk, magnetic field decay and relativistic 2D solutions for stellar configurations for a set of equations of state. We briefly discuss the evolutionary scenarios leading to the formation of these neutron stars and study the influence of the equation of state.

Collisional Penrose process near the horizon of extreme Kerr black holes

Collisions of particles in black holes ergospheres may result in an arbitrarily large center of mass energy. This led recently to the suggestion (Banados et al., 2009) that black holes can act as ultimate particle accelerators. If the energy of an outgoing particle is larger than the total energy of the infalling particles the energy excess must come from the rotational energy of the black hole and hence this must involve a Penrose process. However, while the center of mass energy diverges the position of the collision makes it impossible for energetic particles to escape to infinity. Following an earlier work on collisional Penrose processes (Piran & Shaham 1977) we show that even under the most favorable idealized conditions the maximal energy of an escaping particle is only a modest factor above the total initial energy of the colliding particles. This implies that one shouldnt expect collisions around a black hole to act as spectacular cosmic accelerators.

Keplerian frequency of uniformly rotating neutron stars and quark stars

We calculate Keplerian (mass shedding) configurations of rigidly rotating neutron stars and quark stars with crusts. We check the validity of empirical formula for Keplerian frequency, f_K, proposed by Lattimer & Prakash, f_K(M)=C (M/M_sun)^1/2 (R/10km)^-3/2, where M is the (gravitational) mass of Keplerian configuration, R is the (circumferential) radius of the non-rotating configuration of the same gravitational mass, and C = 1.04 kHz. Numerical calculations are performed using precise 2-D codes based on the multi-domain spectral methods. We use a representative set of equations of state (EOSs) of neutron stars and quark stars. We show that the empirical formula for f_K(M) holds within a few percent for neutron stars with realistic EOSs, provided 0.5 M_sun < M < 0.9 M_max,stat, where M_max,stat is the maximum allowable mass of non-rotating neutron stars for an EOS, and C=C_NS=1.08 kHz. Similar precision is obtained for quark stars with 0.5 M_sun < M < 0.9 M_max,stat. For maximal crust masses we obtain C_QS = 1.15 kHz, and the value of C_QS is not very sensitive to the crust mass. All our Cs are significantly larger than the analytic value from the relativistic Roche model, C_Roche = 1.00 kHz. For 0.5 M_sun < M < 0.9 M_max,stat, the equatorial radius of Keplerian configuration of mass M, R_K(M), is, to a very good approximation, proportional to the radius of the non-rotating star of the same mass, R_K(M) = aR(M), with a_NS approx a_QS approx 1.44. The value of a_QS is very weakly dependent on the mass of the crust of the quark star. Both as are smaller than the analytic value a_Roche = 1.5 from the relativistic Roche model.