Gravitational wave solutions of quadratic curvature gravity using a null coframe formulation

Quadratic curvature gravity equations are projected to a complex null coframe by using the algebra of exterior forms and expressed in terms of the spinor quantities defined originally by Newman and Penrose. As an application, a new family of impulsive gravitational wave solutions propagating in various type D backgrounds is introduced.Comment: Title rephrased, minor changes in the text. Conclusion expanded and new references adde

Short term pulse frequency fluctuations of OAO 1657-415 from RXTE observations

We present new X-ray observations of the high mass X-ray binary (HMXRB) pulsar OAO 1657-415, obtained during one orbital period (10.44 days) with the Rossi X-Ray Timing Explorer (RXTE). Using the binary orbital parameters, obtained from Burst and Transient Source Experiment (BATSE) observations, we resolve the fluctuations in the pulse frequency at time scales on the order of one day for the first time. Recent BATSE results showed that OAO 1657-415 has spin-up/down trends in its pulse frequency time series, without any correlation with the X-luminosity at energies $>$20 keV (Baykal 1997). In the present RXTE observations the source is found to be in an extended phase of spin-down. We also find a gradual increase in the X-ray luminosity which is correlated with a marginal spin-up episode. The marginal correlation between the gradual spin-up (or decrease in spin-down rate) and increase in X-ray luminosity suggests that the OAO 1657-415 is observed during a stable accretion episode where the prograde accretion disk is formed.Comment: MNRAS (2000), vol 313, 63

The Torque and X-Ray Flux Changes of OAO 1657-415

Combining previously published pulse frequencies and BATSE measurements, we estimate the noise strengths (or power density estimates) of angular accelerations by using the root mean square residuals of angular velocity time series of OAO 1657-415 and present the power spectra. The statistical interpretation of the angular velocity fluctuations are consistent with a random walk model. In order to investigate the short term angular velocity fluctuations in detail, a structure function analysis is applied for a two component neutron star model with a solid crust and a superfluid neutron core which is subjected to external white torque noise. No evidence for core-crust coupling on timescales longer than one day is found. The correlations between X-ray flux and angular acceleration ($\dot \Omega$) fluctuations are investigated. These are compared with disk accretion theory (Ghosh & Lamb 1979 a,b) and wind accretion theory (Blondin et al., 1990). It is found that the most natural explanation of X ray flux and angular acceleration fluctuations is the formation of episodic accretion disks in the case of stellar wind accretion.Comment: 39 pages, to appear in "Astronomy and Astrophysics

Energy definition for quadratic curvature gravities

A conserved current for generic quadratic curvature gravitational models is defined, and it is shown that, at the linearized level, it corresponds to the Deser-Tekin charges. An explicit expression for the charge for new massive gravity in three dimensions is given. Some implications of the linearized equations are discussed.Comment: 7 page

Variational derivatives of gravitational actions

A method of calculation for the variational derivatives for gravitational actions in the pseudo-Riemannian case is proposed as a practical variant of the first order formalism with constraints. The method is then used to derive the metric field equations for a generic $f(R)$ model.Comment: References are reordered and updated. Minor changes in the abstrac

An alternative derivation of the Minimal massive 3D gravity

By using the algebra of exterior forms and the first order formalism with constraints, an alternative derivation of the field equations for the Minimal massive 3D gravity model is presented.Comment: 12 pages in IOP article format, improved presentation with minor corrections, an appendix adde

Underdetermined-order recursive least-squares adaptive filtering: The concept and algorithms

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Impulsive gravitational waves in general massive 3D gravity

Impulsive, non-diverging, Petrov-Segre type N gravitational wave solutions to a general massive three-dimensional gravity in the de Sitter, anti-de Sitter and flat Minkowski backgrounds are constructed in a unified manner by using the exterior algebra of differential forms.Comment: Published versio