A helical jet model for OJ287

Context. OJ287 is a quasar with a quasi-periodic optical light curve, with the periodicity observed for over 120 years. This has lead to a binary black hole model as a common explanation of the quasar. The radio jet of OJ287 has been observed for a shorter time of about 30 years. It has a complicated structure that varies dramatically in a few years time scale. Aims. Here we propose that this structure arises from a helical jet being observed from a small and varying viewing angle. The viewing angle variation is taken to be in tune with the binary orbital motion. Methods. We calculate the effect of the secondary black hole on the inner edge of the accretion disk of the primary using particle simulations. We presume that the axis of the helix is perpendicular to the disk. We then follow the jet motion on its helical path and project the jet to the sky plane. This projection is compared with observations both at mm waves and cm waves. Results. We find that this model reproduces the observations well if the changes in the axis of the conical helix propagate outwards with a relativistic speed of about 0.85c. In particular, this model explains at the same time the long-term optical brightness variations as varying Doppler beaming in a component close to the core, i.e. at parsec scale in real linear distance, while the mm and cm radio jet observations are explained as being due to jet wobble at much larger (100 parsec scale) distances from the core.Comment: 5 pages, 7 figures, to be published in Astronomy & Astrophysic

Are the nearby groups of galaxies gravitationally bound objects?

We have compared numerical simulations to observations for the nearby (< 40 Mpc) groups of galaxies (Huchra & Geller 1982 and Ramella et al. 2002). The group identification is carried out using a group-finding algorithm developed by Huchra and Geller (1982). Using cosmological N-body simulation code with the LambdaCDM cosmology, we show that the dynamical properties of groups of galaxies identified from the simulation data are, in general, in a moderate, within 2sigma, agreement with the observational catalogues of groups of galaxies. As simulations offer more dynamical information than observations, we used the N-body simulation data to calculate whether the nearby groups of galaxies are gravitationally bound objects by using their virial ratio. We show that in a LambdaCDM cosmology about 20 per cent of nearby groups of galaxies, identified by the same algorithm as in the case of observations, are not bound, but merely groups in a visual sense. This is quite significant, specifically because estimations of group masses in observations are often based on an assumption that groups of galaxies found by the friends-of-friends algorithm are gravitationally bound objects. Simulations with different resolutions show the same results. We also show how the fraction of gravitationally unbound groups varies when the apparent magnitude limit of the sample and the value of the cosmological constant is changed. In general, a larger value of the Omega_Lambda generates slightly more unbound groups.Comment: 13 figures and 7 tables, Accepted 2007 September 19. Received 2007 September 19; in original form 2007 April

Optical polarization angle and VLBI jet direction in the binary black hole model of OJ287

We study the variation of the optical polarization angle in the blazar OJ287 and compare it with the precessing binary black hole model with a 'live' accretion disk. First, a model of the variation of the jet direction is calculated, and the main parameters of the model are fixed by the long term optical brightness evolution. Then this model is compared with the variation of the parsec scale radio jet position angle in the sky. Finally, the variation of the polarization angle is calculated using the same model, but using a magnetic field configuration which is at a constant angle relative to the optical jet. It is found that the model fits the data reasonably well if the field is almost parallel to the jet axis. This may imply a steady magnetic field geometry, such as a large-scale helical field.Comment: to appear in Monthly Notices of Royal Astronomical Societ

Relativistic Effects on Triple Black Holes II: The Influence of Spin in Burrau's Problem

We continue our study of triple systems of black holes in the context of Burrau's problem by including the effect of spin. Numerical integration of orbits was conducted using ARCcode with relativistic corrections (post-Newtonian) up to the 2.5$^{th}$ order. Pythagorean triangles with with different linear scales were selected where the largest black hole in these systems were given spin vectors in normalised units where maximum is close to unity, ranging from 0 to about 0.95. We also study different masses in scales ranging from 10$^{0}$ M$_{\odot}$ - 10$^{12}$ M$_{\odot}$. It was found that while there was no distinctive effect on the number of two-body encounters nor the fraction of mergers, the lifetimes of the systems may have been affected - particularly in the intermediary mass ranges (10$^{4}$ M$_{\odot}$-10$^{7}$ M$_{\odot}$) in comparison to the zero spin problem. Differences were also found between configurations considered more hierarchical and those of lesser hierarchy. Triple systems ended up moving from the two-dimensional planar problem to the three dimensional one where we see increased motion in the z-axis with increasing spin magnitude for the large mass systems. The argument of periapsis, $\omega$, and the longitude of ascending node, $\Omega$ between the rotating black hole and a non-rotating one within the system, were also affected by the added spin

Gargantuan chaotic gravitational three-body systems and their irreversibility to the Planck length

Chaos is present in most stellar dynamical systems and manifests itself through the exponential growth of small perturbations. Exponential divergence drives time irreversibility and increases the entropy in the system. A numerical consequence is that integrations of the N-body problem unavoidably magnify truncation and rounding errors to macroscopic scales. Hitherto, a quantitative relation between chaos in stellar dynamical systems and the level of irreversibility remained undetermined. In this work we study chaotic three-body systems in free fall initially using the accurate and precise N-body code Brutus, which goes beyond standard double-precision arithmetic. We demonstrate that the fraction of irreversible solutions decreases as a power law with numerical accuracy. This can be derived from the distribution of amplification factors of small initial perturbations. Applying this result to systems consisting of three massive black holes with zero total angular momentum, we conclude that up to five percent of such triples would require an accuracy of smaller than the Planck length in order to produce a time-reversible solution, thus rendering them fundamentally unpredictable.Comment: Accepted for publication in MNRAS. 7 pages, 4 figure

Time-domain behavior of blazar OJ 287 and the binary supermassive black hole conjecture

The proper understanding of blazar variability at the various electromagnetic spectral bands is one goal of multifrequency astrophysics. In this frame a peculiar and controversial phenomenology is the periodicity, postulated for long-term radio or optical flux light curves of about a dozen of blazars. The well-known BL Lac object OJ 287 (PKS 0851+202, S3 0851+20, PG 0851+202, z = 0.306) is not only a high-variable, peculiar, extragalactic source with hints for approximatively cyclical optical outbursts, but it also represents a case of substantial intensive and extensive (longterm) multifrequency observations. This rich database allow us a deeper analysis based on a wide range of variability timescales with some recent results that are highlighted here