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We present a comprehensive analysis of all XMM–Newton spectra of OJ 287 spanning 15 yr of X-ray spectroscopy of this bright blazar. We also report the latest results from our dedicated Swift UVOT and XRT monitoring of OJ 287, which started in 2015, along with all earlier public Swift data since 2005. During this time interval, OJ 287 was caught in extreme minima and outburst states. Its X-ray spectrum is highly variable and encompasses all states seen in blazars from very flat to exceptionally steep. The spectrum can be decomposed into three spectral components: Inverse Compton (IC) emission dominant at low-state, supersoft synchrotron emission that becomes increasingly dominant as OJ 287 brightens, and an intermediately-soft (Γx = 2.2) additional component seen at outburst. This last component extends beyond 10 keV and plausibly represents either a second synchrotron/IC component and/or a temporary disc corona of the primary supermassive black hole (SMBH). Our 2018 XMM–Newton observation, quasi-simultaneous with the Event Horizon Telescope observation of OJ 287, is well described by a two-component model with a hard IC component of Γx = 1.5 and a soft synchrotron component. Low-state spectra limit any long-lived accretion disc/corona contribution in X-rays to a very low value of Lx/LEdd < 5.6 × 10−4 (for MBH, primary = 1.8 × 1010 M⊙). Some implications for the binary SMBH model of OJ 287 are discussed. © 2021 The Author(s)., We would like to thank the Swift and XMM–Newton teams for carrying out our observations and our anonymous referee for their useful comments and suggestions. JLG acknowledges financial support from the Spanish Ministerio de Economía y Competitividad (grants AYA2016-80889-P, PID2019-108995GB-C21), the Consejería de Economía, Conocimiento, Empresas y Universidad of the Junta de Andalucía (grant P18-FR-1769), the Consejo Superior de Investigaciones Científicas (grant 2019AEP112), and the State Agency for Research of the Spanish MCIU through the Center of Excellence Severo Ochoa award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709). SGJ acknowledges financial support from NASA Fermi GI grants 80NSSC20K1565 and 80NSSC20K1566. DH acknowledges funding from the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canada Research Chairs (CRC) program. We acknowledge the use of data we have obtained with the Neil Gehrels Swift mission. We also acknowledge the use of public data from the Swift data archive. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. This research is partly based on observations obtained with XMM–Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. This research has made use of the XRT Data Analysis Software (XRTDAS) developed under the responsibility of the ASI Space Science Data Center (SSDC), Italy.

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 figures

Gene Byrd and Mauri Valtonen describe how radio astronomy helped prove the existence of dark matter.

We have studied short-term variations of the blazar OJ 287, suspected to host a supermassive black hole binary. In this study, we use a two-season optical R-band dataset from 2004--2006 which consists of 3991 data points from the OJ 287 observation campaign. It has sections of dense time coverage, and is largely independent from previously published data. We find that this data confirms the existence of a ~50 day periodic component, presumably related to the half-period of the innermost stable circular orbit (ISCO) of the primary black hole. In addition we find several pseudo-periodic components in the 1 to 7 day range, most prominently at 3.5 days, which are likely Lorentz contracted jet re-emission of the 50 day component. The typical 50 day cycle exhibits a slow rise of brightness and a rapid dimming before the start of the new cycle. We explain this as being due to a spiral wave in the accretion disc which feeds the central black hole in this manner., 8 pages, 12 figures, to be published in Monthly Notices of the Royal Astronomical Society

Two multifrequency campaigns were carried out on OJ287 in 2005: in April when it was in its pre-outburst state, and in November, during the main 12 yr cycle outburst. The wavelength coverage was from radio to X-rays. In the optical-to-UV range the differential spectrum between the observations has a bremsstrahlung spectral shape, consistent with gas at $3 \times 10^{5}K$ temperature. Our result supports the hydrogen column density of the OJ287 host galaxy of $\sim9.3\times 10^{20} cm^{-2}$, the average value found by Gosh & Soundararajaperumal. The $3 \times 10^{5}K$ bremsstrahlung radiation was predicted in the binary black hole model of OJ287, and it arises from a hot bubble of gas which is torn off the accretion disc by the impact of the secondary. As this radiation is not Doppler boosted, the brightness of the outburst provides an estimate for the mass of the secondary black hole, $\sim1.4\times10^{8}$ solar mass. In order to estimate the mass of the primary black hole, we ask what is the minimum mass ratio in a binary system which allows the stability of the accretion disc. By using particle simulations, we find that the ratio is $\sim1.3\times10^{2}$. This makes the minimum mass of the primary $\sim1.8\times10^{10}$ solar mass, in agreement with the mass determined from the orbit solution, $1.84 \times 10^{10}$ solar mass. With this mass value and the measured K-magnitude of the bulge of the host galaxy of OJ287, the system lies almost exactly on the previously established correlation in the black hole mass vs. K-magnitude diagramme. It supports the extension of this correlation to brighter magnitudes and to more massive black holes than has been done previously.

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.

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., 13 figures and 7 tables, Accepted 2007 September 19. Received 2007 September 19; in original form 2007 April 2

Dynamical evolution of 120 000 equal-mass rotating triple systems is investigated. The system rotation is described by the parameter w = -L 2 0 E 0 /G 2 m 5 0 , where G is the gravitational constant, m 0 is the mass of a body, and L 0 and E 0 are the angular momentum and the total energy of the triple system, respectively. We consider the values of w = 0.005, 0.1, 1, 2, 4 and 6. For each w, 20 000 triple systems are studied. The initial coordinates and velocities of the components are randomly chosen. The initial data are chosen in two different ways: the first one assumes a hierarchical structure initially and the second one does not. The evolution of each triple system is calculated until either the escape of one of the bodies occurs or the time exceeds 1000 mean crossing times of the system. The orbital parameters of the final binary and the escaper are recorded for each run. We compare the results of numerical simulations and predictions of a statistical escape theory. The statistical theory is based on the assumption of ergodicity, that is, the only information on the initial conditions remaining at the time of the escape of the third body is contained in the conserved total energy, total angular momentum and the mass values. The distributions of various quantities are derived from the allowable phase-space volumes. The distribution of binary energy agrees with earlier results by Heggie, with the angular momentum dependence of Mikkola & Valtonen being added. The eccentricities are distributed in general accordance with Monaghan's work, while the triple systems break up like in radioactive decay, as was previously found by Valtonen & Aarseth. The escape directions are preferentially perpendicular to the total angular momentum vector; the more so, the greater the angular momentum. The escape-angle distributions are derived from the statistical theory and are found to be in agreement with the numerical data. The relative orientations and magnitudes of the binary and third-body angular momenta are also explained based on the statistical theory.

We identify a sample of 14 planetary nebulae in the nearby starburst disc galaxy NGC 253, using broad- and narrow-band imaging. A simple model of the stellar and dust layers in the galaxy shows that planetary nebula magnitudes are not statistically affected much by extinction. The luminosity function of the planetary nebulae is fitted with models accounting for dust, which leads to a distance to NGC 253 of µ0 = 27.62 +0.16 mag (d = 3.34 +0.26 Mpc). This new estimate is found to be in agreement with the few existing distance estimates for this galaxy when they are all set to a common Large Magellanic Cloud distance of 18.50 mag. A weighted average of the most reliable distance estimates yields a distance of µ0 = 27.7 ± 0.2 mag (d = 3.5 ± 0.2 Mpc). Ke yw ords: galaxies: distances and redshifts ‐ galaxies: individual: NGC 253.

Assuming the Jovian planets experienced smooth orbital migrations, Malhotra successfully explained some important features of the Kuiper Belt by the mechanism of resonance capture. However, the migration should really be stochastic. In this paper we numerically simulate numerous orbital evolutions of test particles under such stochastic planet migrations. The main results are as follows. Given the proper parameters, the concentration of objects in the 3 : 2 resonance is distinct, while very few objects enter the 2 : 1 resonance, and consequently, the orbital distribution of Kuiper Belt objects matches the observational data very well. The stochastic effects excite the orbital eccentricities and inclinations of objects in the non-resonant regions. The introduction of stochastic effects also gives possible explanations of the mass depletion, and of the sources of the classical and scattered Kuiper Belt objects.

ABSTRA C T We test different possibilities for the origin of short-period comets captured from the Oort Cloud. We use an efficient Monte Carlo simulation method that takes into account nongravitational forces, Galactic perturbations, observational selection effects, physical evolution and tidal splittings of comets. We confirm previous results and conclude that the Jupiter family comets cannot originate in the spherically distributed Oort Cloud, since there is no physically possible model of how these comets can be captured from the Oort Cloud flux and produce the observed inclination and Tisserand constant distributions. The extended model of the Oort Cloud predicted by the planetesimal theory consisting of a non-randomly distributed inner core and a classical Oort Cloud also cannot explain the observed distributions of Jupiter family comets. The number of comets captured from the outer region of the Solar system are too high compared with the observations if the inclination distribution of Jupiter family comets is matched with the observed distribution. It is very likely that the Halley-type comets are captured mainly from the classical Oort Cloud, since the distributions in inclination and Tisserand value can be fitted to the observed distributions with very high confidence. Also the expected number of comets is in agreement with the observations when physical evolution of the comets is included. However, the solution is not unique, and other more complicated models can also explain the observed properties of Halley-type comets. The existence of Jupiter family comets can be explained only if they are captured from the extended disc of comets with semimajor axes of the comets a , 5000 au. The original flattened distribution of comets is conserved as the cometary orbits evolve from the outer Solar system era to the observed region.

Time variation in impact probability is studied by assuming that the periodic flux of the Oort Cloud comets within 15 au arises from the motion of the Sun with respect to the Galactic mid-plane. The periodic flux clearly shows up in the impact rate of the captured Oort Cloud cometary population, with a phase shift caused by the orbital evolution. Depending on the assumed flux of comets and the size distribution of comets, the impact rate of the Oort Cloud comets of 1 km in diameter or greater is from 5 to 700 impacts Myr−1 on the Earth and from 0.5 to 70 impacts per 1000 yr on Jupiter. The relative fractions of impacts are 0.09, 0.11, 0.26 and 0.54 for long-period comets, Halley type comets, Jupiter family comets and near-Earth objects, respectively. For Jupiter, the corresponding fractions in the first three categories are 0.18, 0.31 and 0.51. If we consider physical fading of comet activity that is compatible with the observations, then the impact rates of active comets are two orders of magnitude smaller than the total impact rates by all kinds of comets and cometary asteroids of size 1 km or greater.

An ensemble of Newtonian three-body models with close initial separations is investigated by following the evolution of a ‘drop’ in the homology map. The onset of chaos is revealed by the motion and the complex temporal deformation of the drop. In the state of advanced chaos, the drop spreads over almost the whole homology map, quite independently of its initial position on the map. A general quantitative measure of this process is the mean exponential rate of spreading, which bears resemblance to Kolmogorov–Sinai entropy; this is introduced and estimated in terms of the homology mapping. In a similar manner we also estimate the mean exponential rate of divergence of initially close-by trajectories. This is a close analogue to the Lyapunov exponent. These parameters measure two complementary aspects of dynamical instability, which is the basic mechanism of the onset of chaos.

Stars pass the Sun all the time, and many of these stars may have their own planetary systems and their own `Oort cloud' of comets. We consider a straightforward problem in which the planetary system of the passing star is identical to the planetary system of the Sun, and also the cloud of comets is identical to the Oort cloud of the Solar system. We calculate (1) the rate of loss of comets from this other planetary system, (2) the frequency of passage of other stars at a minimum distance r0 and at a constant velocity v0 relative to the Sun, (3) the number and velocity distribution of comets coming from the passing star and impacting our planetary system, and finally (4) the number of cometary collisions with the Earth resulting from this process.

ABSTRA C T We have studied the structure of chaos in three-body dynamics using the concept of intermittency, implying that violent states of a system alternate in time with quasi-regular states producing together a non-stationary and evolving pattern of unpredictable behaviour. Computer simulations are produced to demonstrate explicitly sporadic short violent bursts in quasi-regular hierarchical states of the systems. This is seen both in orbits and in the long time series generated by the system. The time series prove to be similar in shape to what is observed in various physical experiments with laboratory chaotic systems when they reveal the so-called type-III intermittency. The new effective methods of time series analysis enable us to discover a strange attractor with a fractal dimension slightly above 2. This shows that three-body dynamics has the same intrinsic qualitative structure and quantitative measure of chaos as the widely known chaotic system, the Lorenz attractor.

In the neighbourhood of the Local Group of galaxies, some high speeds of relative motion are observed. In particular, the speeds of Maffei 1 and IC 342 relative to the Andromeda galaxy are in the range 250-300 km s −1 , which are rather high velocities for the local neighbourhood. Here we study groups of four galaxies, initially composed of two binaries, and follow their evolution through two mergers until only two galaxies remain. We find that the speed of separation of the two galaxies can reach the above-mentioned range of values if (a) the rotation curves of the galaxies rise to this same range of values, and (b) the mass of the group is not excessive. We use these conditions to estimate the mass-to-light ratio of the galaxies in the local area

Numerical experiments in the framework of the elliptic restricted three-body problem have been applied to obtain the effects of dynamical friction on the evolution of a binary in the field of light particles. Rate coefficients are derived for the evolution of the energy and angular momentum of the heavy binary. The results are compared with an N-body simulation and also with recent theoretical estimates for the case of high-velocity particles. Good agreement is observed in all the comparisons. The angular momentum transfer rate allows us to estimate the evolution of the eccentricity of the binary

On a obtenu des formules donnant la distribution de l'energie en fonction du moment angulaire, ainsi qu'une approximation pour la probabilite de liberation d'une masse donnee dans le cas ou les masses sont inegales. La comparaison avec les simulations numeriques donne un bon accord

Etude du freinage gravitationnel exerce par une galaxie disque sur des galaxies satellites. On a utilise des modeles de simulation par ordinateur: un disque Mestel «froid» sans dispersion des vitesses et un disque «chaud» plus realiste avec une dispersion des vitesses a l'interieur d'un halo spherique inerte