Z. Abraham

Wind-wind collision in the η Carinae binary system: a shell-like event near periastron

ABSTRACT The exact nature of η Carinae is still an open issue. Strict periodicity in the lightcurves at several wavelengths seem to point out to a binary system, but the observedradial velocities, measured from space with high spatial resolution are in conflict withthe ground based observations used to calculate the binary orbit. Also, the observed2-10 keV X-ray flux is much larger that what is expected from a single star, andfavors the wind-wind collision hypothesis, characteristic of high mass binary systems.However, to explain the duration of the dip in the light curve by wind collisions, it isnecessary to postulate a very large increase in the η Carinae mass loss rate. Finally, theoptical and UV light curves are better explained by periodic shell-ejection events. Inthis paper we conciliate the two hypothesis. We still assume a binary system to explainthe strong X-ray emission, but we also take into account that, near periastron andbecause of the highly eccentric orbit, the wind emerging from η Carinae accumulatesbehind the shock and can mimic a shell-like ejection event. For this process to beeffective, at periastron the secondary star should be located between η Carinae andthe observer, solving also the discrepancy between the orbital parameters derived fromground and space based observations. We show that, as the secondary moves in itsorbit, the shell cools down and the number of available stellar ionizing photons is notenough to maintain the shell temperature at its equilibrium value of about 7500 K.The central part of the shell remains cold and under these conditions grain formationand growth can take place in timescales of hours. We also calculated the neutral gascolumn density intercepting the line of sight at each point of the orbit near periastron,and were able to reproduce the form and duration of the X-ray light curve withoutany change in the η Carinae mass loss rate. This same column density can explain theobserved Hα light curve observed during the 2003 event.Key words: stars: individual (η Car) binaries: general stars: variable X-rays: stars

Is the Bardeen-Petterson effect responsible for the warping and precession in NGC 4258?

Strong evidence for the presence of a warped Keplerian accretion disc in NGC 4258 (M 106) has been inferred from the kinematics of water masers detected at subparsec scales. Assuming a power-law accretion disc and using constraints on the disc parameters derived from observational data, we have analysed the relativistic Bardeen‐Petterson effect driven by a Kerr black hole as the potential physical mechanism responsible for the disc warping. We found that the Bardeen‐Petterson radius is comparable to or smaller than the inner radius of the maser disc (independent of the allowed value for the black hole spin parameter). Numerical simulations for a wide range of physical conditions have shown that the evolution of a misaligned disc due to the Bardeen‐Petterson torques usually produces an inner flat disc and a warped transition region with a smooth gradient in the tilt and twist angles. Since this structure is similar to that seen in NGC 4258, we propose that the Bardeen‐Petterson effect may be responsible for the disc warping in this galaxy. We estimated the time-scale necessary for the disc inside of the Bardeen‐Petterson radius to align with the black hole’s equator, as a function of the black hole spin. Our results show that the Bardeen‐Petterson effect can align the disc within a few billion years in the case of NGC 4258. Finally, we show that if the observed curvature of the outer anomalous arms in the galactic disc of NGC 4258 is associated with the precession of its radio jet/counterjet, then the Bardeen‐Petterson effect can provide the required precession period.

Wind-wind collision in the η Carinae binary system — II. Constraints to the binary orbital parameters from radio emission near periastron passage

ABSTRACT In this paper we use the 7 mm and 1.3 mm light curves obtained during the2003.5 low excitation phase of the η Carinae system to constrain the possibleparameters of the binary orbit. To do that we assumed that the mm waveemission is produced in a dense disk surrounding the binary system; duringthe low excitation phase, which occurs close to periastron, the number ofionizing photons decreases, producing the dip in the radio emission. On theother hand, due to the large eccentricity, the density of the shock region atperiastron is very high and the plasma is optically thick for free-free radiationat 7 mm, explaining the sharp peak that was observed at this frequency andlastedforabout10days.Fromtheshapeanddurationofthepeakwewereableto determine the orbital parameters of the binary system, independently ofthe stellar parameters, such as mass loss rates, wind velocities or temperatureat the post-shock region.Key words: stars: individual (η Car) binaries: general stars: variable radiocontinuum: general

Wind–wind collision in the η Carinae binary system – III. The He ii λ4686 line profile

We modelled the Hexa0iiλ4686 line profiles observed in the η Carinae binary system close to the 2003.5 spectroscopic event, assuming that they were formed in the shocked gas that flows at both sides of the contact surface formed by wind–wind collision. We used a constant flow velocity and added turbulence in the form of a Gaussian velocity distribution. We allowed emission from both the primary and secondary shocks but introduced infinite opacity at the contact surface, implying that only the side of the contact cone visible to the observer contributed to the line profile. Using the orbital parameters of the binary system derived from the 7-mm light curve during the last spectroscopic event (Paper II) we were able to reproduce the line profiles obtained with the Hubble Space Telescope at different epochs, as well as the line mean velocities obtained with ground-based telescopes. A very important feature of our model is that the line profile depends on the inclination of the orbital plane; we found that to explain the latitude-dependent mean velocity of the line, scattered into the line of sight by the Homunculus, the orbit cannot lie in the Homunculus equatorial plane, as usually assumed. This result, together with the relative position of the stars during the spectroscopic events, allowed us to explain most of the observational features, like the variation of the ‘Purple Haze’ with the orbital phase, and to conciliate the X-ray absorption with the postulated shell effect used to explain the optical and ultraviolet light curves.

MHD numerical simulations of colliding winds in massive binary systems – I. Thermal versus non‐thermal radio emission

In the past few decades detailed observations of radio and X-ray emission from massive binary systems revealed a whole new physics present in such systems. Both thermal and non-thermal components of this emission indicate that most of the radiation at these bands originates in shocks. O and B-type stars and Wolf–Rayet (WR) stars present supersonic and massive winds that, when colliding, emit largely due to the free–free radiation. The non-thermal radio and X-ray emissions are due to synchrotron and inverse Compton processes, respectively. In this case, magnetic fields are expected to play an important role in the emission distribution. In the past few years the modelling of the free–free and synchrotron emissions from massive binary systems have been based on purely hydrodynamical simulations, and ad hoc assumptions regarding the distribution of magnetic energy and the field geometry. In this work we provide the first full magnetohydrodynamic numerical simulations of wind–wind collision in massive binary systems. We study the free–free emission characterizing its dependence on the stellar and orbital parameters. We also study self-consistently the evolution of the magnetic field at the shock region, obtaining also the synchrotron energy distribution integrated along different lines of sight. We show that the magnetic field in the shocks is larger than that obtained when the proportionality between B and the plasma density is assumed. Also, we show that the role of the synchrotron emission relative to the total radio emission has been underestimated.

Particle acceleration and magnetic field structure in PKS 2155−304: optical polarimetric observations

In this paper, we present multiband optical polarimetric observations of the very-high energy blazar PKS 2155-304 made simultaneously with a HESS/Fermi high-energy campaign in 2008, when the source was found to be in a low state. The intense daily coverage of the data set allowed us to study in detail the temporal evolution of the emission, and we found that the particle acceleration time-scales are decoupled from the changes in the polarimetric properties of the source. We present a model in which the optical polarimetric emission originates at the polarized mm-wave core and propose an explanation for the lack of correlation between the photometric and polarimetric fluxes. The optical emission is consistent with an inhomogeneous synchrotron source in which the large-scale field is locally organized by a shock in which particle acceleration takes place. Finally, we use these optical polarimetric observations of PKS 2155-304 at a low state to propose an origin for the quiescent gamma-ray flux of the object, in an attempt to provide clues for the source of its recently established persistent TeV emission.

Cross‐entropy optimizer: a new tool to study precession in astrophysical jets

Evidence of jet precession in many galactic and extragalactic sources has been reported in the literature. Much of this evidence is based on studies of the kinematics of the jet knots, which depends on the correct identification of the components to determine their respective proper motions and position angles on the plane of the sky. Identification problems related to fitting procedures, as well as observations poorly sampled in time, may influence the follow-up of the components in time, which consequently might contribute to a misinterpretation of the data. In order to deal with these limitations, we introduce a very powerful statistical tool to analyse jet precession: the cross-entropy method for continuous multi-extremal optimization. Only based on the raw data of the jet components (right ascension and declination offsets from the core), the cross-entropy method searches for the precession model parameters that better represent the data. In this work we present a large number of tests to validate this technique, using synthetic precessing jets built from a given set of precession parameters. With the aim of recovering these parameters, we applied the cross-entropy method to our precession model, varying exhaustively the quantities associated with the method. Our results have shown that even in the most challenging tests, the cross-entropy method was able to find the correct parameters within a 1 per cent level. Even for a non-precessing jet, our optimization method could point out successfully the lack of precession.

Constraining the orbital orientation of η Carinae from H Paschen lines

During the past decade, several observational and theoretical works have provided evidence of the binary nature of η Carinae. Nevertheless, there is still no direct determination of the orbital parameters, and the different current models give contradictory results. The orbit is, in general, assumed to coincide with the Homunculus equator although the observations are not conclusive. Among all systems, η Car has the advantage that it is possible to observe both the direct emission of line transitions in the central source and its reflection by the Homunculus, which is dependent on the orbital inclination. In this work, we studied the orbital phase-dependent hydrogen Paschen spectra reflected by the south-east lobe of the Homunculus to constrain the orbital parameters of η Car and determine its inclination with respect to the Homunculus axis. Assuming that the emission excess originates in the wind-wind shock region, we were able to model the latitude dependence of the spectral line profiles. For the first time, we were able to estimate the orbital inclination of η Car with respect to the observer and to the Homunculus axis. The best fit occurs for an orbital inclination to the line of sight of i ∼ 60° ± 10°, and i * ∼ 35° ± 10° with respect to the Homunculus axis, indicating that the angular momenta of the central object and the orbit are not aligned. We were also able to fix the phase angle of conjunction as ∼ ―40°, showing that periastron passage occurs shortly after conjunction.

Modelling spectral line profiles of wind–wind shock emissions from massive binary systems

One of the most intriguing spectral features of Wolf-Rayet (WR) binary stars is the presence of time-dependent line profiles. Long-term observations of several systems revealed the periodicity of this variability, synchronized with the orbital movement. Several partially successful models have been proposed to reproduce the observed data. The most promising model assumes that the origin of the emission is the wind-wind interaction zone. In this scenario, two high velocity and dense winds produce a strong shock layer, responsible for most of the X-rays observed from these systems. As the gas cools down, flowing along the interaction surface, it reaches recombination temperatures and generates the emission lines. Luhrs noted that, as the secondary star moves along its orbital path, the shock region, of conical shape, changes its position with relation to the line of sight. As a consequence, the stream-measured Doppler shift presents time variations resulting in position changes of the spectral line. However, his model requires a very thick contact layer and also fails to reproduce recently observed line profiles of several other WR binary systems. In our work, we present an alternative model, introducing turbulence in the shock layer to account for the line broadening and opacity effects for the asymetry in the line profiles. We showed that the gas turbulence avoids the need of an unnaturally large contact layer thickness to reproduce line broadening. Also, we demonstrated that if the post-shock gas is optically thick at the observed line frequency, the emission from the opposing cone surface is absorbed, resulting in a single-peaked profile. This result fully satisfies the recent data obtained from massive binary systems, and can help in the determination of both the winds and the orbital parameters. We successfully applied this model to the Br22 system and determined its orbital parameters.

Kinematic study of the parsec-scale jet of the quasar PKS 1741–03

We present 23 interferometric images of parsec-scale jet of the quasar PKS 1741--03 at 15, 24 and 43 GHz spanning about 13 yr. We model the images as a superposition of discrete two--dimensional elliptical Gaussian components, with parameters determined by the cross--entropy technique. All the images present a spatially unresolved component (core) and usually two or three components receding from it. The same components were found in simultaneous 24 and 43 GHz maps, showing the robustness of our model-fitting. The core-shift opacity effect between these frequencies is weak. We have identified seven components moving along straight lines at constant apparent superluminal speeds (