Anderson Caproni

Warping and precession in galactic and extragalactic accretion disks

The Bardeen-Petterson general relativistic effect has been suggested as the mechanism responsible for precession in some accretion disk systems. Here we examine separately four mechanisms (tidal, irradiative, magnetic, and Bardeen-Petterson) that can lead to warping and precession. We use a sample of eight X-ray binaries and four active galactic nuclei (AGNs) that present signatures of warping and/or precession in their accretion disks to explore the viability of the different mechanisms. For the X-ray binaries SMC X-1 and 4U 1907+09, all four mechanisms provide precession periods compatible with those observed, while for Cyg X-1 and the active galaxies Arp 102B and NGC 1068, only two mechanisms are in agreement with the observations. The irradiation-driven instability seems to be incapable of producing the inferred precession of the active galaxies in our sample, and the tidally induced precession can probably be ruled out in the case of Arp 102B. Perhaps the best case for a Bardeen-Petterson precession can be achieved for NGC 1068. Our results show that, given the many observational uncertainties that still exist, it is extremely difficult to confirm unambiguously that the Bardeen-Petterson effect has been observed in any of the other sources of our sample.

Precession in the Inner Jet of 3C 345

VLBI observations have shown that the parsec jet of 3C 345 is formed by several components, ejected from the core with superluminal velocities and traveling along bent trajectories on the plane of the sky. We interpret the differences in velocity and position angle among the different features at formation time as the result of parsec-scale precession of the relativistic jet, and we calculate the aperture angle of the precession cone, the angle between the cone axis and the line of sight, and the Lorentz factor associated with the jet bulk motion. We assumed a precession period of 10.1 yr, which is one of the long-term B-band light-curve periods reported in the literature. We propose that boosting of the underlying jet emission, which is time-dependent as a result of precession, is responsible for this long-term optical variability. Jet precession with periods of several years can be produced in supermassive black hole binary systems, when the secondary black hole is in an orbit noncoplanar with the primary accretion disk, inducing torques in the inner parts of the disk. Assuming that this mechanism is responsible for the jet precession in 3C 345, we estimate upper and lower limits for the masses of the two black holes, as well as their mean separation. We found a correlation between the formation of jet components and the occurrence of strong optical flares, as well as a very strong anticorrelation between the intensity of these flares and the time required for the components to reach the maximum flux density at radio frequencies.

Parsec-scale jet precession in BL Lacertae (2200+420)

BL Lacertae is the prototype of the BL Lac class of active galactic nuclei, exhibiting intensive activity on parsec (pc) scales, such as intense core variability and multiple ejections of jet components. In particular, in previous works the existence of precession motions in the pc-scale jet of BL Lacertae has been suggested. In this work we revisit this issue, investigating temporal changes of the observed right ascension and declination offsets of the jet knots in terms of our relativistic jet-precession model. The seven free parameters of our precession model were optimized via a heuristic cross-entropy method, comparing the projected precession helix with the positions of the jet components on the plane of the sky and imposing constraints on their maximum and minimum superluminal velocities. Our optimized best model is compatible with a jet having a bulk velocity of 0.9824c, which is precessing with a period of about 12.1 yr in the observer’s reference frame and changing its orientation in relation to the line of sight between 4 ◦ and 5 ◦ , approximately. Assuming that the jet precession has its origin in a supermassive binary black hole system, we show that the 2.3-yr periodic variation in the structural position angle of the very-long-baseline interferometry (VLBI) core of BL Lacertae reported by Stirling et al. is compatible with a nutation phenomenon if the secondary black hole has a mass higher than about six times that of the primary black hole.

Observational Evidence of Spin-induced Precession in Active Galactic Nuclei

We show that it is possible to explain the physical origin of jet precession in active galactic nuclei through the misalignment between the rotation axes of the accretion disk and of the Kerr black hole. We apply this scenario to quasars, Seyfert galaxies, and also to the Galactic center black hole Sgr A*, for which signatures of either jet or disk precession have been found. The formalism adopted is parameterized by the ratio of the precession period to the black hole mass and can be used to put constraints on the physical properties of the accretion disk as well as on the black hole spin in those systems.

Can long-term periodic variability and jet helicity in 3C 120 be explained by jet precession?

Optical variability of 3C 120 is discussed in the framework of jet precession. Specifically, we assume that the observed long-term periodic variability is produced by the emission from an underlying jet with a time-dependent boosting factor driven by precession. The differences in the apparent velocities of the different superluminal components in the milliarcsec jet can also be explained by the precession model as being related to changes in the viewing angle. The evolution of the jet components has been used to determine the parameters of the precession model, which also reproduce the helical structure seen at large scales. Among the possible mechanisms that could produce jet precession, we consider that 3C 120 harbours a supermassive black hole binary system in its nuclear region, and that torques induced by misalignment between the accretion disc and the orbital plane of the secondary black hole are responsible for this precession; we estimate upper and lower limits for the black holes masses and their mean separation.

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.

Bardeen-Petterson Effect and the Disk Structure of the Seyfert Galaxy NGC 1068

VLBA high spatial resolution observations of the disk structure of the active galactic nucleus (AGN) NGC 1068 have recently revealed that the kinematics and geometry of this AGN is well characterized by an outer disk of H2O maser emission having a compact milliarcsecond- (parsec-) scale structure, which is encircling a thin rotating inner disk surrounding a ~107 M☉ compact mass, likely a black hole. A curious feature in this source is the occurrence of a misalignment between the inner and outer parts of the disk, with the galaxys radio jet being orthogonal to the inner disk. We interpret this peculiar configuration as due to the Bardeen-Petterson effect, a general relativistic effect that warps an initially inclined (to the black hole equator) viscous disk and drives the angular momentum vector of its inner part into alignment with the rotating black hole spin. We estimate the timescale for both angular momenta to get aligned as a function of the spin parameter of the Kerr black hole. We also reproduce the shape of the parsec- and kiloparsec-scale jets, assuming a model in which the jet is precessing with a period and aperture angle that decrease exponentially with time, as expected from the Bardeen-Petterson effect.

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

The mass-loss process in dwarf galaxies from 3D hydrodynamical simulations: the role of dark matter and starbursts

Theoretical

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

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