Zulema Abraham

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.

Millimeter-wave emission during the 2003 low excitation phase of eta Carinae

In this paper we present observations of η Carinae in the 1.3 mm and 7 mm radio continuum, during the 2003.5 low excitation phase. The expected minimum in the light curves was confirmed at both wavelengths and was probably due to a decrease in the number of UV photons available to ionize the gas surrounding the binary system. At 7 mm a very well defined peak was superimposed on the declining flux density. It presented maximum amplitude in 29 June 2003 and lasted for about 10 days. We show that its origin can be free-free emission from the gas at the shock formed by wind-wind collision, which is also responsible for the observed X-ray emission. Even though the shock strength is strongly enhanced as the two stars in the binary system approach each other, during periastron passage the X-ray emission is strongly absorbed and the 7 mm observations represent the only direct evidence of this event.

The Precessing Jet in 3C 279

A model of a precessing jet obtained from radio data was used to explain the optical light curve of the OVV quasar 3C 279. The geometrical parameters of the model and the bulk motion of the jet were obtained from the position angles, velocities, and epoch of formation of the superluminal features in the radio jet. As a consequence, the model provides the Doppler factor time dependence. This factor reached its maximum value at the epoch in which the very strong and fast optical flare was observed. Therefore, the large increases in the emission at optical wavelengths could be the result of beaming, as the jet approaches its minimum separation to the line of sight. Fast variability observed at these epochs would be naturally explained by the shortening of the timescale in the observers reference frame due to relativistic effects. The precessing jet model could also explain the variability in the γ-ray emission assuming that the high-energy radiation is produced by the inverse Compton process on external seed photons. The variability would occur when the precessing jet sweeps the broadline region clouds, which would provide the seed photons.

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.

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.

GEMINI near-infrared spectroscopic observations of young massive stars embedded in molecular clouds

K-band spectra of young stellar candidates in four southern hemisphere clusters have been obtained with the near-infrared spectrograph GNIRS in Gemini South. The clusters are associated with IRAS sources that have colours characteristic of ultracompact HII regions. Spectral types were obtained by comparison of the observed spectra with those of a NIR library; the results include the spectral classification of nine massive stars and seven objects confirmed as background late-type stars. Two of the studied sources have K-band spectra compatible with those characteristic of very hot stars, as inferred from the presence of Civ, Niii, and Nv emission lines at 2.078 � m , 2.116 � m, and 2.100 � m respectively. One of them, I16177 IRS1, has a K-band spectrum similar to that of Cyg OB2 7, an O3If* supergiant star. The nebular K-band spectrum of the associated UC Hii region shows the s-process [Kriii] and [Seiv] high excitation emission lines, previously identified only in planetary nebula. One young stellar object (YSO) was found in each cluster, associated with either the main IRAS source or a nearby resolved MSX component, confirming the results obtained from previous NIR photometric surveys. The distances to the stars were derived from their spectral types and previously determined JHK magnitudes; they agree well with the values obtained from the kinematic method, except in the case of IRAS15408-5356, for which the spectroscopic distance is about a factor two smaller than the kinematic value.

The Young Massive Stellar Cluster Associated with RCW 121

We report near-infrared (NIR) broadband and narrowband photometric observations in the direction of the IRAS 17149-3916 source that reveal the presence of a young cluster of massive stars embedded in an H II region coincident with RCW 121. These observations, together with published radio data and Midcourse Space Experiment and Spitzer images, were used to determine some of the physical parameters of the region. We found 96 cluster member candidates in an area of about 1.5 × 2.0 arcmin2, 30% of them showing excess emission in the near-IR. IRS 1, the strongest source in the cluster with an estimated spectral type of O5 V-O6 V zero-age main sequence based on the color-magnitude diagram, is probably the main ionizing source of the H II region detected at radio wavelengths. Using the integrated Brγ and the 5 GHz flux densities, we derived a mean visual extinction AV = 5.49 ± mag. From the observed size of the Brγ extended emission, we calculated the emission measure E = 4.5 × 1024 cm-5 and the electron density ne = 2.6 × 103 cm-3, characteristic of compact H II regions.