T. P. Dominici

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.

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

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.

Microvariability in the optical polarization of 3C 279

Departamento de Astronomia, Instituto de Astronomia, Geof´isica e Cieˆncias Atmosfe´ricas, Universidade de Sa˜o Paulo, BrasilReceived / AcceptedAbstract. We present results of a microvariability polarization study in the violently variable quasar 3C279. We have resolvedthe polarization curves in the V band for this object down to timescales of minutes. We found two main components in theevolution of the degree of linear polarization, one consisting of a flicker with timescales of several tens of minutes and othercomponent with far more significant variations on timescale s of a few days. The linear polarization descended from ∼ 17 %down to ∼ 8 % in three nights. The polarization angle underwent a sudden change of more that 10 degrees in a few hours,perhaps indicating the injection of a new shock in the jet. The amplitude of the intranight flickering in the degree of pola rizationis at the level of ∼ 1%. These are probably the best sampled polarization data ever obtained for this object. We also performedIR observations and we provide a follow-up of the evolution of this source at such energies after the main polarization outburst.Key words. Galaxies: active: individual: 3C279 – polarization– galax ies: photometry

Multiwavelength flaring activity of PKS 1510-089

Aims. In this work, we analyse the multiwavelength brightness variations and flaring activity of FSRQ PKS 1510-089, aiming to constrain the position of the emission sources. Methods. We report 7 mm (43 GHz) radio and R -band polarimetric observations of PKS 1510-089. The radio observations were performed at the Itapetinga Radio Observatory, while the polarimetric data were obtained at the Pico dos Dias Observatory. The 7 mm observations cover the period between 2011 and 2013, while the optical polarimetric observations were made between 2009 and 2012. Results. At 7 mm, we detected a correlation between four radio and γ -ray flares with a delay of about 54 days between them; the higher frequency counterpart occurred first. Using optical polarimetry, we detected a large variation in polarization angle (PA) within two days associated with the beginning of a γ -ray flare. Complementing our data with other data obtained in the literature, we show that PA presented rotations associated with the occurrence of flares. Conclusions. Our results can be explained by a shock-in-jet model, in which a new component is formed in the compact core producing an optical and/or γ -ray flare, propagates along the jet, and after some time becomes optically thin and is detected as a flare at radio frequencies. The variability in the polarimetric parameters can also be reproduced; we can explain large variation in both PA and polarization degree (PD), in only one of them, or in neither, depending on the differences in PA and PD between the jet and the new component.

Centaurus A: The Nearest Blazar?

Centaurus A (NGC5128), at a distance of 3.4 Mpc is the nearest Active Galaxy, classified as a low luminosity Fanaroff‐Riley class I object. Although the central source is completely obscured at optical wavelengths, VLBI studies at radio frequencies show an unresolved core and an asymmetric jet at sub‐parsec scales. Kinematical studies of the jet components show subluminal expansion velocities, which together with the jet‐counterjet intensity ratio implies that the jet direction forms a large angle with the line of sight (50° to 80°). The nuclear emission is highly variable at all wavelengths, from radio to γ‐rays. Single dish radio observations showed that the stronger, long duration outbursts (months to years) present a correlation at radio and X‐rays, although it is not clear whether the emission mechanism is synchrotron radiation at both frequencies or if the inverse Compton process dominates at high energies. Moreover, no information is available about the correlation between the emission at these two...

Survey of Young Stellar Objects Associated with Molecular Clouds

We present the preliminary results of a survey in the near-infrared of young and massive stellar objects embedded in dense molecular clouds. The survey was conducted at the LNA, Brazil, in the J, H, and nbK bands, in the direction of IRAS sources with colors characteristic of compact HII regions that also present strong CS(2-1) line emission. The coordinates of the IRAS sources were improved to 1.9n resolution with the help of the MSX catalog, allowing us to associate them univocally to the detected stars in our sample. Several IR nebulae were detected, indicating also that young stellar clusters are being formed in the core of the molecular clouds. From the total number of 127 observed regions we found that 105 presented stars in the H band inside the IRAS error ellipse, and were associated to a MSX point source. We obtained the integrated bolometric luminosities and main-sequence spectral types of the detected stars, assuming that the bolometric luminosity coincided with the integrated IR luminosity, and using the fact that for massive stars the evolution towards the main sequence proceeds at constant luminosity. The Initial Mass Function of the sample follows a Salpeter’s law with index 1.6 between spectral types B0 and O3.