I. Andruchow

Optical microvariability of EGRET blazars

We present results of a photometric CCD study of the incidence of microvariability in the optical emission of a sample of 20 blazars detected at gamma-ray energies by the EGRET instrument of the Compton Gamma-Ray Observatory. We have observed strong outbursts in some sources, but many others displayed no significant variability on timescales of hours. The typical minimum timescale results to be of ∼ several hours, not tens of minutes as claimed by some authors. The duty cycle for optical intranight microvariations of gamma- ray blazars, as estimated from our observations, seems to be ∼ 50 %, lower than what is usually assumed. For night-to-night variations, instead, the duty cycle approaches to what is observed in radio-selected BL Lacs and flat-spectrum radio quasars (i.e. ∼ 70 %).

Polarization microvariability of BL Lacertae objects

We present the results of a systematic observational campaign designed to search for microvariability in the optical polarization of BL Lac objects. We observed a sample formed by 8 X-ray-selected and 10 radio-selected sources, looking for rapid changes in both the degree of linear polarization and the corresponding polarization angle. The whole campaign was carried out during the last three years, and most of the objects were observed on at least two consecutive nights. The statistical properties of both classes of BLLac objects are compared, and some general conclusions on the nature of the phenomenon are drawn. In general, radio selected sources seem to display higher duty cycles for polarimetric microvariability and, on average, they have a stronger polarization.

Gamma Ray Bursts with peculiar temporal asymmetry

Based on the study of temporal asymmetry of 631 gamma ray bursts from the BATSE 3B catalog by Link and Epstein [Ap J 466, 764 (1996)], we identify the population of bursts whose rising times are longer than their decays, thus showing atypical profiles. We analyse their sky distribution, morphology, time-space clustering and other average properties and compare them with those associated with the bulk of the bursts. We show how most of the peculiar bursts analysed are consistent with recent fireball models, but a fraction of bursts (� 4% of the total sample) appear to be inconsistent.

Observational constraints for the circumstellar disk of the B[e] star CPD−52 9243

Context. The formation and evolution of gas and dust environments around B[e] supergiants are still open issues. Aims. We intend to study the geometry, kinematics and physical structure of the circumstellar environment (CE) of the B[e] supergiant CPD−52 9243 to provide further insights into the underlying mechanism causing the B[e] phenomenon. Methods. The influence of the different physical mechanisms acting on the CE (radiation pressure, rotation, bi-stability or tidal forces) is somehow reflected in the shape and kinematic properties of the gas and dust regions (flaring, Keplerian, accretion or outflowing disks). To investigate these processes we mainly used quasi-simultaneous observations taken with high spatial resolution optical long-baseline interferometry (VLTI/MIDI), near-IR spectroscopy of CO bandhead features (Gemini/Phoenix and VLT/CRIRES) and optical spectra (CASLEO/REOSC). Results. High angular resolution interferometric measurements obtained with VLTI/MIDI provide strong support for the presence of a dusty disk(ring)-like structure around CPD−52 9243, with an upper limit for its inner edge of ∼ 8m as (∼27.5 AU, considering a distance of 3.44 kpc to the star). The disk has an inclination angle with respect to the line of sight of 46 ± 7 ◦ . The study of CO first overtone bandhead evidences a disk structure in Keplerian rotation. The optical spectrum indicates a rapid outflow in the polar direction. Conclusions. The IR emission (CO and warm dust) indicates Keplerian rotation in a circumstellar disk while the optical line transitions of various species are consistent with a polar wind. Both structures appear simultaneously and provide further evidence for the proposed paradigms of the mass-loss in supergiant B[e] stars. The presence of a detached cold CO ring around CPD–52 9243 could be due to a truncation of the inner disk caused by a companion, located possibly interior to the disk rim, clearing the center of the system. More spectroscopic and interferometric data are necessary to determine a possible binary nature of the star.

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

IN SEARCH FOR NATURAL WORMHOLES

We have investigated 631 time profiles of gamma ray bursts from the BATSE database searching for observable signatures produced by microlensing events related to natural wormholes. The results of this first search of topologically nontrivial objects in the Universe can be used to constrain their number and mass.

Looking for blazars in a sample of unidentified high-energy emitting Fermi sources

Fil: Marchesini, Ezequiel Joaquin. Universidad Nacional de la Plata. Facultad de Ciencias Astronomicas y Geofisicas; Argentina

Polarization and photometric observations of the gamma-ray blazar PG 1553+113

We present the results of an observational photo-polarimetry campaign of the blazar PG 1553+113 at optical wavelengths. The blazar was recently detected at very high energies (> 100 GeV) by the H.E.S.S and MAGIC gamma-ray Cherenkov telescopes. Our high-temporal resolution data show significant variations in the linear polarization percentage and position angle at inter-night time-scales, while at shorter (intra-night) time-scales both parameters varied less significantly, if at all. Changes in the polarization angle seem to be common in gamma-ray emitting blazars. Simultaneous differential photometry (through the B and R bands) shows no significant variability in the total optical flux. We provide B and R magnitudes, along with a finding chart, for a set of field stars suitable for differential photometry.

Incidence of the host galaxy on the measured optical linear polarization of blazars.

We study the incidence of the underlying host galaxy light on the measured optical linear polarization of blazars. Our methodology consists of the implementation of simulated observations obtained under different atmospheric conditions, which are characterised by the Gaussianσ of the seeing function. The simulated host plus active nucleus systems span broad ranges in luminosity, structural properties, redshift, an d polarization; this allows us to test the response of the results against each of these parameters. Our simulations show that, as expected, the measured polarization is always lower than the intrinsic value, due to the contamination by non-polarized star light from the host. This effect is more significant when the host is brighter than the acti ve nucleus, and/or a large photometric aperture is used. On the other hand, if seeing changes along the observing time under certain particular conditions, spurious microvaria bility could be obtained, especially when using a small photometric aperture. We thus give some recommendations in order to minimise both unwanted effects, as well as basic guidelines to estimate a lower limit of the true (nuclear) polarization. As an example, we apply the results of our simulations to real polarimetric observations, with high temporal resolution, of the blazar PKS 0521−365.

Optical flux behaviour of a sample of Fermi blazars

Aims: We aim at investigating the time-behaviour of a sample of gamma-ray blazars. We present the results from a 13 month-long optical photometry monitoring campaign of the blazars PKS 0048-097, PKS 0754+100, HB89 0827+243, PKS 0851+202, PKS 1253-055, PKS1510-089, PKS 1749+096, PKS 2230+114 and PKS 2251+158. Methods: We analyse the variability of each object, focusing on different time-scales (long term, short term, and microvariability), in an attempt to achieve a statistical comparison of the results. Results: After applying a geometric model to explain the variability results, we found that it is possible that a slight change in the direction of the jet generates the variations detected in some objects during this campaign.