Abstract. We present results of a search for optical microvariability in a selected sample of 23 southern AGNs, which includes both radio-quiet and radio-loud objects. Microvariations were clearly detected in 60% of the radioloud sources, with amplitudes from 2.2% up to 8% within a single night. Night-to-night variability with amplitudes of ∼ 20% was also observed in the BL Lac object 0537 − 441. On the contrary, no rapid variability was detected at all in 8 radio-quiet quasars. We have used microvariability data previously reported for northern objects along with our new results for southern AGNs in order to estimate duty cycles of each class of objects from the largest possible sample.Most of the microvariability in radio-loud objects could be originating in interactions between relativistic shocks and features in the inner jets, although contributions from superluminal microlensing and accretion disk instabilities can be present in some sources. It is possible that the latter phenomenon is responsible for the microvariability observed in northern radio-quiet quasars. We suggest that the difference in the microvariability behaviour of radio and X-ray selected BL Lacs could be due to the effect of stronger magnetic fields in the latter group of objects, fields that can prevent the formation of features like density inhomogeneities and bends in the base of the jets by Kelvin-Helmholtz macroscopic instabilities.

Context. The nature of the hard X-ray source XSS J12270-4859 is still unclear. It was claimed to be a possible magnetic cataclysmic variable of the Intermediate Polar type from its optical spectrum and a possible 860 s X-ray periodicity in RXTE data. However, recent observations do not support the latter variability, leaving this X-ray source still unclassified. Aims. To investigate its nature we present a broad-band X-ray and gamma ray study of this source based on a recent XMM-Newton observation and archival INTEGRAL and RXTE data. Using the Fermi/LAT 1-year point source catalogue, we tentatively associate XSS J12270-4859 with 1FGL J1227.9-4852, a source of high-energy gamma rays with emission up to 10 GeV. We further complement the study with UV photometry from XMM-Newton and ground-based optical and near-IR photometry. Methods. We have analysed both timing and spectral properties in the gamma rays, X-rays, UV and optical/near-IR bands of XSS J12270-4859. Results. The X-ray emission is highly variable, showing flares and intensity dips. The flares consist of flare-dip pairs. Flares are detected in both X-rays and the UV range, while the subsequent dips are present only in the X-ray band. Further aperiodic dipping behaviour is observed during X-ray quiescence, but not in the UV. The broad-band 0.2-100 keV X-ray/soft gamma ray spectrum is featureless and well described by a power law model with Γ = 1.7. The high-energy spectrum from 100 MeV to 10 GeV is represented by a power law index of 2.45. The luminosity ratio between 0.1-100 GeV and 0.2-100 keV is ∼0.8, indicating that the GeV emission is a significant component of the total energy output. Furthermore, the X-ray spectrum does not greatly change during flares, quiescence and the dips seen in quiescence. The X-ray spectrum however hardens during the post-flare dips, where a partial covering absorber is also required to fit the spectrum. Optical photometry acquired at different epochs reveals a period of 4.32 hr that could be ascribed to the binary orbital period. Near-IR, possibly ellipsoidal, variations are detected. Large amplitude variability on shorter (tens mins) timescales is found to be non-periodic. Conclusions. The observed variability at all wavelengths together with the spectral characteristics strongly favour a low-mass atypical low-luminosity X-ray binary and are against a magnetic cataclysmic variable nature. The association with a Fermi/LAT high-energy gamma ray source further strengths this interpretation.

Abstract. 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 is found 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 that observed in radio-selected BL Lacs and flat-spectrum radio quasars (i.e. ∼70%).

We present the first colour–magnitude relation (CMR) of early‐type galaxies in the central region of the Antlia cluster, obtained from CCD wide‐field photometry in the Washington photometric system. Integrated (C−T1) colours, T1 magnitudes, and effective radii have been measured for 93 galaxies (i.e. the largest galaxies sample in the Washington system till now) from the FS90 Antlia Group catalogue. Membership of 37 objects can be confirmed through new radial velocities and data collected from the literature. The resulting colour–magnitude diagram shows that early‐type FS90 galaxies that are spectroscopically confirmed Antlia members or that were considered as definite members by FS90, follow a well‐defined CMR that spans 9 mag in brightness with no apparent change of slope. This relation is very tight for the whole magnitude range but S0 galaxies show a larger dispersion, apparently due to a separation of ellipticals and S0s. Antlia displays a slope of −13.6 in a T1 versus (C−T1) diagram, in agreement with results for clusters like Fornax, Virgo, Coma and Perseus, which are dynamically different to Antlia. This fact might indicate that the build‐up of the CMR in cluster of galaxies is more related to galaxies internal processes than to the influence of the environment. Interpreting the CMR as a luminosity–metallicity relation of old stellar systems, the metallicities of the Antlia galaxies define a global relation down to MV≈−13. We also find, for early‐type dwarfs, no clear relation between luminosity and effective radius, indicating a nearly constant mean effective radius of ∼1 kpc. This value is also found in several samples of dwarf galaxies in Virgo and Coma.

Using 8 telescopes in the northern and southern hemispheres, plus archival data from two on-line sky surveys, we performed a systematic optical spectroscopic study of 39 putative counterparts of unidentified or poorly studied INTEGRAL sources in order to determine or at least better assess their nature. This was implemented within the framework of our campaign to reveal the nature of newly-discovered and/or unidentified sources detected by INTEGRAL. Our results show that 29 of these objects are active galactic nuclei (13 of which are of Seyfert 1 type, 15 are Seyfert 2 galaxies and one is possibly a BL Lac object) with redshifts between 0.011 and 0.316, 7 are X-ray binaries (5 with high-mass companions and 2 with low-mass secondaries), one is a magnetic cataclysmic variable, one is a symbiotic star and one is possibly an active star. Thus, the large majority (74%) of the identifications in this sample belongs to the AGN class. When possible, the main physical parameters for these hard X-ray sources were also computed using the multiwavelength information available in the literature. These identifications further underscore the importance of INTEGRAL in studying the hard X-ray spectra of all classes of X-ray emitting objects, and the effectiveness of a strategy of multi-catalogue cross-correlation plus optical spectroscopy to securely pinpoint the actual nature of still unidentified hard X-ray sources.

The incidence of a host galaxy in aperture photometry of active galactic nuclei is studied by means of actual and simulated CCD observations. Our goal is to evaluate the importance of spurious variations, introduced by seeing Ñuctuations during the observations, in the di †erential light curves used to study optical microvariability. Repeated CCD observations during two consecutive nights were used to obtain time-resolved aperture photometry for the BL Lac object PKS 2316[423, which is located at the center of a conspicuous elliptical galaxy, and for several Ðeld stars. The blazar seems to be variable according to standard variability criteria ; however we show that the observed di †erential magnitude variations are strongly correlated with seeing Ñuctuations during the nights. Moreover, another galaxy within the same CCD Ðeld shows nearly identical variations, clearly indicating that such variations are artifacts of the aperture photometry. Simulated observations of quasars within host galaxies of di †erent morphologies and spanning a wide range of luminosities were also used to evaluate the e †ects of changing seeing conditions. The results show that spurious di †erential magnitude variations due to seeing Ñuctuations are larger for active nuclei within brighter hosts, particularly when small photometric apertures (about the seeing FWHM in radius) are used. According to our results, several recommendations are given to future observers.

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 BL Lac 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.

No abstract