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.

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 provide an account of the possible acceleration of iron nuclei up to energies ∼ 300 EeV in the nearby, metally-rich starburst galaxy NGC 253. It is suggested that particles can escape from the nuclear region with energies of ∼ 10 15 eV and then could be reaccelerated at the terminal shock of the galactic superwind generated by the starburst, avoiding in this way the photodisintegration expected if the nuclei were accelerated in the central region of high photon density. We have also made estimates of the expected arrival spectrum, which displays a strong dependency with the energy cutoff at the source. PACS number(s): 96.40 -98.70.S -95.85.R -13.85.T

Abstract. We have used the Hubble Space Telescope to measure the motion in the sky and compute the galactocentric orbit of the black hole X-ray binary GRO J1655-40. The system moves with a runaway space velocity of 112 ± 18 km s −1 in a highly eccentric (e = 0.34 ± 0.05) orbit. The black hole was formed in the disk at a distance greater than 3 kpc from the Galactic centre and must have been shot to such an eccentric orbit by the explosion of the progenitor star. The runaway linear momentum and kinetic energy of this black hole binary are comparable to those of solitary neutron stars and millisecond pulsars. GRO J1655-40 is the first black hole for which there is evidence for a runaway motion imparted by a natal kick in a supernova explosion.

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.

Abstract. We present a study of 40 low-latitude unidentified 3EG gamma-ray sources which were found to be not positionally coincident with any known class of potential gamma-ray emitters in the Galaxy (Romero et al. 1999). We have performed a variability analysis which reveals that many of these 40 sources are variable. These sources have, in addition, a steep mean value of the gamma-ray spectral index, < Γ > = 2.41 ± 0.2, which, combined with the high level of variability, seems to rule out a pulsar origin. The positional coincidences with uncatalogued candidates to supernova remnants were also studied. Only 7 sources in the sample are spatially coincident with these candidates, a result that is shown to be consistent with the expected level of pure chance association. A complementary search for weak radio counterparts was also conducted and the results are presented as an extensive table containing all significant point-like radio sources within the 40 EGRET fields. We argue that in order to produce the high variability, steep gamma-ray spectra, and absence of strong radio counterparts observed in some of the gamma-ray sources of our sample, a new class of objects should be postulated, and we analyze a viable candidate.

The HEGRA Cherenkov telescope array group recently reported a steady and extended unidentified TeV gamma-ray source lying at the outskirts of Cygnus OB2. This is the most massive stellar association known in the Galaxy, estimated to contain ~2600 OB type members alone. It has been previously argued that the large scale shocks and turbulence induced by the multiple interacting supersonic winds from the many young stars in such associations may play a role in accelerating Galactic cosmic rays. Indeed, Cyg OB2 also coincides with the nonvariable MeV-GeV range unidentified EGRET source, 3EG 2033+4118. We report on the near-simultaneous follow-up observations of the extended TeV source region with the CHANDRA X-ray Observatory and the Very Large Array (VLA) radio telescope obtained in order to explore this possibility. Analysis of the CO, HI, and IRAS 100 mm emissions shows that the TeV source region coincides with an outlying sub-group of powerful OB stars which have evacuated or destroyed much of the ambient atomic, molecular and dust material and which may be related to the very high-energy emissions. An interesting SNR-like structure is also revealed near the TeV source region in the CO, HI and radio emission maps. Applying a numerical simulation which accurately tracks the radio to gamma-ray emission from primary hadrons as well as primary and secondary e ± , we find that the broadband spectrum of the TeV source region favors a predominantly nucleonic -rather than electronic -origin of the highenergy flux, though deeper X-ray and radio observations will help confirm this. A very reasonable, ~0.1%, conversion efficiency of Cyg OB2's extreme stellar wind mechanical luminosity to nucleonic acceleration to ~PeV (10 15 eV) energies is sufficient to explain the multifrequency emissions. Corresponding author Y.M.B.

The ultra-high energy cosmic rays recently detected by several air shower experiments could have an extragalactic origin. In this case, the nearest active galaxy Centaurus A might be the source of the most energetic particles ever detected on Earth. We have used recent radio observations in order to estimate the arrival energy of the protons accelerated by strong shock fronts in the outer parts of this southern radio source. We expect detections corresponding to particles with energies up to ∼ 2.2 × 10 21 eV and an arrival direction of (l ≈ 310• , b ≈ 20 • ) in galactic coordinates. The future Southern Hemisphere PierreAuger Observatory might provide a decisive test for extragalactic models of the origin of the ultra-high energy cosmic rays.