The Optical Emission from Gamma-ray Quasars

We present photometric observations of six radio-loud quasars that were detected by the COMPTEL gamma-ray telescope. The data encompasses seven wavebands in the optical and near-infrared. After correction for Galactic extinction, we find a wide range in optical slopes. Two sources are as blue as optically-selected quasars, and are likely to be dominated by the accretion disc emission, while three others show colours consistent with a red synchrotron component. We discuss the properties of the COMPTEL sample of quasars, as well as the implications our observations have for multi-wavelength modelling of gamma-ray quasars.Comment: 12 pages, 3 figures, accepted for publication in P.A.S.A; minor typos correcte

Dissipative N - body code for galaxy evolution

The evolving galaxy is considered as a system of baryonic fragments embedded into the static dark nonbaryonic (DH) and baryonic (BH) halo and subjected to gravitational and viscous interactions. Although the chemical evolution of each separate fragment is treated in the frame of one -- zone close box model with instantaneous recycling, its star formation (SF) activity is a function of mean local gas density and, therefore, is strongly influenced by other interacting fragments. In spite of its simplicity this model provides a realistic description of the process of galaxy formation and evolution over the Hubble timescale.Comment: 11 pages, LaTeX, 7 figures, using the article.sty, expected in A&ApTr, 18, 83

Molecular Dynamics of Solids at Constant Pressure and Stress Using Anisotropic Stochastic Cell Rescaling

Molecular dynamics simulations of solids are often performed using anisotropic barostats that allow the shape and volume of the periodic cell to change during the simulation. Most existing schemes are based on a second-order differential equation that might lead to undesired oscillatory behaviors and should not be used in the equilibration phase. We recently introduced stochastic cell rescaling, a first-order stochastic barostat that can be used for both the equilibration and production phases. Only the isotropic and semi-isotropic variants have been formulated and implemented so far. In this paper, we develop and implement the equations of motion of the fully anisotropic variant and test them on Lennard-Jones solids, ice, gypsum, and gold. The algorithm has a single parameter that controls the relaxation time of the volume, results in the exponential decay of correlation functions, and can be effectively applied to a wide range of systems

Radio-to-UV monitoring of AO 0235+164 by the WEBT and Swift during the 2006--2007 outburst

The blazar AO 0235+164 was claimed to show a quasi-periodic behaviour in the radio and optical bands. Moreover, an extra emission component contributing to the UV and soft X-ray flux was detected, whose nature is not yet clear. A predicted optical outburst was observed in late 2006/early 2007. We here present the radio-to-optical WEBT light curves during the outburst, together with UV data acquired by Swift in the same period. We found the optical outburst to be as strong as the big outbursts of the past: starting from late September 2006, a brightness increase of 5 mag led to the outburst peak in February 19-21, 2007. We also observed an outburst at mm and then at cm wavelengths, with an increasing time delay going toward lower frequencies during the rising phase. Cross-correlation analysis indicates that the 1 mm and 37 GHz flux variations lagged behind the R-band ones by about 3 weeks and 2 months, respectively. These short time delays suggest that the corresponding jet emitting regions are only slightly separated and/or misaligned. In contrast, during the outburst decreasing phase the flux faded contemporaneously at all cm wavelengths. This abrupt change in the emission behaviour may suggest the presence of some shutdown mechanism of intrinsic or geometric nature. The behaviour of the UV flux closely follows the optical and near-IR one. By separating the synchrotron and extra component contributions to the UV flux, we found that they correlate, which suggests that the two emissions have a common origin.Comment: 9 pages, 7 figures, in press for Astronomy and Astrophysic

Breathing in Low Mass Galaxies: A Study of Episodic Star Formation

We simulate the collapse of isolated dwarf galaxies using SPH + N-Body simulations including a physically motivated description of the effects of supernova feedback. As the gas collapses and stars form, the supernova feedback disrupts enough gas to temporarily quench star formation. The gas flows outward into a hot halo, where it cools until star formation can continue once more and the cycle repeats. The star formation histories of isolated Local Group dwarf galaxies exhibit similar episodic bursts of star formation. We examine the mass dependence of the stellar velocity dispersions and find that they are no less than half the velocity of the halos measured at the virial radius.Comment: 5 pages, 3 figures, accepted ApJ. Full resolution figures and movies available at http://hpcc.astro.washington.edu/feedbac

A new activity phase of the blazar 3C 454.3. Multifrequency observations by the WEBT and XMM-Newton in 2007-2008

We present and analyse the WEBT multifrequency observations of 3C 454.3 in the 2007-2008 observing season, including XMM-Newton observations and near-IR spectroscopic monitoring, and compare the recent emission behaviour with the past one. In the optical band we observed a multi-peak outburst in July-August 2007, and other faster events in November 2007 - February 2008. During these outburst phases, several episodes of intranight variability were detected. A mm outburst was observed starting from mid 2007, whose rising phase was contemporaneous to the optical brightening. A slower flux increase also affected the higher radio frequencies, the flux enhancement disappearing below 8 GHz. The analysis of the optical-radio correlation and time delays, as well as the behaviour of the mm light curve, confirm our previous predictions, suggesting that changes in the jet orientation likely occurred in the last few years. The historical multiwavelength behaviour indicates that a significant variation in the viewing angle may have happened around year 2000. Colour analysis reveals a complex spectral behaviour, which is due to the interplay of different emission components. All the near-IR spectra show a prominent Halpha emission line, whose flux appears nearly constant. The analysis of the XMM-Newton data indicates a correlation between the UV excess and the soft-X-ray excess, which may represent the head and the tail of the big blue bump, respectively. The X-ray flux correlates with the optical flux, suggesting that in the inverse-Compton process either the seed photons are synchrotron photons at IR-optical frequencies or the relativistic electrons are those that produce the optical synchrotron emission. The X-ray radiation would thus be produced in the jet region from where the IR-optical emission comes.Comment: 10 pages, 12 figures (7 included in the text, 5 in GIF format), accepted for publication in A&