arXiv subjects

G. Natali

Publications and source records attributed to G. Natali.

3 recordsLinked to original sources

arXiv2000

Was the Whirlpool (M51) a Spiral Galaxy before the Encounter with the Companion NGC 5195?

Using the B-I color index in the radial photometry of M51, applied to defined azimuthal sectors, we reveals asymmetries in the shape of the density pattern of the galactic arms. The anomalous shape is present only in the side close to the companion and not in the far side. Two more non-interacting check-spirals do not show similar behaviour. The anomalous sawtooth-like shape is similar to that produced by a progressive wave propagating in a preexisting density pattern. In this hypothesis we have to conclude that the galaxy M51 could be a spiral galaxy formerly, i.e. before the encounter with the companion.

astro-ph↗

arXiv1998

BVI Microvariability in BL LAC during the summer 1997 outburst

BL Lac has been observed during the outburst occurred in the 1997 summer (Noble et al. 1997; Bloom et al. 1997) in three optical bands (BVI), with the aim of studying the intranight colour variations on very short timescales. Due to the high luminosity of the source we could perform quasi-simultaneous measurements in the three bands with a resolving time of about 5 minutes using one single telescope. We have detected several microvariability events occurring on timescales ranging from 30 min up to 3.5 hours. The magnitude changes had different amplitudes in the three bands being larger for shorter wavelenghts, and the source showed a marked trend to became bluer when brigther. There was no evidence of quasi-periodic oscillations in our multicolour light curves.

astro-ph↗

arXiv1995

The Variability of Quasars. II. Frequency Dependence

The variability of 30 PG quasars has been observed in the red band during three years. A rest-frame structure function analysis shows an increase of variability with the time interval up to 0.2 mag r.m.s. after two years. A comparison with the IUE data available for PG quasars shows that the variability increases with the rest-frame frequency at each time interval. Evidence for this effect is supported by the analysis of the average variability of other published quasar samples. The effect can completely account for the increase of variability with redshift found in other studies.

astro-ph↗