Franco Giovannelli

Ultraviolet and Optical Observations of HDE 245770/ A 0535+26 System during an X-Ray Flare

We present high and low dispersion UV spectra of HDE 245770,which is the optical counterpart of the recurrent transient X-ray pulsar A 0535+26,during a decay of an X-ray flare. UBV photometric measurements and medium dispersion optical spectra were simultaneously obtained.

Multifrequency monitoring of the pre-main-sequence star RU Lupi

Within the framework of a five-year campaign of coordinated multifrequency observations of the 3 Tauri star RU Lupi, we present results obtained in X-ray, UV, optical, and IR spectral regions. In five cases, simultaneous data from UV to IR were collected. Data analysis shows a large variability at different wavelengths both in the continuum and lines, stronger in the UV and weaker in the IR. Correlations among variations in the UV and optical continua have been found. Moreover flare-like events (FLEs), probably due to strong activity in the surface layers of the star and burst phenomena on large scale, were detected. No forbidden coronal lines were detected in UV and optical regions with IUE and ESO 1.4m CAT telescope, respectively. This means either the used instruments have not enough sensitivity to detect the predicted forbidden lines, or the low X-ray detected intensity — with ASTRON satellite (2–25 keV) — is due to the absorption of the X-rays in the circumstellar envelope. In this paper we will discuss these results within the framework of the current models.

Time lag in transient cosmic accreting sources

Models are developed for time lag between maxima of the source brightness in different wave- lengths during a transient flash of luminosity connected with a short period of increase of the mass flux onto the central compact object. A simple formula is derived for finding the time delay among events in different wavelengths, valid in general for all disk accreting cosmic sources. In close binaries with accretion disks the time lag is connected with effects of viscosity defining a radial motion of matter in the accretion disk. In AGN flashes, the falling matter has a low angular momentum, and the time lag is defined by the free fall time to the gravitating center. The validity of these models is shown by means of several examples of galactic and extragalactic accreting sources.

SIXE: An X-ray experiment for the MINISAT platform

SIXE (Spanish Italian X-ray Experiment) is an X-ray payload with geometric area of ~3200 cm2 , formed by four identical gas-filled Multiwire Proportional Counters devoted for a long term spectroscopy of selected X-ray sources in the energy range ~3-60 KeV. The spectral resolution of ~5% for E>35 KeV and ?E/E=60/E1/2 for E<35 KeV, the continuum sensitivity (3o in 105 s) of ~3•10-6ph/cm2 •KeV, the line detection sensitivity (3o in 105 s) of ~3.5•10-6ph /cm2 •KeV, the time accuracy of 0.1 ms, the experiment weight of ~90 Kg, the full dimensions including the electronic box of ~660x660x400 mm3, the power budget of -40 W, the telemetry rate of :=100 Kbit/s and on-board memory of ~2 Gbytes makes this experiment fully compatible for a minisatellite mission, such as for istance the spanish MINISAT 02. The main scientific goal of SIXE is the study of the short and long term spectral and intensity variabilities of few selected extragalactic and galactic X-ray sources, such as quasars, Seyfert galaxies, high and low mass X-ray binaries, etc. Since the sensitivity and the energy resolution of SIXE are rather good, it is possible to study the behaviour of the eventual Fe lines with respect to the continuum emission. This possibility of SIXE is unique for the present generation of X-ray experiments, since no one is dedicated to long term monitoring of X-ray sources.SIXE (Spanish Italian X-ray Experiment) is an X-ray detector withgeometric area of 2300 cm2, formed by four identical gas-filledMulticell Proportional Counters, and devoted to study the long termspectroscopy of selected X-ray sources in the energy range 3–50 keV. Themain characteristics of SIXE are: time accuracy of 1 microsecond,spectral resolution of 5% for E > 35 keV and 46/√E% for E <35 keV, continuum sensitivity (3σ in 105 s) of 2 ×10-6 ph cm-2 s-1 keV-1, and line sensitivity (3σin 105 s) of 8 × 10-6 ph cm-2 s-1. The size of theinstruments and the requirements of the payload (weight 103 kg, fulldimensions 660 × 660 × 450 mm3, power budget < 60 W,on-board memory 2 Gbits, telemetry rate < 100 kbps) make this experimentfully compatible with the MINISAT platform.The main scientific goal of SIXE is the study of short and long termvariability of some of the most important X-ray sources. To do that a fewselected extragalactic and galactic X-ray sources will be selected toperform a dedicated and extensive monitoring program. The mission willprovide in this way the unique opportunity for the study of X-ray sourceswith a temporal accuracy of 1 microsecond all through the time range10-5 :107 s.

SIXE: An X-ray experiment for a minisatellite

SIXE (Spanish Italian X-ray Experiment) is an X-ray detector with geometric area of 2300 cm2, formed by four identical gas-filled Multicell Proportional Counters, and devoted to study the long term spectroscopy of selected X-ray sources in the energy range 3–50 keV. The main characteristics of SIXE are: time accuracy of 1 microsecond, spectral resolution of 5% for E>35 keV and 46/E% for E<35 keV, continuum sensitivity (3σ in 105 s) of 2×10−6 ph/cm2⋅s−1⋅keV−1, and line sensitivity (3σ in 105 s) of 8×10−6 ph/cm2⋅s−1. The size of the instruments and the requirements of the payload (weight 103 kg, full dimensions 660×660×450 mm3, power budget <60 W, on-board memory 2 Gbits, telemetry rate <100 kbps) make this experiment fully compatible with a minisatellite mission. The experiment, whose feasibility study has just been finished, has been proposed for flying on the Spanish MINISAT-02 satellite, in a 3 years long mission starting about 2002–2004. The main scientific goal is the study of the short and long term ...

SIXE: A Payload for MINISAT-02

A description of the X-ray instrument SIXE proposed for the MINISAT-02 mission is presented. The description includes the scientific goals and instrument performances.

The unusual star V439 Cyg

V439 Cyg, a strange star belonging to the Berkeley 87 cluster, changed its spectrum in the last decades from a late type to an early type. The present spectral type is (O9.5-B0) with narrow Hα and Hβ emission lines and broad, partially filled-in, Balmer absorption lines, at shorter wavelengths, very similar to those of several X-ray sources. An extremely strong IR excess (5812 Jy at 60 μm) and the anomalous reddening of the star render the case of V439 Cyg one of the most exciting theoretical puzzle of the astrophysics.

Multifrequency behayiour of the dwarf nova SS Cygni

Several years of coordinated multifrequency observations of the dwarf nova SS Cygni have been performed in order to study the accretion behaviour of this system in particular, and in general to clarify this crucial problem for the cataclysmic variables.From IUE measurements, together with those from optical and IR ground based telescopes, we have found orbital modulations in the continuum and in the equivalent widths of the emission lines. During quiescent phases the behaviour of SS Cygni are dependent on the type of the preceding outburst (long-type or short-type).In this paper we will discuss the physical implications one can derive from these results.