E. K. Sheffer

The x-ray experiment on-board the “astron”

Abstract This paper describes the X-ray instrumentation, its capabilities and preliminary results which have been obtained for first two months operation.

Multifrequency Monitoring of RU Lupi: Observational Results and a Model

A large campaign of multifrequency observations of the extreme T Tauri star RU Lupi was performed from 1983 to 1988.

Fourteen years of multifrequency-coordinated observations of the X-ray/Be system A 0535+26/HDE 245770

Within the framework of a very long multifrequency campaign of coordinated, and very often simultaneous, observations of the transient X-ray/Be system A 0535+26/HDE 245770, we present a summary of this large collection of data taken with IUE and ASTRON satellites as well as with many ground-based telescopes in optical and IR regions.In this paper we emphasize the study of the physics and dynamics of the mass transfer at periastron and subsequent X-ray flaring, which is a typical problem of the X-ray/Be binary system class.

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.

Fast X-ray and IR variability of A 0535+262

X-ray bursts of the source A 0535+262 with a characteristic time-scale ∼ 5 min have been discovered using the observational data of the Soviet ‘Astron’ satellite obtained on 7 April, 1983. Similar temporal variations were observed in the IR range on 26 September, 1981, 26–27 October and 16–17 December, 1983. The presence of such bursts gives evidence of strong inhomogeneity in the electron shell around the optical component and an accreting flow of matter in the vicinity of the neutron star. The parameters of the inhomogeneities are estimated.

Multi-Waveband Behaviour Of the T Tauri-Star RU Lupi

RU Lupi is an extreme TTS, late K spectral type, Teff = 4400 K, Lbol=2 L⊙, R*=2.4 R⊙, d≃150 pc. Its reddening is E(B-V)=0.10 ±0.02 mag. The optical spectrum is dominated by emission lines of H, Ca II, He I, Fe I, and Fe II. Type-III P Cygni profiles in B aimer lines allow to evaluate an expansion velocity of ≈ 100 Km/s and a density of ~ 1010 cm-3. Also, forbidden lines of [O I] and [S II] are present, indicating an extenser region at lower density (n≃106 cm-3, Te ≃ 2 × 104 K). RU Lupi shows also a strong IR excess and a strong UV excess. In the IUE band (1200–3200 A) it shows a luminosity L UV ≃ 0.3 L⊙, indicating a dense chromosphere. The fluxes of the strong UV emission lines of C III, C IV, Si III, Si IV, and N V are ≈ 3 × 105 times the solar values. X-ray observations made by HEAO-B (0.5–4.5 KeV) and ASTRON (2–6 KeV) satellites did not detect any flux greater than 1.2 × 10-13 erg/cm2 s and 3 × 10-11 erg/cm2 s, respectively, which are too low if scaled to the solar values. Among the many results obtained during our six-year-campaign of observations, one of the most exciting is the detection of two flare-like events (FLEs) (April 17, 1984, and June 30, 1986) with simultaneous observations from UV to IR. As example, the FLE of June 30, 1986, is characterized by a total variation of ~ 87% with respect to the energy distribution in a “quiescent” state obtained on 27 June. The peak of this variation (≈ 300%) is in the UV range 1200–3600 A. The longer are the wavelengths, the smaller are the variations: i.e., in the range 1.25–5 µm the variation is ≈ 42%. A positive correlation, best fitted by a straight line with a slope equal to 0.28, has been found between the total line fluxes and the total flux of the continuum in the short wavelength (SW) IUE range.

Observational Contrains on Collimation Mechanisms in RU Lupi

A large campaign of multifrequency observations of extreme T Tauri star RU Lupi was performed from 1983 to 1988. We present here main observational results of the campaign and their preliminary interpretation in terms of disk accretion onto an young star with moderately strong global magnetic field.