At its 1994 General Assembly, the International Astronomical Union resolved to introduce new arrangements to establish and maintain an accessible and authoritative set of constants, algorithms and procedures that implement standard models used in fundamental astronomy. The initiative was the responsibility of the Working Group on Astronomical Standards (WGAS), part of Division 1 of the IAU, and the set of procedures and constants was named SOFA, for “Standards Of Fundamental Astronomy”. The background to the IAU Resolution is given by Fukushima (1995) and Wallace (1996).

The main strengths of ROSAT [1] are its high quality imaging of extended sources and its large sensitivity for the detection of point sources. In the all-sky survey the diffuse X-ray emission was mapped with a resolution of 12 arcmin [2]. The number of point sources detected in the survey is ≈ 80 000 [3]. In addition more than 80 000 sources have been found in the ≈ 6 500 fields observed with the PSPC and HRI until now in the course of the pointed programme. A wealth of information about source variabilities has been extracted both from the all-sky survey and from the pointed observations, as well as from a combination of both.

RCMA SWG was appointed by the IAU WGAS (Working Group on Astronomical Standards) in accordance with IAU Resolution C6 (1994) with the aim ‘to provide definitions of the astronomical units, of the quantities linking these astronomical units to the units of the International System (SI), and of other astronomical quantities, compatible with the theory of General Relativity’. It is evident that the relativistic aspects of units of measurement cannot be isolated from the more general problem of astronomical constants and fundamental astronomy concepts in the relativistic framework. Therefore, along with the problem of units the main topics of discussion of RCMA SWG concerned also the IAU (1991) Resolutions on References Systems (RSs) and Time Scales (TSs) and their interpretation in IERS Standards (1992) and IERS Conventions (1996). In what follows we tried to summarize the results of these discussions.

We discuss recent detections of time-variable gamma-ray sources near the Galactic plane. A new bright gamma-ray transient was detected by EGRET in June 1995 near the Galactic center (GRO J1838-04). Also one of the most interesting unidentified gamma-ray sources in the plane, 2CG 135+1, was recently shown to be variable. Both GRO J1838-04 and 2CG 135+1 share many characteristics: variability of the gamma-ray flux within days/weeks, occasional peak gamma-ray emission of comparable flux (˜ 4 x 10-6ph cm-2s-1), absence of radio-loud spectrally-flat AGNs or prominent radio pulsars within their error boxes, lack of strong X-ray and/or hard X-ray counterparts. These characteristics do not match those of either gamma-ray blazars or isolated pulsars. Therefore, GRO J1838-04 and 2CG 135+1 provide strong evidence for the existence of a new class of variable gamma-ray sources.

The discovery of the association between relativistic radio jets and X-ray events in GRS 1915+105 (Mirabel and Rodriguez, 1994) and GRO J1655-40 showed that strong radio flares (Harmon et al, 1994) in general, and jet ejection (Hjellming and Rupen, 1994) in particular, could be identified with features in hard X-ray events. This has caused us to re-think the interpretation of the radio events in X-ray transients, and has made multi-wavelength observations even more important. In this paper the focus is on the connection between X-ray and radio events as seen since 1994 in GRS 1915+105, GRO J1655-40, GRO J1719-24, GRO J1739-278, Cyg X-3, Cyg X-l, and Sco X-l.

We study the formation of low-mass X-ray binaries with a black hole as accreting object. The performed semi-analytic analysis reveals that the formation rate of black holes in low-mass X-ray binaries is about two orders of magnitude smaller than that of systems with a neutron star as accretor. This is contradicted by the six observed systems, which are all transients, which suggest that the majority of these systems has not been seen jet. The birthrate for both type of objects are expected to be similar (for reviews see Cowley 1992, Tanaka & Lewin 1995).

Binary relativistic stars merging — the most powerful high energy transient in the Universe: Lmerging ˜ Mc2/Rg/c = c5/G ˜ 1058 erg/s. That is equal to Planckian luminosity (Lipunov 1992) Lplanck = EPlanck/tplanck = C5/G.

Since those reviews there have been very important new developments. First, a surprisingly new animal, GRO J1744—28 (better known as the Bursting Pulsar), made its debut in 1995. Second, but NOT last, in 1996/97 kHz pulsations were discovered in the persistent emission of several burst sources, and near coherent pulsations were detected in type I X-ray bursts.

It has long been suggested that turbulence provide viscous torques to transport angular momentum outward and flow mass inward in accretion disks (von Weizsäcker 1948, Shakura & Sunyaev 1973). Recent advances in subject of understanding of accretion disk turbulence are mach linked with magnetised disks (cf. Vishniac & Diamond 1992, Balbus, Gammie & Hawley 1994, Brandenburg et al. 1995, Stone et al 1996). However, not all the disks are magnetically coupled (see Balbus, Hawley & Stone 1996). Two different sources that are able to sustain turbulence in not magnetised accretion disk are the following:

Supersoft X-ray sources are a new class of luminous X-ray binaries discovered with the X-ray telescopes of the Einstein and ROSAT satellites and extensively studied in the optical with ground based telescopes, in the UV with IUE and HST and in X-rays with ROSAT, Beppo-SAX and ASCA (cf. Kahabka & van den Heuvel 1997, van Teeseling 1997). The luminosities derived for a first sample of supersoft sources studied with moderate resolution X-ray spectroscopy (using Beppo-SAX LECS and ASCA SIS detectors, Parmar et al. 1997, Ebisawa et al. 1997) have been predicted to follow Iben’s stability line (Iben 1982), i.e. the location in the Hertzsprung-Russell diagram which separates the plateau phase from the cooling phase. This is not unreasonable as any system experiencing steady-state accretion, i.e. accretion at a rate equalling about the nuclear burning rate will be found close to the stability line. If the accretion rate exceeds this limit then the white dwarf gets bloated and disappears in X-rays. If the accretion rate falls below this limit the white dwarf envelope cools, the luminosity as well as the temperature ceases and the source enters unstable recurrent nuclear burning. From the population synthesis calculations of Yungelson (1996) follows that there exit for the Milky Way a few sources at any epoch which are more massive than 1.2 MQ. They are expected to be extremely X-ray bright and may be standard candles (cf. Table 1 and Figure 1 for the brightest known supersoft sources per galaxy Milky Way to NGC 55). Their spectral distribution is expected to be similar to that of the extremely hot galactic source RXJ0925.7-4758 (it peaks at 1 keV and the flux is distributed from 0.5 to 2 keV, see Figure 2 for the ASCA spectrum of RX J0925.7-4758 (and CAL 87) as derived by Ebisawa et al. 1997).

We discuss how soft X-ray transients exhibit light curves associated with two accretion-disc instabilities that are well known to operate in cataclysmic variables. A thermal instability that is due to hydrogen ionization leads to the transient nature but is modified by X-ray irradiation of the disc. Growth of the disc between the ensuing outbursts leads to a 3:1 commensurability instability at the outer edge of the disc that drives superoutbursts and produces superhumps in the lightcurve if the mass ratio is sufficiently small. Superhumps are observed only in the short period SU UMa CVs but black hole primaries allow SXTs to exhibit superoutbursts and superhumps at much larger periods. We point out similarities between SXT superhump shapes and those of the small class of ER UMa CVs, The similarity between these systems stems from the fact that the mass-transfer rates are almost large enough to stabilize the disc against the thermal instability in both cases.

Exept Geminga there could be other gamma ray pulsars with no radioemission, which are exhibited in EGRET observations as ordinary point-like sources [6]. Determination of pulsations in a hard gamma ray source is complicated by rareness of arriving quanta δt ≫ P, and their small total number. When the value of the period is known from other observations (radio or X-ray), timing analysis gives the possibility to find this periodicity also in a gamma region [1].