ROSAT observations of nearby galaxies

First results of pointed and All Sky Survey observations of galaxies with the X-ray observatory satellite ROSAT are reported. During observations of the Magellanic Clouds and the Andromeda galaxy new super-soft X-ray sources have been detected. This new class of luminous X-ray sources may help to solve the millisecond pulsar progenitor problem. Due to the improved sensitivity and longer observation times of ROSAT new X-ray point sources have been resolved in several nearby galaxies. The diffuse emission of the LMC that was already reported by EINSTEIN has been mapped in detail. It shows a lot of fine structure and temperatures around 5 × 10⁶ K. The improved low energy response of ROSAT led to the discovery of 10⁶ K gas from the spiral galaxy M101 and the halo of the starburst galaxy NGC 253. No diffuse emission was detected from the halo of the edge-on spiral galaxy NGC 5907.     

ROSAT observations of galaxy clusters and cosmological implications

Clusters of galaxies are filled with a hot teneous plasma which is emitting soft X-rays over very extended regions. The ROSAT observatory with its high resolution and high sensitivity X-ray telescope allows to image nearby clusters in great detail and is therefore ideally suited to study the structure and morphology of galaxy clusters. In particular the distribution of the intracluster gas can be mapped, some information on the temperature structure of the gas can be obtained, the gravitational mass can be infered, and the dynamical state of the clusters can be investigated. Results of such studies for some nearby clusters as Virgo, Perseus and Coma are presented. Gas mass fractions of the total gravitational mass of the cluster in excess of 10 to 15% are infered from X-ray observations where the cluster emission can be traced out to an Abell radius (3 Mpc). If these values are taken to be representative for the matter composition in the Universe it has interesting consequences for cosmology as for example for the nucleosythesis models and the mean density of the Universe.     

ROSAT PSPC observations of two starburst galaxies

We present results from ROSAT observations of NGC 1808 and NGC 2903. Exposures of 10 ksec each with the PSPC detector show X-ray sources at the central positions of both galaxies which are classified as nuclear starburst galaxies. Both targets, NGC 1808 and NGC 2903 appear slightly extended in X-ray maps in the energy band 0.1–2.4 keV. The X-ray spectrum of NGC 1808 shows almost complete absorption below 0.5 keV, indicating an extremely high hydrogen column density towards that source (N_H ≈ 8 × 10²¹cm⁻² resulting from model fits on the PSPC spectrum). In case of NGC 2903, the number of counts in the ROSAT band is significantly lower than expected from a previous EINSTEIN investigation of the source.     

ROSAT observations of X-ray sources in globular clusters

We review the first observations of globular clusters obtained with the X-ray telescope on board of the ROSAT satellite. In the All-Sky Survey, all known bright sources and two new transient sources were detected. In addition a super-soft source was found in NGC5272. Concerning the dim sources, the survey suggests that those outside the core of ω Cen are not related to the cluster. The survey further improved by one or two magnitudes on many upper limits previously obtained by the HEAO-1 satellite. Pointed observations have improved the positional accuracy of a number of sources, and added to the number of known dim sources. In 47 Tuc, the known central source is resolved into 4 separate sources, which we argue are all soft X-ray transients. A comparison with observations of the old open cluster M67 leads us to suggest that some of the dim sources may be conglomerates of a large number of RS Can Ven type binaries.     

The structure of the IC443 supernova remnant complex derived from ROSAT and Ginga observations

Radio observations have shown that the emission from IC443 can be attributed to three, partially incomplete shells. The two smaller diameter shells are bright in X-rays, whereas the X-ray extent of the largest one has been discovered only recently in the ROSAT All-Sky-Survey. This new X-ray shell — G189.6+3.3 — is well explained by a separate old ( 10⁵yr) supernova remnant (SNR), in contrast to the IC443 proper SNR, the age of which has been estimated to 10³yr. Spatial structure of the X-ray absorption observed with ROSAT suggests that G189.6+3.3 is located in front of a molecular cloud while the two smaller subshells previously known as IC443 are behind it. This geometry is confirmed by the observation of an additional absorption ridge across IC443, which is produced by a shell of cool and dense matter associated with G189.6+3.3.     

Active galactic nuclei in the ROSAT survey

One of the main objectives of the ROSAT mission turned out to be the study of active galactic nuclei. The soft energy range of the X-Ray Telescope combined with the good energy resolution of the PSPC detector allows an investigation of the spectral properties of sources in this energetically important energy band. The high sensitivity of the instrument in the All Sky Survey will yield more than 25000 previously unknown X-ray AGN for the statistical and morphological exploration of these objects.

A short overview is given of the actual ongoing research which ranges from the detailed spectral study of some well known objects and correlations of samples of AGN with existing catalogues at other wavelengths to identification programmes for large numbers of suspected AGN candidates.     

Design and construction of the ROSAT 5 arcsec mirror assembly

The ROSAT mission, which is currently being prepared in W.-Germany, will perform the first soft X-ray all-sky survey by means of a large imaging X-ray telescope. Detailed calculations under the cost, volume and mass constraint of the satellite being a Shuttle payload have led to a design of the imaging optics with optimized geometry. The mirror system is of the Wolter type I configuration and includes four nested shells with a maximum aperture of 835 mm and a focal length of 2400 mm. The on-axis angular resolution of the mirror assembly has been specified to 5 arcsec with a scattering level as low as 3% for single reflection at 1.5 keV photon energy. Construction and technology studies have been completed by now and manufacturing of the first mirror shell has begun.     

The ROSAT view of the cataclysmic variable sky

The ROSAT All-Sky Survey has for the first time permitted a synoptic view of the soft X-ray sky with high sensitivity. In this paper, we discuss the X-ray properties of known cataclysmic variables (CVs) as observed in the Survey and present a status report on programs to identify CVs among the newly discovered ROSAT X-ray sources. Of 170 CVs with known orbital period, 92 were detected in the Survey and 22 of these fall in the bright-source category with more than 0.5 PSPC cts/s. Among the new bright sources, so far 19 have been identified as CVs and 3 as CV candidates, about doubling the census. We present spectra and light curves of known and new systems and discuss the origin of the X-ray emission in the different subclasses of CVs.     

X-ray emission from the planetary nebula NGC 1360

The EXOSAT observatory has detected the nucleus of NGC 1360 in four photometric energy bands. The data rules out that the emission is from blackbody origin. Initial fits made with LTE model atmosphere spectra require the presence of highly ionized Oxygen and Neon in the stellar atmosphere.     

New magnetic cataclysmic variables discovered in the ROSAT wide field camera all-sky survey

Seven new magnetic cataclysmic variables have been found amongst the EUV sources discovered in the ROSAT Wide Field Camera all-sky survey. Follow-up studies of these objects have now been made, yielding the orbital periods and classifications for all but one. Six objects are confirmed, or likely, polar systems, one is an intermediate polar. The properties of this new, EUV-selected sample are reviewed and compared with those of known polars and intermediate polars. The unusual features of the most interesting objects in the sample are briefly discussed.     

The location and intrinsic luminosity of the hidden nucleus of NGC 1068

Two key problems on the ‘hidden’ nucleus of NGC 1068 are discussed, based on the archival Hubble Space Telescope images. First, we discuss the accurate location of the nucleus. We have found that the most probable location is only ∼ 0.″1 (∼ 7 pc) south of the UV brightest cloud. Second, we consider the intrinsic luminosity of the hidden nucleus. We show that its lower limit is as large as ∼ 2 × 10⁴⁵ erg/sec, suggesting that the luminosity is almost at the Eddington limit.     

Analysis of the ROSAT pointed observation of the hot DA white dwarf HS1234+4811

The ROSAT data of the hot DA white dwarf HS1234+4811 were analyzed in the framework of three different assumptions for the structure of the white dwarf's atmosphere. It could be excluded that the star has a homogeneously mixed photosphere consisting of helium and hydrogen only, but from ROSAT observations alone it is not possible to decide if the object has a stratified atmosphere with an ultrathin hydrogen layer (M_H ≈ 7 · 10⁻¹⁴M) on top of the He envelope or if heavier elements are responsible for the absorption in the soft X-ray region.     

The nuclear region of NGC 3079

We report on high resolution studies of the nuclear region of NGC 3079 using H₂O maser, continuum emission and HI absorption with both Japan VLBI Netowrk and global VLBI Network including the VLBA, the phased VLA and Effelsberg 100-m telescope. We identify the nucleus and rotation axis of the central 10 pc region, and suggest a rotating torus to explain the results.     

On the origin of cosmic rays in the spiral galaxy NGC 3310

The problem of cosmic ray production in the spiral galaxy NGC 3310 is addressed by analysing and comparing optical and radio continuum data. Tentative results indicate that on global scales relativistic electrons may be produced in the shock front associated with teh density wave while on local scales extreme population I objects may be producing them. It is inferred that the same conclusions apply to all cosmic rays produced in the disk.     

Time series analysis of active galactic nuclei: The case of Arp 102B, 3C 390.3, NGC 5548 and NGC 4051

We used the Z-transformed Discrete Correlation Function (ZDCF) and the Stochastic Process Estimation for AGN Reverberation (SPEAR) methods for the time series analysis of the continuum and the Hα$\alpha$ and Hβ$\beta$ line fluxes of a sample of well known type 1 active galactic nuclei (AGNs): Arp 102B, 3C 390.3, NGC 5548, and NGC 4051, where the first two objects are showing double-peaked emission line profiles. The aim of this work is to compare the time lag measurements from these two methods, and check if there is a connection with other emission line properties. We found that the obtained time lags from Hβ are larger than those derived from the Hα analysis for Arp 102B, 3C 390.3 and NGC 5548. This may indicate that the Hβ   line originates at larger radii in these objects. Moreover, we found that the ZDCF and SPEAR time lags are highly correlated (r∼0.87$r\sim 0.87$), and that the error ranges of both ZDCF and SPEAR time lags are correlated with the FWHM of used emission lines (r∼0.7$r\sim 0.7$). This increases the uncertainty of the black hole mass estimates using the virial theorem for AGNs with broader lines.     

On the physical state in the narrow-line region of the classical Seyfert galaxy NGC 7469

Based on the spectrophotometric data, by decomposition of the observed continuum, the power - law continuum characteristics of the central source are obtained. The behaviour of both electron temperature T_e and density n_e in the Narrow - Line Region (NLR) of NGC 7469 is discussed on the thermal and ionization equilibria calculations. T_e in NLR of the Sy1 galaxies are higher than the Sy2 ones, and the possible explanation is the lack of the dense (n_e 10¹⁰ cm⁻³) zone close to the central source in the Sy2 galaxies.     

Non-LTE analysis of a new pulsating PG 1159-type central star detected in the ROSAT XRT all sky survey

We summarize the analysis of a new PG 1159 star, i.e., a hydrogen deficient pre-white dwarf detected in the ROSAT XRT all sky survey /1/. The X-ray source RX J2117.1+3412 is relatively faint (≈0.33 cnt s⁻¹) and was selected for optical identification on the basis of its extreme X-ray softness. With V = 13.2, the counterpart of the X-ray source is in the optical the second brightest member of the PG1159 class. CCD imagery reveals that the star is surrounded by an old highly excited planetary nebula of faint surface brightness. Optical line profiles of HeII, CIV, and OVI and the overall optical and ROSAT PSPC (0.1–2.4 keV) energy distributions are compared to predictions of NLTE model atmospheres. We find an excellent agreement between the atmospheric parameters derived from optical and soft X-ray data. The effective temperature of the star is extremely high (150,000 K). Abundances of He, C and O are found to be consistent with those derived in other PG1159 stars whereas the surface gravity is significantly smaller in RXJ2117.1+3412. New optical spectra presented here show the presence of ultra-high excitation lines of O VIII.     

X-ray broad line emission in Seyfert galaxies: Mrk 766 and NGC 4051

Long (>100 ks) observations of the bright Seyfert galaxies Mrk 766 and NGC 4051 have been obtained using XMM-Newton. The RGS 5–38 Å spectra reveal evidence of broad features. These can be modelled with relativistic emission lines coming from the immediate vicinity of a massive rotating black hole. Lines of OVIII, NVII and CVI are required to reproduce the spectrum of Mrk 766, whereas the spectrum of NGC 4051 can be modelled using a single, even broader OVIII line. Both Seyferts also exhibit broad iron line emission in the 2–8 keV range, and the data available thus far suggest that the strength of the low-energy emission lines and the strength of the iron line may be correlated.     

EXOSAT observations of the Perseus cluster

EXOSAT CMA and GSPC observations of the Perseus Cluster of galaxies are reported. The soft X-ray image data confirm the earlier Einstein Observatory results in terms of the density distribution radially outward from NGC1275. We have used the GSPC spectral data at higher energies to study the strength of the X-ray continuum and of the Fe line emission over the cluster. An asymmetry is found in the distribution of the hot gas around NGC1275, which is consistent with the E-W direction of the ellipticity discovered at soft X-rays by the Einstein Observatory.