Search

The eROSITA telescope array aboard the Spektrum Roentgen Gamma (SRG) satellite began surveying the sky in December 2019, with the aim of producing all-sky X-ray source lists and sky maps of an unprecedented depth. Here we present catalogues of both point-like and extended sources using the data acquired in the first six months of survey operations (eRASS1; completed June 2020) over the half sky whose proprietary data rights lie with the German eROSITA Consortium. We describe the observation process, the data analysis pipelines, and the characteristics of the X-ray sources. With nearly 930000 entries detected in the most sensitive 0.2-2.3 keV energy range, the eRASS1 main catalogue presented here increases the number of known X-ray sources in the published literature by more than 60%, and provides a comprehensive inventory of all classes of X-ray celestial objects, covering a wide range of physical processes. A smaller catalogue of 5466 sources detected in the less sensitive but harder 2.3-5 keV band is the result of the first true imaging survey of the entire sky above 2 keV. We show that the number counts of X-ray sources in eRASS1 are consistent with those derived over narrower fields by past X-ray surveys of a similar depth, and we explore the number counts variation as a function of the location in the sky. Adopting a uniform all-sky flux limit (at 50% completeness) of F_{0.5-2 keV} > 5 \times 10^{-14}$ erg\,s$^{-1}$\,cm$^{-2}$, we estimate that the eROSITA all-sky survey resolves into individual sources about 20% of the cosmic X-ray background in the 1-2 keV range. The catalogues presented here form part of the first data release (DR1) of the SRG/eROSITA all-sky survey. Beyond the X-ray catalogues, DR1 contains all detected and calibrated event files, source products (light curves and spectra), and all-sky maps. Illustrative examples of these are provided., Comment: 39 pages, 23 figures. Accepted for publication in A&A. Accompanying eROSITA-DE Data Release 1

Radiative transfer calculations in strong (few $\times 10^{12}$ G) magnetic fields, observed in X-ray pulsars, require accurate resonant differential scattering cross sections. Such cross sections exist, but they are quite cumbersome. Here we compare the classical (non-relativistic) with the quantum-mechanical (relativistic) resonant differential scattering cross sections and offer a prescription for the use of the much simpler classical expressions with impressively accurate results. We have expanded the quantum-mechanical differential cross sections and kept terms up to first order in $\epsilon \equiv E/m_ec^2$ and $B \equiv {\cal B}/{\cal B}_{cr}$, where $E$ is the photon energy and ${\cal B}_{cr}$ is the critical magnetic field, and recovered the classical differential cross sections plus terms that are due to spin flip, which is a pure quantum-mechanical phenomenon. Adding by hand the spin-flip terms to the polarization-dependent classical differential cross sections, we find that they are in excellent agreement with the quantum mechanical ones for all energies near resonance and all angles. We have plotted both of them and the agreement is impressive. We give a prescription for the use of the classical differential cross sections that guarantees very accurate results.

Accretion onto magnetic neutron stars results in X-ray spectra that often exhibit a cyclotron resonance scattering feature (CRSF) and, sometimes, higher harmonics of it. Two places are suspect for the formation of a CRSF: the surface of the neutron star and the radiative shock in the accretion column. Here we explore the first possibility: reflection at the neutron-star surface of the continuum produced at the radiative shock. It has been proposed that for high-luminosity sources, as the luminosity increases, the height of the radiative shock increases, thus a larger polar area is illuminated, and as a consequence the energy of the CRSF decreases because the dipole magnetic field decreases by a factor of two from the pole to the equator. We used a Monte Carlo code to compute the reflected spectrum from the atmosphere of a magnetic neutron star, when the incident spectrum is a power-law one. We restricted ourselves to cyclotron energies $\ll m_ec^2$ and used polarization-dependent scattering cross sections, allowing for polarization mode change. As expected, a prominent CRSF is produced in the reflected spectra if the incident photons are in a pencil beam, which hits the neutron-star surface at a point with a well-defined magnetic field strength. However, the incident beam from the radiative shock has a finite width and thus various magnetic field strengths are sampled. As a result of overlap, the reflected spectra have a CRSF, which is close to that produced at the magnetic pole, independent of the height of the radiative shock. Reflection at the surface of a magnetic neutron star cannot explain the observed decrease in the CRSF energy with luminosity in the high-luminosity X-ray pulsar V 0332+53.

eROSITA (extended ROentgen Survey with an Imaging Telescope Array) is the primary instrument on the Spectrum-Roentgen-Gamma (SRG) mission, which was successfully launched on July 13, 2019, from the Baikonour cosmodrome. After the commissioning of the instrument and a subsequent calibration and performance verification phase, eROSITA started a survey of the entire sky on December 13, 2019. By the end of 2023, eight complete scans of the celestial sphere will have been performed, each lasting six months. At the end of this program, the eROSITA all-sky survey in the soft X-ray band (0.2--2.3\,keV) will be about 25 times more sensitive than the ROSAT All-Sky Survey, while in the hard band (2.3--8\,keV) it will provide the first ever true imaging survey of the sky. The eROSITA design driving science is the detection of large samples of galaxy clusters up to redshifts $z>1$ in order to study the large-scale structure of the universe and test cosmological models including Dark Energy. In addition, eROSITA is expected to yield a sample of a few million AGNs, including obscured objects, revolutionizing our view of the evolution of supermassive black holes. The survey will also provide new insights into a wide range of astrophysical phenomena, including X-ray binaries, active stars, and diffuse emission within the Galaxy. Results from early observations, some of which are presented here, confirm that the performance of the instrument is able to fulfil its scientific promise. With this paper, we aim to give a concise description of the instrument, its performance as measured on ground, its operation in space, and also the first results from in-orbit measurements., Comment: 16 pages, 19 figures, accepted by Astronomy & Astrophysics

Cyclotron lines, also called cyclotron resonant scattering features (CRSF) are spectral features, generally appearing in absorption, in the X-ray spectra of objects containing highly magnetized neutron stars, allowing the direct measurement of the magnetic field strength in these objects. Cyclotron features are thought to be due to resonant scattering of photons by electrons in the strong magnetic fields. The main content of this contribution focusses on electron cyclotron lines as found in accreting X-ray binary pulsars (XRBP) with magnetic fields on the order of several 1012 Gauss. Also, possible proton cyclotron lines from single neutron stars with even stronger magnetic fields are briefly discussed.With regard to electron cyclotron lines, we present an updated list of XRBPs that show evidence of such absorption lines. The first such line was discovered in a 1976 balloon observation of the accreting binary pulsar Hercules X-1, it is considered to be the first direct measurement of the magnetic field of a neutron star. As of today (mid 2018), we list 36 XRBPs showing evidence of one ore more electron cyclotron absorption line(s). A few have been measured only once and must be confirmed (several more objects are listed as candidates). In addition to the Tables of objects, we summarize the evidence of variability of the cyclotron line as a function of various parameters (especially pulse phase, luminosity and time), and add a discussion of the different observed phenomena and associated attempts of theoretical modeling. We also discuss our understanding of the underlying physics of accretion onto highly magnetized neutron stars. For proton cyclotron lines, we present tables with seven neutron stars and discuss their nature and the physics in these objects., Comment: 32 pages, 15 figures, 8 Tables, accepted by A&A 2018

We present the second ROSAT all-sky survey source catalogue, hereafter referred to as the 2RXS catalogue. This is the second publicly released ROSAT catalogue of point-like sources obtained from the ROSAT all-sky survey (RASS) observations performed with the PSPC between June 1990 and August 1991, and is an extended and revised version of the bright and faint source catalogues. We used the latest version of the RASS processing to produce overlapping X-ray images of 6.4x6.4 degrees sky regions. To create a source catalogue, a likelihood-based detection algorithm was applied to these, which accounts for the PSF across the PSPC field of view. Improvements in the background determination compared to 1RXS were also implemented. We obtained about 135,000 X-ray detections in the 0.1-2.4 keV energy band down to a likelihood threshold of 6.5. Our simulations show that the expected spurious content of the catalogue is a strong function of detection likelihood, and the full catalogue is expected to contain about 30% spurious detections. X-ray images and overlaid X-ray contour lines provide an additional user product to evaluate the detections visually, and we performed our own visual inspections to flag uncertain detections. Intra-day variability in the X-ray light curves was quantified based on the normalised excess variance and a maximum amplitude variability analysis. X-ray spectral fits were performed using three basic models, a power law, a thermal plasma emission model, and black-body emission. Thirty-two large extended regions with diffuse emission and embedded point sources were identified and excluded from the present analysis. The 2RXS catalogue provides the deepest and cleanest X-ray all-sky survey catalogue in advance of eROSITA., Comment: 27 pages, 37 figures, Graphical illustrations for light curves, spectral fits and X-ray images are available within CDS

We present the results from the analysis of the broad-band X-ray spectra of 5 Anomalous X-ray Pulsars (AXPs) and Soft $\gamma$-ray Repeaters (SGRs). We fit their Suzaku and INTEGRAL spectra with models appropriate for the X-ray emission from the accretion flow onto a pulsar. We find that their X-ray spectra can be well described with this model. In particular we find that: (a) the radius of the accretion column is $\sim150-350$ m resulting in a transverse optical depth of $\sim 1$; (b) the vertical Thompson optical depth is $\approx 50-400$, and (c) their luminosity translates in accretion rates $\approx10^{15}\rm{g\, s^{-1}}$. These results are in good agreement with the predictions from the fall-back disk model, providing further support in the interpretation of AXPs and SGRs as accreting pulsars., Comment: Accepted for publication in MNRAS, 10 pages, 2 figures

In the fallback disk model for the persistent emission of Anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs), the hard X-ray emission arises from bulk- and thermal Comptonization of bremsstrahlung photons, which are generated in the accretion column. The relatively low X-ray luminosity of these sources implies a moderate transverse optical depth to electron scattering, with photons executing a small number of shock crossings before escaping sideways. We explore the range of spectral shapes that can be obtained with this model and characterize the most important parameter dependencies. We use a Monte Carlo code to study the crisscrossing of photons in a radiative shock in an accretion column and compute the resulting spectrum. As expected, high-energy power-law X-ray spectra are produced in radiative shocks with photon-number spectral index larger than or about 0.5. We find that the required transverse optical depth is between 1 and 7. Such spectra are observed in low-luminosity X-ray pulsars. We demonstrate here with a simple model that Compton upscattering in the radiative shock in the accretion column can produce hard X-ray spectra similar to those seen in the persistent and transient emission of AXPs and SGRs. In particular, one can obtain a high-energy power-law spectrum, with photon-number spectral index ~ 1 and a cutoff at 100 - 200 keV, with a transverse Thomson optical depth of ~ 5, which is shown to be typical in AXPs/SGRs., Comment: Accepted for publication in A&A

We investigate the X-ray enhancement and the long-term evolution of the recently discovered, second "low-B magnetar" Swift J1822.3-1606 in the frame of the fallback disk model. During a soft gamma burst episode, the inner disk matter is pushed back to larger radii forming a density gradient at the inner disk. Subsequent relaxation of the inner disk could account for the observed X-ray enhancement light curve of Swift J1822.3-1606. We obtain model fits to the X-ray data with basic disk parameters similar to those employed to explain the X-ray outburst light curves of other AXPs and SGRs. The long period (8.4 s) of the neutron star can be reached by the effect of the disk torques in the long-term accretion phase (1-3 x 10^5 yrs). The currently ongoing X-ray enhancement could be due to a transient accretion epoch, or the source could still be in the accretion phase in quiescence. Considering these different possibilities, we determine the model curves that could represent the long-term rotational and the X-ray luminosity evolution of Swift J1822.3-1606, which constrain the strength of the magnetic dipole field to the range of 1-2 x 10^12 G on the surface of the neutron star., Comment: 21 pages, 3 figures, Accepted for publication in the Astrophysical Journal

We propose that the quiescent emission of AXPs/SGRs is powered by accretion from a fallback disk, requiring magnetic dipole fields in the range 10^{12}-10^{13} G, and that the luminous hard tails of their X-ray spectra are produced by bulk-motion Comptonization in the radiative shock near the bottom of the accretion column. This radiation escapes as a fan beam, which is partly absorbed by the polar cap photosphere, heating it up to relatively high temperatures. The scattered component and the thermal emission from the polar cap form a polar beam. We test our model on the well-studied AXP 4U 0142+61, whose energy-dependent pulse profiles show double peaks, which we ascribe to the fan and polar beams. The temperature of the photosphere (kT~0.4 keV) is explained by the heating effect. The scattered part forms a hard component in the polar beam. We suggest that the observed high temperatures of the polar caps of AXPs/SGRs, compared with other young neutron stars, are due to the heating by the fan beam. Using beaming functions for the fan beam and the polar beam and taking gravitational bending into account, we fit the energy-dependent pulse profiles and obtain the inclination angle and the angle between the spin axis and the magnetic dipole axis, as well as the height of the radiative shock above the stellar surface. We do not explain the high luminosity bursts, which may be produced by the classical magnetar mechanism operating in super-strong multipole fields., Comment: ApJ in press, 14 pages, 5 figures

Recent measurements showed that the period derivative of the 'high-B' radio pulsar PSR J1734-3333 is increasing with time. For neutron stars evolving with fallback disks, this rotational behavior is expected in certain phases of the long-term evolution. Using the same model as employed earlier to explain the evolution of anomalous X-ray pulsars and soft gamma-ray repeaters, we show that the period, the first and second period derivatives and the X-ray luminosity of this source can simultaneously acquire the observed values for a neutron star evolving with a fallback disk. We find that the required strength of the dipole field that can produce the source properties is in the range of 10^{12} - 10^{13} G on the pole of the neutron star. When the model source reaches the current state properties of PSR J1734-3333, accretion onto the star has not started yet, allowing the source to operate as a regular radio pulsar. Our results imply that PSR J1734-3333 is at an age of ~ 3 x 10^4 - 2 x 10^5 years. Such sources will have properties like the X-ray dim isolated neutron stars or transient AXPs at a later epoch of weak accretion from the diminished fallback disk., Comment: 7 pages, 5 figures. Accepted for publication in MNRAS

We propose to perform a continuously scanning all-sky survey from 200 keV to 80 MeV achieving a sensitivity which is better by a factor of 40 or more compared to the previous missions in this energy range. The Gamma-Ray Imaging, Polarimetry and Spectroscopy (GRIPS) mission addresses fundamental questions in ESA's Cosmic Vision plan. Among the major themes of the strategic plan, GRIPS has its focus on the evolving, violent Universe, exploring a unique energy window. We propose to investigate $\gamma$-ray bursts and blazars, the mechanisms behind supernova explosions, nucleosynthesis and spallation, the enigmatic origin of positrons in our Galaxy, and the nature of radiation processes and particle acceleration in extreme cosmic sources including pulsars and magnetars. The natural energy scale for these non-thermal processes is of the order of MeV. Although they can be partially and indirectly studied using other methods, only the proposed GRIPS measurements will provide direct access to their primary photons. GRIPS will be a driver for the study of transient sources in the era of neutrino and gravitational wave observatories such as IceCUBE and LISA, establishing a new type of diagnostics in relativistic and nuclear astrophysics. This will support extrapolations to investigate star formation, galaxy evolution, and black hole formation at high redshifts., Comment: to appear in Exp. Astron., special vol. on M3-Call of ESA's Cosmic Vision 2010; 25 p., 25 figs; see also www.grips-mission.eu

AXPs and SGRs constitute a special population of young neutron stars, which are thought to be magnetars, i.e., neutron stars with super-strong magnetic fields (10^14 - 10^15 G). Assuming that AXPs and SGRs accrete matter from a fallback disk, we attempt to explain the energy-dependent pulse profiles of AXP 4U 0142+61, as well as its phase-dependent energy spectra. In the fallback disk model, the Thomson optical depth along the accretion funnel is significant and bulk-motion Comptonization operates efficiently. This is enhanced by resonant cyclotron scattering. The thus scattered photons escape mainly sideways and produce a fan beam, which is detected as a main pulse up to energies of ~160 keV. The approximately isotropic emission from the stellar surface (soft thermal photons and reflected hard X-ray ones) is detected as a secondary pulse. This secondary pulse shows a bump at an energy of ~60 keV, which may be interpreted as resonant cyclotron scattering of fan-beam photons at the neutron-star surface, implying a dipole magnetic field strength B ~7 x 10^12 (1+z) G, where z is the gravitational redshift. Our model explains the soft and hard X-ray spectra of 4U 0142+61 and its energy dependent pulse profiles of the quiescent emission, while the short bursts are due to magnetar-type processes taking place in superstrong multiple fields., Comment: 6 pages, 3 figures, submitted to A&A

Assuming that AXPs and SGRs accrete matter from a fallback disk, we attempt to explain both the soft and the hard X-ray emission as the result of the accretion process. We also attempt to explain their radio emission or the lack of it. We test the hypothesis that the power-law, hard X-ray spectra are produced in the accretion flow mainly by bulk-motion Comptonization of soft photons emitted at the neutron star surface. Fallback disk models invoke surface dipole magnetic fields of $10^{12} - 10^{13}$ G, which is what we assume here. Unlike normal X-ray pulsars, for which the accretion rate is highly super-Eddington, the accretion rate is approximately Eddington in AXPs and SGRs and thus the bulk-motion Comptonization operates efficiently. As an illustrative example we reproduce both the hard and the soft X-ray spectra of AXP 4U 0142+61 well using the XSPEC package compTB. Our model seems to explain both the hard and the soft X-ray spectra of AXPs and SGRs, as well as their radio emission or the lack of it, in a natural way. It might also explain the short bursts observed in these sources. On the other hand, it cannot explain the giant X-ray outbursts observed in SGRs, which may result from the conversion of magnetic energy in local multipole fields., Comment: 6 pages, 2 figures, minor corrections, accepted for publication in A&A

Context: Isolated cooling neutron stars with thermal X-ray emission remain rarely detected objects despite many searches investigating the ROSAT data. Aims: We simulate the population of close-by young cooling neutron stars to explain the current observational results. Given the inhomogeneity of the neutron star distribution on the sky it is particularly interesting to identify promising sky regions with respect to on-going and future searches. Methods: Applying a population synthesis model the inhomogeneity of the progenitor distribution and the inhomogeneity of the X-ray absorbing interstellar medium are considered for the first time. The total number of observable neutron stars is derived with respect to ROSAT count rates. In addition, we present sky maps of neutron star locations and discuss age and distance distributions of the simulated neutron stars. Implications for future searches are discussed. Results: With our advanced model we can successfully explain the observed logN - logS distribution of close-by neutron stars. Cooling neutron stars will be most abundant in the directions of rich OB associations. New candidates are expected to be identified behind the Gould Belt, in particular in the Cygnus-Cepheus region. They are expected to be on average younger and then hotter than the known population of isolated cooling neutron stars. In addition, we propose to use data on runaway stars to search for more radio-quiet cooling neutron stars., Comment: 18 pages, 14 figures; added Erratum after bug in code was discovered, updated results in Appendix, main conclusions do not change

We present our new advanced model for population synthesis of close-by cooling NSs. Detailed treatment of the initial spatial distribution of NS progenitors and a detailed ISM structure up to 3 kpc give us an opportunity to discuss the strategy to look for new isolated cooling NSs. Our main results in this respect are the following: new candidates are expected to be identified behind the Gould Belt, in directions to rich OB associations, in particular in the Cygnus-Cepheus region; new candidates, on average, are expected to be hotter than the known population of cooling NS. Besides the usual approach (looking for soft X-ray sources), the search in 'empty' $\gamma$-ray error boxes or among run-away OB stars may yield new X-ray thermally emitting NS candidates., Comment: 3 pages, 2 figures, proceedings of the conference "40 Years of Pulsars ", 12-17 August 2007, Montreal, Canada

The Magnificent Seven have all been discovered by their exceptional soft X-ray spectra and high ratios of X-ray to optical flux. They all are considered to be nearby sources. Searching for similar neutron stars with larger distances, one expects larger interstellar absorption resulting in harder X-ray counterparts. Current interstellar absorption treatment depends on chosen abundances and scattering cross-sections of the elements as well as on the 3D distribution of the interstellar medium. After a discussion of these factors we use the comprehensive 3D measurements of the Local Bubble by Lallement et al. 2003 to construct two simple models of the 3D distribution of the hydrogen column density. We test these models by using a set of soft X-ray sources with known distances. Finally, we discuss possible applications for distance estimations and population synthesis studies., Comment: 8 pages, 7 figures; to appear in Astrophysics and Space Science, in the proceedings of "Isolated Neutron Stars: from the Interior to the Surface", edited by D. Page, R. Turolla and S. Zane

Observations of cooling neutron stars allow to measure photospheric radii and to constrain the equation of state of nuclear matter at high densities. In this paper we concentrate on neutron stars, which show thermal (photospheric) X-ray emission and have measured distances. After a short summary of the radio pulsars falling into this category we review the observational data of the 7 radio quiet isolated neutron stars discovered by ROSAT which have been studied in detail by Chandra, XMM-Newton and optical observations. Their spectra show blackbody temperatures between 0.5 and 1 million Kelvin and an optical excess of a factor of 5-10 over the extrapolation of the X-ray spectrum. Four of these sources show periodicities between 3.45 and 11.37 s, indicating slow rotation. The pulsed fractions are small, between 6 and 18 %. The magnetic fields derived from spin down and/or possible proton cyclotron lines are of the order 10$^{13}-10^{14}$ G. We then discuss RX J1856.5-3754 in detail and suggest that the remarkable absence of any line features in its X-ray spectrum is due to effects of strong magnetic fields ($\sim 10^{13}$ G). Assuming blackbody emission to fit the optical and X-ray spectrum we derive a conservative lower limit of the ``apparent'' neutron star radius of 16.5 km $\times$ (d/117 pc). This corresponds to the radius for the ``true'' radius of 14 km for a 1.4 M$_{\odot}$ neutron star, indicating a stiff equation of state at high densities. A comparison of the result with mass-radius relations shows that in this case a quark star or a neutron star with a quark matter core can be ruled out with high confidence., Comment: 15 pages, 5 figures, to appear in ASI proceedings of The Electromagnetic Spectrum of Neutron Stars, Marmaris 2004, very minor revision

We discuss recent Chandra and XMM-Newton observations of the bright isolated neutron star RX J1856.5-3754 and suggest that the absence of any line features is due to effects of a high magnetic field strength (~10^13 G). Using different models for the temperature distribution across the neutron star surface assuming blackbody emission to fit the optical and X-ray spectrum and we derive a conservative lower limit of the "apparent" neutron star radius of 16.5 km x (d/117 pc). This corresponds to the radius for the "true" (de-redshifted) radius of 14 km for a 1.4 Msun neutron star, indicating a stiff equation of state at high densities. A comparison of the result with mass-radius diagrams shows that quark stars and neutron stars with quark matter cores can be ruled out with high confidence., Comment: 6 page, 2 figures, "The Restless High-Energy Universe" Proceedings of the symposium dedicated to six years of successful BeppoSAX operations Amsterdam, May 5-8, 2003

The XMM-Newton spectra of the isolated neutron star RX J0720.4-3125 show deviations from a Planckian energy distribution below 400 eV, similar to the spectra of RBS1223, another long-period X-ray pulsar, as reported recently by Haberl et al. (2003). For a Gaussian-shaped absorption line we derive an energy of 271 eV and an equivalent width of -40 eV from the phase-averaged spectra of RX J0720.4-3125. We investigate the spectral variations seen in hardness ratios as function of pulse phase and find that they are best described by changes in the depth of the absorption line. The line equivalent width changes between -31 eV around intensity maximum of the pulse and -58 eV at the declining part of the pulse. Small variations (<20 eV) of the line energy with pulse phase may still be caused by statistical fluctuations. On the other hand, the black-body temperature varies significantly by 2.5 eV (statistical 90% errors typically 0.7 eV) reaching the highest value at pulse maximum. One possible interpretation for the absorption line is cyclotron resonance scattering of protons in a magnetic field with B about 5x10^13 G. This field strength is compatible with estimates inferred from recent spin down measurements of the pulsar., Comment: Accepted for publication in A&A

We present results of the analysis of data collected in 57-ks XMM-Newton and 505-ks Chandra observations of the nearby ~= 120 pc isolated neutron star RX J1856.5-3754. We confirm most of the statements made by Burwitz et al. (2001) who discussed the original 55-ks Chandra data. Detailed spectral analysis of the combined X-ray and optical data rules out the currently available nonmagnetic light and heavy element neutron star atmosphere (LTE) models with hydrogen, helium, iron and solar compositions. We find that strongly magnetized atmosphere models also are unable to represent the data. The X-ray and optical data show no spectral features and are best fitted with a two-component blackbody model with kT_{bb,X}^{\infty} ~= 63.5 eV and R_{bb,X}^{\infty} ~= 4.4(d/120 pc) km for the hot X-ray emitting region, and kT_{bb,opt}^{\infty}<33 eV and R_{bb,opt}^{\infty}>17(d/120 pc) km for the rest of the neutron star surface responsible for the optical flux. The large number of counts collected with XMM-Newton allows us to reduce the upper limit on periodic variation in the X-ray range down to 1.3% (at a 2\sigma confidence level) in the 10^{-3}-50 Hz frequency range. In an attempt to explain this small variability, we discuss an one-component model with kT_{bb}^{\infty} ~= 63 eV and R_{bb}^{\infty} ~= 12.3(d/120 pc) km. This model requires a low radiative efficiency in the X-ray domain, which may be expected if the neutron star has a condensed matter surface., Comment: 6 pages, (including 3 figures) accepted for publication in Astronomy and Astrophysics (A&A)

We report on spectral and timing observations of the nearest millisecond pulsar J0437-4715 with Chandra. The pulsar spectrum, detected up to 7 keV, cannot be described by a simple one-component model. We suggest that it consists of two components, a nonthermal power-law spectrum generated in the pulsar magnetosphere, with a photon index about 2, and a thermal spectrum emitted by heated polar caps, with a temperature decreasing outwards from 2 MK to 0.5 MK. The lack of spectral features in the thermal component suggests that the neutron star surface is covered by a hydrogen (or helium) atmosphere. The timing analysis shows one X-ray pulse per period, with a pulsed fraction of about 40% and the peak at the same pulse phase as the radio peak. No synchrotron pulsar-wind nebula is seen in the X-rays., Comment: to appear in ApJ; 13 pages, 5 figures (1 in color)

We report the first detection of a sharp spectral feature in a Narrow-Line Seyfert 1 galaxy. Using XMM-Newton we have observed 1H0707-495 and find a drop in flux by a factor of more than 2 at a rest-frame energy of ~7 keV without any detectable narrow Fe K alpha line emission. The energy of this feature suggests a connection with the neutral iron K photoelectric edge, but the lack of any obvious absorption in the spectrum at lower energies makes the interpretation challenging. We explore two alternative explanations for this unusual spectral feature: (i) partial covering absorption by clouds of neutral material and (ii) ionised disc reflection with lines and edges from different ionisation stages of iron blurred together by relativistic effects. We note that both models require an iron overabundance to explain the depth of the feature. The X-ray light curve shows strong and rapid variability, changing by a factor of four during the observation. The source displays modest spectral variability which is uncorrelated with flux., Comment: 5 pages incl. 6 figures, accepted for publication in MNRAS

We present the Chandra LETGS X-ray spectrum of the nearby (~60 pc) neutron star RX J1856.5-3754. Detailed spectral analysis of the combined X-ray and optical data rules out the nonmagnetic neutron star atmosphere models with hydrogen, helium, iron and solar compositions. We also conclude that strongly magnetized atmosphere models are unable to represent the data. The data can be explained with a two-component blackbody model. The harder component with temperature of kT_bb~63 eV and a radius R_bb~2.2 km of the emitting region well fits the X-ray data and can be interpreted as radiation from a hot region on the star's surface., Comment: 4 pages, 3 color figures; acceped by A&A Letters; http://www.xray.mpe.mpg.de/~burwitz/burwitz_refereed.html

This paper reports the results of the analysis of the second ROSAT PSPC survey of M31 performed in summer 1992. We compare our results with those of the first survey. Within the ~10.7 deg^2 field of view, 396 individual X-ray sources are detected in the second survey data, of which 164 are new detections. When combined with the first survey, this result in a total of 560 X-ray sources in the field of M31. Their (0.1 keV - 2.0 keV) fluxes range from 7 x 10^-15 to 7.6 x 10^-12 erg cm^-2 s^-1, and of these 560 sources, 55 are tentatively identified with foreground stars, 33 with globular clusters, 16 with supernova remnants, and 10 with radio sources and galaxies (including M32). A comparison with the results of the Einstein M31 survey reveals 491 newly detected sources, 11 long term variable sources, and 7 possible transient sources. Comparing the two ROSAT surveys, we come up with 34 long term variable sources and 8 transient candidates. For the M31 sources, the observed luminosities range from 4 x 10^35 to 4 x 10^38 erg s^-1. The total (0.1 keV - 2.0 keV) luminosity of M31 is (3.4+-0.3) x 10^39 erg s^-1, distributed approximately equally between the bulge and disk. Within the bulge region, the luminosity of a possible diffuse component combined with faint sources below the detection threshold is (2.0+-0.5) x 10^38 erg s^-1. An explanation in terms of hot gaseous emission leads to a maximum total gas mass of (1.0+-0.3) x 10^6 M_sun., Comment: 39 pages, 3 figures (figure 2 on page 5 should be printed in color). Accepted (03.04.2001) for publication in A&A

We describe in this paper the ROSAT Ultra Deep Survey (UDS), an extension of the ROSAT Deep Survey (RDS) in the Lockman Hole. The UDS reaches a flux level of 1.2 x 10E-15 erg/cm2/s in 0.5-2.0 keV energy band, a level ~4.6 times fainter than the RDS. We present nearly complete spectroscopic identifications (90%) of the sample of 94 X-ray sources based on low-resolution Keck spectra. The majority of the sources (57) are broad emission line AGNs (type I), whereas a further 13 AGNs show only narrow emission lines or broad Balmer emission lines with a large Balmer decrement (type II AGNs) indicating significant optical absorption. The second most abundant class of objects (10) are groups and clusters of galaxies (~11%). Further we found five galactic stars and one ''normal'' emission line galaxy. Eight X-ray sources remain spectroscopically unidentified. The photometric redshift determination indicates in three out of the eight sources the presence of an obscured AGN in the range of 1.2 < z < 2.7. These objects could belong to the long-sought population of type 2 QSOs, which are predicted by the AGN synthesis models of the X-ray background. Finally, we discuss the optical and soft X-ray properties of the type I AGN, type II AGN, and groups and clusters of galaxies, and the implications to the X-ray backround., Comment: 26 pages, 8 figures (finding charts and spectra in JPEG), accepted for publication by A & A

We present the XMM-Newton first light image, taken in January 2000 with the EPIC pn camera during the instrument's commissioning phase, when XMM-Newton was pointing towards the Large Magellanic Cloud (LMC). The field is rich in different kinds of X-ray sources: point sources, supernova remnants (SNRs) and diffuse X-ray emission from LMC interstellar gas. The observations are of unprecedented sensitivity, reaching a few 10^32 erg/s for point sources in the LMC. We describe how these data sets were analysed and discuss some of the spectroscopic results. For the SNR N157B the power law spectrum is clearly steeper than previously determined from ROSAT and ASCA data. The existence of a significant thermal component is evident and suggests that N157B is not a Crab-like but a composite SNR. Most puzzling is the spectrum of the LMC hot interstellar medium, which indicates a significant overabundance of Ne and Mg of a few times solar., Comment: to be published in A&A 365

The XMM-Newton first-light image revealed X-ray point sources which show heavily absorbed power-law spectra. The spectral indices and the probable identification of a radio counterpart for the brightest source suggest AGN shining through the interstellar gas of the Large Magellanic Cloud (LMC). The column densities derived from the X-ray spectra in combination with HI measurements will allow to draw conclusions on HI to H_2 ratios in the LMC and compare these with values found for the galactic plane., Comment: 4 pages, LaTex, 4 figures, Accepted for publication in A&A Letters

We report on the first deep X-ray survey with the XMM-Newton observatory during the performance verification phase. The field of the Lockman Hole, one of the best studied sky areas over a very wide range of wavelengths, has been observed. A total of ~100 ksec good exposure time has been accumulated. Combining the images of the European Photon Imaging Camera (EPIC) detectors we reach a flux limit of 0.31, 1.4 and 2.4 X 10^{-15} erg cm^{-2} s^{-1}, respectively in the 0.5-2, 2-10, and 5-10 keV band. Within an off-axis angle of 10 arcmin we detect 148, 112 and 61 sources, respectively. The log(N)-log(S) relation in the three bands is compared with previous results. In particular in the 5-10 keV band these observations present the deepest X-ray survey ever, about a factor 20 more sensitive than the previous BeppoSAX observations. Using X-ray spectral diagnostics and the set of previously known, spectroscopically identified ROSAT sources in the field, the new sources can be classified. XMM-Newton detects a significant number (~40%) of X-ray sources with hard, probably intrinsically absorbed X-ray spectra, confirming a prediction of the population synthesis models for the X-ray background., Comment: 6 pages, based on the new A&A style file (included), to be published in A&A 365 (2001, special XMM issue), one page contains color images

We have analyzed the timing properties of the Narrow-line Seyfert 1 galaxy Mrk 766 observed with XMM-Newton during the PV phase. The source intensity changes by a factor of 1.3 over the 29,000 second observation. If the soft excess is modeled by a black body component, as indicated by the EPIC pn data, the luminosity of the black body component scales with its temperature according to L ~ T^4. This requires a lower limit 'black body size` of about 1.3*10^25 cm^2. In addition, we report the detection of a strong periodic signal with 2.4*10^-4 Hz. Simulations of light curves with the observed time sequence and phase randomized for a red noise spectrum clearly indicate that the periodicity peak is intrinsic to the distant AGN. Furthermore, its existence is confirmed by the EPIC MOS and RGS data. The spectral fitting results show that the black body temperature and the absorption by neutral hydrogen remain constant during the periodic oscillations. This observational fact tends to rule out models in which the intensity changes are due to hot spots orbiting the central black hole. Precession according to the Bardeen-Petterson effect or instabilities in the inner accretion disk may provide explanations for the periodic signal., Comment: 6 pages, 8 Figures, accepted for publication in the special A&A Letters issue for XMM-Newton; corrections in Section 2.2 required

The central source of the supernova remnant PKS 1209-52 was observed with the Advanced CCD Imaging Spectrometer aboard Chandra X-ray observatory on 2000 January 6-7. The use of the Continuos Clocking mode allowed us to perform the timing analysis of the data with time resolution of 2.85 ms and to find a period P=0.42412927+/-2.3e-7 s. The detection of this short period proves that the source is a neutron star. It may be either an active pulsar with unfavorably directed radio beam or a truly radio-silent neutron star whose X-ray pulsations are caused by a nonuniform distribution of surface temperature. To infer the actual properties of this neutron star, the period derivative should be measured., Comment: submitted to ApJ Letters

The ROSAT Ultradeep HRI survey in the Lockman Hole contains a complete sample of 91 X-ray sources with fluxes in the 0.5-2 keV band larger than 1.2 times 10e-15 erg cm-2 s-1, where over about 75 per cent of the sources are quasars or Seyfert galaxies. During the course of our optical identification work, we have obtained optical spectra of 67 narrow emission line galaxies (NELG), which are physically not associated with the X-ray sources. We have derived the equivalent width (EW) and the full width at half maximum (FWHM) for the most prominent emission lines of 41 quasars and Seyfert galaxies taken from the ROSAT Deep Survey (RDS), which has a flux limit of 5.5 times 10e-15 erg cm-2 s-1 in the 0.5-2.0 keV band. Furthermore we have obtained the EW and FWHM values of the field NELGs. Here we present the spectroscopic discrimination between RDS Seyfert galaxies and field galaxies (NELG). The analysis of the emission lines has revealed that a single object out of 69 spectroscopically identified AGN fits the optical criteria of Narrow-Line Seyfert 1 galaxies (NLS1). This may indicate that NLS1 contribute only marginally to the soft X-ray background, but we can not exclude a possible larger contribution., Comment: Invited talk presented at the Joint MPE,AIP,ESO workshop on NLS1s, Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also available at http://wave.xray.mpe.mpg.de/conferences/nls1-workshop

In the construction of an X-ray selected sample of galaxy clusters for cosmological studies, we have assembled a sample of 495 X-ray sources found to show extended X-ray emission in the first processing of the ROSAT All-Sky Survey. The sample covers the celestial region with declination $\delta \ge 0\deg $ and galactic latitude $|b_{II}| \ge 20\deg $ and comprises sources with a count rate $\ge 0.06$ counts s$^{-1}$ and a source extent likelihood of 7. In an optical follow-up identification program we find 378 (76%) of these sources to be clusters of galaxies. ..., Comment: 61 pages; ApJS in press; fixed bug in table file; also available at (better image quality) http://www.xray.mpe.mpg.de/theorie/NORAS/

We present a summary of an identification program of the more than 2000 X-ray sources detected during the ROSAT All-Sky Survey (Voges et al. 1999) at high galactic latitude, |b| > 30 degr, with countrate above 0.2 s-1. This program, termed the ROSAT Bright Survey RBS, is to more than 99.5% complete. A sub-sample of 931 sources with countrate above 0.2 s-1 in the hard spectral band between 0.5 and 2.0 keV is to 100% identified. The total survey area comprises 20391 sq.deg at a flux limit of 2.4 \times 10^{-12} erg/cmsq/s in the 0.5 -- 2.0 keV band. About 1500 sources of the complete sample could be identified by correlating the RBS with SIMBAD and the NED. The remaining 500 sources were identified by low-resolution optical spectroscopy and CCD imaging utilizing telescopes at La Silla, Calar Alto, Zelenchukskaya and Mauna Kea. Apart from completely untouched sources, catalogued clusters and galaxies without published redshift as well as catalogued galaxies with unusual high X-ray luminosity were included in the spectroscopic identification program. Details of the observations with an on-line presentation of the finding charts and the optical spectra will be published separately. Here we summarize our identifications in a table which contains optical and X-ray information for each source. As a result we present the most massive complete sample of X-ray selected AGNs with a total of 669 members and a well populated X-ray selected sample of 302 clusters of galaxies with redshifts up to 0.70. Three fields studied by us remain without optical counterpart (RBS0378, RBS1223, RBS1556). While the first is a possible X-ray transient, the two latter are isolated neutron star candidates (Motch et al. 1999, Schwope et al. 1999)., Comment: Also available at http://www.aip.de/~preprint/preprints/2000/2000_03.html

The central pointlike X-ray source of the Cas A supernova remnant was discovered in the Chandra First Light Observation and found later in the archival ROSAT and Einstein images. The analysis of these data does not show statistically significant variability of the source. The power-law fit yields the photon index 2.6-4.1, and luminosity (2-60)e34 erg/s, for d=3.4 kpc. The power-law index is higher, and the luminosity lower, than those observed fromvery young pulsars. One can fit the spectrum equally well with a blackbody model with T=6-8 MK, R=0.2-0.5 km, L=(1.4-1.9)e33 erg/s. The inferred radii are too small, and the temperatures too high, for the radiationcould be interpreted as emitted from the whole surface of a uniformly heated neutron star. Fits with the neutron star atmosphere models increase the radius and reduce the temperature, but these parameters are still substantially different from those expected for a young neutron star. One cannot exclude, however, that the observed emission originates from hot spots on a cooler neutron star surface. Because of strong interstellar absorption, the possible low-temperature component gives a small contribution to the observed spectrum; an upper limit on the (gravitationally redshifted) surface temperature is < 1.9-2.3 MK. Amongst several possible interpretations, we favor a model of a strongly magnetized neutron star with magnetically confined hydrogen or helium polar caps on a cooler iron surface. Alternatively, the observed radiation may be interpreted as emitted by a compact object (more likely, a black hole) accreting from a fossil disk or from a late-type dwarf in a close binary., Comment: 12 pages, 2 figures, submitted to ApJL

The ROSAT Deep Survey in the Lockman Hole contains a complete sample of 50 X-ray sources with fluxes in the 0.5-2.0 keV band larger than 5.5E-15 erg/cm2/s. Previous work has provided optical identification of 46 of the 50 X-ray sources; over 75% of the sources are AGNs (Schmidt et al. 1998). We present now the atlas of optical finding charts and the full description of the spectra, including emission line properties of the optical counterparts, which are important for the object classification. New optical/infrared observations of three of the four unidentified sources show that one source is an AGN and two sources with an unusually large ratio of X-ray to optical flux have counterparts in the K'-band suggesting that they are obscured AGNs. Furthermore, we found evidence from radio emission that the remaining unidentified source is a powerful radio galaxy (AGN). We thus obtain a 100% completeness. During the course of our optical identification work, we obtained optical spectr a of 83 field galaxies, of which 67 were narrow-emission line galaxies (NELG). We demonstrate that it is highly unlikely that a significant number of NELG are physically associated with X-ray sources., Comment: 21 pages, 5 figures (finding charts and spectra in JPEG), accepted for publication by A&A. The paper including all better quality finding charts and optical spectra can be obtained from http://www.aip.de:8080/~preprint/preprints/1999/1999_052.ps.gz (2.65 Mbyte)

We derive an X-ray luminosity function for active galactic nuclei (AGN) that accounts for the X-ray source counts in the 0.5-2.0 and 2-10 keV energy ranges, the redshift distribution of AGNs in the ROSAT Deep Survey (RDS), as well as the X-ray background (XRB) from 1-10 keV. We emphasize the role of X-ray absorption, which has a large effect on the faint end of the 2-10 keV source counts, as well as on the integrated X-ray background., Comment: 5 pages, 3 figures, to appear in "Highlights in X-ray Astronomy", proceedings of the workshop in honour of Joachim Truemper's 65th birthday, June 1998, Garching

The ROSAT Deep Survey in the Lockman Hole is the most sensitive X-ray survey performed to date, encompassing an exposure time of 200 ksec with the PSPC and 1.2 Msec with the HRI. The source counts reach a density of ~1000/deg2 at a limiting flux of ~1E-15 erg/cm2/s. At this level 70-80% of the 0.5-2 keV X-ray background is resolved into discrete sources. Because of the excellent HRI positions, 83 X-ray sources with fluxes (0.5-2 keV) above 1.2E-15 erg/cm2/s could be optically identified so far utilizing deep optical CCD images, NIR photometry and Keck spectroscopy. Only 11 sources above this flux limit remain unidentified. The majority of objects turned out to be active galactic nuclei (AGN) with minority contributions of clusters of galaxies, stars and some individual galaxies. These deep pencil beam data together with a number of shallower ROSAT surveys define the source counts over six orders of magnitude in flux and provide a unique tool of unprecedented quality to study the cosmological evolution of AGN, which can easily provide the bulk of the extragalactic X-ray background and could give an important contribution to the total background light in the universe., Comment: To appear in "Highlights in X-ray Astronomy", proceedings of the workshop in honour of Joachim Truemper's 65th birthday, June 1998, Garching. 10 pages, 7 postscript figures. Slightly revised version

There are several supernova remnants which contain unresolved X-ray sources close to their centers, presumably radio-quiet neutron stars. To prove that these objects are indeed neutron stars, to understand the origin of their X-ray radiation, and to explain why they are radio-quiet, one should know their periods and period derivatives. We searched for pulsations of the X-ray flux from the radio-quiet neutron star candidate RX J0822-4300 near the center of the Puppis A supernova remnant observed with the ROSAT PSPC and HRI. A standard timing analysis of the separate PSPC and HRI data sets does not allow one to detect the periodicity unequivocally. However, a thorough analysis of the two observations separated by 4.56 yr enabled us to find a statistically significant period $P\simeq 75.3$ ms and its derivative $\dot{P}\simeq 1.49\times 10^{-13}$ s s$^{-1}$. The corresponding characteristic parameters of the neutron star, age $\tau=P/(2\dot{P})=8.0$ kyr, magnetic field $B=3.4\times 10^{12}$ G, and rotational energy loss $\dot{E}=1.4\times 10^{37}$ erg s$^{-1}$, are typical for young radio pulsars. Since the X-ray radiation has a thermal-like spectrum, its pulsations may be due to a nonuniform temperature distribution over the neutron star surface caused by anisotropy of the heat conduction in the strongly magnetized crust., Comment: 9 pages, 2 postscript figures, to appear in ApJ Letters; an acknowledgment is added

The ROSAT Bright Survey (RBS) aims to completely optically identify the more than 2000 brightest sources detected in the ROSAT all-sky survey at galactic latitudes |b| > 30 degr (excluding LMC, SMC, Virgo cluster). This paper presents a subsample of 66 bright point-like ROSAT survey sources with almost hard PSPC spectra, the hardness ratio HR1 is > 0.5 for most of the sources. The subsample could be nearly completely identified by low-resolution optical spectroscopy with the following breakdown into object classes: 31 Seyfert galaxies, 22 BL Lac candidates, 5 clusters of galaxies 1 cataclysmic variable, and 5 bright stars. Only one object remained unidentified and one X-ray source was a spurious detect ion. The redshift distribution peaks around 0.06 for the Seyferts and around 0.13 for the BL Lac candidates. Observations with medium spectral resolution were obtained for most of the new S eyfert galaxies. A large fraction (20 objects) are type 1 Seyfert galaxies, the other fraction includes Seyfert galaxies of type 1.5 - 1.8 (5 objects), two LINERs, and 4 possible narrow-line Seyfert 1 galaxies (NLS1). About one third of the new Seyfert's have nearby companion galaxies displaying either emission or absorption lines at the same redshift. Among them are a couple of systems showing direct morphological evidence for interaction. The large fraction of interacting galaxies among our sample suggests a scenario where interaction is the main trigger of AGN activity., Comment: Astronomische Nachrichten, in press, 11 pages, 4 figures, full paper, 22 pages, with 8 figures available at http://www.aip.de:8080/~preprint/preprints/1998/1998_rhs1.html

An unusual double-lobed extended X-ray source (RX J105343+5735) is detected in the ROSAT ultra-deep HRI image of the Lockman Hole. The angular size of the source is 1.7 X 0.7 arcmin^2 and its X-ray flux is 2 X 10^-14 erg cm^-2 s^-1. R-band imaging from the Keck telescope revealed a marginal excess of galaxies brighter than R=24.5, but Keck LRIS spectroscopy of 24 objects around the X-ray centroid did not yield a significant number of concordant redshifts. The brightest galaxy close to the centre of the eastern emission peak appears to be a gravitationally lensed arc at z=2.570, suggesting that the X-ray object is associated with the lens, most likely a cluster of galaxies. Based on a comparison of lensing surface mass density, X-ray luminosity, morphology and galaxy magnitudes with clusters of known distance, we argue that RX J105343+5735 is a cluster at a redshift around 1. Future X-ray, ground-based optical/NIR and high resolution HST observations of the system will be able to clarify the nature of the object., Comment: Accepted for publication in A&A letters, 4 pages, LaTeX text, 2 postscript figures, psfig.sty and l-aa.sty

Until now X-radiation from nine millisecond pulsars has been detected. In the present paper we summarize the observations and show the results of a re-analysis of archival ROSAT data. In addition we present the results of recent observations of PSR J0437-4715 with the ROSAT PSPC and HRI detectors. We show that the pulsed fraction is independent of energy in the range 0.1-2.4 keV. The pulse width as measured at X-ray energies is comparable with that observed in the radio domain. An upper limit for the X-ray luminosity of the pulsar's bow-shock visible in H_alpha is found to be L_x < 2 x 10^29 erg/s (d/180pc)^2. We further report on the discoveries of PSR J1024-0719, J1744-1134 and J2124-3358 with the ROSAT-HRI, making them the first solitary galactic millisecond pulsars detected at X-ray energies. The pulse profile of PSR J2124-3358 shows marginal evidence for a double peak structure. We finally discuss the observed emission properties of the detected millisecond pulsars and conclude that the measured power-law spectra and pulse profiles together with the close correlation between the pulsar's spin-down energy and the observed X-ray luminosity suggests a non-thermal origin for the bulk of the observed X-rays., Comment: 16 pages. To appear in Astronomy and Astrophysics (in press). A preprint with higher resolution postscript images is available via ftp://mpeu13.rosat.mpe-garching.mpg.de/outgoing/web/ms.ps.gz

We re-analyzed soft X-ray data collected with the ROSAT and ASCA observatories on a candidate neutron star (NS) near the center of the supernova remnant PKS 1209-52. We fitted the observed spectra with NS atmosphere models. The hydrogen atmosphere fits yield more realistic parameters of the NS and the intervening hydrogen column than the traditional blackbody fit. In particular, for a NS of mass 1.4 M_\odot and radius 10 km, we obtained the NS surface temperature T_{eff}=(1.4-1.9)*10^6 K and distance d=1.6-3.3 kpc versus T=(4.2-4.6)*10^6 K and (implausible) d=11-13 kpc for the blackbody fit, at a 90% confidence level. Our fits suggest that the surface magnetic field is either very weak, B < 10^{10} G, or it exceeds 2*10^{12} G. The hydrogen column density inferred from the atmosphere fits, n_H=(0.7-2.2)*10^{21} cm^{-2}, agrees fairly well with independent estimates obtained from UV observations of nearby stars, radio data, and X-ray spectrum of the shell of the supernova remnant, whereas the blackbody and power-law fits give considerably lower and greater values, n_H=(0.2-0.4)*10^{21} and (5.2-7.0)*10^{21} cm^{-2}, respectively. The inferred NS surface temperature is consistent with standard NS cooling models., Comment: 4 figures, l-aa.sty; submitted to A&A

As a result of recent observations with ROSAT and ASCA the number of rotation-powered pulsars seen at X-ray energies has increased substantially. In this paper we review the phenomenology of the observed X-ray emission properties. At present 27 pulsars are detected, representing a wide range of ages (10^3 - 7 x 10^9 yrs), magnetic field strength (10^8 - 10^13 G) and spin periods (1.6 - 530 ms). Despite these dispersions in parameters all pulsars show an X-ray luminosity closely correlated with the rotational energy loss. This suggests that most of the observed X-rays are produced by magnetospheric emission originating from the co-rotating magnetosphere. Only for three middle aged pulsars (PSR 0656+14, Geminga and PSR 1055-52) and probably for the Vela-pulsar an additional thermal component is detected which can be attributed to thermal emission from the neutron stellar surface., Comment: 12 pages, LaTex, 5 figures in eps-format, 4 tables one in eps-format, to appear in Astronomy and Astrophysics (in press), MPE-Preprint 384

We report phase-resolved optical polarimetric, photometric and spectroscopic observations of RX J2115.7-5840 (= EUVE J2115-58.6, Craig 1996) which confirms the system to be a magnetic cataclysmic binary of the polar (AM Herculis) subclass. The optical light curve is sometimes flat and occasionally displays a pronounced bright phase, reminiscent of the self-eclipse of a small accretion spot by the revolving white dwarf, as seen in self-eclipsing polars. Our period search reveals ambiguous results only which can be interpret assuming that the white dwarf is not synchronously rotating with the binary orbit. We find circularly polarized cyclotron radiation with V/I ranging from 0% to -15% on one occasion, from -8% to +15% on another occasion. Compared with other polars, the self-occulted accretion region of RXJ2115 had an extreme red cyclotron spectrum. In addition, the system has an extreme hard X-ray colour during the ROSAT all-sky survey observation. Both properties suggest a low value of the magnetic field strength, and our best estimate gives B = 11+-2 MG. Due to the absence of significant M-star features in our low-resolution spectra we estimate the minimum distance to RXJ2115 to be d > 250pc (for an M5+ secondary star)., Comment: 8 pages, 9 figures, Astron. Astrophys., in press

Results from a cross-correlation analysis between the COBE DMR 4 year, and ROSAT PSPC All-Sky Survey data are presented. Statistical comparisons between microwave and X-ray maps can probe interesting astrophysical environments and processes, such as e.g. the Sunyaev-Zel'dovich effect in clusters of galaxies or the Integrated Sachs-Wolfe effect. In order to test the diffuse, extragalactic X-ray background as probed by ROSAT, against the COBE DMR large-scale CMB structure, our analysis was performed in most detail in a ROSAT selected region of the sky (40 deg < b, 70 < l < 250 deg) in an X-ray energy range with minimal Galactic structure and residual X-ray contamination. Comparing to other regions of the sky and neighbouring energies and frequencies, we find indication for a positive Galactic correlation on large scales. Removing a quadrupole term on a sky map with a Galactic cutout or related gradients in the selected regions leaves no correlations above a 1-sigma level on smaller angular scales. We conclude that there is no significant extragalactic correlation on scales for which the combined data are sensitive (7 deg - 40 deg) and that Galactic correlation is significant only on angular scales of the order of the quadrupole. In the context of removing large angular scale gradients we give results on best fit X-ray dipoles from various ROSAT data and discuss these with respect to the difficulty of finding a cosmological dipole. The lowest correlation upper limits we can place are about 15 % of COBE CMB fluctuations and about 10 % of the ROSAT extragalactic XRB fluctuations. We discuss these results with respect to possible correlation mechanisms., Comment: 12 pages, LaTeX file, 3 PS figures, uses l-aa.sty and psfig.tex, accepted for publication in Astronomy and Astrophysics

We show that an analysis of multiwavelength observations of isolated neutron stars based on neutron star atmosphere models can be used not only to evaluate the neutron star effective temperature, but also to determine chemical composition of its surface. To demonstrate how this new method can be applied to a specific object, we chose the old isolated neutron star candidate RXJ1856.5-3754, whose soft X-ray radiation has been studied recently by Walter et al. (1996). We fitted the soft X-ray spectrum of this object with neutron star atmosphere models of different chemical compositions and used these fits to calculate the source spectrum over a broad wavelength range. We showed, in particular, that the optical/UV flux expected from this object depends drastically on the composition of its surface. In particular, the neutron star covered with hydrogen would be 5-6 magnitudes brighter than the neutron star with an iron surface. The object should also be observable with $EUVE$; the EUV flux is expected to be almost twice higher for the iron surface than for the hydrogen one. Thus, multiwavelength observations of this object would enable one to examine, for the first time, chemical composition of the neutron star surface. The method proposed can be applied to other nearby isolated neutron stars., Comment: a few misprints corrected, aaspp4.sty, accepted for publication in ApJ Letters

X-ray observations of SN 1987A in the Magellanic Clouds have been performed throughout the ROSAT mission, using the PSPC and the HRI detectors. We present the X-ray light curve based on all observations in the years 1991-1995. For the first time a significant increase of the X-ray flux from SN 1987A can be seen, corresponding to X-ray luminosities (0.5-2 keV) of 0.8 - 2.2 X 10**34 erg/s about 4 - 8 years after the explosion. SN 1987A is surrounded by a ringlike nebula, which is thought to form the interface between the blue-supergiant wind and the denser red-supergiant wind of the progenitor. The X-ray data can constrain the density of the matter inside the ring to about 30 amu/cm**3 and the date at which the blast wave will reach the ring to about AD 2003, when a dramatic brightening is expected to occur. Nevertheless, other interpretations of the X-ray emission are possible., Comment: Accepted for A&A (Letters), 4 pages, LaTex text, 4 postscript figures, l-aa.sty and psfig.sty files are tar'd, gzip'ed and uuencoded

We report on the observation of PSR 2334+61 in the soft X-ray domain with ROSAT. Assuming a power-law spectrum $dN/dE\propto E^{-\alpha}$ with photon-index $\alpha=2$ we derive an X-ray flux of $f_x=(7.1 \pm 0.2) \times 10^{-14} \hbox{erg s}^{-1}\hbox{cm}^{-2}$ within the $0.1$--$2.4$ keV energy range. An upper limit for the neutron star's surface temperature is put at $T_s^\infty \sim 1.2\times 10^6$ K for a neutron star with a medium stiff equation of state (FP-model with $M=1.4$ \Mo, $R=10.85$ km). Slightly different values for $T_s^\infty$ are computed for the various neutron star models available in the literature, reflecting differences in the equation of state. No soft X-ray emission is detected from the supernova remnant G114.3+0.3 associated with PSR 2334+61., Comment: 4 pages in uuencoded and compressed ps-format, accepted for publication in Astonomy \& Astrophysics

We report on the detection of PSR 1706-44 in two ROSAT-PSPC observations. The recorded source counts are unpulsed with a $2\sigma$ pulsed fraction upper limit of 18%. Spectral analysis did not distinguish between black-body and power law models; however, we argue that the lack of pulsations and the similarity in the pulsar's spin parameters to those of the Vela pulsar favour a power law model $dN/dE\propto E^{-2.4\pm 0.6}$ and indicate synchrotron emission from a pulsar-powered nebula as the origin of the detected X-radiation. The X-ray flux derived for the power law model is f_x=3.2^{+6.5}_{-1.8} x E-12 erg/ s/ cm^2 within the 0.1-2.4 keV energy range. An upper limit for the neutron star's surface temperature is put at Log T_s^\infty \sim 6.03 K for a neutron star with a medium stiff equation of state (FP-model with M=1.4 Mo, R=10.85 km). Slightly different values for $T_s^\infty$ are computed for the various neutron star models available in the literature, reflecting the difference in the equation of state. No soft X-ray emission is detected from the supernova remnant G343.1-2.3, proposed to be associated with PSR 1706-44., Comment: 7 pages and picture in tared uuencoded and compressed ps-format