The fundamental properties of neutron stars provide a direct test of the equation of state of cold nuclear matter, a relationship between pressure and density that is determined by the physics of the strong interactions between the particles that constitute the star. The most straightforward method of determining these properties is by measuring the gravitational redshift of spectral lines produced in the neutron star photosphere. The equation of state implies a mass-radius relation, while a measurement of the gravitational redshift at the surface of a neutron star provides a direct constraint on the mass-to-radius ratio. Here we report the discovery of significant absorption lines in the spectra of 28 bursts of the low-mass X-ray binary EXO0748-676. We identify the most significant features with the Fe XXVI and XXV n = 2-3 and O VIII n = 1-2 transitions, all with a redshift of z = 0.35, identical within small uncertainties for the respective transitions. For an astrophysically plausible range of masses (M approximately 1.3-2.0 solar masses; refs 2-5), this value is completely consistent with models of neutron stars composed of normal nuclear matter, while it excludes some models in which the neutron stars are made of more exotic matter.

Abstract. The ESA X-ray Multi Mirror mission, XMM-Newton, carries two identical Reflection Grating Spectrometers (RGS) behind two of its three nested sets of Wolter I type mirrors. The instrument allows highresolution (E/∆E = 100 to 500) measurements in the soft X-ray range (6 to 38Å or 2.1 to 0.3 keV) with a maximum effective area of about 140 cm 2 at 15Å. Its design is optimized for the detection of the K-shell transitions of carbon, nitrogen, oxygen, neon, magnesium, and silicon, as well as the L shell transitions of iron. The present paper gives a full description of the design of the RGS and its operational modes. We also review details of the calibrations and in-orbit performance including the line spread function, the wavelength calibration, the effective area, and the instrumental background.

High-sensitivity wide-band X-ray spectroscopy is the key feature of the Suzaku X-ray observatory, launched on 2005 July 10. This paper summarizes the spacecraft, in-orbit performance, operations, and data processing that are related to observations. The scientific instruments, the high-throughput X-ray telescopes, X-ray CCD cameras, non-imaging hard X-ray detector are also described.

Abstract. We present the first high resolution X-ray spectrum of the bright O4Ief supergiant star ζ Puppis, obtained with the Reflection Grating Spectrometer on-board XMM-Newton. The spectrum exhibits bright emission lines of hydrogen-like and helium-like ions of nitrogen, oxygen, neon, magnesium, and silicon, as well as neon-like ions of iron. The lines are all significantly resolved, with characteristic velocity widths of order 1000-1500 km s −1 . The nitrogen lines are especially strong, and indicate that the shocked gas in the wind is mixed with CNO-burned material, as has been previously inferred for the atmosphere of this star from ultraviolet spectra. We find that the forbidden to intercombination line ratios within the helium-like triplets are anomalously low for N VI, O VII, and Ne IX. While this is sometimes indicative of high electron density, we show that in this case, it is instead caused by the intense ultraviolet radiation field of the star. We use this interpretation to derive constraints on the location of the X-ray emitting shocks within the wind that are consistent with current theoretical models for this system.

Abstract. The RS CVn binary system HR 1099 was extensively observed by the XMM-Newton observatory in February 2000 as its first-light target. A total of 570 ks of exposure time was accumulated with the Reflection Grating Spectrometers (RGS). The integrated X-ray spectrum between 5-35Å is of unprecedented quality and shows numerous features attributed to transitions of the elements C, N, O, Ne, Mg, Si, S, Fe, and Ni. We perform an in-depth study of the elemental composition of the average corona of this system, and find that the elemental abundances strongly depend on the first ionisation potential (FIP) of the elements. But different from the solar coronal case, we find an inverse FIP effect, i.e., the abundances (relative to oxygen) increase with increasing FIP. Possible scenarios, e.g., selective enrichment due to Ne-rich flare-like events, are discussed.

We present the results of six Suzaku observations of the recurrent black hole transient 4U 1630−472 during its decline from outburst from February 8 to March 23 in 2006. All observations show the typical high/soft state spectral shape in the 2-50 keV band, roughly described by an optically thick-disk spectrum in the soft energy band plus a weak power-law tail that becomes dominant only above ∼ 20 keV. The disk temperature decreases from 1.4 keV to 1.2 keV as the flux decreases by a factor of 2, consistent with a constant radius, as expected for disk-dominated spectra. All of the observations reveal significant absorption lines from highly ionized (H-like and He-like) iron Kα's at 7.0 keV and 6.7 keV. The energies of these absorption lines suggest a blue shift with an outflow velocity of ∼ 1000 km s −1 . The H-like iron Kα equivalent width remains approximately constant at ∼ 30 eV over all of the observations, while that of the He-like Kα line increases from 7 eV to 20 eV. Thus, the ionization state of the material decreases, as expected from the decline in flux. By fitting the profile with Voigt functions (curve of growth) together with detailed photo-ionization calculations, the total absorbing column, and the ionization parameter were estimated to be (1.0-0.7) × 10 23 cm −2 and (6-4) × 10 4 , respectively, for a velocity dispersion of 500 km s −1 . This in turn constrains the size of the plasma to be ∼ 10 10 cm, assuming a source distance of 10 kpc.

Type-I X-ray bursts are thermonuclear flashes that take place on the surface of accreting neutron stars. The wait time between consecutive bursts is set by the time required to accumulate the fuel needed to trigger a new burst; this is at least one hour. Sometimes secondary bursts are observed, approximately 10 min after the main burst. These short wait-time bursts are not yet understood. We observed the low-mass X-ray binary and X-ray burster EXO 0748-676 with XMM-Newton for 158 h, during 7 uninterrupted observations lasting up to 30 h each. We detect 76 X-ray bursts. Most remarkably, 15 of these bursts occur in burst triplets, with wait times of 12 min between the three components of the triplet, T1, T2, and T3. We also detect 14 doublets with similar wait times between the two components of the doublet, D1 and D2. We characterize this behavior to try and obtain a better understanding of bursts with short wait times. We measure the burst peak flux, fluence, wait time and time profile, and study correlations between these parameters and with the persistent flux representing the mass accretion rate. (i) For all bursts with a long wait time, the fluence is tightly correlated with the wait time, whereas burst with short wait times generally have higher fluences than expected from this relationship; (ii) wait times tend to be longer after doublets and triplets; (iii) the time profile of single bursts, S1, and of the first burst in a double or triple burst, D1 and T1, always contains a slow component which is generally absent in the D2, T2 and T3 bursts; (iv) the peak flux is highest for S1, D1 and T1 bursts, but this is still a factor of 7 lower than the highest peak flux ever seen for a burst in this system; (v) the persistent flux, representing the mass accretion rate onto the neutron star, is about 1% of Eddington, which is among the lowest value so far measured for this system. The amount of energy per gram of accreted mass liberated during bursts is consistent with a fuel mixture of hydrogen-rich material. The characteristics of the bursts indicate that possibly all bursts in this system are hydrogen-ignited, in contrast with most other frequent X-ray bursters in which bursts are helium-ignited, but consistent with the low mass accretion rate in EXO 0748-676. Possibly the hydrogen ignition is the determining factor for the occurrence of short wait-time bursts. For example the 12 min wait time may be associated with a nuclear beta decay timescale.

An analysis of the iron L-shell emission in the publicly available spectrum of the Capella binary system, as obtained by the High Energy Transmission Grating Spectrometer on board the Chandra X-ray Observatory, is presented. The atomic-state model, based on the HULLAC code, is shown to be especially adequate for analyzing high-resolution x-ray spectra of this sort. Almost all of the spectral lines in the 10 -18 Å wavelength range are identified. It is shown that, for the most part, these lines can be attributed to emission from L-shell iron ions in the Capella coronae. Possibilities for electron temperature diagnostics using line ratios of Fe 16+ are demonstrated. It is shown that the observed iron-L spectrum can be reproduced almost entirely by assuming a single electron temperature of kT e = 600 eV. This temperature is consistent with both the measured fractional ion abundances of iron and with the temperature derived from ratios of Fe 16+ lines. A volume emission measure of 10 53 cm -3 is calculated for the iron L-shell emitting regions of the Capella coronae indicating a rather small volume of 10 29 cm 3 for the emitting plasma if an electron density of 10 12 cm -3 is assumed.Subject headings: atomic processes ---line: identification ---x-rays: stars ---stars: individual (Capella)