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Register a new userAn X-ray Survey of Wolf-Rayet Stars in the Magellanic Clouds. II. The ROSAT PSPC and HRI Datasets
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Wolf-Rayet (WR) stars in the Magellanic Clouds (MCs) are ideal for studying the production of X-ray emission by their strong fast stellar winds. We have started a systematic survey for X-ray emission from WR stars in the MCs using archival Chandra, ROSAT, and XMM-Newton observations. In Paper I, we reported the detection of X-ray emission from 29 WR stars using Chandra ACIS observations of 70 WR stars in the MCs. In this paper, we report the search and analysis of archival ROSAT PSPC and HRI observations of WR stars. While useful ROSAT observations are available for 117 WR stars in the MCs, X-ray emission is detected from only 7 of them. The detection rate of X-ray emission from MCs WR stars in the ROSAT survey is much smaller than in the Chandra ACIS survey, illustrating the necessity of high angular resolution and sensitivity. LMC-WR 101-102 and 116 were detected by both ROSAT and Chandra, but no large long-term variations are evident.
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Wolf-Rayet (WR) stars are evolved massive stars with strong fast stellar winds. WR stars in our Galaxy have shown three possible sources of X-ray emission associated with their winds: shocks in the winds, colliding stellar winds, and wind-blown bubbles; however, quantitative analyses of observations are often hampered by uncertainties in distances and heavy foreground absorption. These problems are mitigated in the Magellanic Clouds (MCs), which are at known distances and have small foreground and internal extinction. We have therefore started a survey of X-ray emission associated with WR stars in the MCs using archival Chandra, ROSAT, and XMM-Newton observations. In the first paper of this series, we report the results for 70 WR stars in the MCs using 192 archival Chandra ACIS observations. X-ray emission is detected from 29 WR stars. We have investigated their X-ray spectral properties, luminosities, and temporal variability. These X-ray sources all have luminosities greater than a few times 10^32 ergs s^-1, with spectra indicative of highly absorbed emission from a thin plasma at high temperatures typical of colliding winds in WR+OB binary systems. Significant X-ray variability with periods ranging from a few hours up to ~20 days is seen associated with several WR stars. In most of these cases, the X-ray variability can be linked to the orbital motion of the WR star in a binary system, further supporting the colliding wind scenario for the origin of the X-ray emission from these stars.
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.
We present an in-flight calibration of the ROSAT PSPC using the incident spectra of the hot white dwarf HZ43 and the polar AM Her. We derive an absolute flux calibration of the PSPC using the accurately known soft X-ray spectrum of HZ43. Corrections to the PSPC response matrix are derived from a comparison of predicted and observed PSPC spectra of HZ43, supplemented by results for AM Her. The calibration of the PSPC for photon energies E < 0.28 keV is found to be accurate to better than 5% refuting earlier reports of a major miscalibration. Our corrections to the detector response matrices remove systematic residuals in the pulse height spectra of soft sources.
We present an X-ray investigation of the elliptical galaxy NGC 507. We make use of archival Rosat HRI, PSPC and Chandra data, to connect the large-scale structure of the halo to the core morphology. Our analysis shows that the halo core (r<2-3 r_e) and the external halo (r>3 r_e) are characterised by different dynamical properties and suggests a different origin of the two components. The halo core has a complex morphology with a main X-ray emission peak, coincident with the center of the optical galaxy, and several secondary peaks. The spatial and spectral analysis of the central peak shows that this feature is produced by denser hot gas in the galaxy core. Our data support the scenario where the gas is kinetically heated by stellar mass losses. Comparison with previously published studies suggest that the core of X-ray extended galaxies is associated to the stellar distribution and has similar properties to the X-ray halo of compact galaxies. The secondary peaks are due instead to interactions between the radio-emitting plasma and the surrounding ISM. We found that the energy input by the central radio source in the ISM may be large enough to prevent gas from cooling. The total mass profile shows the presence of a dark matter halo extending on cluster scales which is likely to be associated with the whole cluster rather than with NGC 507. This structure is typical of many X-ray bright Early-Type galaxies and may explain the spatial and spectral differences with X-ray compact galaxies largely debated in literature. Finally we discuss the nature of the point sources detected in the HRI FOV.
Using XMM-Newton, we undertook a dedicated project to search for X-ray bright wind-wind collisions in 18 WR+OB systems. We complemented these observations with Swift and Chandra datasets, allowing for the study of two additional systems. We also improved the ephemerides, for these systems displaying photometric changes, using TESS, Kepler, and ASAS-SN data. Five systems displayed a very faint X-ray emission ($log [L_{rm X}/L_{rm BOL}]<-8$) and three a faint one ($log [L_{rm X}/L_{rm BOL}]sim-7$), incompatible with typical colliding wind emission: not all WR binaries are thus X-ray bright. In a few other systems, X-rays from the O-star companion cannot be excluded as being the true source of X-rays (or a large contributor). In two additional cases, the emission appears faint but the observations were taken with the WR wind obscuring the line-of-sight, which could hide a colliding wind emission. Clear evidence of colliding winds was however found in the remaining six systems (WR19, 21, 31, 97, 105, 127). In WR19, increased absorption and larger emission at periastron are even detected, in line with expectations of adiabatic collisions.
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