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Register a new userA New View of the Supersoft X-Ray Source Cal 87 Observed with XMM-Newton
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Cal 87 was observed with XMM-Newton in April of 2003. The source shows a rich emission line spectrum, where lines can be identified if they are red-shifted by 700-1200 km/s. These lines seem to have been emitted in a wind from the system. The eclipse is observed to be shifted in phase by 0.03 phi(orb), where phi(orb) is the phase of the optical light curve.
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We present and discuss 25 spectra obtained in November 1996, covering all phases of the CAL 87 binary system. These spectra are superior both in signal-to-noise and wavelength coverage to previously published data so that additional spectral features can be measured. Photometry obtained on the same nights is used to confirm the ephemeris and to compare with light curves from previous years. Analysis of the color variation through the orbital cycle has been carried out using archival MACHO data. When a barely resolved red field star is accounted for, there is no (V-R)-color variation, even through eclipse. There have been substantial changes in the depth of minimum light since 1988; it has decreased more than 0.5 mag in the last several years. The spectral features and radial velocities are also found to vary not only through the 0.44-day orbit but also over timescales of a year or more. Possible interpretations of these long-term changes are discussed. The 1996 spectra contain phase-modulated Balmer absorption lines not previously seen, apparently arising in gas flowing from the region of the compact star. The changes in emission-line strengths with orbital phase indicate there are azimuthal variations in the accretion disk structures. Radial velocities of several lines give different amplitudes and phasing, making determination of the stellar masses difficult. All solutions for the stellar masses indicate that the companion star is considerably less massive than the degenerate star. The Balmer absorption-line velocities correspond to masses of ~1.4Msun for the degenerate star and ~0.4Msun for the mass donor. However, the strong He II emission lines indicate a much more massive accreting star, with Mx>4Msun.
Compact binary supersoft X-ray sources (CBSS) are explained as being associated with hydrostatic nuclear burning on the surface of a white dwarf with high accretion rate. This high mass transfer rate has been suggested to be caused by dynamical instability, expected when the donor star is more massive than the accreting object. When the orbital period is smaller than ~6 hours, this mechanism does not work and the CBSS with such periods are believed to be fed by a distinct mechanism: the wind-driven accretion. Such a mechanism has been proposed to explain the properties of objects like SMC 13, T Pyx and V617 Sgr. One observational property that offers a critical test for discriminating between the above two possibilities is the orbital period change. As systems with wind-driven accretion evolve with increasing periods, some of them may reach quite long orbital periods. The above critical test may, therefore, also be applied to orbital periods longer than 6 hours. CAL 87 is an eclipsing system in the LMC with an orbital period of 10.6 hours that could provide the opportunity for testing the hypothesis of the system being powered by wind-driven accretion. We obtained eclipse timings for this system and show that its orbital period increases with a rate of P/Pdot = +7.2(+/-1.3) X 10^{6} years. Contrary to the common belief, we conclude that CAL 87 is the first confirmed case of a wind-driven CBSS with an orbital period longer than 6 hours. The system is probably an evolved object that had an initial secondary mass of M2i=0.63 solar masses but is currently reduced to about M2=0.34 solar masses. We discuss evidence that other CBSS, like CAL 83 and V Sge stars, like WX Cen, are probably also wind-driven systems. This may in fact be the rule, and systems with inverted mass ratio, the exception.
A new ephemeris has been determined for the supersoft X-ray binary CAL 83 using MACHO photometry. With an improved orbital period of 1.047568 days, it is now possible to phase together photometric and spectroscopic data obtained over the past two decades with new far ultraviolet spectra taken with FUSE. We discuss the properties of the orbital and longterm optical light curves as well as the colors of CAL 83. In the far ultraviolet the only well-detected stellar feature is emission from the O VI resonance doublet. The radial velocity of this emission appears to differ from that of HeII in the optical region, although we only have partial phase coverage for the O VI line. The FUSE continuum variations are similar to the optical light curve in phase and amplitude.
We present the global X-ray properties of the point source population in the grand-design spiral galaxy M101, as seen with XMM-Newton. 108 X-ray sources are detected within the D25 ellipse of M101, of which ~24 are estimated to be background sources. Multiwavelength cross-correlations show that 20 sources are coincident with HII regions and/or supernova remnants (SNRs), 7 have identified/candidate background galaxy counterparts, 6 are coincident with foreground stars and one has a radio counterpart. We apply an X-ray colour classification scheme to split the source population into different types. Approximately 60 per cent of the population can be classified as X-ray binaries (XRBs), although there is source contamination from background AGN in this category as they have similar spectral shapes in the X-ray regime. Fifteen sources have X-ray colours consistent with supernova remnants (SNRs), three of which correlate with known SNR/HII radio sources. We also detect 14 candidate supersoft sources, with significant detections in the softest X-ray band (0.3-1 keV) only. Sixteen sources display short-term variability during the XMM-Newton observation, twelve of which fall into the XRB category, giving additional evidence of their accreting nature. Using archival Chandra & ROSAT HRI data, we find that ~40 per cent of the XMM sources show long-term variability over a baseline of up to ~10 years, and eight sources display potential transient behaviour between observations. Sources with significant flux variations between the XMM and Chandra observations show a mixture of softening and hardening with increasing luminosity. The spectral and timing properties of the sources coincident with M101 confirm that its X-ray source population is dominated by accreting XRBs (abridged).
We present the first results of an XMM-Newton EPIC observation of the luminous X-ray source population in the supergiant spiral galaxy M101. We have studied the properties of the fourteen most luminous sources, all of which have intrinsic X-ray luminosities exceeding the Eddington limit for a 1.4 solar mass neutron star, with a subset in the ultraluminous X-ray source (ULX) regime. Eleven sources show evidence of short-term variability, and most vary by a factor of ~2-4 over a baseline of 11-24 yrs, providing strong evidence that these sources are accreting X-ray binary (XRB) systems. The sources show a variety of spectral shapes, with no apparent spectral distinction between those above and below the ULX threshold. Nine are well-fit with either simple absorbed disc blackbody/powerlaw models. However for three of the four sources best-fit with powerlaw models, we cannot exclude the disc blackbody fits and therefore conclude that, coupled with their high luminosities, eight out of nine single-component sources are possibly high state XRBs. The nuclear source has the only unambiguous powerlaw spectrum (photon index~2.3), which may be evidence for a low-luminosity AGN. The remaining five sources require at least two-component spectral fits. We have compared the spectral shapes of nine sources covered by both this observation and an archival 100ks Chandra observation of M101; the majority show behaviour typical of Galactic XRBs i.e. softening with increasing luminosity. We find no definitive spectral signatures to indicate that these sources contain neutron star primaries, and conclude that they are likely to be stellar-mass black hole XRBs, with black hole masses of ~2-23 solar masses if accreting at the Eddington limit (abridged).
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