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MASS AND SPECTRAL TYPE OF THE COMPANION STAR

STATE TRANSITIONS IN LMC X-3. Roberto Soria, Mat Page, Kinwah Wu (MSSL/UCL).

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MASS AND SPECTRAL TYPE OF THE COMPANION STAR

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  1. STATE TRANSITIONS IN LMC X-3 Roberto Soria, Mat Page, Kinwah Wu (MSSL/UCL) X-RAY SPECTRAL STATE TRANSITIONS Most black-hole candidates (BHC) show transitions between soft and hard X-ray spectral states. In the soft state, the X-ray spectrum consists of a thermal component (disk blackbody) and a power-law component (Comptonised emission); in the hard state, the thermal component is insignificant and the power law is harder. The BHC LMC X-3 (M >5 Msun) is normally found in the soft state; a rare transition to the hard state occurred in 2000 April. We used XMM to study its spectral behaviour over this transition. The thermal disk component disappeared in the low-hard state, but became dominant again as the system returned to the high-soft state. The inner-disk temperature changed as shown in the figures below. The emitted luminosity in the 0.3-10 keV band varied by 3 orders of magnitude. The optical/UV luminosity increased by a factor of 2 (0.8 mag) in the high-soft state. 2000 Feb 02 2000 Mar 07 nH~4 1020 cm-2 nH~4 1020 cm-2 Confidence contours: 68%, 95%, 99% 2000 Jun 09 2000 Nov 24 nH~4 1020 cm-2 nH~4 1020 cm-2 Nov 24 Lx~6 x 1038 erg/s WIND ACCRETION RULED OUTHigh-resolution RGS spectra allowed us to determine the absorbing column density for the X-ray emitting region. From the depth of the O I absorption edge, weinfer an intrinsicnH <~ 1020 cm-2 (figure above). This rules out wind accretion as the main mechanism of mass transfer, and suggests that accretion is instead due to Roche-lobe overflow. Tin~1.3 keV Feb 02 Nov 24 Lx~3 x 1037 erg/s Mar 07 Tin~0.2 keV Jun 09 Jun 09 Apr 19 Lx~5 x 1035 erg/s Apr 19 disk not detected XMM/PN data XMM/RGS data MASS AND SPECTRAL TYPE OF THE COMPANION STAR Observations of LMC X-3 in its low state allowed us to determine the mass and spectral type of the companion. The system was observed with XMM/OM on 2000 April 19, in a low state. From the U,B,V brightness and colours we infer that the companion is a subgiant of mass 4.5 < M < 5.0 Msun and temperature 15500 < Teff < 16500 (spectral type B5 IV). No significant wind is expected from such a star, in agreement with the low column density inferred from the X-ray data. The companion was previously thought to be a main sequence B3 star (M~ 7 Msun). MASS TRANSFER VIA ROCHE LOBE OVERFLOW The mean mass density in the Roche lobe (RL) of the companion star is uniquely determined by the binary period. We plot here the evolutionary tracks in the (MV,r) plane (for Z=0.008), compared with the mean density inside the RL. Stars with mass M~ 4.5 Msunwould be very close to filling their RL. Mass transfer would occur mainly via RL overflow, in agreement with our X-ray observations. A more massive companion would not fill its RL, and the mechanism of mass transfer would have to be a stellar wind. This is ruled out by the UV/optical colours and by the RGS data.

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