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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.
Lx ~ 3 x
erg/s
Tin ~ 0.2 keV
Feb 02
Nov 24
Mar 07
Jun 09
Jun 09
Apr 19
Lx ~ 5 x 1035 erg/s
disk not detected
XMM/PN data
nH ~ 4 1020 cm-2
2000 Mar 07
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
Tin ~ 1.3 keV
1037
2000 Feb 02
Apr 19
XMM/RGS data
WIND ACCRETION RULED OUT
High-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, we infer an intrinsic nH <~ 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.
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
Msun would 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.