Twelve years of INTEGRAL observations of the symbiotic star RT Crucis
RT Crucis (or RT Cru) is a binary system composed of a red giant and a white dwarf.
It is the prototype of a new subclass of symbiotic stars characterised by high X-ray
luminosity and hard X-ray emission, detectable up to about 100 keV.
A recent study has used about twelve years of INTEGRAL observations of RT Crucis
(complemented with Swift/XRT and Suzaku data) to determine the properties of the white
dwarf. The resulting X-ray spectrum (top panel) can be explained by two distinct
scenarios: (1) optically thin boundary layer, or (2) accretion onto the polar caps of
a magnetised white dwarf.
Scenario 1 (bottom-left panel): The material lost by the red giant is gravitationally
captured by the white dwarf and forms an accretion disc. If the magnetic field of the
white dwarf is weak (B << 106 G), it cannot disrupt the accretion disc, which
can then extend close to the surface of the white dwarf. The material of the disc spirals
in towards the WD until it reaches the transition region between the disc and the white
dwarf surface, called "boundary layer". The hard X-ray emission in this case originates
from a hot, optically thin plasma located in this transition region.
The spectral coverage at high energies delivered by INTEGRAL allowed to better constrain
the spectral parameters obtained in previous studies of this source
(
Luna & Sokoloski 2007, ApJ 671, 741;
Kennea et al. 2009, ApJ 701, 1992 )
and estimate a mass of the white dwarf of about 1.2 M⊙. Theoretical studies show
that for a high mass accretion rate like that required by the high X-ray luminosity of
RT Cru, the boundary layer might be optically thick and the X-ray spectrum soft
(kT < 0.1 keV). Although the optically thin boundary layer scenario cannot be ruled out,
an alternative explanation for the X-ray emission of RT Cru is favoured.
Scenario 2 (bottom-right panel): If the magnetic field at the surface of the white dwarf
is sufficiently strong (B ~ 106-107 G), it can disrupt the accretion
flow geometry at some distance from the star surface, and channel the matter onto the polar
caps. Shocks form above the polar caps, where the hot and optically thin post-shock region
(PSR) emits hard X-ray photons. The PSR model of
Suleimanov et al. (2005, A&A 435, 191),
explains well the hard X-ray emission of RT Cru and its high X-ray luminosity
(L ~ 1034 erg/s), assuming that the mass of the WD is in the range
0.9-1.1 M⊙. The lack of detection of pulsation (usually observed in magnetic
white dwarfs) can be explained by aligned or nearly aligned rotation and magnetic axes
of the white dwarf.
