3. Accretion in the propeller regime
A0538-66 is a transient X-ray source, the outburst of which likely derive from an enhanced emission of its Be companion star (hard X-ray transients). In the previous section we presented the first detection of A0538-66 during quiescence. The detected luminosity could in principle derives from different mechanisms (cf. Stella et al. 1994), its emission level is however sufficiently high not to be accounted by the Be companion star emission (which should emit erg s-1 at most; Meurs et al. 1992), nor by the emission of the underlying neutron star either due to cooling or by non-thermal processes. The most likely explanation is that the observed luminosity derives from accretion. Two different regimes are possible in this case (e.g. Illarionov & Sunyaev 1975). The motion of the matter falling in the potential well of a neutron star becomes dominated by the rapidly increasing magnetic pressure at the magnetospheric radius, . At smaller radii the accretion flow follows the magnetic field lines and is enforced to corotate with the neutron star. If the magnetospheric radius lies inside the corotation radius (), i.e. the radius at which a test particle in Keplerian orbit would corotate with the neutron star, the accreting matter can proceed down to the neutron star surface.
Due to the scaling of with the accretion rate a minimum value, , exists below which centrifugal acceleration is strong enough to expels the infalling matter, preventing the accretion. This limiting accretion rate corresponds to a minimum accretion luminosity down to the neutron star surface of
where is the neutron star spin period in units of 69 ms and its mass in units of . The neutron star magnetic moment is in units of G cm3, being cm the neutron star radius and B the surface magnetic field (e.g. Stella et al. 1994).
The high X-ray luminosity ( erg s-1 ; Skinner et al. 1980) and the presence of pulsations testify that during the bright outbursts the centrifugal barrier is open and accretion onto the neutron star surface takes place. As noted by Skinner et al. (1982), the lower range of X-ray luminosities observed during bright outbursts implies an upper limit of . On the other hand, the presence of X-ray pulsations indicates that a small magnetosphere is still present (i.e. ); this implies .
Below the minimum luminosity , the accretion flow is halted at the magnetospheric boundary, which behaves like a closed barrier. Entering the propeller regime a drop in the accretion-induced luminosity is expected to occur down to
If the minimum detected X-ray luminosity ( erg s-1, for a black body model) derives from accretion onto the neutron star surface, we have from eq. 1 an upper limit to the magnetic moment of . This field is likely too low to explain the observed pulsations during outburst. On the other hand, no stringent limitations on the magnetic moment exist if the accretion matter is stopped at the magnetospheric radius by the centrifugal barrier (one has to require so that a radio pulsar cannot turn on; see Campana et al. 1995).
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998