Abstract for Proposal 084466
Observation of the unusual Be X-ray transient 1A0538-66 in outburst
1A0538-66 is a Be/X-ray transient containing a 69 ms X-ray pulsar, one
of the fastest known in high-mass X-ray binaries. Its X-ray luminosity
reaches 1E39 erg/s. We propose a 25 ks TOO observation to perform the
first detailed study of 1A0538-66 in the super-Eddington regime. This
observation will allow us to measure the current spin frequency to
independently confirm the neutron star origin of 1A0538-66 and measure
the spin period derivative. The transient behaviour of 1A0538-66 has
been so far explained as a result of the interaction of the accretion
flow with the magnetosphere of the neutron star. This scenario implies
a surface magnetic field of about B=1E11 G, a prediction which will be
tested by the detection of a cyclotron resonance scattering feature in
the X-ray spectrum.
Details on Observing Strategy and Trigger Criteria
The XMM-Newton (pn in timing mode) observation should be performed if
1A0538-66 shows, during A018, a visual magnitude V < 13 or undergoes
an X-ray outburst above a flux of F(0.3-12 keV) > 2e-10 erg/cm2/s
announced with an ATel or GCN, or if the target is observed by
INTEGRAL or Swift/BAT with a flux F(20-40 keV) > 4.6e-11
erg/cm2/s. Based on the rate of bright X-ray and optical outbursts
shown by 1A0538-66 and thanks to the continuous daily monitoring of
this source performed by the REM telescope in the optical band, we
estimated that the trigger probability during A018 is about 14%. We
ask the ToO to be planned as soon as possible to catch the source in
its brightest state.