Shahbaz, T.; Linares, M.; Nevado, S. P.; Rodríguez-Gil, P.; Casares, J.; Dhillon, V. S.; Marsh, T. R.; Littlefair, S.; Leckngam, A.; Poshyachinda, S.
Bibliographical reference
Monthly Notices of the Royal Astronomical Society, Volume 453, Issue 4, p.3461-3473
Description
We present time-resolved optical photometry of the binary millisecond
`redback' pulsar PSR J1023+0038 (=AY Sex) during its low-mass X-ray
binary phase. The light curves taken between 2014 January and April show
an underlying sinusoidal modulation due to the irradiated secondary star
and accretion disc. We also observe superimposed rapid flaring on
time-scales as short as ˜20 s with amplitudes of ˜0.1-0.5
mag and additional large flare events on time-scales of ˜5-60 min
with amplitudes of ˜0.5-1.0 mag. The power density spectrum of the
optical flare light curves is dominated by a red-noise component,
typical of aperiodic activity in X-ray binaries. Simultaneous X-ray and
UV observations by the Swift satellite reveal strong correlations that
are consistent with X-ray reprocessing of the UV light, most likely in
the outer regions of the accretion disc. On some nights we also observe
sharp-edged, rectangular, flat-bottomed dips randomly distributed in
orbital phase, with a median duration of ˜250 s and a median
ingress/egress time of ˜20 s. These rectangular dips are similar
to the mode-switching behaviour between disc `active' and `passive'
luminosity states, observed in the X-ray light curves of other redback
millisecond pulsars. This is the first time that the optical analogue of
the X-ray mode-switching has been observed. The properties of the
passive- and active-state light curves can be explained in terms of
clumpy accretion from a trapped inner accretion disc near the corotation
radius, resulting in rectangular, flat-bottomed optical and X-ray light
curves.
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The study of binary stars is essential to stellar astrophysics. A large number of stars form and evolve within binary systems. Therefore, their study is fundamental to understand stellar and galactic evolution. Particularly relevant is that binary systems are still the best source of precise stellar mass and radius measurements. Research lines
Black holes, neutron stars, white dwarfs and their local environment
Accreting black-holes and neutron stars in X-ray binaries provide an ideal laboratory for exploring the physics of compact objects, yielding not only confirmation of the existence of stellar mass black holes via dynamical mass measurements, but also the best opportunity for probing high-gravity environments and the physics of accretion; the most
