Bibcode
de Martino, D.; Casares, J.; Mason, E.; Buckley, D. A. H.; Kotze, M. M.; Bonnet-Bidaud, J.-M.; Mouchet, M.; Coppejans, R.; Gulbis, A. A. S.
Referencia bibliográfica
Monthly Notices of the Royal Astronomical Society, Volume 444, Issue 4, p.3004-3014
Fecha de publicación:
11
2014
Número de citas
48
Número de citas referidas
46
Descripción
The peculiar low-mass X-ray binary XSS J12270-4859, associated with the
Fermi/LAT source 2FGL J1227.7-4853, was in an X-ray, gamma-ray and
optical low-luminosity persistent state for about a decade until the end
of 2012, when it entered into the dimmest state ever observed. The
nature of the compact object has been controversial until the detection
of a 1.69 ms radio pulsar early 2014. We present optical spectroscopy
and optical/near-IR photometry during the previous brighter and in the
recent faint states. We determine the first spectroscopic orbital
ephemeris and an accurate orbital period of 6.912 46(5) h. We infer a
mid G-type donor star and a distance d = 1.8-2.0 kpc. The donor spectral
type changes from G5 V to F5 V between inferior and superior
conjunctions, a signature of strong irradiation effects. We infer a
binary inclination 45° ≲ i ≲ 65° and a highly
undermassive donor, M2 ˜ 0.06-0.12 M⊙,
for a neutron star mass in the range 1.4-3 M⊙. Thus, this
binary joins as the seventh member the group of `redbacks'. In the high
state, the emission lines reveal the presence of an accretion disc. They
tend to vanish at the donor star superior conjunction, where also flares
are preferentially observed together with the occurrence of random dips.
This behaviour could be related to the propeller mechanism of the
neutron star recently proposed to be acting in this system during the
high state. In the low state, the emission lines are absent in all
orbital phases indicating that accretion has completely switched-off and
that XSS J12270-4859 has transited from an accretion-powered to a
rotation-powered phase.
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