Bibcode
Russell, D. M.; Lewis, F.; Roche, P.; Clark, J. S.; Breedt, E.; Fender, R. P.
Referencia bibliográfica
Monthly Notices of the Royal Astronomical Society, Volume 402, Issue 4, pp. 2671-2681.
Fecha de publicación:
3
2010
Número de citas
18
Número de citas referidas
17
Descripción
Three years of optical monitoring of the low-mass X-ray binary (LMXB) 4U
1957+11 is presented. The source was observed in V, R and i bands using
the Faulkes Telescopes North and South. The light curve is dominated by
long-term variations which are correlated (at the >3σ level)
with the soft X-ray flux from the All Sky Monitor on board the Rossi
X-ray Timing Explorer. The variations span 1mag in all three filters. We
find no evidence for periodicities in our light curves, contrary to a
previous short-time-scale optical study in which the flux varied on a
9.3-h sinusoidal period by a smaller amplitude. The optical spectral
energy distribution is blue and typical of LMXBs in outburst, as is the
power-law index of the correlation β = 0.5, where
Fν,OPT ~ FβX. The discrete
cross-correlation function reveals a peak at an X-ray lag of 2-14 days,
which could be the viscous time-scale. However, adopting the
least-squares method we find the strongest correlation at a lag of 0 +/-
4 d, consistent with X-ray reprocessing on the surface of the disc. We
therefore constrain the optical lag behind X-ray to be between -14 and
+4 d. In addition, we use the optical-X-ray luminosity diagram for LMXBs
as a diagnostic tool to constrain the nature of the compact object in 4U
1957+11, since black hole and neutron star sources reside in different
regions of this diagram. It is found that if the system contains a black
hole (as is the currently favoured hypothesis), its distance must exceed
~20 kpc for the optical and X-ray luminosities to be consistent with
other soft-state black hole systems. For distances <20 kpc, the data
lie in a region of the diagram populated only by neutron star sources
(black hole systems are 10 times optically brighter for this X-ray
luminosity). 4U 1957+11 is unique: it is either the only black hole LMXB
to exist in an apparent persistent soft state or a neutron star LMXB
which behaves like a black hole.