TY - JOUR
T1 - A long-term optical–X-ray correlation in 4U 1957+11
AU - Roche, Paul
AU - Clark, J.S.
AU - Breedt, E.
AU - Fender, R.P.
AU - Russell, D.M.
AU - Lewis, F.
PY - 2009/11/20
Y1 - 2009/11/20
N2 - ABSTRACT 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 andgt;3s level) with the soft X-ray flux from the All Sky Monitor on board the Rossi X-ray Timing Explorer. The variations span 1 mag 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 ß = 0.5, where F?,OPT ? Fß X. 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 andlt;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.
AB - ABSTRACT 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 andgt;3s level) with the soft X-ray flux from the All Sky Monitor on board the Rossi X-ray Timing Explorer. The variations span 1 mag 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 ß = 0.5, where F?,OPT ? Fß X. 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 andlt;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.
KW - accretion
KW - accretion discs – black hole physics – stars
KW - individual: 4u 1957+11 –x-rays
KW - binaries.
U2 - 10.1111/j.1365-2966.2009.16098.x
DO - 10.1111/j.1365-2966.2009.16098.x
M3 - Article
VL - 402
SP - 2671
EP - 2681
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
SN - 0035-8711
ER -