The Library
A long-term optical-X-ray correlation in 4U 1957+11
Tools
Tools
Tools
Russell, D. M., Lewis, F., Roche, P., Clark, J. S., Breedt, E. and Fender, R. P. (2010) A long-term optical-X-ray correlation in 4U 1957+11. Monthly Notices of the Royal Astronomical Society, Vol. 402 (No. 4). pp. 2671-2681. doi:10.1111/j.1365-2966.2009.16098.x
Research output not available from this repository, contact author.
Official URL: http://dx.doi.org/10.1111/j.1365-2966.2009.16098.x
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 >3σ 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 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.
| Item Type: | Journal Article | ||||
|---|---|---|---|---|---|
| Divisions: | Faculty of Science > Physics | ||||
| Journal or Publication Title: | Monthly Notices of the Royal Astronomical Society | ||||
| Publisher: | Wiley | ||||
| ISSN: | 0035-8711 | ||||
| Official Date: | 2010 | ||||
| Dates: |
|
||||
| Volume: | Vol. 402 | ||||
| Number: | No. 4 | ||||
| Page Range: | pp. 2671-2681 | ||||
| DOI: | 10.1111/j.1365-2966.2009.16098.x | ||||
| Status: | Peer Reviewed | ||||
| Publication Status: | Published | ||||
| Access rights to Published version: | Restricted or Subscription Access |
Request changes or add full text files to a record
Repository staff actions (login required)
 |
View Item |
