MBCAA Observatory

SW Sex observation and BinaryMaker3 model

Observations: 10, 11, 14, 15 January 2005

Michel Bonnardeau
15 Feb 2005
Updated: 21 Feb 2005
Updated and revised: 23 Feb 2005
Updated: 4 Feb 2006

Abstract

SW Sex is an accreting binary with deep, non symmetrical eclipses. The observations are modelled with the BinaryMaker3 program.

Introduction

SW Sextantis is a cataclysmic star, that is an accreting binary where the primary is a white dwarf. It is classified as a nova-like (NL): it has a stable accretion disk, presumably due to a high enough mass transfer. It has eclipses with the period P=0.134938 day.

SW Sextantis is the prototype of the SW Sex stars, a subset of the NL that are characterized by a peculiar velocity distribution (from spectroscopy). Models to explain them can be very different, e.g.:

  • some of the accreted material penetrates deep inside the accretion disk (Hellier 1999);
  • the accreting white dwarf is magnetized and is spinning rapidly (much like an IP). The inner part of the accretion disk is also magnetized and acts as a propeller to launch an outflow of material (Horne 1999).
  • Observations

    The observations were carried out with a 203mm f/6.3 SC telescope fitted with a Clear filter and SBIG ST7E camera (KAF401E CCD). 769 images were acquired in 4 sessions. Each exposure is 60s long.

    The comparison star is GSC 4907-01182 (RA=153.81002 DEC=-03.08191) with an assumed unfiltered magnitude of 11.75. The check star is GSC 4907-01166 (RA=153.82489 DEC=-03.12261) with a measured magnitude of 13.749 and a standard deviation of 0.016. An example of an image is here.

    The light curve for the 15 January session shows 2 deep eclipses:


    Red: SW Sex, Green: the check star shifted by +4.5 magnitudes. The error bars are the +/- 1 sigma statistical uncertainties.

    All the light curves are here.

    Analysis

    Six eclipses are observed at: 381.608, 381.744, 382.688, 385.657, 386.467, 386.602. With the above period, the epoch t0 of the eclipses is then:
    t0=2,453,386.602 +/- 0.001 HJD

    Between the eclipses, the light curves are steady. There may be faint bumps but there are not very significant.

    The eclipses are V shaped, and not symmetrical, with the ingress steeper than the egress. According to Rutten et al (1992) this reveals a hot spot on the accretion disk. This is modelled with the BinaryMaker3 program:

    BinaryMaker3 model

    To model the light curves with BinaryMaker3, data from Rutten et al (1992) are used:

  • mass of the primary (the accreting white dwarf) and of the secondary (in solar masses):
    M1=0.44    M2=0.30
  • inclination: 79
  • distance to the L1 Lagrangian point: RL1=0.54 solar radii
  • radius of the accretion disk (intensity fallen to 10%): R01=0.32 solar radii
  • distance to the bright spot of the accretion disk: Rspot=0.28 solar radii
  • azimuthal position of the bright spot: 35
  • The distances to the center of mass can then be computed (in solar radii):
    R1=0.40    R2=0.59
    and the amplitudes of the velocities (in km/s):
    V1=152    V2=224

    These parameters are used as input to BinaryMaker3, other parameters are ajusted by hand so that the model fits well with the light curve. All the parameters and the results are here:

  • the model is very sensitive to the temperature factor of the bright spot;
  • the temperature of the secondary is somewhat low to explain the depth of the eclipses;
  • the secondary does not overflow its Roche lobe.
  • The t0 of the eclipses may also be adjusted for a good fit with BinaryMaker3:
    t0=2,453,386.6013 +/- 0.0001 HJD

    An animation showing the result (the bright spot is in red on the accretion disk):

    A fairly good fit is obtained, however there may be small unaccounted features in the residuals, especially around phase 0.25.

    There is no reflection from the secondary. This suggests that the disk rim is flared enough so that it shadows its light. (J. Patterson, B. Gaensicke, personal communications).

    References

    C. Hellier (1999) Proceedings of the Warner Symposium on Cataclysmic Variables, [arXiv:astro-ph/9906089].

    K. Horne (1999) Annapolis Workshop on Magnetic Cataclysmic Variables, ASP Conference Series, vol 157.

    R.G.M. Rutten, J. van Paradijs, J. Tinbergen (1992) A&A 260 226.

    Technical notes

    Telescope and camera configuration.

    Computer and software configuration.

    Model with BinaryMaker3.

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