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SW Sextantis
(14 February 2009) |
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SW Sextantis (p = 0.1349385 days, 194 minutes) is the prototype of a class of eclipsing binary stars with a white dwarf component and nova-like variability.
Novalike variables are insufficiently studied objects resembling novae by the characteristics of their light changes or
by spectral features. This type includes, in addition to variables showing novalike outbursts, objects with no bursts ever
observed; the spectra of novalike variables resemble those of old novae, and small light changes resemble those typical
for old novae at minimum light. However, quite often a detailed investigation makes it possible to reclassify some
representatives of this highly inhomogeneous group of objects into other types. These eclipsing binary systems have
orbital planes so close to the observer's line of sight (the inclination i of the orbital plane to the plane orthogonal
to the line of sight is close to 90 deg) that the components periodically eclipse each other. Consequently, the
observer finds changes of the apparent combined brightness of the system with the period coincident with that of the
components' orbital motion.
Light amplitudes are also quite different and may reach several magnitudes. In the case of SW Sextantis, the amplitude of primary minimum is about 1.8 magnitudes, and there does not appear to be a secondary minimum. This binary is a very well studied one, with many references in the literature. The time of primary minimum is predicted by the equation: HJD = 2452500.0550 + 0.1349385 x CycleNo. |
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Finding chart for SW Sextantis
SW Sex: RA(2000) = 10h 15m 09.38s Dec(2000) = -3° 08' 33.5"
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Eclipse curves for SW Sextantis
Observation of SW Sextantis was carried out on one night for a period of about three hours, almost the same length as the period of variability. Each data point represents a 50-second integration exposure using an ST-7XME CCD camera cooled to -30°C and a "Clear" filter. Each point is separated by about one minute. The ten-second difference represents the download and processing time needed before the next exposure begins. The telescope is continuously autoguided. After observing, all of the CCD images in the series are calibrated in the standard way to remove dust donuts and vignetting. The figure below shows the eclipse curve for SW Sextantis, along with three reference stars and a check star. This is data from 01-02 February 2009 EST (cycle no. 17,524), and the observed geocentric time of minimum (middle of eclipse minimum) was approximately 02 Feb 2009 05:05:41 UT, or HJD = 2454864.71737. The image on the right is a single CCD frame near the time of mid-eclipse, #41 from a series of 172 images. The positions of SW Sextantis and the reference stars are indicated in the image and on the graphic.
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Things to Note
These are preliminary results, based only on inspection of the light curves and corresponding CCD images. The
observed geocentric UT time of primary minimum is determined from the light curve
and corrected to provide the heliocentric Julian date of mid-eclipse. The magnitudes shown on the graphics are
instrumental magnitudes that have not been transformed to a standard photometric system.
Nevertheless, the "constant" values for the magnitudes of the reference stars show that the telescope-CCD system
and evening conditions were stable. There's greater variability in the check star, and to some extent in the binary
itself because they are fainter.
This light curve has some similarities to the typical Algol-type eclipsing binaries, but the period is so very short. With a 194 minute period, a deep primary minimum appearing to be partial, and no observed secondary minimum, it's easy to conclude that the secondary star is a low luminosity object. All available data are shown in the figure below. The eclipse minimum is occuring about 3½ minutes earlier than predicted.
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References
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This data and information on this page are Copyright © 2008-2013, Richard A. Berg, Washington, DC
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