U Gem:

Eclipse observations have astrophysical importance

W J Worraker

Eclipses of this star can be observed when in quiescence and possibly on the rise to outburst, and are are of considerable astrophysical interest. An eclipse ephemeris is given below. Note that V-filtered CCD eclipse observations made before an outburst and, if possible, on the rise to outburst and also towards the end and afterwards would be very valuable. Light curves starting well before a predicted eclipse are best in order to cover any orbital hump (which is quite large in U Gem in quiescence) as well as the eclipse itself.

I stress that observations of eclipses are needed mainly BEFORE an outburst and DURING THE RISE; they are not likely to be seen once an outburst is established (though it may be worth checking then as well). They should reappear towards the end of the decline, and comparisons between eclipses (or better, whole orbital light curves) before and after an outburst are of great interest.

These notes apply throughout the year, though realistically good coverage (i.e. complete orbital cycles) can only be expected from November to mid-March.

Lightcurve Details

The orbital period of U Gem is 4h 15m. It's V magnitude in quiescence is typically 14.6 about 2 hours before an eclipse is due. The orbital hump starts about 1.7 hours before the eclipse; the light level increases steadily to a maximum of V = 14.0, 30 minutes before mid-eclipse. The eclipse begins about 15 minutes later with V = 14.2; the initial drop in brightness is very sharp. Minimum light is at V = 15.1, after which the light level recovers quickly to V = 14.4. The total eclipse duration is about 20 to 25 minutes. In the next 2 hours the system fades back to V = 14.6.

The above details differ in different orbital cycles, and the hump and eclipse practically disappear when U Gem goes into outburst. Details of how the orbital lightcurve changes during the rise are of special interest. Eclipse observations should be made as frequently as photometric accuracy allows, up to one estimate per minute if an eclipse is being followed visually.

The eclipse ephemeris given below should adequately predict mid-eclipse times while U Gem is at minimum light. In this state it is mainly the hot spot which is being eclipsed; the white dwarf primary is not eclipsed at all. However when an outburst begins and the light from the accretion disc becomes stronger, mid-eclipse is expected to occur earlier by up to 5 minutes compared with the ephemeris. This change is reversed at the end of an outburst. Eclipses also become shallower on the rise to outburst and practically disappear at maximum light. The total eclipse duration is considerably longer after an outburst than before because of the increase in radius of the accretion disc during the outburst.

Long term lightcurve of U Gem

Early alert of the start of an outburst would be appreciated,
contact: Bill Worraker or Gary Poyner