Astro-Drawings 2001

by Gerhard Niklasch

Our Galaxy: Variables

WZ Sagittae (WZ Sge), a recurrent dwarf nova

This is a prime representative of a very rare class of cataclysmic binary systems. Members of this class show no evidence of frequent low-amplitude outbursts, which distinguishes them from other dwarf novae of the SU Uma family. However, they do exhibit strong superoutbursts, during which they brighten by up to 9 magnitudes or so, and which happen very rarely - in the present case, the last three recorded events were (Nova Cygni) 1913, (Nova Cygni) 1946, and 1978.

Current theoretical models assume an accretion disk around the dwarf star which is very slowly fueled by material overflowing the Roche lobe of its unevolved partner. After many years, the disk grows large enough to reach a resonance with the 82-minute orbit, and becomes geometrically unstable. A fairly large amount of mass and angular momentum can now be shed, and a similarly large amount of material can get rid of its angular momentum and be deposited on the dwarf. Then all sorts of interesting things begin to happen!

To add to the excitement, our line of sight is close to the orbital plane in this case, so we can see eclipses - in addition to the "superhumps" which are "normal" for this type of object (and which depend on our viewing geometry onto the hottest and brightest parts of the whole affair).

For more info, see Gianluca Masi's page with many detailed photometric light curves, as well as the VSNET page about the ongoing campaign (with finder charts and links to pages about other similar objects).

Alerted by a posting by Gianluca Masi to sci.astro.amateur, I first caught sight of the star, almost at the height of the outburst, in the early morning hours of July 26. I tried a few estimates, using the 7m70 star HD 190720 20' to the WSW for comparison (not very suitable since it is much yellower than the nova, at spectral type K2!). I looked at it far too infrequently, not yet having realized how fast the variations were and how large the amplitude was. At least I noted it as being brighter than the comparison star three times, and fainter at other times! At any rate, dawn stopped the observation short. Later comparison to Gianluca's curves for JD2452116.4-116.6 was encouraging: I had indeed seen three successive maxima at intervals of about 80 minutes.

Two nights later, I returned for a slightly more methodical attack. The following curve is still too sparsely sampled and too crude, but again, I could see some features which match up with the professional photometry on Gianluca's page. The time axis is labeled with Julian Dates (2452000+x). There were several longish gaps in my observing run, one caused by clouds, the other by looking at other targets, so I had XEphem draw separate curves in succession instead of connecting them by a single unbroken line. The labels "A" and "B" correspond to isolated data points.

WZ Sge light curve, 2001 July 28></p>

<p>All observing was done with my 8m0, 9m32 pair NW of the Nova as an indication of a 0m3 brightness difference, and used the 8m76 star HD 191083 = GSC 01621-01830 = BD+174225 just 3' SSW of WZ as a base point. I frequently defocused the eyepiece to spread out the light.

Three nights later, on the morning of July 31, I got another chance at a somewhat more systematic run. By now, the nova was dipping occasionally below the magnitude of HD 191083. The moon was interfering somewhat. Sometimes, I had the impression of variations on a time scale of a few ten seconds, but some of this may have been seeing and atmospheric scintillation. Here's the July 31 curve, which extends somewhat past the end of Gianluca's corresponding curve:

WZ Sge light curve, 2001 July 31></p>

<p>On the morning auf August 2, after the moon had disappeared from
direct view, I obtained a very closely spaced run of more than 2 hours
(with inter-estimate gaps between 30 seconds and no more than 3 minutes).
I used the same comparison stars as on the preceding two occasions,
and continued until dawn drowned out the field. Compare the first
section of the curve, from JD2452123.495 to 123.515, to <a href=
Gianluca's curve. My very first point, where the "A" is, may be somewhat spurious. The broad maximum and descent to a minimum do match up. My data points show another maximum after his run ended, but rather earlier (after 123.54) than one would have expected it (after 123.56, where my curve dips to a pronounced minimum).

WZ Sge light curve, 2001 August 2></p>

<p>On the evening of the same day (JD2452124.374), the clouds only
allowed one brief glance, showing WZ much fainter than HD 191083 -
about 9<sup>m</sup>2 or so. This corresponds to a minimum on
Gianluca's JD2452124 curve.</p>

<p>(August 13) A campaign involving amateur and professional observers is
being staged to track the further evolution of this remarkable star.
See <a href=D. Steeghs' Page. Space-borne observatories including Chandra and FUSE and the HST have taken a look, more are to follow.- Other related pages are due to:

Drawings created using XEphem

G. Niklasch / Sun, 25 Nov 2001 22:51:27 MET