[vsnet-superoutburst 767] WZ Sge: note on the close companion and data analysis

[Taichi Kato <tkato>]

WZ Sge: note on the close companion and data analysis

Dear Colleagues,

   WZ Sge is currently experiencing the most unknown period of its
superoutburst.  The object may rise or fall with very short intervals,
as have been experienced in EG Cnc (six post-superoutburst rebrightenings),
AL Com (looking most similar to the present WZ Sge outburst; AL Com in
1995 showed undulations with time-scales of hours to days, whose nature
have not been yet clear) and WZ Sge in 1978 (rather sparcely sampled visual
data suggest day-scale variations).  Please keep your eye on this object
as long as you can -- we may not have a next similar chance within some
decades.

   However, as Arne Henden has commented, there is close companion (10 arcsec
away) which may partly degrade CCD photometry (esp. in red or unfiltered
ones).  One may include this companion within the same aperture, and
subtract its contribution.  However, this process has a caveat: the
companion is red, and is affected by differential extinction differently
from WZ Sge and/or comparison stars.  One must also know exactly the system
color response to exactly determine the contribution.  The contribution
from the companion (and the sky background when including both stars in
a large aperture) dominates when WZ Sge fades.  The combination of these
effects causes both systematic errors and a reduced S/N.  It is therefore
recommended to treat these measurements only as preliminary ones (to be
used as real-time assessment of the behavior of the object).  Observers are
thus strongly requested to store all images (please don't erase them!), and
make more elaborate analysis (PSF photometry) to separate these two stars.
The VSNET Collaboration team is fully prepared to receive these raw images,
and to make necessary PSF photometry.  We can receive the data either
via ftp or CD-ROM (please consult us if you use other media).

   For those who are familiar with IRAF/DAOPHOT or other professional
softwares may perform PSF photometry by themselves.  We recommend to use
GSC 1621.1830 (200733.72 +174000.1) = SAO 105719 as the primary PSF standard,
if the star is not saturated.  The second best selection is GSC 1621.1758
(200733.52 +173916.0).  Please don't use the "automatic centering"
function of the software [even the combination of professional softwares
and long focal length images can easily fail], but use calculated PSF
positions from the rest of the stars in the frame.  Please don't rely on
old positions (e.g. in the past literature, from DSS scans, etc.) since
WZ Sge has a high proper motion.

   A sample of such PSF photometry (separation of two stars) in quiescence
can be seen at:

   http://ftp.kusastro.kyoto-u.ac.jp/pub/vsnet/DNe/WZ_Sge/wzq.gif

   for which we used a frontside-illuminated (low Q.E.) CCD attached to a
60-cm telescope, V-filter and 10 sec exposures.  If your analysis cannot
attain this level of constancy for the companion, it may be possible
that something is wrong with the analysis.  Please refer to this light
curve in assessing your analysis.

Regards,
Taichi Kato
VSNET Collaboration team

VSNET Home Page

---

vsnet-adm@kusastro.kyoto-u.ac.jp

VSNET Home Page

Return to Daisaku Nogami

---

vsnet-adm@kusastro.kyoto-u.ac.jp