WZ Sge and Friends

I have WZ Sge classified as a "possible" IP for many years. Then I decided to classify V455 And as a "confimed" IP in the 2014 version of the catalog, based on the persistence and the coherence of the 67.6197 s signal. That left me thinking: am I being inconsistent in how I categorize V455 And and WZ Sge?

Quite possibly.

Ideally, we would like to have a persistent and stable spin periodicity that does not disappear - at least not without a good reason. Additionally, we would love to have an explanation for every period that is a recurring feature of the power spectra. WZ Sge and other objects discussed in this page do not fulfill these ideal conditions.

After some soul-searching, I've decided to call WZ Sge a "probable" IP in the 2014 version of my catalog. In reality, I vacillate between "possible" and "confirmed", and maybe beyond, when it comes to WZ Sge.

Notes

V455 And

The 67.619704 s period is persistent and stable; it surely must be the white dwarf spin period. The added presence of the 67.2 s photometric period is a puzzle, but at leaset we also see a 3.5 hr signal (not strictly coherent) in spectroscopy, and this beats with the spin period to generate the 67.2 s signal. So, while mysterious, it is a self-consistent mystery. We know these twin periods are not due to white dwarf pulsations, because the pulsation periods are detected in the 5-6 min range. The 1.35 hr orbital period is securely known from the shallow eclipses, and the system also shows a permanent superhump period.

WZ Sge

The 27.87 s clock, when observed, appears to be very stable, which is a strong indication that it is, indeed, the white dwarf spin period. The satellite signals primarily at 28.96 s is an enigma; however, based on the behavior of V455 And, it may not be a deal breaker. The interpretation of these as due to white dwarf pulsations hasn't been ruled out, but it is increasingly strained since the characteristics of the multiple periods of WZ Sge are different from those of true pulsators such as GW Lib, as well as V455 And, not to mention the discrepancy relative to the theoretical expectations.

Nevertheless, I am less confident that WZ Sge is an IP, compared to how I view V455 And. For one thing, WZ Sge often fails to show the 27.87 s signal. It was present during 1971-1978, disappeared for 16 years, and reappeared in 1995-1996 (including the detection with ASCA), according to Patterson et al. (1998). The 27.87 s oscillation again disappred after the 2001 outburst. What is the explanation for this behavior? But my strongest concern is the fact that the X-ray period switched from 27.87 s in the 1996 ASCA data to 28.96 s in the 2003 XMM-Newton data. It's not simply a switch in the accretion mode (disk/stream overflow/diskless), since the separation of the two is not obviously related to the binary orbit. While it is plausible to produce a modulation in the optical between the spin period and a non-orbital period, I cannot think of a good way to do so in the X-rays.

V553 Her

This star once exhibited a coherent 63.633 s period, which, however, has not been seen in many years. Other than hoping for the reappearance of this period, and wishing there was an explanation for the disappearance, I have no strong reason to doubt the IP credential. In a comparison with WZ Sge, I think the evidence for V533 Her being an IP is weaker (fewer detection of the 63.6 s period) and evidence against is also weaker. I have therefore reclassified this as a "probable" IP.

V842 Cen

The 56.5 s oscillation is detected repeatedly in optical photometry, but has not been detected in X-rays or UV. At least, the period has not disappeared (yet). While I do not consider the X-ray and UV non-detections to be a strong evidence against the IP interpretaton, they certainly did not advance its credentials.

FS Aur

This system exhibits a plethora of periods, with some similarities to V455 And in that regard. However, this system lacks a credible spin period candidate. For now, I have classified it as "doubtful."

Please send your comments, suggestions etc. to Koji.Mukai@nasa.gov and/or Koji.Mukai@umbc.edu

National Aeronautics and Space Administration

Goddard Space Flight Center

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Full Catalog

Related Systems

WZ Sge and Friends

Notes