Superhumps in Cataclysmic Binaries. XV. EG Cancri, King of the Echo Outbursts

Abstract

We report photometry and spectroscopy of the dwarf nova EG Cancri in its 1996/1997 episode of eruptions. The main eruption was clearly a superoutburst featuring common superhumps with a period of 0.06036(2) days, establishing the star as a new member of the SU UMa class of dwarf novae. At the end of the main eruption, the superhumps were replaced by another wave of slightly longer period (0.06045 days), which endured for at least another 90 days. The properties of the latter wave suggest identification as a "late superhump." The recurrence time for superoutbursts is long, probably in the range 7-20 yr. After the 15 day superoutburst, the star displayed six remarkable surges in light, with an average interval of 7 days; these "echo outbursts" strongly resemble ordinary dwarf nova eruptions. This suggests that a high accretion rate persisted in the disk for ∼40 days after the main superoutburst. By a year after outburst, the star faded to , with most of the V ϭ 18.7 continuum light from a white dwarf of K. We estimate a distance of 320 pc and a for T ∼ 15,000 AM S ϭ ϩ11.5 v accretion light. The extreme faintness, the rarity of eruptions, the enormous duration of superhumps, and the quiescent spectrum together establish membership in the WZ Sge subclass, the most sluggish of dwarf novae. Spectroscopy at quiescence and a transient wave in early outburst establish an orbital period of 0.05997(9) days. This suggests a secondary star with M , , yet . A binary can reach such a state after the M ∼ 0.02 R ∼ 0.08 R , secondary is forced to lose mass on a timescale shorter than its Kelvin-Helmholtz timescale.

The remarkable outburst of the highly-evolved post period-minimum dwarf nova SSS J122221.7−311525

“…This is in general agreement with the previous observations of other WZ Sge-type objects (see Patterson et al 1998;Kato et al 2008;Kato 2015, and references therein), although in SSS J122222 these late superhumps were seen for significantly longer time than in any other WZ Sge-type system. Moreover, the stable modulations still persisted even about 420 d after the superoutburst, but their period has further increased by about 2 per cent.…”

“…For example, a similarly strong cool continuum source was reported for EG Cnc by Patterson et al (1998), which also showed the red colours (B-I ≃ 1.4). However, multicolour observations after the active stage of superoutbursts are rare and sporadic, and no detailed study of the above phenomenon has ever been presented.…”

“…Although the sodium doublet is one of the strongest interstellar lines, it is not expected to be seen in the spectra of CVs because of their relatively close proximity. Nonetheless, Na D in absorption was noticed in spectra of several WZ Sge-type systems at different stages of their superoutbursts (Patterson et al 1998;Nogami & Iijima 2004;N o g a m ie ta l .2009), but it was not seen in other dwarf novae showing large amplitude outbursts (e.g. BZ UMa; .…”

The nature of V359 Centauri revealed: New long-period SU UMa-type dwarf nova

“…a few percent longer than P orb ), but have phases of ∼0.5 different from those of ordinary superhumps. Since this signal was observed long after then cessation of the 1999 outburst, Woudt & Warner (2001) suggested that V359 Cen may have shown a long persistence of late superhumps as was observed in a WZ Sge-type dwarf nova in EG Cnc Patterson et al 1998). …”

Research on AM CVn stars: ASASSN-14ei and CR Boo

“…The 1996 superoutburst of the WZ Sge-type object EG Cnc also showed multiple rebrightenings [20], [19] (see figure 2). As you can see, the rebrightenings of ASASSN-14ei are very similar to ones of EG Cnc except the time scale.…”