The orbital period of the bright WZ Sge-type dwarf nova TCP J21040470+4631129

ATel #13009: V. Neustroev (U. Oulu), D. Boyd, P. Berardi (ARAS), S. Zharikov, A. L. Medina (IA UNAM), K. L. Page, J. P. Osborne (U. Leicester), N. P. M. Kuin (UCL-MSSL), C. Knigge (U. Southampton), T. R. Marsh, B. T. Gaensicke (U. Warwick), L. Franco, F. Teyssier (ARAS), G. Sjoberg (AAVSO)

We report optical and X-ray follow-up observations of the bright WZ Sge-type dwarf nova TCP J21040470+4631129 discovered on July 12, 2019 (ATel #12936, #12947). Our optical spectroscopic data were obtained with the 2.1-m telescope at the OAN-SPM, the 2.5-m Isaac Newton Telescope on La Palma, and other smaller telescopes, while photometric observations were mostly performed using 30-cm class telescopes.

Our observations show that during about three weeks after the discovery, the transient monotonically declined to the level of ~11.7 mag. On August 3, it started to fade more rapidly reaching ~13.5V on August 7. It was still more than 4 mag brighter than in quiescence (ATel #12936). However, on August 8.8 we found that TCP J21040470+4631129 rebrightened again to the level of ~11.9V confirming thus the observations of Y. Sato and H. Nishimura (vsnet-alert 23465). Such a behavior resembles that of another WZ Sge-type dwarf nova, the solid period-bounce candidate SSS J122221.7-311525 (Kato et al., 2013, PASJ, 65, L11; Neustroev et al., 2017, MNRAS, 467, 597). Time-resolved photometric observations obtained during the first days of the superoutburst revealed the presence of early superhumps with the period of 77.04+/-0.58 min (ATel #12947), suggesting a very short orbital period. Normal, full-grown superhumps appeared on July 23 (vsnet-alert 23422). Their period is not stable; using the observations obtained between July 23-29, we measured a period of normal superhumps to be Psh=78.9+/-0.4 min.

Between July 13 and 31, we obtained 18 nights of time-resolved spectroscopic observations. In general, the appearance of spectra was similar to that reported by F. Teyssier (ATel #12936). We note, however, that the He II 4686 line and the Bowen blend were quite strong during the first days of the superoutburst peaking at 30% and 15% above the continuum, respectively, but then almost disappeared on July 19-20. Although the emission lines of TCP J21040470+4631129 are relatively narrow, the spectral resolution of our time-resolved data (R=1000-6000) is enough to trace the variability of line profiles. For consistency, we extracted two sets of quantities from the H-alpha line – the radial velocities measured using the double-Gaussian method, and the line asymmetry measured as the ratio between the areas of the blue and red parts of the line – and calculated periodograms for them. Both sets of quantities revealed the presence of the orbital variability with the period of Porb=77.07+/-0.02 min, which is consistent with the period of early superhumps.

Such a short orbital period places TCP J21040470+4631129 at the very short-period end of the orbital period distribution of cataclysmic variables where the main-sequence donor stars transform into brown dwarf-like objects. However, it seems that TCP J21040470+4631129 is still at the start of this transition. Indeed, the fractional superhump period excess eps = (Psh-Porb)/Porb is ~2.3%. Employing the empirical relation between eps and the mass ratio q (Patterson et al. 2005, PASP, 117, 1204), we find q to be ~0.1. Such a large q suggests that the donor star is still on the main sequence.

We also continued monitoring TCP J21040470+4631129 with Swift. Between July 29 and August 5, Swift/XRT obtained four additional 1 ks observations during which an XRT count rate of the transient was at the level of 0.025+/-0.004 cts/s, that is about 3 times lower than during the previous observation on July 17 (ATel #12947).

We thank the Swift PI, Brad Cenko, for approving the observations, and the Swift planning and operations teams for their ongoing support.

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