Swift X-ray detection and ground-based observations of the new WZ Sge-type dwarf nova TCP J21040470+4631129

ATel #12947: K. Sokolovsky (MSU), V. Neustroev (U. Oulu), K. L. Page (U. Leicester), J. Leahy-McGregor, M. Bundas, H. Gallamore, C. Wicklund, B. McIntyre, C. Conner, E. Zobel, E. Aydi, L. Chomiuk, A. Kawash, J, Strader (MSU), K. Mukai (NASA/GSFC), N. P. M. Kuin (UCL-MSSL), J. P. Osborne (U. Leicester), C. Knigge (U. Southampton), D. Denisenko (SAI)

The 9.2 mag optical transient TCP J21040470+4631129 was discovered on 2019-07-12.49 UT by H. Nishimura on images obtained with a DSLR camera and a 200 mm telephoto lens. It was spectroscopically confirmed on 2019-07-12.9 as a dwarf nova outburst (ATel #12936). Swift observed TCP J21040470+4631129 for 5.1 ks between 2019-07-13 and 2019-07-17. During the first observation, Swift/XRT detected a relatively bright X-ray source at the position of the dwarf nova, with an XRT count rate of 0.64 +/- 0.03 cts/s. In the following days, the X-ray flux declined before stabilizing at an average level of 0.09 +/- 0.02 cts/s.

The total X-ray spectrum can be approximated by a combination of two optically thin emission components (the APEC model in XSPEC) having kT = 11.5 +13.8/-4.5 keV and 0.19 +/- 0.02 keV, respectively, and negligible absorption (n_H1 < 7e19 cm^-2). We note, however, that the spectra, extracted separately from the first and remaining observations are statistically different. While the lower APEC temperature component is consistent in both spectra (kT~0.2 keV), the hotter APEC component has cooled from >23.6 keV to 5.4 +4.6/-1.5 keV. The peak 0.3-10 keV flux of 1.5e-11 ergs/cm^2/s corresponds to the X-ray luminosity of 2.1e31 ergs/s at 109 pc (see below). The object is still too bright for UVOT photometry or spectroscopy.

We also observed TCP J21040470+4631129 with the 0.6 m telescope of the Michigan State University Campus Observatory (MPC code 766) on 2019-07-13, 15, 16 and 17 for ~7 h each night. The object declined from V=8.59 +/-0.01 on 2019-07-13.1145 to V=10.25 +/-0.01 on 2019-07-18.1111. The period search (after detrending) revealed the presence of superhumps with the period of 0.0535 +/-0.0003 d, close to the value reported by T. Vanmunster in vsnet-alert 23388, and shorter than the periods suggested by T. Kato in vsnet-alert 23396. The peak-to-peak amplitude of the superhumps is about 0.04 mag.

We used 10 best-seeing images to measure the position of TCP J21040470+4631129 relative to UCAC3 (Zacharias et al. 2010 AJ, 139, 2184) stars in the field: 21:04:04.691 +46:31:13.68 J2000 +/-0.05″; thus confirming the identification of Gaia DR2 2163612727665972096 (g=17.77, parallax 9.13 +/-0.12 mas) with the transient. The large outburst amplitude combined with the presence of superhumps suggests that TCP J21040470+4631129 is a WZ Sge-type dwarf nova. A dramatic increase in X-ray luminosity accompanying the optical outburst has been observed in other WZ Sge systems: GW Lib (Byckling et al. 2009, MNRAS, 399, 1576), SSS J122221.7-311525 (Neustroev et al. 2018, A&A, 611, 13), V805 Aur (ATel #4954), ASASSN-18fs (Parikh & Wijnands 2018, RNAAS, 2d, 201), TCP J06373299-0935420 (ATel #12531), ASASSN-19hl (ATel #12629).

Further Swift observations are planned to monitor this object. We thank the Swift PI, Brad Cenko, for approving the observations, and the Swift planning and operations teams for rapid scheduling of these ToO observations and for their ongoing support.

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