The broadening of emission lines in TCP J21040470+4631129 occurs during a day at the end of rapid fading from the superoutburst
ATel #13646 (17 Apr 2020): V. Neustroev (U. Oulu), F. Sims (ARAS), K. L. Page (U. Leicester), T. Tordai (MCSE), A. V. Moiseev (SAO RAS), D. Boyd (ARAS), J. V. Echevarria (IA UNAM), B. T. Gaensicke (U. Warwick), C. Knigge (U. Southampton), T. R. Marsh (U. Warwick), R. Michel (IA UNAM), A. Oksanen (Hankasalmi Obs), J. P. Osborne (U. Leicester), S. Zharikov (IA UNAM)
We report the results of our optical and X-ray observations of a new superoutburst of the bright WZ Sge-type dwarf nova TCP J21040470+4631129 (hereafter TCP2104), discovered on July 12, 2019. At the maximum light, it was one of the brightest objects of this type ever observed (V~8.6 mag). However, the pre-outburst V magnitude of this object was ~18.1 (ATel #13122), thus the total amplitude of the outburst was ~9.5 mag. After the main superoutburst of a total duration of 22 days, the object has never returned to its original low state but instead experienced two smaller superoutbursts (V~10.8-11 mag at the maximums) and three rebrightenings (for previous reports, see ATel #12936, #12947, #13009, #13122, #13297).
On 2020 March 31, D. Denisenko reported that TCP2104 went into a new outburst (vsnet-alert 24120) which, after the detection of superhumps, appeared to be the third small superoutburst. Our first spectrum of TCP2104 was taken with the 6-m telescope of the SAO RAS within 14.5 hours after the observation by D. Denisenko. Using other smaller telescopes, we were also able to obtain spectra during all but one day of this superoutburst, and also during the rapid fading stage. Our photometric time-resolved observations were performed using 30-cm class telescopes.
The new superoutburst closely resembles the two previous small superoutbursts. At the maximum, the transient reached V~10.8 mag, it then slowly declined for ~9 days. Rapid fading started when the transient has reached V~11.85 mag – the exact same level as in all previous outbursts. As of 2020 April 16, TCP2104 is at V~15.3 mag.
We also obtained a series of Halpha images using a narrow-band filter, attempting to detect a nebulosity in emission (Hernandez Santisteban et al., 2019, 486, 2631). No sign of such a nebulosity was visible.
These new spectroscopic observations allowed us to trace a broadening of emission lines, which was first reported in ATel #13297. We found that the lines were narrow from the very beginning of the outburst to an initial part of the rapid fading stage (the FWHM of the Halpha line was ~450 km/s). However, during a single day at the end of rapid fading the lines became very broad (~2000 km/s, see Figure).
We also continued monitoring TCP2104 with Swift, getting a relatively dense coverage of this entire superoutburst. Before the first optical signature of an outburst, the XRT (0.3-10 keV) count rate of the transient was ~0.08+/-0.01 cts/s, consistent with (though slightly lower than) the inter-outburst observations obtained on 2019 December 24-30 after the previous superoutburst (0.10+/-0.01 cts/s). At the beginning of the optical outburst, during ~6.4 h the count-rate decreased sharply to 0.012+/-0.004 cts/s. 16 hours later the transient was found at 0.032+/-0.006 cts/s, staying at this level during the rest of the optical outburst. After the superoutburst ended, the count-rate returned to the pre-outburst level (see Figure).
We thank the Swift PI, Brad Cenko, for approving the observations, and the Swift planning and operations teams for their ongoing support.
Light curves and spectra (Followup Reports page for TCP2104)
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