The bright WZ Sge-type dwarf nova TCP J21040470+4631129 (hereafter TCP2104) was discovered on July 12, 2019. We are monitoring this object since the discovery. Our optical spectroscopic data are 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 are mostly performed using 30-cm class telescopes.
The observations show that the object is exhibiting remarkable, very unusual behaviour. After the end of the plateau stage on August 3, TCP2104 experienced 2 short rebrightenings on August 8-10 and 14-16, after which the object underwent the second superoutburst with a relatively short plateau stage of about 9 days (from August 25 to September 3). After the second plateau ended, TCP2104 showed another short rebrightening on September 15-17. We note that the fading after the rebrightening has again decelerated suggesting that another rebrightening is possible. TCP2104 is still about 3.4 mag brighter than in quiescence (according to PAN-STARRS1 measurements), and ~1.3 mag brighter than just before the discovery (according to ASAS-SN). We also note that superhump modulations have never disappeared (see also ATel #12947 and #13009), and they are still present in a light curve after the end of the most recent rebrightening. Using the observations obtained between September 18-21, we measured a period of modulations to be P=77.15+/-0.3 min, which is consistent with the orbital period Porb=77.07+/-0.02 min (ATel #13009).
Our extensive optical spectroscopic monitoring showed a notable difference between the spectra obtained during the first and the second plateau stages. During the second plateau, the flux in the emission lines was much reduced, and especially higher-order Balmer lines were significantly weakened.
The most dramatic spectral changes were observed during the recent rebrightening. The spectrum obtained on September 15.84, just before the rebrightening maximum, shows only Balmer absorption lines, while all the emission lines completely disappeared. However, another spectrum obtained 9 hours later (September 16.23) shows very strong emission lines again.
We are also monitoring TCP J21040470+4631129 with Swift. The observations obtained between rebrightenings showed a very stable XRT count rate at the level of 0.11+/-0.02 cts/s. However, during rebrightenings it has dropped by about 10 times.
This observation shows a remarkable change in TCP2104's emission line profiles. All the Balmer and He I lines are now much broader than they were during the superoutbursts and rebrightenings. For example, the FWHM of the Halpha line is now ~1550 km/s, 3-4 times larger than it was during the superoutburst (350-500 km/s). In the blue part of the spectrum, the Balmer emission lines are now superposed on the broad absorption lines of the white dwarf.
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.
The 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).
We also continued monitoring TCP2104 with Swift. 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.