Monitoring of TCP J21040470+4631129

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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).

The optical light curve of TCP2104. The times of spectroscopic observations are marked by blue and red ticks.

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 spectra obtained in the beginning of both superoutbursts and in rebrightenings (shown in blue), and also two spectra from both superoutbursts but when the object's brightness was closer to the last observation. To the right from the spectra their JD times (-2450000) are shown.

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.

The spectra are normalised to continuum and shifted according to the V magnitude of TCP2104 during those observations.

The spectroscopic observations from the previous plot are marked by orange ticks over the light curve.

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.


Dramatic broadening of emission lines during the decline from superoutburst (ATel #13297)

Our new optical spectroscopic data were obtained on November 6, 50 days after the most recent rebrightening observed on September 15-17 (ATel #13122). We used the ALFOSC spectrograph at the Nordic Optical Telescope (NOT) on La Palma.

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.

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