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.


The broadening of emission lines in TCP J21040470+4631129 occurs during a day at the end of rapid fading from the superoutburst (ATel #13646)

TCP2104 went into a new superoutburst on 2020 March 30 (Denisenko, vsnet-alert 24120). Our first spectrum of the transient 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.

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 light curve of the third small superoutburst of TCP2104. The times of spectroscopic observations are marked by blue.

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.

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