Study of the rapid variability of a dwarf nova SS Cyg at different brightness levels

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Abstract

Observations of the dwarf nova SS Cyg were made in the period 2019–2021 at different brightness values (V ~ 10–12m) both at the stage of falling radiation flux after the flare maximum, and in the inactive state between flares. Data were obtained in filters Rc (~8650 observations, 3 sets), and V (~50 000 points, 22 sets). The value of the system’s orbital period in 2019–2021 (Porb = 0.27408(2)d) used in this study is 0.4% less than the value obtained in 1983–1996. The time resolution between two successive measurements is 6–14 s depending on the equipment used. An extensive database of new observational data allowed us to perform a quantitative analysis of observations. Analysis of the data after taking into account orbital variability and other trends associated with changes in the system’s radiation flux during the night showed the presence of cyclic fluctuations in brightness, usually 4–10 events per orbital cycle — flickering. For most series of observations, the Lafleur-Kinman method determined such a value of the oscillation period at which convolution of observations with it showed a single wave. The obtained values of the characteristic flickering times and their amplitudes show their dependence on the average brightness level of the system. With increasing luminosity of the system, both of these quantities decreased linearly. From the component size ratios SS Cyg it was shown that the source of flickering is located in the region of interaction of the gas flow with the near-disk halo: only this region in the SS Cyg system with parameters (q, i, Rd), defined by the authors earlier, can be eclipsed at large radii disk, and is clearly visible in all other orbital phases of the system.

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About the authors

T. S. Khruzina

Lomonosov Moscow State University, Sternberg Astronomical Institute

Author for correspondence.
Email: kts@sai.msu.ru
Russian Federation, Moscow

I. B. Voloshina

Lomonosov Moscow State University, Sternberg Astronomical Institute

Email: vib@sai.msu.ru
Russian Federation, Moscow

V. G. Metlov

Lomonosov Moscow State University, Sternberg Astronomical Institute; Crimean Astronomical Station, Lomonosov Moscow State University, Sternberg Astronomical Institute

Email: kts@sai.msu.ru
Russian Federation, Moscow; Nauchnyi

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Supplementary files

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2. Fig. 1. Periodograms of observations in the Rc filter (upper panel) and V (lower panel). The arrow shows the frequency of the previously determined orbital period (P = 0.2751302d),  = 3.6346 day–1 [42]. F is a parameter in the Lafleur-Kinman method, analogous to the power of the Fourier spectrum.

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3. Fig. 2. Convolution of SS Cyg brightness deviations from the nightly mean Rc and V in observational sets with ephemerides (2) in the Rc (upper panel) (JD 245 8655 — red, JD 245 8656 — blue, JD 245 8658 — green dots) and V (lower panel) filters, here color legends were not used due to the large number of observations, n ~ 50 000). The black curve (in the upper figure) and the yellow one (in the lower one) are the average deviation curves constructed from the corresponding combined data with a step of 0.01 in the orbital phase (Table 3).

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4. Fig. 3. Distribution of SS Cyg observations over time: (a) in the Rc filter; (b) in the V filter, the colored rectangle shows the region of observation of the system by the TESS observatory, shown in the figure (c), the radiation fluxes here are given in arbitrary units.

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5. Fig. 4. Outburst V-light curves of the dwarf nova SS Cyg according to AAVSO data in June 2019 (a) and November 2019 (b). Arrows mark the moments of our observations.

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6. Fig. 5. Observed light curves of SS Cyg, convolved with ephemerides (2) in the June (a) and November (b) seasons of 2019. The numbers next to the curves indicate the set numbers according to the table. 2: 1 - 06/20/2019 (JD 8655), 2 - 06/21/2019 (JD 8656), 3 - 06/23/2019 (JD 8658), 4 - 06/25/2019 (JD 8660), 5 - 06/26/2019 (JD 8661), 6 - 06/27/2019 (JD 8662), 7 - 11/08/2019 (JD 8796), 8 - 11/10/2019 (JD 8798), 9 - 11/12/2019 (JD 8800).

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7. Fig. 6. Outburst light curves of the dwarf nova SS Cyg according to AAVSO data in May 2020 (a). Arrows mark the moments of our observations. The dashed line with an arrow shows the moment of the May X-ray outburst, observed in the system from April 27 to May 6, 2020 (JD 8967–8976). On the right (b) are the light curves of SS Cyg in May 2020, folded with the ephemeris (2). The numbers next to the curves indicate the set numbers according to Table 2: 10 — 18.05.2020 (JD 8988), 11 — 23.05.2020 (JD 8993), 12 — 31.05.2020 (JD 9001).

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8. Fig. 7. Outburst light curves of SS Cyg according to AAVSO data in September–November 2020 (a). Arrows mark the moments of our observations. The lower panel (b) shows the light curves of the system for this period of time, folded with the ephemerides (2). The numbers next to the curves indicate the set numbers according to the table. 2: 13 - 01.09.2020 (JD 9094), 14 - 02.09.2020 (JD 9095), 15 - 02.10.2020 (JD 9125), 16 - 27.10.2020 (JD 9150), 17 - 11.11.2020 (JD 9165), 18 - 19.11.2020 (JD 9173), 19 - 02.01.2021 (JD 9217).

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9. Fig. 8. Outburst light curves of SS Cyg according to AAVSO data in September-October 2021 (top). Arrows mark the moments of our observations. Below are the light curves of the system for this time period, folded with the ephemerides (2). The numbers next to the curves indicate the set numbers according to Table 2: 20 - 09/06/2021 (JD 9464), 21 - 09/07/2021 (JD 9465), 22 - 10/20/2021 (JD 9508), 23 - 10/21/2021 (JD 9509), 24 - 10/22/2021 (JD 9510), 25 - 10/26/2021 (JD 9514).

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10. Fig. 9. a) Time dependence of the Rc data deviations obtained in the JD 8655 set on the mean orbital light curve (Table 3); the secondary wave mdef(t) is shown by the solid (orange) line. The result of its subtraction from the Rc observations, the residual deviations, ′Rc(t), are shown in panel (b); the orbital phases of the observations according to (2) are indicated on the upper axis of panels (a) and (b); c) the power spectrum for the residual deviations shown in (b), obtained by the Lafler-Kinman method; d) convolution of the residual deviations ′Rc with the period P = 0.0335301d ( = 29.8239 day–1), the initial epoch is arbitrary.

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11. Fig. 10. The same as in Fig. 9, for observations in the JD 8656 network. The convolution of the residual deviations in panel (d) is performed with a period of 0.037137d ( = 26.9273 day–1), the initial epoch is arbitrary.

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12. Fig. 11. The same as in Fig. 9, for observations in the JD 8658 network. The convolution of the residual deviations in panel (d) is performed with a period of 0.040656d ( = 24.5966 day–1), the initial epoch is arbitrary.

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13. Рис. 12. Вверху — зависимость от времени остаточных отклонений ′V(t), полученных путем вычитания вторичной волны из данных V (аналог рис. 10 (б)), в центре — спектр мощности для этих остаточных отклонений и внизу — свертка ′V() c полученным периодом (указаны на рисунках) для трех сетов июньской вспышки 2019 г.

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14. Fig. 13. Dependences of the characteristic flickering time P′ (a) and their total amplitude A′ (b) on the average nightly brightness level of the system m on the descending branch of the outburst in SS Cyg in June 2019 (JD 8655–8662, see Fig. 4 (a)).

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15. Fig. 14. Same as Fig. 12, for three sets obtained at the end of the November 2019 outbreak.

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16. Fig. 15. The same as in Fig. 13, for observations on the descending branch of the SS Cyg outburst in November 2019 near its minimum (see Fig. 4 (b)).

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17. Fig. 16. Same as Fig. 12, for three sets obtained during the May 2020 outbreak.

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18. Fig. 17. The same as in Fig. 13, for observations on the descending branch of the SS Cyg flare curve during the May 2020 outburst (see Fig. 6a) for sets obtained near the brightness maximum, in the middle of the descending wing and at the brightness minimum after the end of the outburst.

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19. Fig. 18. Same as Fig. 12, but for six sets obtained during periods of minimum brightness between the short autumn outbursts of 2020 (see Fig. 7(a)).

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20. Fig. 19. Same as Fig. 12, but for a short set of JD 9217 observations taken before the SS Cyg outburst maximum in January 2021 (see Fig. 7(a)). The convolution of residuals in the panel (bottom) is performed with a period of 0.009734d ( = 102.7327 day–1), the initial epoch is arbitrary.

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21. Fig. 20. Same as Fig. 13, for sets on the descending branch of the SS Cyg flare curve in autumn 2020 (see Fig. 7 (a)).

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22. Fig. 21. Same as Fig. 12, but for six sets of observations obtained in the quiet state of the system between the autumn outbursts of 2021 (see Fig. 8(a)).

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23. Fig. 22. Same as Fig. 13, for six sets at brightness minima between outbursts in the system in autumn 2021 (see Fig. 8 (a)).

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24. Fig. 23. Schematic representation of the SS Cyg system constructed with the parameters from [43]: q = 1.5 and i = 52.5 for three values ​​of the disk radii Rd = 0.42, 0.64 and 0.75 in orbital phases  = 0.0 and 0.5. The lower panel shows images of the system in the picture plane, i = 0 for  = 0.0. Different colors show the elementary areas on the surfaces: the optical star (blue), the inner (orange) and lateral (green) parts of the disk, the gas flow (crimson), the hot spot (brown) and the white dwarf in the center of the disk (black).

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25. Fig. 24. Dependencies for all 5 observation groups in the period 2019–2021, numbers indicate the group number.

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