Nova-like cataclysmic variable TT Arietis^*

QPO behaviour coming back from positive superhumps

¹ University Observatory, Chungbuk National University, 361-763, Cheongju, Korea
² Institute for Basic Science Research, Chungbuk National University, 361-63, Korea
³ Odessa National Maritime University, Mechnikov str., 34, 65029 Odessa, Ukraine e-mail: tt_ari@ukr.net; uavso@pochta.ru; i_andronov@yahoo.com
⁴ Astronomical Observatory, Odessa National University, T.G. Shevchenko Park, 65014 Odessa, Ukraine

Received: 18 April 2008
Accepted: 1 October 2008

Abstract

Aims. We study the variability of the nova-like cataclysmic variable TT Ari, on time-scales of between minutes and months.

Methods. The observations in the filter R were obtained at the 40-cm telescope of the Chungbuk National University (Korea), 51 observational runs cover 226 h. The table of individual observations is available electronically. In our analysis, we applied several methods: periodogram, wavelet, and scalegram analysis.

Results. TT Ari remained in a “negative superhump” state after its return from the “positive superhump” state, which lasted for 8 years. The ephemeris for 12 of the best pronounced minima is The phases of minima may reach 0.2, which reflects the non-eclipse nature of these minima. The quasi-periodic oscillations (QPO) are present with a mean “period” of 21.6 min and mean semi-amplitude of 36 mmag. This value is consistent with the range 15-25 min reported for previous “negative superhump” states and does not support the hypothesis of secular decrease in the QPO period. Either the period, or the semi-amplitude show significant night-to-night variations. According to the position at the two-parameter diagrams (i.e. diagrams of pairs of parameters: time, mean brightness of the system, brightness of the source of QPO, amplitude, and timescale of the QPOs), the interval of observations was divided into 5 parts, showing different characteristics: 1) the “pre-outburst” stage; 2) the “rise to outburst”; 3) “top of the outbursts”; 4) “post-outburst QPO” state; and 5) “slow brightening”. The the QPO source was significantly brighter during the 10-day outburst, than during the preceding interval. However, after the outburst, the large brightness of the QPO source still existed for about 30 days, producing the stage “4”. The diagram for exhibits two groups in the brightness range 106-108, which correspond to larger and smaller amplitudes of the QPO. For the group “5” only, statistically significant correlations were found, for which, with increasing mean brightness, the period, amplitude, and brightness of the of QPO source also increase. The mean brightness at the “negative superhump state” varies within 103-112, so the system is brighter than at the “positive superhump” (113), therefore the “negative superhump” phenomenon may be interpreted by a larger accretion rate. The system is an excellent laboratory for studying processes resulting in variations on timescales of between seconds and decades and needs further monitoring at various states of activity.