[vsnet-campaign 1642] TT Ari: Recent Negative Superhumps and QPO's

[Ivan L.Andronov]

TT Ari: Recent Negative Superhumps and QPO's
(submitted to www.astronomerstelegram.org)

I.L. Andronov (ONU, Ukraine), J.N. Yoon (CBNU, Korea), S.M.Cha (CBNU, Korea), L.L.Chinarova (ONU, Ukraine), Y. Kim (CBNU, Korea)

TT Ari has been recently discovered to be at a relatively unstable phase by
Andronov, Gazeas & Niarchos (2005, VSNET-Campaign <a href="http://uavso.pochta.ru/TTAri2005"> 1630</a>).
Its prolonged "positive superhump" state from 1997 to 2004 has changed.
In October-November, 2005, the star was fainter, and the superhumps are not pronounced. 
The dominating type of variability was quasi-periodic oscillations (QPO) with a time-scale of 10-35 minutes and amplitude 0.07 mag (B). This state is distincly different from both the bright "negative superhump" state and the "positive superhump" state (Andronov et al. IBVS, 2005, N <a href="http://www.konkoly.hu/cgi-bin/IBVS?5664">5664</a>). Further monitoring had been carried out at the 40-cm telescope of the University Observatory, Chungbuk National University, Cheongju, Korea. From October 25, 2005 to January 24, 2005, alltogether 11245 data points have been obtained during 37 nights using the R filter with a time resolution 34-70 sec.

Yoon, Andronov, Cha & Kim (2005, VSNET-Campaign <a href="http://uavso.pochta.ru/vsnet1636"> 1636</a>, ATel <a href="http://www.astronomerstelegram.org/?read=698">698</a>) reported on the "outburst-like" event with an amplitude of 0.9m (R) and characteristic timescales dt/dm=15.87(11)days/mag (rise at JD 2453704-715) and dt/dm=4.95(7)days/mag (fall at HJD 2453721-724). Then there was a standstill with minor fluctuations of the mean brightness.

Yoon, Andronov, Cha, Chinarova & Kim (2005, VSNET-Campaign <a href="http://uavso.pochta.ru/vsnet1637"> 1637</a> reported on a standstil with a mean level <R>=10.732 and a scatter of 0.107 mag, which is much 
larger than an error estimate 0.020 mag.
The "Lambda-scalegram" analysis (Andronov 2003, ASP Conf.Ser. 292, 391) shows a most prominent peak 
corresponding to the QPO with an effective period of 
0.01439d=20.7min and semi-amplitude of 0.061mag. The superhump variability is less prominent (semi-amplitude 0.033mag and period 0.128d).
This is typical for the "negative superhump" state observed before 1997 
(Tremko et al. 1996, AA 312, 121; Andronov et al. 1999, AJ 117, 574). Similar periods (0.01486d, 0.1166d) are present at the sine-based periodogram (<a href="http://il-a.pochta.ru/oap7_049.ps"> Andronov 1994</a>, OAP 7, 49), but the corresponding peaks are low enough arguing for a quasi-periodic character of variations, rather than truly periodic.

The long-term <a href="http://uavso.pochta.ru/TT_Ari-37.gif">light curve</a> and 
<a href="http://uavso.pochta.ru/TT_Ari.dat">nightly mean</a> are presented.

The character of variability seems to be changed again. A recent <a href="http://uavso.pochta.ru/TTAri742.gif">light curve</a> started at JD 2453742.8 shows both negative superhumps and QPOs. The <a href="http://uavso.pochta.ru/TTAri742p.gif">periodogram</a> shows the higher semi-amplitude of superhumps (77+-5mmag) then  that of the QPOs (50+-4mmag). The ephemeris for this night is Max.HJD=2453743.00362(21)+0.015098(44)*E corresponding to unexpectedly high coherence of these 20-min pulses during this night. From 16 individual timings of maxima, we determined a consistent value 0.0158d. One should note the presence of other maxima, which are often more thin and occur at different phases of the QPO period, sometimes splitting the maxima.

For the 3-hour variability, we have determined 12 minima timings, which have been analyzed using the PERMIN algorithm (Andronov 1991, KFNT 7, 2, 78). The corresponding <a href="http://uavso.pochta.ru/TTAri-pm.gif"> periodogram</a> has a minimal phase scatter for the elements Min.HJD=2453747.0700(47)+0.132322(53)*E.
This is slightly different (by 4.5 sigma) from the period obtained from the sine fit P=0.1217(22)d, but the  light curve definitely shows cycle-to-cycle changes, so this difference seems not to be statistically significant.

Spectral and multi-color photometric observations of TT Ari at this state of multi-component variability are requested.