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ATEL # 2254; Koji Mukai (NASA/GSFC/CRESST and UMBC), Joe Patterson (Columbia U.), Bob Koff (CBA-Colorado), Etienne Morelle (CBA-France), William Stein (CBA-Las Cruces) and Arto Oksanen (CBA-Norway)
on 23 Oct 2009; 18:36 UT
Password Certification: Koji Mukai (firstname.lastname@example.org)
Subjects: Optical, X-ray, Cataclysmic Variables
We observed the nova-like cataclysmic variable (CV) TT Ari, which is currently undergoing a fading episode, with Swift between 2009 October 16 01:31 UT and 09:02 UT. The X-ray telescope (XRT) was operated in photon counting mode and accumulated 4,400 s of good exposure during this interval. The UV/optical telescope (UVOT) was used with the U filter in imaging mode, with an integration time also of 4,400 s.
Despite fading by ~3.7 magnitudes in the optical, TT Ari was detected strongly with the XRT, at 0.24 counts s-1. Multi-temperature thermal plasma model can provide an adequate description of the data (single temperature models result in a poor fit), as was the case with the ASCA high state observation (Mauche & Mukai 2002, ApJLett, 566, L33), with similar parameters. The Swift/XRT flux during the current fading episode is 1.5x10-11 ergs cm-2s-1 (0.3-10 keV) or 9x10-12 ergs cm-2s-1(2-10 keV), corresponding to a 2-10 keV luminosity of 1.2x1032 ergs s-1 for a distance of 335 pc (Gaensicke et al. 1999, A&A 347, 184). This is similar to the luminosity in the ASCA data (1.4x1032 ergs s-1, 2-10 keV).
TT Ari was clearly variable in X-rays during the Swift observation. However, simultaneous optical light curves, obtained by several telescopes of the Center for Backyard Astrophysics, showed no obvious correlation. The latter did show large-amplitude waves with a period or quasi-period of ~0.4 days, not previously seen in any band. The average Swift/UVOT U magnitude is estimated to be ~11.9 after correction for coincidence losses (approximately a factor of 4).
Although accretion must have declined severely in the current "low state," its most telltale signatures - X-rays and fast variability - remain quite strong. In fact, TT Ari remains one of the most luminous X-ray sources among non-magnetic CVs, along with SS Cyg and V603 Aql. In case of a significant further decline, it will be interesting to test these limits further. Eventually the X-rays should turn off, but we still do not know when that happens.
Plots of X-ray spectrum and X-ray/optical light curves of TT Ari on 2009 October 16