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ATEL # 1438; T. Lanz (U Maryland), M. Audard (ISDC, Geneva Obs), F. Walter (SUNY)
on 20 Mar 2008; 18:21 UT
Password Certification: Thierry Lanz (email@example.com)
Subjects: Optical, X-ray, Binaries, Transients
We report a detection of CAL 83 with the Swift X-ray telescope during a short exposure (490 sec) started on 2008 March 20 at 10:27 UT, with a count rate of about 0.024 ct/s, corresponding to about 15% of the normal X-ray flux of CAL 83. The latest X-ray off-state of CAL 83 was discovered with Swift on 2008 Jan 2 (Greiner et al. 2008, ATel #1357), following an increase of brightness in the optical a week earlier. We started a monitoring campaign with Swift on 2008 Jan 24 in order to define the duration of the off-state and to investigate the correlations between the X-ray and optical states. As late as 2008 March 18, 00:24 UT, CAL 83 remained undetected with Swift in a 1.07 ks exposure. The inferred 3-sigma upper limit on March 18 was 0.0028 ct/s, which is consistent with the upper limit reported by Greiner (ATel #1357) for their longer exposure (4.7 ks) on Jan 2. CAL 83 was last detected with Swift XRT on 2007 Nov 24, 00:48 UT, and with XMM-Newton on 2007 Nov 24, 20:44 UT. The off-state therefore lasted at least 76 days, but was in any case shorter than 116 days. The duration of the 2008 off-state is similar to the duration of the original 1996 Apr off-state (less than 120 days).
The detection of CAL 83 by Swift XRT follows a drop in optical brightness about 7 days earlier. Our SMARTS photometry shows a dimming of 0.54 mag in B and 0.43 mag in I between 2008 March 13, 00:32 UT, and March 14, 01:37 UT. The current data therefore suggest that the X-ray variations lag the optical by less than a week. In the last few days, CAL 83 brightened back by about 0.2 mag in B, but remained fainter than during the optical high state. Swift and ground-based photometric observations will continue to monitor the transition back to CAL 83 X-ray bright state.