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Supersoft X-ray source CAL 83 in an optical-high, X-ray off state

ATEL # 1357; . Greiner (MPE Garching), R. Schwarz (AIP Potsdam), G. Sala (MPE), J.-U. Ness (ASU Tempe), R. Mennickent (Concepcion)
on 12 Jan 2008; 17:21 UT
Password Certification: J. Greiner (jcg@mpe.mpg.de)

Subjects: Optical, Ultra-Violet, X-ray, Binaries, Variables

Between Dec. 20 and 27, 2007, the optical (B&R) brightness of CAL 83 jumped up by 0.5 mag, and stayed at that level since then. With the anti-correlation in mind of optical and X-ray flux as indicated by MACHO data and earlier Chandra and XMM observations (Greiner & DiStefano 2002, A&A 387, 944), we performed a 4.7 ksec Swift ToO observation on January 2, 2008, between 0:00--6:00 UT. As expected, we find no X-ray emission from CAL 83, with a 2-sigma upper limit of 6.1*10-4 cts/s in the <1.0 keV band. In contrast, during the previous 3 Swift observations of CAL 83 in November 2007, supersoft X-ray emission was detected at a level of 0.2 cts/s, with all photons below 0.7 keV. Our data demonstrate that X-ray off-states of CAL 83 can indeed be reliably predicted by the optical brightness level, and that the lag of X-ray variations, if it exists, is less than about one week.
The UV flux as measured by Swift has also increased with respect to the earlier Swift observations, by about 0.8 mag, see table below:

   Filter   Nov. 2007         Jan 2, 2008      
------------------------------------------
UVW1 15.59 +- 0.03 14.84 +- 0.02
UVM2 -- 14.54 +- 0.02
UVW2 15.25 +- 0.03 14.42 +- 0.02
We note that the amplitude is larger in the UV than in the optical band. The flux increase is consistent with an earlier IUE observation of an ultraviolett bright state (Crampton et al. 1987, ApJ 321, 745) during an optical bright phase (Bianchi & Pakull 1988, ESA-SP 281, 145). A denser monitoring at all wavelengths would be required to constrain the spectral shape changes, the speed of the transition as well as the relative timing of changes at different wavelengths in order to understand the cause of these emission changes.
We are extremely grateful to the Swift team for the very rapid response on a holiday.


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