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
Credential Certification: J. Greiner (jcg@mpe.mpg.de)
Subjects: Optical, Ultra-Violet, X-ray, Binary, Variables
Referred to by ATel #: 1438
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.
