[vsnet-alert 9819] (fwd) Supersoft X-ray source CAL 83 in an optical-high, X-ray off state

[Taichi Kato]

   CAL 83 (=LMC V4218) is located at: 05:43:33.5 -68:22:23 (J2000.0)

------------------------------------------------------------------------------ 
The Astronomer's Telegram                   http://www.astronomerstelegram.org 
------------------------------------------------------------------------------
Posted: Sat Jan 12 01:30:01 EST 2008 -- Sun Jan 13 01:30:01 EST 2008
==============================================================================
ATEL #1357							     ATEL #1357

Title:		Supersoft X-ray source CAL 83 in an optical-high, X-ray off
		state
Author:	. Greiner (MPE Garching), R. Schwarz (AIP Potsdam), G. Sala
		(MPE), J.-U. Ness (ASU Tempe), R. Mennickent (Concepcion) 
Queries:	jcg at mpe.mpg.de
Posted:	12 Jan 2008;  17:21  UT
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<sup>-4</sup> 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. 

------------------------------------------------------------------------------
Password Certification:  J. Greiner (jcg at mpe.mpg.de)
http://www.astronomerstelegram.org/?read=1357
==============================================================================