[vsnet-j 2096] V4641 Sgr: eclipse!?

[Taichi Kato <tkato@kusastro.kyoto-u.ac.jp>]

V4641 Sgr: eclipse!?

Dear Colleagues,

    We have received two new data sets from Berto Monard (see vsnet-alert
7342).  On May 21/22, the system underwent a deep fading between May 22.0
and 22.1 UT.  This timing agrees with the expected inferior conjunction
(ephemeris from Orosz et al. 2001; updated by Uemura et al. 2002).
During this fading, strong flickering activity virtually disappeared!
Such behavior could be reasonably explained if the (compact) flickering
body was eclipsed by the secondary (first case in microquasars, if
confirmed).  There was an indication that flickers rapidly emerged
when the system brightened from this fading.  During the declining branch,
flickering was less apparent than on the brightening branch.

    On May 22/23, the strong ultra-short-period flickering were always
present in high-speed photometry (5-s integratrations), as on May 20-21.

    Expected times of inferior conjunctions (likely eclipses), in geocentric
UT.  Times are approximate (to 1 hr or so) owing to the unavoidable
uncertainty in the epoch and period.

  20020519 06:39:14
  20020522 02:15:53
  20020524 21:52:33
  20020527 17:29:15
  20020530 13:05:57

    If this interpretation is correct, the object is expected to remain
active until late May 24.  Continuous coverage is most urgently needed.
More detailed analysis, together with newly available data, will be
later presented by Uemura-san.  The Kyoto team is planning to make
high-speed (1s) photometry at Ouda tonight, depending on the conditions.
You will hear more from Uemura-san about the progress of the observation
and data analysis.

Regards,
Taichi Kato
VSNET Collaboration team

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