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[vsnet-campaign-news 154] Outburst of V4641 Sgr continues: twinkling black hole!




  ====================================================================
        Outburst of the Galactic microquasar V4641 Sgr Continues
        --- Naked-Eye Visibility of the Twinkling Black Hole ---
  ====================================================================

    The outburst of the Galactic microqusar V4641 Sgr follows a very
complex course.  The object violently varies with various time-scales.
At the time of this writing, the object undergoes its third outburst
during the present activity phase.  This course of the outburst strongly
resembles that observed in the 1999 September event
(http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/Xray/gmsgr.html), which suggests
that the present activity may be a precursor to a giant event in near
future.  Please keep the cloest watch to see what is ongoing!

    Further to this, the object shows "naked-eye visibility of flickering
of a black hole".  A comment from Mike Linnolt (Hawaii) follows:

[vsnet-campaign-v4641sgr 8]

I watched this object over a half hour period and saw it varied
chaotically from mag 12.5 to 12.8 with a "period" of just a few seconds! I
have never seen such a behavior in a variable star before.

[vsnet-alert 7340]

I observed this amazing object again this evening, and noticed it has
brightened significantly over past 24 hours. In addition, the chaotic
ultra-short variations in brightness seems to have increased its range to
>0.5 mag compared to ~0.4 yesterday. The periodicity remains on the
order of a second to a few seconds, but the mean brightness tends toward
mag 12.0, on the higher end of the range 12.5-11.8: which I observed in
15mins of continuous visual monitoring between 20020522.4097 and
20020522.4201

The following observations are just representative points, the actual
variations occur at an unprecedented rate for even 4 decimal CV reporting.

SGRV4641 20020522.4132 11.8: LMK
SGRV4641 20020522.4133 12.3 LMK

Note that visual observation or video recording is the only way to monitor
the full range of the brightness variations. CCD would be limited by the
integration times and thus yield only a mean value, unless one has
sufficient aperture to do sub-second integrations.

---

    The light variation was also recorded with CCD photometry by Berto Monard
and other contributors:

[vsnet-campaign-v4641sgr 9]

   We have received time-series CCD photometry on May 20-21 from Berto Monard.
The light curve is very complex and very short-term fluctuations up to
0.4 mag is observed (the light curve even resembles a X-ray light curve
of some black hole binaries!).

[vsnet-campaign-v4641sgr 13]

    V4641 Sgr is not in a monotonic fading, but still very active!
As already reported by B. Monard in [vsnet-alert 7339], by 
M. Linnolt in [vsnet-alert 7340], and by A. Pearce in 
[vsnet-obs 39944], the object has been showing short-term modulations.  

    Furthermore, the combined data set shows that the object is now 
oscillating between 11.5 - 13 mag with a time-scale of 1 day.  
After the first maximum observed by R. Stubbings on May 19, the 
object once faded.  Then, B. Monard's observations detected a 
rising phase to the second maximum on May 21.

    Our CCD observations at Kyoto last night shows a fading from 
the second maximum.  It was 12.68mag (average) at that time.  
The short-term modulations seem to be less prominent during our 
observations.  A. Pearces observations on May 22 shows that the 
object again brightened, indicating the third maximum.  He reports 
that short-term modulations again appear.

    Durations of each oscillation is short, no longer than 1 day. 
Multi-longitude observations are essential to study the correlation 
between short-term modulations and larger-scale oscillations.

---

    The activity has been detected in X-rays:

[vsnet-campaign-v4641sgr 11]

   According to IAUC 7906 (Markwardt and Swank), V4641 Sgr is also reported
to be active in X-ray.  X-ray flux from RXTE PCA monitoring (mCrab):
4.5 +/- 0.3 (May 17.6), 10.0 +/- 0.5 (May 20.9).

---

    We are conducting an extensive campaign on this source.  Those who
are able to obtain high-speed (i.e. seconds to several tens of seconds)
CCD photometry are most welcomed to join the present campaign to fully
record the current exotic activity of this black hole binary.  Other
modalities of observations are also highly encouraged.

Regards,
Taichi Kato
VSNET Collaboration team

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  VSNET (Variable Star Network) is an international variable star observing
network, covering various areas of novae, supernovae, cataclysmic variables
(CVs), X-ray transients, and other classical eruptive, pulsating, and
eclipsing variables.  VSNET is one of invited contributing organizations
to the SkyPub AstroAlert system.

       VSNET Home Page: http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/

CCD observations of such targets are a relatively easy task for a 20-40cm
telescope; simply take as many CCD frames (with exposure times 10-30 sec)
as possible, spanning several hours per night.  The only requirements are
the weather and your patience!  If you need more help on the observing
technique, please feel free to ask on the vsnet-campaign list.

   We would sincerely appreciate volunteers who would join the VSNET
Collaboration team to study the wonders of these exotic variable stars.
To join the VSNET campaign collaborative list, send an e-mail to

         vsnet-adm@kusastro.kyoto-u.ac.jp (VSNET administrator)

with a line "SUBSCRIBE vsnet-campaign." or your comment to join the
collaboration team.

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