[vsnet-campaign-v4641sgr 1] V4641 Sgr outburst!!!

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

V4641 Sgr outburst!!!

   Rod Stubbings reports that the microquasar V4641 Sgr = SAX J1819.3-2525
(formerly called GM Sgr) is in outburst as indicated by the following
observations.

V4641 Sgr    020517.569    13.4
V4641 Sgr    020519.699    11.5
             020519.704    11.5
             020519.716    11.5
             020519.723    11.5

   The object underwent a giant outburst in 1999 September (Stubbings
and Pearce (1999) IAUC 7253), preceded by optical brightening (Kato et al.
(1999) IBVS 4777) which was first detected by T. Watanabe.

[Excerpr from Uemura et al. (2002) PASJ 54, 95]

Microquasars are a class of X-ray novae with relativistic radio jets.
Their X-ray and optical emissions are thought to be from an accretion disk
around a neutron star or a black hole, however their relationship with the 
jet production mechanism is poorly understood.  Here we report on the 
unprecedented discovery of a giant optical outburst in the X-ray nova 
V4641 Sagittarii (= SAX J1819.3-2525 = XTE J1819-254), which enabled
subsequent X-ray observations accurately pinpointing an intense X-ray flare
reaching 12 Crab and establishing it as a microquasar at a small
distance. The decay with an e-folding time of 0.2 day is the shortest
ever observed for any X-ray novae, implying a new category.  Observations
revealed the first ever exemplification of the detailed optical behavior
prior to the X-ray maximum, and moreover, the totally unexpected 7 hour
precedence of the optical maximum to the X-ray maximum.  The seemingly
anti-correlated light curves in X-ray and optical can be best understood as
a short episode of supercritical accretion producing a relativistic jet.

X-ray novae are binary systems which exhibit luminous X-ray and 
optical outburst which lasts for a few tens of days.  
They uniquely provide the most compelling evidence for the 
existence of steller mass black holes using radial 
velocity studies, giving mass functions exceeding the maximum mass 
of a stable neutron star (~3 Msolar).  Microquasars are 
X-ray novae with superluminal jets whose mechanism is poorly unknown 
while a number of observational results and models have been discussed.

Supercritical accretion disks have recently been discussed for 
black hole candidates shining at the Eddington luminosity which 
frequently show jets.  The accretion disks theoretically become 
geometrically and optically thick when the mass accrete over a 
critical rate, on the other hand, the geometrically thin disks are 
applied for the subcritical accretion rate.  We can consider 
that supercritical accretion occurs in persistent jet sources, 
for example, active galactic nuclei or SS433 whose mass accretion 
rate is observationally suggested to be near critical.  
On the other hand, during any transient jet source outbursts, 
we had detected no implication that supercritical accretion occurs. 
However, a new atypical microquasar, V4641 Sgr showed an evidence of 
this for the first time.  

20th February 1999, the Dutch-Italian X-ray astronomy satellite 
(BeppoSAX) detected a faint flaring X-ray transient.  There 
was a known variable star V4641 Sgr [Goranskij (1978, 1990) discovered
an eruptive variable  star in the close vicinity of the nominal position
of Luyten's variable (HV 4048).  Goranskij's object took over the GCVS
nomenclature, GM Sgr, of HV 4048.  However, it was established that they
are two independent variables after the giant outburst on 1999 September, 
and then, the object which is the optical counterpart of SAX 
J1819.3-2525 is named as V4641 Sgr.  When the small flare on February 
1999 was reported, the object was thereby called GM Sgr, however we only 
use V4641 Sgr for simplicity in this paper], whose visual magnitude in
quiescence is 13.8,  within the error box at the position of this transient.
Ironically the object had been overlooked until 8th August 1999 when
V4641 Sgr brightened to 12.9 mag.

 [...]

Following the first small outburst on 8th August 
(JD 2451399), the brightness of V4641 Sgr modulated between 12 and 14 mag 
for about one month, and then, on 9th September (JD 2451430), 
rose to 11.7 mag.  We detected the quasi-periodic modulation of an 
amplitude of ~1 mag with a period of 2.5 days for six days from 
this second outburst, and then, V4641 Sgr suddenly flared to 8.8 mag on
15th September (JD 2451436).  The optical flux then began an exponential
decay with the e-folding time of 0.2 day and returned to the pre-outburst
level within 12 hours after the maximum. 
This demonstrates that this high energy event lasted only 1 day. 
The magnitude returned to the quiescent level within one week, and then, 
no other activity has been detected.  

---

   The object was further studied in detail, and it is now established
to be a close binary containing a black hole (Orosz et al. (2001)
ApJ 555, 489).  Radio jets (with superluminal motion ~9 c?) was detected
(Hjellming et al. (2000) ApJ 544, 977), indicating that highly relativistic
ejection was taking place at the time of optical/Xray flare-up.

   If the current outburst follow the course of the 1999 outburst,
the source may brighten up to a binocular object in near future (tonight?
in several day? or more? .. no one knows!).  This outburst provides
excellent opprotunity to follow full evolution of the expected flare-up
in all wavelengths.  In optical, time-resolved photometry and spectrocopy
are highly recommended.  Prompt ToO X-ray observations, as well as
radio interferometry, are also very wanted.  In order to record this
rare event, we have set up a new list

   vsnet-campaign-v4641sgr

   Those who wish to subscribe to this list should send an e-mail with
a line

   SUBSCRIBE vsnet-campaign-v4641sgr

   to vsnet-adm@kusastro.kyoto-u.ac.jp

   For the details of the 1999 event, see also

   http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/Xray/gmsgr.html

   Good luck!

Regards,
Taichi Kato
VSNET Collaboration team

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