SAGITTARIUS

The 2003 outburst of Galactic microquasar V4641 SGR:

The Galactic microquasar (close binary system containing a black hole, characterized by the presence of jet ejection) V4641 Sgr is now extremely active.

The object is reported to vary between mag 10 and 13 with short time scales. The object has undergone a giant eruption (reaching at least mag 8.8) in 1999 September. The flickering light observed now is possibly originating from the inner(most) region of the accretion disk, close to the black hole. This means that you may even directly "see" the light from the matter just falling into the black hole!

 

This picture was taken in Singapore at 15:00 GMT on 7 August 2003.

V : Microquasar

C : Comparaison star GSC 6848:3606

K : Calibration star GSC 6848:3245

Light curve:

 

The following light curve was drawn using CCDSoft software on a set of 130 pictures, 30 seconds each, taken with a 12" LX200GPS and SBIG ST-8 camera, between 15: and 17:00 GMT on 7 August 2003.

Observations by Jean-Yves Beninger, Singapore.

This image shows the rise and decrease in brightness of V4641 during a two hours period

 

 

Note the fast variation in luminosity of the object : each step in magnitude equals a difference in brightness of 2.5 times !

 

 

This animation shows the microquasar (top left) at its maximum magnitude and then one hour later at its minimum magnitude : a drop of 2 magnitudes.

 

Then back to normal three days later ...

 

For updates on variable stars, visit VSNET

 

[Excerpt 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.