SAGITTARIUS

The 2003 outburst of Galactic microquasar V4641 SGR:

Publication in the Astronomical Society of Japan, Vol.56, No.5, pp. 823-829 HERE

[Excerpt from Uemura et al. (2002)PASJ 54, 95] Microquasars are a class of X-raynovae with relativistic radio jets. Their X-ray and optical emissionsare thought to be from an accretion disk around a neutron star or ablack hole, however their relationship with the jet production mechanismis poorly understood. Here we report on the unprecedenteddiscovery 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 anintense X-ray flare reaching 12 Crab and establishing it as a microquasarat a small distance. The decay with an e-folding timeof 0.2 day is the shortest ever observed for any X-ray novae, implyinga new category. Observations revealed the first ever exemplificationof the detailed optical behavior prior to the X-ray maximum, and moreover,the totally unexpected 7 hour precedence of the optical maximum to theX-ray maximum. The seemingly anti-correlated light curves in X-ray andoptical can be best understood as a short episode of supercritical accretionproducing a relativistic jet. X-ray novae are binary systems whichexhibit luminous X-ray and optical outburst which lasts for a few tensof days. They uniquely provide the most compelling evidence for theexistence of steller mass black holes using radial velocity studies,giving mass functions exceeding the maximum mass of a stable neutronstar (~3 Msolar). Microquasars are X-ray novae with superluminal jetswhose mechanism is poorly unknown while a number of observational resultsand models have been discussed. Supercritical accretion disks haverecently been discussed for black hole candidates shining at the Eddingtonluminosity which frequently show jets. The accretion disks theoreticallybecome geometrically and optically thick when the mass accrete overa critical rate, on the other hand, the geometrically thin disks areapplied for the subcritical accretion rate. We can consider that supercriticalaccretion occurs in persistent jet sources, for example, active galacticnuclei or SS433 whose mass accretion rate is observationally suggestedto be near critical. On the other hand, during any transient jet sourceoutbursts, we had detected no implication that supercritical accretionoccurs. However, a new atypical microquasar, V4641 Sgr showed an evidenceof this for the first time.

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 ...

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