Peculiar outbursts of a black hole X-ray transient, V4641 Sgr

Details Peculiar outbursts of a black hole X-ray transient, V4641 Sgr Peculiar outbursts of a black hole X-ray transient, V4641 Sgr

Apr 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007iaus..238..465u&link_type=abstract

Black Holes from Stars to Galaxies -- Across the Range of Masses. Edited by V. Karas and G. Matt. Proceedings of IAU Symposium #

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V4641 Sgr is a black hole X-ray binary (BHXB), which has shown mysterious outbursts almost once a year. We pay attention to this object, not only because of its peculiar behaviour, but also because it provides us with a number of information about topics which have recently received much attention in BHXBs, such as the jet-disk interaction, the synchrotron emission in the optical and IR regions, and rapid optical variations. We keep on monitoring V4641 Sgr since 1999, and observed 5 optical outbursts. Here, we summarize optical properties of V4641 Sgr outbursts. Its outbursts are quite unique in the points of short durations and rapid time-evolutions. Due to these characteristics, the number of data during outbursts is so small that it had been difficult to give a unified description for them even phenomenologically. Based on our long monitoring, however, we have finally found common properties in outbursts in 1999, 2002, 2003, 2004, and 2005. The state of V4641 Sgr can be divided in 5 phases based on its optical variability; i) the initial rising phase, ii) the high variability phase, iii) the main flare, iv) the post-outburst active phase, and v) the quiescence. During the phases ii) and iv), the object exhibits rapid optical variations with time-scales of a few seconds. No periodic or quasi-periodic signal has been found in these phases. While X-ray luminosities were small in the phases ii) and iv), it has reached super-Eddington levels during the main flare, at least in 1999 and 2005, and possibly in 2002. Relativistic jets were associated to those flares. Our long monitoring also enabled us to accurately determine the orbital parameters from ellipsoidal modulations at quiescence. We suggest that the distance and luminosity of the object are indeed much smaller than those we had believed before.

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