MAXI/GSC Discovery of the Black-Hole Candidate MAXI J1305–704

Abstract

We present the first results on a new black-hole candidate, MAXI J1305–704, observed by MAXI/GSC. The new X-ray transient, named as MAXIJ1305–704, was first detected by the MAXI-GSC all-sky survey on 2012 April 9 in the direction to the outer Galactic bulge at $(l,b)=(304_\cdot^\circ 2,-7_\cdot^\circ 6)$. A Swift/XRT follow-up observation confirmed the uncatalogued point source, and localized it to the position at $(13^\textrm h06^\textrm m56_\cdot^\textrm s44,-70^\circ27^\prime4.^{\prime\prime}91)$. The source continued its activity for about five months, until 2012 August. The MAXI/GSC light curve in the 2–10 keV band and the variation of the hardness ratio of the 4–10 keV to the 2–4 keV flux revealed the hard-to-soft state transition on the the sixth day (April 15) in the brightening phase and the soft-to-hard transition on the ∼60th day (June 15) in the decay phase. The luminosity at the initial hard-to-soft transition was significantly higher than that at the soft-to-hard transition in the decay phase. The X-ray spectra in the hard state are represented by a single power-law model with a photon index of ∼2.0, while those in the soft state need such an additional soft component as represented by a multi-color disk blackbody emission with an inner disk temperature of ∼0.5–1.2 keV. All of the obtained features support the source identification of a Galactic black-hole binary located in the Galactic bulge.