MAXI GSC Observations of a Spectral State Transition in the Black Hole Candidate XTE J1752–223

Abstract

Abstract We present the first results on the black-hole candidate XTE J1752$-$223 from the Gas Slit Camera (GSC) aboard the Monitor of All-sky X-ray Image (MAXI) on the International Space Station. Including the onset of an outburst reported by the Proportional Counter Array aboard the Rossi X-ray Timing Explorer on 2009 October 23, MAXI / GSC has been monitoring this source approximately 10 times per day with high sensitivity in the 2–20 keV band. XTE J1752$-$223 was initially in a low / hard state during the first 3 months. An anti-correlated behavior between the 2–4 keV and 4–20 keV bands was observed around 2010 January 20, indicating that the source exhibited a spectral transition to the high / soft state. A transient radio jet may have been ejected when the source was in the intermediate state where the spectrum was roughly explained by a power-law with a photon index of 2.5–3.0. The unusually long period in the initial low / hard state implies a slow variation in the mass-accretion rate, and a dramatic soft X-ray increase may be explained by a sudden appearance of the accretion disk component with a relatively low innermost temperature (0.4–0.7 keV). Such a low temperature might suggest that the maximum accretion rate was just above the critical gas-evaporation rate required for the state transition.