The identification of MAXI J1659-152 as a black hole candidate

We report on the analysis of all 65 pointed Rossi X-ray Timing Explorer (RXTE) observations of the recently discovered soft X-ray transient MAXI J1659-152 (initially referred to as GRB 100925A). The source was studied in terms of its evolution through the hardness-intensity diagram (HID), as well as its X-ray variability properties. MAXI J1659-152 traced out a counterclockwise loop in the HID, which is commonly seen in transient low-mass X-ray binaries. The variability properties of the source, in particular the detection of type-B and type-C low-frequency quasi-periodic oscillations, and the way they evolve along the HID track, indicate that MAXI J1659-152 is a black hole candidate. The spectral and variability properties of MAXI J1659-152 imply that the source was observed in the hard and soft intermediate states during the RXTE observations, with several transitions between these two states.