A new X-ray nova MAXI J1910-057 (= Swift J1910.2-0546) and mass accretion inflow

Abstract

We report on long-term monitoring of a newly discovered X-ray nova, MAXI J1910-057 (= Swift J1910.2-0546) by MAXI and Swift. The new X-ray transient was first detected on 2012 May 31 by the MAXI Gas Slit Camera (GSC) and the Swift Burst Alert Telescope (BAT) almost simultaneously. We analyzed X-ray and UV data for 270 d following the outburst onset taken by repeated MAXI scans and Swift pointing observations. The obtained X-ray light curve for the inital 90 d is roughly represented by a fast-rise and exponentialdecay profile. However, it re-brightened at ∼110 d after the onset and finally went down below both GSC and BAT detection limits after 240 d. All the X-ray energy spectra are fitted well with a model consisting of a multi-color-disk blackbody and its Comptonized hard tail. During the soft-state periods, the inner-disk radius of the best-fit model was almost constant. If the radius represents the innermost stable circular orbit of a nonspinning black hole and the soft-to-hard transitions occur at 1%-4% of the Eddington luminosity, the mass of the compact object is estimated to be > 2.9 M⊙ and the distance to be > 1.70 kpc. The inner-disk radius became larger in the hard/hard-intermediate state. This suggests that the accretion disk would be truncated. We detected an excess of the UV flux over the disk blackbody component extrapolated from the X-ray data, which can be modeled as reprocessed emission irradiated by the inner disk. We also found that the UV light curve mostly traced the X-ray curve, but a short dipping event was observed in both the UV and the X-ray bands with a 3.5 d X-ray time lag. This can be interpreted as the radial inflow of accreting matter from the outer UV region to the inner X-ray region.