State Transitions of GRS 1739\(-\)278 in the 2014 Outburst

Abstract

We report on the X-ray spectral analysis and time evolution of GRS 1739\(-\)278 during its 2014 outburst based on MAXI/GSC and Swift/XRT observations. Over the course of the outburst, a transition from the low/hard state to the high/soft state and then back to the low/hard state was seen. During the high/soft state, the innermost disk temperature mildly decreased, while the innermost radius estimated with the multi-color disk model remained constant at \(\sim18\ (\frac{D}{8.5\ \mathrm{kpc}}) \ {(\frac{\cos i}{\cos 30^{\circ}})}^{-1/2}\) km, where \(D\) is the source distance and \(i\) is the inclination of observation. This small innermost radius of the accretion disk suggests that the central object is more likely to be a Kerr black hole rather than a Schwardzschild black hole. Applying a relativistic disk emission model to the high/soft state spectra, a mass upper limit of \(18.3\ \mathrm{M_{\odot}}\) was obtained based on the inclination limit \(i<60^{\circ}\) for an assumed distance of 8.5 kpc. Using the empirical relation of the transition luminosity to the Eddington limit, the mass is constrained to \(4.0-18.3\ \mathrm{M_{\odot}}\) for the same distance. The mass can be further constrained to be no larger than \(9.5\ \mathrm{M_{\odot}}\) by adopting the constraints based on the fits to the NuSTAR spectra with relativistically blurred disk reflection models (Miller et al. 2015).