  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).
