We report MAXI and Swift observations of short-term spectral softenings of the galactic black-hole X-ray binary Swift J1753.5-0127 in the low/hard state .
These softening events are characterized by a simultaneous increase of soft X-rays ( 2–4 keV ) and a decrease of hard X-rays ( 15–50 keV ) lasting for a few tens of days .
The X-ray energy spectra during the softening periods can be reproduced with a model consisting of a multi-color disk blackbody and its Comptonized component .
The fraction of the Comptonized component decreased from 0.30 to 0.15 when the spectrum became softer ; meanwhile the inner disk temperature ( T { { } _ { in } } ) increased from 0.2 to 0.45 keV .
These results imply that the softening events are triggered by a short-term increase of the mass accretion rate .
During the observed spectral softening events , the disk flux ( F _ { disk } ) and T { { } _ { in } } did not obey the F { { } _ { disk } } \propto T { { } _ { in } } { { } ^ { 4 } } relation , suggesting that the inner disk radius does not reach the innermost stable circular orbit .