  Active regions often show S-shaped structures in the corona called sigmoids.
These are highly sheared and twisted loops formed along the polarity inversion
line. They are considered to be one of the best pre-eruption signatures for
CMEs. Here, we investigate the thermodynamic evolution of an on-disk sigmoid
observed during December 24-28, 2015. For this purpose, we have employed
Emission Measure (EM) and filter-ratio techniques on the observations recorded
by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics
Observatory (SDO) and X-ray Telescope (XRT) onboard Hinode. The EM analysis
showed multi-thermal plasma along the sigmoid and provided a peak temperature
of 10-12.5 MK for all observed flares. The sigmoidal structure showed emission
from Fe XVIII (93.93 {\AA}) and Fe XXI 128.75 {\AA}) lines in the AIA 94 and
131 {\AA} channels, respectively. Our results show that the hot plasma is often
confined to very hot strands. The temperature obtained from the EM analysis was
found to be in good agreement with that obtained using the XRT, AIA, and GOES
filter-ratio methods. These results provide important constraints for the
thermodynamic modeling of sigmoidal structures in the core of active regions.
Moreover, this study also benchmarks different techniques available for
temperature estimation in solar coronal structures.
