  We perform the first measurement of the thermal and ionization state of the
intergalactic medium (IGM) across 0.9 < z < 1.5 using 301 \lya absorption lines
fitted from 12 HST STIS quasar spectra, with a total pathlength of \Delta
z=2.1. We employ the machine-learning-based inference method that uses joint
b-N distributions obtained from \lyaf decomposition. Our results show that the
HI photoionization rates, \Gamma, are in good agreement with the recent UV
background synthesis models, with \log (\Gamma/s^{-1})={-11.79}^{0.18}_{-0.15},
-11.98}^{0.09}_{-0.09}, and {-12.32}^{0.10}_{-0.12} at z=1.4, 1.2, and 1
respectively. We obtain the IGM temperature at the mean density, T_0, and the
adiabatic index, \gamma, as [\log (T_0/K), \gamma]= [{4.13}^{+0.12}_{-0.10},
{1.34}^{+0.10}_{-0.15}], [{3.79}^{+0.11}_{-0.11}, {1.70}^{+0.09}_{-0.09}] and
[{4.12}^{+0.15}_{-0.25}, {1.34}^{+0.21}_{-0.26}] at z=1.4, 1.2 and 1
respectively. Our measurements of T_0 at z=1.4 and 1.2 are consistent with the
expected trend from z<3 temperature measurements as well as theoretical
expectations that, in the absence of any non-standard heating, the IGM should
cool down after HeII reionization. Whereas, our T_0 measurements at z=1 show
unexpectedly high IGM temperature. However, because of the relatively large
uncertainty in these measurements of the order of \Delta T_0~5000 K, mostly
emanating from the limited redshift path length of available data in these
bins, we can not definitively conclude whether the IGM cools down at z<1.5.
Lastly, we generate a mock dataset to test the constraining power of future
measurement with larger datasets. The results demonstrate that, with redshift
pathlength \Delta z \sim 2 for each redshift bin, three times the current
dataset, we can constrain the T_0 of IGM within 1500K. Such precision would be
sufficient to conclusively constrain the history of IGM thermal evolution at z
< 1.5.
