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We perform the first measurement of the thermal and ionization state of theintergalactic medium (IGM) across 0.9 < z < 1.5 using 301 \lya absorption linesfitted from 12 HST STIS quasar spectra, with a total pathlength of \Deltaz=2.1. We employ the machine-learning-based inference method that uses jointb-N distributions obtained from \lyaf decomposition. Our results show that theHI photoionization rates, \Gamma, are in good agreement with the recent UVbackground 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 1respectively. We obtain the IGM temperature at the mean density, T_0, and theadiabatic 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 1respectively. Our measurements of T_0 at z=1.4 and 1.2 are consistent with theexpected trend from z<3 temperature measurements as well as theoreticalexpectations that, in the absence of any non-standard heating, the IGM shouldcool down after HeII reionization. Whereas, our T_0 measurements at z=1 showunexpectedly high IGM temperature. However, because of the relatively largeuncertainty in these measurements of the order of \Delta T_0~5000 K, mostlyemanating from the limited redshift path length of available data in thesebins, 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 futuremeasurement with larger datasets. The results demonstrate that, with redshiftpathlength \Delta z \sim 2 for each redshift bin, three times the currentdataset, we can constrain the T_0 of IGM within 1500K. Such precision would besufficient to conclusively constrain the history of IGM thermal evolution at z< 1.5.

Measurements of the Thermal and Ionization State of the Intergalactic Medium during the Cosmic Afternoon