We derive constraints on the thermal and ionization states of the intergalactic medium ( IGM ) at redshift \approx 9.1 using new upper limits on the 21-cm power spectrum measured by the LOFAR radio-telescope and a prior on the ionized fraction at that redshift estimated from recent cosmic microwave background ( CMB ) observations .
We have used results from the reionization simulation code grizzly and a Bayesian inference framework to constrain the parameters which describe the physical state of the IGM .
We find that , if the gas heating remains negligible , an IGM with ionized fraction \gtrsim 0.13 and a distribution of the ionized regions with a characteristic size \gtrsim 8 ~ { } h ^ { -1 } comoving megaparsec ( Mpc ) and a full width at the half maximum ( FWHM ) \gtrsim 16 ~ { } h ^ { -1 } Mpc is ruled out .
For an IGM with a uniform spin temperature T _ { S } \gtrsim 3 K , no constraints on the ionized component can be computed .
If the large-scale fluctuations of the signal are driven by spin temperature fluctuations , an IGM with a volume fraction \lesssim 0.34 of heated regions with a temperature larger than CMB , average gas temperature 7-160 K and a distribution of the heated regions with characteristic size 3.5-70 h ^ { -1 } Mpc and FWHM of \lesssim 110 h ^ { -1 } Mpc is ruled out .
These constraints are within the 95 per cent credible intervals .
With more stringent future upper limits from LOFAR at multiple redshifts , the constraints will become tighter and will exclude an increasingly large region of the parameter space .