| Abstract
| - The thermal state of the intergalactic medium (IGM) provides an indirect probe of both the H i and He ii reionization epochs. Current constraints on the IGM temperature from the Lyα forest are restricted to the redshift range 2 ≤z≤ 4.5, limiting the ability to probe the thermal memory of H i reionization towards higher redshift. In this work, we present the first direct measurement of the IGM temperature around a z= 6 quasar by analyzing the Doppler widths of Lyα absorption lines in the proximity zone of SDSS J0818+1722. We use a high-resolution (R= 40 000) Keck/HIRES spectrum in combination with detailed numerical modelling to obtain the temperature at mean density, T0= 23 600 ±50006900 K (±92009300 K) at 68 (95) per cent confidence assuming a prior probability following H i and He ii reionization. This enables us to place an upper limit on the redshift of H i reionization, zH, within 33 comoving Mpc of SDSS J0818+1722. If the quasar reionizes the He ii in its vicinity, then in the limit of instantaneous reionization we infer zH< 9.0(11.0) at 68 (95) per cent confidence assuming photoheating is the dominant heat source and that H i reionization is driven by ionizing sources with soft spectra, typical of Population II stars. If the H i and He ii in the IGM around SDSS J0818+1722 are instead reionized simultaneously by a population of massive metal-free stars, characterized by very hard ionizing spectra, we obtain a tighter upper limit of zH< 8.4(9.4). Initiating reionization at higher redshifts produces temperatures which are too low with respect to our constraint unless the H i ionizing sources or the quasar itself has spectra significantly harder than typically assumed.
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