We investigate the \gamma -ray and X-ray properties of the flat spectrum radio quasar PKS 2149 - 306 at redshift z = 2.345 .
A strong \gamma -ray flare from this source was detected by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope satellite in 2013 January , reaching on January 20 a daily peak flux of ( 301 \pm 36 ) \times 10 ^ { -8 } ph cm ^ { -2 } s ^ { -1 } in the 0.1–100 GeV energy range .
This flux corresponds to an apparent isotropic luminosity of ( 1.5 \pm 0.2 ) \times 10 ^ { 50 } erg s ^ { -1 } , comparable to the highest values observed by a blazar so far .
During the flare the increase of flux was accompanied by a significant change of the spectral properties .
Moreover significant flux variations on a 6-h time-scale were observed , compatible with the light crossing time of the event horizon of the central black hole .

The broad band X-ray spectra of PKS 2149 - 306 observed by Swift -XRT and NuSTAR are well described by a broken power-law model , with a very hard spectrum ( \Gamma _ { 1 } \sim 1 ) below the break energy , at E _ { break } = 2.5–3.0 keV , and \Gamma _ { 2 } \sim 1.4 –1.5 above the break energy .
The steepening of the spectrum below \sim 3 keV may indicate that the soft X-ray emission is produced by the low-energy relativistic electrons .
This is in agreement with the small variability amplitude and the lack of spectral changes in that part of the X-ray spectrum observed between the two NuSTAR and Swift joint observations .
As for the other high-redshift FSRQ detected by both Fermi -LAT and Swift -BAT , the photon index of PKS 2149 - 306 in hard X-ray is 1.6 or lower and the average \gamma -ray luminosity higher than 2 \times 10 ^ { 48 } erg s ^ { -1 } .