Confirmation of the Esrcpeak-Eiso(AMATI) relation from the X-ray flash XRF 050416A by the Swift Burst Alert Telescope

We report Swift Burst Alert Telescope ( BAT) observations of the X-ray flash (XRF) XRF 050416A. The fluence ratio between the 15-25 and 25-50 keV energy bands of this event is 1.1, thus making it the softest gamma-ray burst (GRB) observed by BAT so far. The spectrum is well fitted by a Band function with of k(peak)(obs) 15.0(-2.7)(+2.3) keV. Assuming the redshift of the host galaxy (E-peak(src)), the isotropic equivalent radiated energy E-iso and the peak z = 0.6535 energy at the GRB rest frame (E-peak(src)) of XRF 050416A are not only consistent with the correlation found by Amati (E-peak(src)) et al. and extended to XRFs by Sakamoto et al. but also fill in the gap of this relation around the 30-80 keV range of. This result tightens the validity of the relation from XRFs to GRBs. We also find that the jet break time estimated using the empirical relation between E-peak(src) and the collimation corrected energy E-gamma is inconsistent with the afterglow observation by the Swift X-Ray Telescope. This could be due to the extra external shock emission overlaid around the jet break time or to the nonexistence of a jet break feature for XRFs, which might be a further challenge for GRB jet emission models and XRF/GRB unification scenarios.