Confirmation of the $Epeaksrc − Eiso$ (Amati) relation from the X‐ray flash XRF 050416A observed by Swift/BAT

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 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 the Band function with $Epeakobs$ of $15.6−2.7+2.3 keV$⁠. Assuming the redshift of the host galaxy (z=0.6535), the isotropic‐equivalent energy E_iso and the E_peak energy at the GRB rest frame $(Epeaksrc)$ of XRF 050416A are not only consistent with the correlation found by Amati et al. and extended to XRFs by Sakamoto et al., but also fill‐in the gap of this relation around 30–80 keV range of $Epeaksrc$⁠. This result tightens the validity of the $Epeaksrc−Eiso$ relation form XRFs to GRBs.