Confirmation of the $\eps$ -- $\eiso$ (Amati) relation from the X-ray flash XRF 050416A observed by Swift/BAT

TL;DR

This study confirms the Amati relation for X-ray flashes using Swift/BAT data, showing that XRF 050416A fits the established E_{peak} - E_{iso} correlation, thereby supporting a unified view of XRFs and GRBs.

Contribution

The paper provides observational evidence that XRF 050416A adheres to the Amati relation, extending its validity to include XRFs and filling the gap in the E_{peak} range, thus strengthening the unification model.

Findings

XRF 050416A's spectral properties are consistent with the Amati relation.

The E_{peak} and E_{iso} values fill the gap in the relation around 30-80 keV.

Jet break time estimates are inconsistent with observations, challenging existing models.

Abstract

We report Swift Burst Alert Telescope (BAT) observations of the X-ray Flash (XRF) XRF 050416A. The fluence ratio between the 15-25 keV and 25-50 keV energy bands of this event is 1.5, thus making it the softest gamma-ray burst (GRB) observed by BAT so far. The spectrum is well fitted by the Band function with E^{\rm obs}_{\rm peak} of 15.0_{-2.7}^{+2.3} keV. Assuming the redshift of the host galaxy (z = 0.6535), the isotropic-equivalent radiated energy E_{\rm iso} and the peak energy at the GRB rest frame (E^{\rm src}_{\rm peak}) 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 the 30 - 80 keV range of E^{\rm src}_{\rm peak}. This result tightens the validity of the E^{\rm src}_{\rm peak} - E_{\rm iso} relation from XRFs to GRBs. We also find that the jet break time estimated using the empirical relation between E^{\rm src}_{\rm peak} and the collimation corrected energy E_{\gamma} is inconsistent with the afterglow observation by Swift X-ray Telescope. This could be due to the extra external shock emission overlaid around the jet break time or to the non existence of a jet break feature for XRF, which might be a further challenging for GRB jet emission, models and XRF/GRB unification scenarios.