We report HETE-2 WXM/FREGATE observations of the X-ray flash , XRF 020903 .
This event was extremely soft : the ratio \log ( S _ { X } / S _ { \gamma } ) = 0.7 , where S _ { X } and S _ { \gamma } are the fluences in the 2-30 and 30-400 keV energy bands , is the most extreme value observed so far by HETE-2 .
In addition , the spectrum has an observed peak energy E ^ { obs } _ { peak } < 5.0 keV ( 99.7 % probability upper limit ) and no photons were detected above \sim 10 keV .
The burst is shorter at higher energies , which is similar to the behavior of long GRBs .
We consider the possibility that the burst lies at very high redshift and that the low value of E _ { peak } ^ { obs } is due to the cosmological redshift , and show that this is very unlikely .
We find that the properties of XRF 020903 are consistent with the relation between the fluences S ( 7 - 30 ~ { } { keV } ) and S ( 30 - 400 ~ { } { keV } ) found by Barraud et al .
for GRBs and X-ray-rich GRBs , and are consistent with the extension by a decade of the hardness-intensity correlation ( Mallozzi et al .
1995 ) found by the same authors .
Assuming that XRF 020903 lies at a redshift z = 0.25 as implied by the host galaxy of the candidate optical and radio afterglows of this burst , we find that the properties of XRF 020903 are consistent with an extension by a factor \sim 300 of the relation between the isotropic-equivalent energy E _ { iso } and the peak E _ { peak } of the \nu F _ { \nu } spectrum ( in the source frame of the burst ) found by Amati et al .
for GRBs .
The results presented in this paper therefore provide evidence that XRFs , X-ray-rich GRBs , and GRBs form a continuum and are a single phenomenon .
The results also impose strong constraints on models of XRFs and X-ray-rich GRBs .