High Energy Transient Explorer 2 Observations of the Extremely Soft X-Ray Flash XRF 020903

• Published in: The Astrophysical journal Vol. 602; no. 2; pp. 875 - 885
• Main Authors: Sakamoto, T, Lamb, D. Q, Graziani, C, Donaghy, T. Q, Suzuki, M, Ricker, G, Atteia, J.-L, Kawai, N, Yoshida, A, Shirasaki, Y, Tamagawa, T, Torii, K, Matsuoka, M, Fenimore, E. E, Galassi, M, Tavenner, T, Doty, J, Vanderspek, R, Crew, G. B, Villasenor, J, Butler, N, Prigozhin, G, Jernigan, J. G, Barraud, C, Boer, M, Dezalay, J.-P, Olive, J.-F, Hurley, K, Levine, A, Monnelly, G, Martel, F, Morgan, E, Woosley, S. E, Cline, T, Braga, J, Manchanda, R, Pizzichini, G, Takagishi, K, Yamauchi, M
• Format: Journal Article
• Language: English
• Published: IOP Publishing 20.02.2004
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/381232

We report High Energy Transient Explorer 2 (HETE-2) Wide Field X-Ray Monitor/French Gamma Telescope observations of the X-ray flash XRF 020903. This event was extremely soft: the ratio log(S sub(X)/S sub(gamma)) = 0.7, where S sub(X) and S sub(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 of E[image] < 5.0 keV (99.7% probability upper limit), and no photons were detected above [approx]10 keV. The burst is shorter at higher energies, which is similar to the behavior of long gamma-ray bursts (GRBs). We consider the possibility that the burst lies at very high redshift and that the low value of E[image] 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 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 [approx]300 of the relation between the isotropic-equivalent energy E sub(iso) and the peak E sub(peak) of the nu F sub( 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 X-ray flashes (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.