Swift observations of the prompt X-ray emission and afterglow from GRB050126 and GRB050219A

• Published in: Astronomy and astrophysics (Berlin) 2006-04, Vol.449 (1), p.89-100
• Main Authors: Goad, M. R., Tagliaferri, G., Page, K. L., Moretti, A., Osborne, J. P., Kobayashi, S., Kumar, P., Mészáros, P. I., Chincarini, G., Sakamoto, T., Zhang, B., Barthelmy, S. D., Beardmore, A. P., Burrows, D. N., Campana, S., Capalbi, M., Cominsky, L., Cusumano, G., Gehrels, N., Giommi, P., Godet, O., Hill, J. E., Kennea, J. A., Krimm, H., La Parola, V., Mangano, V., Mineo, T., Morris, D. C., Mukerjee, K., Nousek, J. A., O'Brien, P. T., Pagani, C., Perri, M., Romano, P., Wells, A. A.
• Format: Article
• Language: English
• Subjects: Astronomy; black hole physics; Earth, ocean, space; Exact sciences and technology; gamma rays: bursts; gamma rays: observations
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361:20054457

We report on the temporal and spectral characteristics of the early X-ray emission from the Gamma Ray Bursts GRB050126 and GRB050219A as observed by Swift . The X-ray light-curves of these 2 bursts both show remarkably steep early decays ($F(t)\propto t^{-3}$), breaking to flatter slopes on timescales of a few hundred seconds. For GRB050126 the burst shows no evidence of spectral evolution in the 20–150 keV band, and the spectral index of the γ-ray and X-ray afterglows are significantly different suggesting a separate origin. By contrast the BAT spectrum of GRB050219A displays significant spectral evolution, becoming softer at later times, with Γ evolving toward the XRT photon index seen in the early X-ray afterglow phase. For both bursts, the 0.2–10 keV spectral index pre- and post-break in the X-ray decay light-curve are consistent with no spectral evolution. We suggest that the steep early decline in the X-ray decay light-curve is either the curvature tail of the prompt emission; X-ray flaring activity; or external forward shock emission from a jet with high density regions of small angular size (${>}\Gamma^{-1}$). The late slope we associate with the forward external shock.