Swift X-ray Afterglows: Where Are the X-ray Jet Breaks?

— We examine the Swift/X-ray Telescope (XRT) light curves from the first ∼ 150 gamma-ray burst (GRB) afterglows. Although we expected to find jet breaks at typical times of 1-2 days after the GRB, we find that these appear to be extremely rare. Typical light curves have a break in the slope at about 10 4 s, followed by a single power-law decay whose slope is much too shallow to be consistent with expectations for jet breaks. X-ray light curves typically extend out to ∼ 10 days without any further breaks, until they become too faint for the XRT to detect. In some extreme cases, light curves extend out to more than two months without evidence for jet breaks. This raises concerns about our understanding of afterglow and jet dynamics, and of GRB energetics. PACS 98.70.Rz – γ-ray sources; γ-ray bursts. PACS 95.85.Nv – X-ray observations. The beaming factors of Gamma-Ray Bursts (GRBs) are critically important for understanding their overall energetics. For GRBs at known redshift, the GRB fluence can be converted to a total radiated energy assuming isotropic radiation. The values found for E γ,iso can range up to 10 54 ergs, a value that is difficult to explain unless the radiation is actually concentrated into a narrow beam, or jet, pointed towards us. The jet beaming factor must then be measured in order to determine the actual beamed energy radiated by the GRB. Fortunately, the jet opening angle can be determined directly from detailed measurements of the light curves of GRBs. The power law decay indices of GRB afterglows are expected to steepen to F ν ∝ t −p when the jet has decelerated to the point that the bulk Lorentz factor, Γ, is given by Γ ∼ θ −1 j , where p is the power law index of the electron energy distribution and θ j is the opening angle of the collimated jet [1]. The jet opening angle for a uniform jet can be expressed as θ j = 1 6 t j 1 + z 3/8 nη γ