Outlook First Results of the Swift Mission

The successful launch of NASA's Swift gamma-ray burst mission [28] in November 2004 and the smooth calibration/verification phase and operation since then has brought several exciting news. Swift carries three instruments: (1) the burst alert telescope (BAT) is a coded-mask telescope with a two steradian field of view, imaging the sky in the 15-150 keV band. (2) the X-ray telescope (XRT) is a Woltertype telescope operating in the 0.2-10 keV band, and (3) the ultraviolet and optical telescope (UVOT). Within seconds of detecting a burst with BAT, Swift relays the burst's location to ground stations, allowing both ground-based and space-based telescopes around the world to observe the burst's afterglow. At the same time, Swift is autonomously slewing its XRT and UVOT instruments to the burst position, allowing to start observing the afterglow emission in the X-ray/UV/optical bands at typically 1 min after the burst onset. In the following, only few highlights are mentioned.

Because of the technical limitations of BeppoSAX to trigger on GRBs with duration shorter than 1 s, expectations were high that Swift would localize short GRBs, investigate its afterglow properties, and ultimately determine the nature of short GRBs. Swift detected the first short GRB on Feb 2, 2005, but the location was too close to the Sun to allow repointing Swift for afterglow observations. However, Swift's second short burst, GRB 050509B was better suited, and afterglow observations started within 1 min. While no optical/UV emission was detected, a fading X-ray afterglow was seen over the first 1000 s, marking the first afterglow of a short GRB [29]. The X-ray localization near an elliptical galaxy at low redshift with no sign of star formation as well as several other constraints are consistent with the merger scenario [14]. Soon after, HETE-2 also localized a short burst, GRB 050709, and its afterglow properties [21] are surprisingly similar to that of GRB 050509B. If further observations confirm these findings, then short GRBs may indeed be powered by mergers of compact objects.

The temporal behavior of the early X-ray afterglow is certainly one of the major surprises from Swift. Starting at 50-70 s after the GRB onset, a rapidly decaying X-ray afterglow is seen which often can be described with a power law slope steeper than 3. Thereafter, the afterglow emission turns into a very shallow decay, sometimes staying even constant for 100-1000 s. Finally, the decay slope gets steeper again (to —1.2), consistent with the standard fireball scenario. The early rapid decline, in nearly all cases, smoothly connects to the spectrally extrapolated BAT light curve, and therefore this component is now generally interpreted as the tail of the GRB. For the shallow decay, the interpretation is not yet settled. To keep the X-ray emission on for that long, the total energy in the fireball needs to increase with time, refreshing the fireball for a duration much longer than the burst duration. This could be due to different physical mechanisms, namely (1) the central engine keeps injecting energy, (2) the energy injection is short but involves a wide range of Lorentz factors, or (3) the outflow is poynting-flux dominated, so that the magnetic field energy takes a longer time to be transferred into the medium. Independent of this interpretation, the most surprising finding is that occasionally a bright X-ray flare is observed during that same time period, with a duration of few hundred seconds (Fig. 24.12). While late central engine activity is the presently favored interpretation, a clear physical mechanism still needs to be found. Most confusing in this respect is that such X-ray flares have also been seen in short bursts [7]! Thus, besides being a major problem for the merger scenario, it also seems to be independent of the rather different physical conditions of the central engine or the burst surrounding.

Finally, it is worth mentioning that despite Swift X-ray observations within less than 60 sec after the GRB trigger, no spectral lines have been found yet. It remains to be seen whether or not this is due to the generally faint X-ray afterglow intensity.

There are many more topics for which Swift is expected to significantly contribute to our understanding. With the Swift satellite and its instruments working oo o c o c


Fig. 24.12 The X-ray flux (0.3-10 keV in the observer frame) as a function of the observed time for selected GRBs showing prominent X-ray flares [66]

perfectly, the near-term future for GRB research, particularly the GRB afterglow study, is very bright.

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