3. RECENT RESULTS FROM SWIFT AND FOLLOW-UP OBSERVATIONS

Compared to previous missions, the Swift results represent a significant advance on two main accounts. First, the sensitivity of the Burst Alert Detector (BAT, in the range 20-150 keV) is somewhat higher than that of the corresponding instruments in CGRO-BATSE, BeppoSAX and HETE-2 [21]. Second, Swift can slew in less then 100 seconds in the direction determined by the BAT instrument, positioning its much higher angular resolution X-ray (XRT) and UV-Optical (UVOT) detectors on the burst [156]

As of December 2005, at an average rate of 2 bursts detected per week, over 100 bursts had been detected by BAT in about a year (compared to 300/year by BATSE; note the BAT field of view is 2 sr, versus 4 in BATSE). Of these ~ 100 bursts, 90% were detected and followed with the XRT within 350 s from the trigger, and about half within 100 s [58], while ~ 30% were detected also with the UVOT [411]. Of the total, over 23 resulted in redshift determinations. Included in this total sum are nine short GRB, of which five had detected X-ray afterglows, three had optical, and one had a radio afterglow, with five host galaxy detections and redshift determinations. These were the first ever short GRB afterglows detected and followed.

The new observations bring the total redshift determinations to over 50 since 1997, when BeppoSAX enabled the first one. The median redshift of the Swift bursts is z 2, which is a factor ~ 2 higher than the median of the BeppoSAX and HETE-2 redshifts [40]. This is a statistically significant difference between the Beppo-SAX and Swift redshift samples [213, 16]. This may in part be ascribed to the better sensitivity of the BAT detector, but mostly to the prompt and accurate positions from XRT and UVOT, making possible ground-based detection at a stage when the afterglow is much brighter, by a larger number of robotic and other telescopes. As of January 2006 the highest Swift-enabled redshift is that of GRB 050904, obtained with the Subaru telescope, z = 6.29 [220], and the second highest is GRB 050814 at z = 5.3, whereas the previous Beppo-SAX era record was z = 4.5. The relative paucity of UVOT detections versus XRT detections may be ascribed in part to this higher median redshift, and in part to the higher dust extinction at the implied shorter rest-frame wavelenghts for a given observed frequency [411], although additional effects may be at work too.

In some of the bursts, both of the "long" (t 2 s) and "short" (t 2 s) categories as defined by BATSE, the Swift BAT results show faint soft gamma-ray extensions or tails, which extend the duration by a substantial factor beyond what BATSE would have detected [156]. A rich trove of information on the burst and afterglow physics has come from detailed XRT light curves, starting on average 100 seconds after the trigger, together with the corresponding BAT light curves and spectra. This suggests a canonical X-ray afterglow picture [338, 528, 67] which includes one or more of the following:
1) an initial steep decay F_X t^-₁ with a temporal index 3 ₁ 5, and an energy spectrum F ^-₁ with energy spectral index 1 ₁ 2 (or photon number index 2 + 1 3), extending up to a time 300 s t₁ 500 s;
2) a flatter decay portion F_X t^-₂ with temporal index 0.2 ₂ 0.8 and energy index 0.7 ₂ 1.2, at times 10³ s t₂ 10⁴ s;
3) a "normal" decay F_X t^-₃ with 1.1 ₃ 1.7 and 0.7 ₃ 1.2 (generally unchanged the previous stage), up to a time t₃ ~ 10⁵ s, or in some cases longer;
4) In some cases, a steeper decay F_X t^-₄ with 2 ₄ 3, after t₄~ 10⁵ s;
5) In about half the afterglows, one or more X-ray flares are observed, sometimes starting as early as 100 s after trigger, and sometimes as late as 10⁵ s. The energy in these flares ranges from a percent up to a value comparable to the prompt emission (in GRB 050502b). The rise and decay times of these flares is unusually steep, depending on the reference time t₀, behaving as (t - t₀)^{± _fl} with 3 _fl 6, and energy indices which can be also steeper (e.g. _fl 1.5) than during the smooth decay portions. The flux level after the flare usually decays to the value extrapolated from the value before the flare rise (see Figure 3). The above characteristics are derived mainly from long bursts, but interestingly, at least one of the short bursts shows similar features. However, the evidence for late time activity is more sketchy in short bursts, so that the analogy must be considered with caution.

Figure 3. Schematic features seen in early X-ray afterglows detected with the Swift XRT instrument (e.g. [528, 338] (see text).

An exciting result from Swift was the detection of the long burst GRB 050904, which broke through the astrophysically and psychologically significant redshift barrier of z ~ 6, which is thought to mark the approximate end of the "dark ages", when re-ionization of the intergalactic medium by the first generation of light sources approaches completion. This burst was very bright, both in its prompt -ray emission (E_{, iso} ~ 10⁵⁴ erg) and in its X-ray afterglow. Prompt ground-based optical/IR upper limits and a J-band detection suggested a photometric redshift z > 6 [188], and spectroscopic confirmation soon followed with the 8.2 m Subaru telescope, giving z = 6.29 [220]. There are several striking features to this burst. One is the enormous X-ray brightness, exceeding for a full day the X-ray brightness of the most distant X-ray quasar know to-date, SDSS J1030+0524, by up to a factor 10⁵ in the first minutes [491]. The implications as a tool for probing the intergalactic medium are thought-provoking. Another feature is the extremely variable X-ray light curve, showing many large amplitude flares extending up to at least a day. A third exciting feature is the report of a brief, very bright IR flash [54], comparable in brightness to the famous m_V ~ 9 optical flash in GRB 990123.

A third major advance from Swift was the discovery and localization of short GRB afterglows. As of December 2005 nine short bursts had been localized by Swift, while in the same period HETE-2 discovered two, and one was identified with the IPN network. In five of the short bursts, GRB 050509b, 050709, 050724 and 051221a an X-ray afterglow was measured and followed up, with GRB 050709, 050724 and 051221a showing also an optical afterglow, and 050724 also a radio afterglow, while 040924 had an optical afterglow but not an X-ray one [131]. These are the first afterglows detected for short bursts. Also, for the first time, host galaxies were identified for these short bursts, which in a number of cases are early type (ellipticals) and in other cases are irregular galaxies (e.g. [383]. The redshifts of four of them are in the range z ~ 0.15-0.5, while another one was determined to be z = 0.8 (and less securely, it has been argued that this latter may instead be z 1.8 [39]). The median z is 1/3-1/2 that of the long bursts. There is no evidence for significant star formation in these host environments (except for GRB 050709, [130, 383], which is compatible with what one expects for an old population, such as neutron star mergers or neutron star-black hole mergers, the most often discussed progenitor candidates (although it would also be compatible with other progenitors involving old compact stars). While the evidence for a neutron star or black merger is suggestive, the evidence is not unequivocal. E.g. the observations suggest a typical time delay of at least several Gyr between the start formation epoch and the explosion of short GRBs [328, 530]. There are a number of unresolved issues related to this (see Section 7). The first short burst afterglow followed up by Swift, GRB 050509b, was a rather brief (~ 30 ms), moderate luminosity (L_iso ~ 10⁵⁰ erg s^-1 but low fluence (E_iso ~ 2 × 10⁴⁸ erg) burst with a simple power-law X-ray afterglow which could only be followed for several hundreds of seconds [155]. The third one, GRB 050724, was brighter, E_iso ~ 3 × 10⁵⁰ erg, and could be followed in X-rays with Swift for at least 10⁵ s [26], and with Chandra up to 2 × 10⁶ s [157]. The remarkable thing about this burst's X-ray afterglow is that it shows some similarities to the typical X-ray light curves described above for long GRB - except for the lack of a slow-decay phase, and for the short prompt emission which places it the the category of short bursts, as well as the elliptical host galaxy candidate. It also has also bumps in the X-ray light curve at 100 s and at 3 × 10⁴ s, which resemble some of the long burst X-ray flares and whose origin is unclear. The first bump or flare has the same fluence as the prompt emission, while the late one has ~ 10% of that. The interpretation of these pose interesting challenges, as discussed below and in Section 7.

A fourth exciting result from Swift was the detection of a long burst, GRB 060218, which was seen also with the XRT and UVOT instruments [61], and which is associated with SN 2006aj [317, 316, 454, 69]. The redshift is z = 0.033, and the contribution to the optical light curve as well as the spectrum are similar to those of the Ic type SN 1998bw. The result this is so exciting is that it is the first time that a GRB/SN has been observed minutes after the -ray trigger at X-ray and UV/optical wavelengths. This is discussed further in Section 8.2.