The Afterglow

Over the first two decades, GRB coordinates came with two mutually exclusive properties: accurate localization, e.g., arcmin accuracy, as provided by the Interplanetary Network [42] or rapid localization, e.g., short delay between GRB event and distribution of coordinates, as provided by the BATSE Coordinate Distribution Network system [6]. The launch of the Italian-Dutch "Satellite per Astronomia X," SAX (nick-named BeppoSAX after the Italian high-energy astronomy pioneer Giuseppe Occhialini), in 1996, combined a better localization accuracy of the y-ray burst itself (of order 5 arcmin) by coded-mask imaging in the 2-35 keV range, and rapid notification within minutes of the GRB. This and the fast slewing capability of the BeppoSAX satellite with its co-aligned X-ray telescopes to the GRB position, on the timescale of few hours, allowed to discover long-lasting, fading afterglow emission of GRBs, marking a breakthrough in our understanding of GRBs. The discovery of afterglows in 1997 (see Fig. 24.3 for the first GRB afterglow [18,94]) demonstrated their high redshifts (median of z = 1. 1 as of end of 2005, see Fig. 24.4 for the present redshift distribution) unambiguously and proved GRBs to be the most luminous objects known. The spectral energy distribution, from radio to X-rays, suggested that the afterglow emission is predominantly synchrotron radiation.

Fig. 24.3 Sequence of error circles from y-rays to optical for GRB 970228, the first GRB for which long-wavelength afterglow emission has been identified. Left: The underlying image is from a 34 ks ROSAT high-resolution imager observation [24], with the large circle showing the 3c error circle of the X-ray afterglow as determined with the BeppoSAX wide-field camera (WFC), the smaller circle being the arcmin error circle of the fading source SAX J0501.7+1146 as found with the two Narrow-Field instrument (NFI) pointings, and the two straight lines marking the triangulation circle derived from the BeppoSAX and Ulysses timings [43]. Right: Optical image taken on Feb 28, 1997 [94] at the William Herschel Telescope (Canary Islands) with the WFC error circle marked as dashed segment, the NFI error circle with the dotted segment, and the 10" ROSAT/HRI error box as full circle. The optical transient (OT) falls right into the ROSAT error box

Fig. 24.3 Sequence of error circles from y-rays to optical for GRB 970228, the first GRB for which long-wavelength afterglow emission has been identified. Left: The underlying image is from a 34 ks ROSAT high-resolution imager observation [24], with the large circle showing the 3c error circle of the X-ray afterglow as determined with the BeppoSAX wide-field camera (WFC), the smaller circle being the arcmin error circle of the fading source SAX J0501.7+1146 as found with the two Narrow-Field instrument (NFI) pointings, and the two straight lines marking the triangulation circle derived from the BeppoSAX and Ulysses timings [43]. Right: Optical image taken on Feb 28, 1997 [94] at the William Herschel Telescope (Canary Islands) with the WFC error circle marked as dashed segment, the NFI error circle with the dotted segment, and the 10" ROSAT/HRI error box as full circle. The optical transient (OT) falls right into the ROSAT error box

Fig. 24.4 Redshift distribution of 86 GRBs as of July 2006. (Courtesy S. Klose, priv. comm.; see www.mpe.mpg.de/~jcg/grbrsh.html for an up-to-date version) Because of, among other effects, the higher sensitivity of Swift's GRB detector, the mean redshift of Swift GRBs is above 2, higher than in the pre-Swift era

The detection of the first optical afterglow(s) sparked an international observing effort, which is unique, except perhaps for SN 1987A. All major ground-based telescopes were used at optical, infrared as well as radio wavelengths, and basically every space-born observatory since then has observed GRBs. The HETE-2 satellite [75], launched in October 2000, continued to provide rapid and arcmin sized GRB localizations at a rate of about 2 per month after BeppoSAX had been switched off in April 2003. Over the last 8 years (Feb 1997-Feb 2005) a total of 261 GRBs have been localized within a day to less than one square-degree size, and X-ray afterglows have been detected basically for each of those bursts for which X-ray observations have been done within a few days (total of 58; see http://www.mpe.mpg.de/~jcg/grb.html). Note that because of GRB detector properties only long-duration GRBs have been measured, and so no afterglow of a short GRB was known until the Swift era (see Sect. 24.5).

Interestingly, for only ~50% of the GRB (and X-ray afterglows) an optical afterglow was found, and radio afterglows for ~30%. While the low detection rate at radio wavelength can be easily explained by insufficient sensitivity, the case is more open for the optical afterglows.

Originally, those GRBs with X-ray but without optical afterglows have been coined "dark GRBs." This darkness in the optical can be due to several reasons [26]: the afterglow could (i) have an intrinsically low luminosity, e.g., due to a low-density environment or low explosion energy, (ii) be strongly absorbed by intervening material, either very local around the GRB, or along the line-of-sight through the host galaxy, or (iii) be at high redshift (z > 6) so that Lya blanketing and absorption by intervening Lyman-limit systems would prohibit detection in the usually used R

Fig. 24.4 Redshift distribution of 86 GRBs as of July 2006. (Courtesy S. Klose, priv. comm.; see www.mpe.mpg.de/~jcg/grbrsh.html for an up-to-date version) Because of, among other effects, the higher sensitivity of Swift's GRB detector, the mean redshift of Swift GRBs is above 2, higher than in the pre-Swift era

redshift

redshift band. An analysis of a subsample of GRBs, namely those with particularly accurate positions provided with the Soft X-ray Camera on HETE-2, shows that optical afterglows were found for 10 out of 11 GRBs [96]. This suggests that the majority of dark GRBs are neither at high redshift nor strongly absorbed, but just faint, i.e., the spread in afterglow brightness at a given time after the GRB is much larger than previous observations indicated.

It is probably fair to say that the progress in the GRB field since 1997 has mostly occurred in understanding the afterglow emission and the GRB surroundings. Observational X-ray astronomy has played a vital role in this progress as it allowed the identification by drastically improving the position accuracy as well as allowing to place severe constraints from measurements of the X-ray spectra and X-ray flux variability.

In addition to the nearly-default repointings of BeppoSAX for 0.5-10 keV follow-up observations with the narrow-field instruments, X-ray afterglow observations have also been done with ROSAT (8 GRBs, 5 days fastest response) [36], ASCA (7 GRBs, 1.2 days), XMM (11 GRBs until Sep. 2004 and counting, 4h), Chandra (20 GRBs until Sep. 2004 and counting, 16 h) [30].

0 0

Post a comment