Stroh and Kennea (GCN Circ. 15468) reported the initial Swift results. Swift-XRT has observed the error circle of the Fermi/LAT GRB 131108A (Racusin et al., GCN Circ. 15464, Evans, GCN Circ. 15465) in a series of observations tiled on the sky. Table 1 contains the best reported positions from Swift.
Xu et al. (GCN Circ. 15471) reported the position from NOT for the optical afterglow of this GRB. Chester and Stroh (GCN Circ. 15476) reported the detection with UVOT of an optical afterglow. de Ugarte Postigo et al. (GCN Circ. 15470) determined a redshift of 2.40 from GTC. Table 2 is a summary of GCN Circulars about this GRB from observatories other than Swift.
BAT results are not available.
We have analysed 16 ks of XRT data for the Fermi/LAT-detected burst: GRB 131108A, from 25.9 ks to 83.6 ks after the Fermi/LAT trigger. The data are entirely in Photon Counting (PC) mode. An X-ray source is detected within the Fermi/LAT error circle.
The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.77 (+0.29, -0.27).
A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.25 (+0.32, -0.29). The best-fitting absorption column is 9.4 (+6.6, -5.7) x 1020 cm-2, in excess of the Galactic value of 3.2 x 1020 cm-2 (Kalberla et al. 2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.2 x 10-11 (4.4 x 10-11) erg cm-2 count-1.
A summary of the PC-mode spectrum is thus:
Total column: 9.4 (+6.6, -5.7) x 1020 cm-2
Galactic foreground: 3.2 x 1020 cm-2
Excess significance: 1.8 σ
Photon index: 2.25 (+0.32, -0.29)
The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00020326.
The Swift/UVOT began settled observations of the field of the Fermi/LAT-detected GRB 131108A 25920 s after the LAT trigger (Racusin et al., GCN Circ. 15464) (Chester and Stroh GCN Circ. 15476). A source consistent with the XRT position (Stroh and Kennea GCN Circ. 15474) is detected in the initial UVOT exposures. Table 3 gives preliminary magnitudes using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc., 1358, 373). No correction has been made for the expected extinction in the Milky Way corresponding to a reddening of EB-V of 0.028 mag. in the direction of the GRB (Schlegel et al. 1998).
Figure 1. The BAT light curve is not available.
Figure 2. The XRT light curve.
Any data from a cross-hatched region are not included in the fit.
| RA (J2000) | Dec (J2000) | Error | Note | Reference |
|---|---|---|---|---|
| 10h26m00.45s | +09°39'44.1" | 0.58" | UVOT-refined | Chester and Stroh GCN Circ. 15476 |
| 10h26m00.43s | +09°39'44.9" | 1.6" | XRT | Stroh and Kennea GCN Circ. 15474 |
| Band | Authors | GCN Circ. | Subject | Observatory | Notes |
|---|---|---|---|---|---|
| Optical | Gorosabel et al. | 15469 | 1.23m CAHA optical candidate | CAHA | detection |
| Optical | de Ugarte Postigo et al. | 15470 | Redshift from 10.4m GTC | GTC | redshift |
| Optical | Xu et al. | 15471 | NOT optical observations | NOT | detection |
| Optical | Klotz et al. | 15473 | TAROT-Calern optical observation | TAROT | detection |
| Optical | Troja et al. | 15475 | RATIR Optical and NIR Observations | RATIR | detection |
| Optical | Troja et al. | 15482 | Continued RATIR Optical and NIR Observations | RATIR | detection |
| Optical | Volnova et al. | 15484 | optical observations | Abastumani | detection |
| Optical | Volnova et al. | 15485 | ISON Blagoveshensk upper limit | Searchlight Obs. Network | upper limits |
| Radio | Perley | 15478 | CARMA 3mm detection | CARMA | detection |
| Radio | Corsi et al. | 15502 | VLA Observations | VLA | |
| Gamma-ray | Racusin et al. | 15464 | Fermi-LAT onboard detection of a burst | Fermi LAT | detection |
| Gamma-ray | Hurley et al. | 15466 | IPN triangulation | IPN | |
| Gamma-ray | 15467 | IPN triangulation of GRB 131108A - correction |
IPN | ||
| Gamma-ray | Vianello et al. | 15472 | Fermi-LAT refined analysis | Fermi LAT | light curve |
| Gamma-ray | Younes | 15477 | Fermi GBM observation | Fermi GBM | Epeak = 373 ± 14 keV |
| Gamma-ray | Giuliani et al. | 15479 | AGILE/GRID observation | AGILE | |
| Gamma-ray | Golenetskii et al. | 15480 | Konus-Wind observation | Konus-Wind | Epeak = 340 ± 11 keV |
| Filter | Tstart(s) | Tstop(s) | Exp(s) | Mag |
|---|---|---|---|---|
| white | 27746 | 28046 | 295 | 19.70 ± 0.10 |
| b | 26833 | 27133 | 295 | 19.67 ± 0.16 |
| u | 25920 | 26220 | 295 | 19.10 ± 0.14 |
| w1 | 31699 | 32599 | 886 | 20.96 ± 0.33 |
| m2 | 37465 | 37914 | 442 | >20.6 |
| w2 | 45446 | 49794 | 1069 | >21.4 |
Table 3. UVOT observations reported by Chester and Stroh (GCN Circ. 15476). The start and stop times of the exposures are given in seconds since the BAT trigger. The preliminary detections and 3-σ upper limits are given. No correction has been made for extinction in the Milky Way.