A.Y. Lien (GSFC/UMBC), P.A. Evans (U. Leicester) and S.J. LaPorte (PSU) for the Swift team
At 22:57:08 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210807C (trigger=1064421) (Lien et al. GCN Circ. 30610). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 68° from the Sun (0.4 hours East) and 75° from the 0%-illuminated Moon. Table 1 contains the best reported positions from Swift, and the latest XRT position can be viewed at http://www.swift.ac.uk/xrt_positions.
Table 2 is a summary of GCN Circulars about this GRB from observatories other than Swift.
Standard analysis products for this burst are available at https://gcn.gsfc.nasa.gov/swift_gnd_ana.html.
As reported by Barthelmy et al. (GCN Circ. 30625),
the BAT ground-calculated position is RA, Dec = 143.540, -51.409 deg which is RA(J2000) = 0
The BAT light curve (Figure 1) showed a complex structure with a duration of about ~100 s.
The time-averaged spectrum from T-31.36 to T+78.57 s is best fit by a simple power-law model.
The power law index of the time-averaged spectrum is 1.65 ± 0.06.
The fluence in the 15-150 keV band is 1.2 ± 0.0 x 1
The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/1064421/BA/.
Analysis of the initial XRT data was reported by Evans and Lien (GCN Circ. 30621). We have analysed 10 ks of XRT data for GRB 210807C, from 166 s to 149.0 ks after the BAT trigger. The data comprise 203 s in Windowed Timing (WT) mode (the first 9 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 30615).
The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=4.1 (+1.2, -1.0), followed by a break at T+324 s to an α of 1.28 (+0.08, -0.07).
A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.28 (+0.15, -0.14). The best-fitting absorption column is 4.9 ± 0.4 x 1
A summary of the PC-mode spectrum is thus:
Total column: 6.0 (+1.8, -1.5) x 1
Galactic foreground: 8.5 x 1
Excess significance: 5.5 σ
Photon index: 2.5 ± 0.6
The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01064421.
The Swift/UVOT began settled observations of the field of GRB 210807C 180 s after the BAT trigger
(LaPorte and Lien GCN Circ. 30619).
No optical afterglow consistent with the XRT position 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
Figure 1. The BAT
mask-weighted light curve in the four individual and total
energy bands. The units are counts
Figure 2. The XRT light curve.
Any data from a crosshatched region are not included in the fit.
| RA (J2000) | Dec (J2000) | Error | Note | Reference |
|---|---|---|---|---|
| 0 |
-51°24'03.8" | 2.0" | XRT-final | UKSSDC |
| 0 |
-51°24'03.8" | 2.0" | XRT-enhanced | Goad et al. GCN Circ. 30615 |
| 0 |
-51°24'32.8" | 1.0' | BAT-refined | Barthelmy et al. GCN Circ. 30625 |
| Band | Authors | GCN Circ. | Subject | Observatory | Notes |
|---|---|---|---|---|---|
| Optical | Lipunov et al. | 30612 | Fermi GRB 210807C: Global MASTER-Net observations report |
MASTER | |
| Gamma-ray | Fermi | 30609 | Fermi GBM Final Real-time Localization | Fermi GBM | |
| Gamma-ray | Poolakkil and Meegan | 30620 | Fermi GBM detection | Fermi GBM | Fluence=56.43±0.54x1 (95.8 percentile for long GRBs) |
| Filter | Exp(s) | Mag | ||
|---|---|---|---|---|
| v | 180 | 852 | 596 | >19.4 |
Table 3. UVOT observation reported by LaPorte and Lien (GCN Circ. 30619). The start and stop time of the exposure are given in seconds since the BAT trigger. The preliminary 3-σ upper limit is given. No correction has been made for extinction in the Milky Way.
August 11, 2021