J.D. Gropp (PSU), J.A. Kennea (PSU) and S.R. Oates (U.Birmingham) for the Swift team
At 06:19:20 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220518A (trigger= 1107050) (Gropp et al. GCN Circ. 32060). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 67° from the Sun (-0.0 hours West) and 101° from the 94%-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.
Oates and Gropp (GCN Circ. 32063) reported the detection with UVOT of an optical afterglow. 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.
Analysis of the BAT data was reported by Laha and Barthelmy (GCN Circ. 32069).
The light curve (Figure 1) has a peak at the trigger time and potential precursor
The time-averaged spectrum from T-8.36 to T+7.58 s is best fit by a simple
The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/1107050/BA/.
Analysis of the initial XRT data was reported by Kennea et al. (GCN Circ. 32067). We have analysed 11 ks of XRT data for GRB 220518A, from 133 s to 144.1 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 32062).
The late-time light curve (Figure 2) (from T0+7.3 ks) can be modelled with a power-law decay with a decay index of α=1.40 (+0.23, -0.21).
A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.85 (+0.15, -0.13). The best-fitting absorption column is 2.7 (+3.1, -1.7) x 1
A summary of the PC-mode spectrum is thus:
Total column: 2.7 (+3.1, -1.7) x 1
Galactic foreground: 9.3 x 1
Excess significance: <1.6 σ
Photon index: 1.85 (+0.15, -0.13)
The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01107050.
The Swift/UVOT began settled observations of the field of GRB 220518A 129 s after the BAT trigger
(Oates and Gropp GCN Circ. 32063).
A source consistent with the enhanced XRT position (Beardmore et al. GCN Circ. 32062) 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 |
-47°35'00.3" | 0.55" | UVOT-refined | Oates and Gropp GCN Circ. 32063 |
| 0 |
-47°35'01.5" | 2.2" | XRT-enhanced | Beardmore et al. GCN Circ. 32062 |
| 0 |
-47°35'36.3" | 2.0' | BAT-refined | Laha and Barthelmy GCN Circ. 32069 |
| Band | Authors | GCN Circ. | Subject | Observatory | Notes |
|---|---|---|---|---|---|
| Optical | Lipunov et al. | 32061 | Swift GRB 220518A: Global MASTER-Net observations report |
MASTER |
| Filter | Exp(s) | Mag | ||
|---|---|---|---|---|
| whit |
129 | 279 | 147 | 20.00 ± 0.25 |
| white | 621 | 1717 | 334 | 20.49 ± 0.23 |
| v | 671 | 1767 | 136 | >19.0 |
| b | 597 | 1861 | 131 | >20.1 |
| 341 | 591 | 246 | >19.7 | |
| u | 341 | 1841 | 324 | >19.9 |
| w1 | 1103 | 1296 | 39 | >18.4 |
Table 3. UVOT observations reported by Oates and Gropp (GCN Circ. 32063). 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.
May 20, 2022