A. D'Ai (INAF-IASFPA), J.P. Osborne (U. Leicester) and S.J. LaPorte (PSU) for the Swift team
At 20:02:01 UT, the Swift Burst Alert Telescope (BAT) triggered and located a GRB 210515C (trigger=1048618) (D'Ai et al. GCN Circ. 30020). 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 Palmer et al. (GCN Circ. 30021),
the BAT ground-calculated position is RA, Dec = 1.106, -14.972 deg which is RA(J2000) = 0
The mask-weighted light curve (Figure 1) shows a weak pulse that starts at ~T-3 s, peaks at ~T+1 s, and ends at ~T+11 s, with some tail emission lasting until ~T+60 s.
The time-averaged spectrum from T-2.53 to T+56.90 s is best fit by a simple power-law model.
The power law index of the time-averaged spectrum is 1.26 ± 0.28.
The fluence in the 15-150 keV band is 5.9 ± 1.1 x 1
The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/1048618/BA/.
Analysis of the initial XRT data was reported by Osborne et al. (GCN Circ. 30023). We have analysed 21 ks of XRT data for GRB 210515C, from 76 s to 178.2 ks after the BAT trigger. The data comprise 98 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode.
The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=2.63 (+0.13, -0.12), followed by a break at T+772 s to an α of 0.81 (+0.10, -0.09).
A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 3.06 (+0.24, -0.21). The best-fitting absorption column is 1.7 ± 0.4 x 1
A summary of the PC-mode spectrum is thus:
Total column: 6.7 (+5.7, -4.1) x 1
Galactic foreground: 2.6 x 1
Excess significance: <1.6 σ
Photon index: 2.12 (+0.26, -0.24)
The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01048618.
The Swift/UVOT began settled observations
(LaPorte and D'Ai GCN Circ. 30039) of the field of GRB 210515C 40 s after the BAT trigger (A.
D'Ai et al., GCN Circ. 30020).
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 |
-14°57'12.0" | 2.3" | XRT-final | UKSSDC |
0 |
-14°57'12.0" | 2.3" | XRT-refined | Osborne et al. GCN Circ. 30023 |
0 |
-14°58'18.2" | 2.0' | BAT-refined | Palmer et al. GCN Circ. 30021 |
Band | Authors | GCN Circ. | Subject | Observatory | Notes |
---|---|---|---|---|---|
Optical | Strausbaugh and Cucchiara | 30028 | LCO Optical Upper Limit | LCO | upper limits |
Filter | Exp(s) | Mag | ||
---|---|---|---|---|
b | 486 | 851 | 58 | >18.60 |
uvm2 | 758 | 5041 | 216 | >18.99 |
u | 231 | 5285 | 311 | >19.33 |
v | 561 | 4836 | 235 | >18.60 |
uvw1 | 610 | 5246 | 235 | >18.89 |
uvw2 | 4432 | 4631 | 196 | >18.90 |
white | 72 | 942 | 271 | >20.12 |
Table 3. UVOT observations reported by LaPorte and D'Ai (GCN Circ. 30039). The start and stop times of the exposures are given in seconds since the BAT trigger. The preliminary 3-σ upper limits are given. No correction has been made for extinction in the Milky Way.
May 18, 2021