V. D'Elia (SSDC) and K.K. Simpson (PSU) for the Swift team
At 03:53:57 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210306A (trigger=1035994) (D'Elia et al. GCN Circ. 29597). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 119° from the Sun (9.6 hours East) and 127° from the 49%-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.
D'Elia et al. (GCN Circ. 29597) reported the discovery 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.
As reported by Laha et al. (GCN Circ. 29617),
the BAT ground-calculated position is RA, Dec = 129.964, 60.203 deg which is RA(J2000) = 0
The BAT light curve (Figure 1) showed a complex structure with a duration of about ~10 s.
The time-averaged spectrum from T-0.18 to T+13.16 s is best fit by a power law with an exponential cutoff.
This fit gives a photon index 1.16 ± 0.19, and
The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/1035994/BA/.
Analysis of the initial XRT data was reported by D'Elia et al. (GCN Circ. 29622). We have analysed 19 ks of XRT data for GRB 210306A, from 65 s to 417.9 ks after the BAT trigger. The data comprise 576 s in Windowed Timing (WT) mode (the first 7 s were taken while Swift was slewing) 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=0.435 ± 0.027, followed by a break at T+1636 s to an α of 1.61 (+0.06, -0.05).
A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.89 (+0.05, -0.04). The best-fitting absorption column is 1.53 (+0.15, -0.14) x 1
A summary of the PC-mode spectrum is thus:
Total column: 1.7 ± 0.4 x 1
Galactic foreground: 4.2 x 1
Excess significance: 5.2 σ
Photon index: 1.98 (+0.14, -0.13)
The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01035994.
The Swift/UVOT began settled observations of the field of GRB 210306A 85 s after the BAT trigger
(Simpson and D'Elia GCN Circ. 29636).
A source 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 |
+60°12'19.2" | 0.42" | UVOT-refined | Simpson and D'Elia GCN Circ. 29636 |
| 0 |
+60°12'18.9" | 1.9" | XRT-final | UKSSDC |
| 0 |
+60°12'18.8" | 1.4" | XRT-refined | D'Elia et al. GCN Circ. 29622 |
| 0 |
+60°12'11.5" | 1.0' | BAT-refined | Laha et al. GCN Circ. 29617 |
| Band | Authors | GCN Circ. | Subject | Observatory | Notes |
|---|---|---|---|---|---|
| Optical | Watson et al. | 29598 | DDOTI Afterglow Confirmation | Deca-Degree Optical Imager | detection |
| Optical | 29599 | iTelescope optical afterglow observations |
iTelescope | ||
| Optical | Strausbaugh and Cucchiara | 29600 | LCO Optical Afterglow Detection | LCO | detection |
| Optical | Hentunen and Nissinen | 29603 | iTelescope T24 optical observations | iTelescope | detection |
| Optical | Stecklum et al. | 29604 | Tautenburg observations | Tautenburg | |
| Optical | Lipunov et al. | 29606 | Swift GRB 210306A: Global MASTER-Net observations report |
MASTER | |
| Optical | Zhu et al. | 29607 | NEXT optical observations | Xinjiang Astro. Obs. | detection |
| Optical | Hosokawa et al. | 29608 | MITSuME Akeno optical observation | MITSuME Akeno | marginal detection |
| Optical | Belkin et al. | 29616 | Mondy optical observations | Mondy | detection |
| Optical | Dimple et al. | 29618 | 1.3m DFOT observations | DFOT | detection |
| Optical | Belkin et al. | 29620 | continued Mondy optical observations | Mondy | |
| Optical | Belkin et al. | 29635 | continued Mondy optical observations | Mondy | detection |
| Optical | Smith et al. | 29638 | Early Spectroscopy of the Optical Afterglow |
spectroscopy | |
| Optical | Klose et al. | 29644 | Tautenburg observations | Tautenburg | |
| Optical | Rossi | 29645 | LBT observations | LBT | detection |
| Optical | Kumar et al. | 29654 | HCT optical upper limit | Himalayan Chandra Telescope | upper limits |
| Optical | Kumar et al. | 29657 | GIT optical follow-up | GROWTH-India | detection |
| Gamma-ray | Ohno et al. | 29602 | Fermi-LAT detection | Fermi LAT | |
| Gamma-ray | Veres | 29605 | Fermi GBM observation | Fermi GBM | Fluence=9.82±0.20x1 (7 |
| Gamma-ray | Ridnaia et al. | 29642 | Konus-Wind detection | Konus-Wind | Duration=~11 seconds Fluence=7.01(-0.45,+0.48)x1 |
| Filter | Exp(s) | Mag | ||
|---|---|---|---|---|
| white | 85 | 235 | 147 | 16.23 ± 0.03 |
| v | 628 | 648 | 20 | 17.42 ± 0.24 |
| b | 553 | 573 | 20 | 17.32 ± 0.13 |
| u | 297 | 547 | 246 | 16.86 ± 0.04 |
Table 3. UVOT observations reported by Simpson and D'Elia (GCN Circ. 29636). 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 27, 2021