S. Dichiara (NASA/GSFC/UMCP), K.L. Page (U. Leicester) and M.H. Siegel (PSU) for the Swift team
At 07:13:51 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210321A (trigger=1037828) (Dichiara et al. GCN Circ. 29677). Swift did not immediately slew due to an observing constraint. At the time of the trigger, the initial BAT position was 89° from the Sun (5.8 hours East) and 45° from the 47%-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.
Siegel and Dichiara (GCN Circ. 29679) reported the detection with UVOT of an optical afterglow. (GCN Circ. 29694) reported the position from Deca-Degree Optical Imager for the optical afterglow of this GRB. de Ugarte Postigo et al. (GCN Circ. 29685) determined a redshift of 1.487 from GTC. 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 Ukwatta et al. (GCN Circ. 29691),
the BAT ground-calculated position is RA, Dec = 87.887, 70.109 deg which is RA(J2000) = 0
The mask-weighted light curve (Figure 1) shows a single, fairly symmetrical peak from approximately T-8 to T+4 s.
The time-averaged spectrum from T-7.00 to T+1.84 s is best fit by a simple power-law model.
The power law index of the time-averaged spectrum is 2.15 ± 0.20.
The fluence in the 15-150 keV band is 6.7 ± 0.8 x 1
The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/1037828/BA/.
Analysis of the initial XRT data was reported by Page et al. (GCN Circ. 29686). We have analysed 12 ks of XRT data for GRB 210321A, from 2.7 ks to 147.5 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. 29681).
The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.56 (+0.12, -0.10).
A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.00 (+0.18, -0.17). The best-fitting absorption column is 4.8 (+3.4, -2.9) x 1
A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.1 x 1
Intrinsic column: 4.8 (+3.4, -2.9) x 1
Photon index: 2.00 (+0.18, -0.17)
The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01037828.
The Swift/UVOT began settled observations of the field of GRB 210321A 2735 s after the BAT trigger
(Siegel and Dichiara GCN Circ. 29679).
A source consistent with the XRT position (Perri et al., GCN Circ. 29678) 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
The Swift/UVOT began settled observations of the field of GRB 210321A 2735 s after the BAT trigger
(Laporte and Dichiara GCN Circ. 29697).
A source consistent with the XRT position (Beardmore et al. GCN Circ. 29681) 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 |
+70°07'49.8" | 0.49" | UVOT-refined | Laporte and Dichiara GCN Circ. 29697 |
| 0 |
+70°07'50.0" | 1.5" | XRT-final | UKSSDC |
| 0 |
+70°07'50.1" | 1.7" | XRT-enhanced | Beardmore et al. GCN Circ. 29681 |
| 0 |
+70°06'33.4" | 1.6' | BAT-refined | Ukwatta et al. GCN Circ. 29691 |
| Band | Authors | GCN Circ. | Subject | Observatory | Notes |
|---|---|---|---|---|---|
| Optical | Butler et al. | 29680 | DDOTI Afterglow Confirmation | Deca-Degree Optical Imager | detection |
| Optical | Lipunov et al. | 29683 | MASTER optical observation | MASTER | |
| Optical | Heintz et al. | 29684 | NOT optical afterglow detection and redshift limit |
NOT | redshift |
| Optical | de Ugarte Postigo et al. | 29685 | Redshift from OSIRIS/GTC | GTC | redshift |
| Optical | Xin et al. | 29687 | Xinglong GWAC-F60A optical observation | Xinglong Obs. | detection |
| Optical | Pankov et al. | 29689 | Mondy optical observations | Mondy | detection |
| Optical | Pankov et al. | 29692 | CrAO optical upper limit | CrAO | upper limits |
| Optical | 29694 | DDOTI Position Correction | Deca-Degree Optical Imager | ||
| Optical | Kann et al. | 29725 | OSN Upper Limit | Obs.de Sierra Nevada | upper limits |
| Gamma-ray | Frederiks et al. | 29696 | Konus-Wind detection | Konus-Wind |
| Filter | Exp(s) | Mag | ||
|---|---|---|---|---|
| white | 2735 | 2885 | 147 | 18.69 ± 0.09 |
| v | 2892 | 4465 | 331 | 18.44 ± 0.16 |
| b | 3713 | 3913 | 197 | 18.64 ± 0.11 |
| u | 3507 | 3706 | 197 | 18.29 ± 0.12 |
| w1 | 3302 | 3502 | 197 | 18.48 ± 0.17 |
| m2 | 3097 | 3296 | 197 | >18.9 |
| w2 | 4124 | 4324 | 197 | >19.8 |
Table 3. UVOT observations reported by Laporte and Dichiara (GCN Circ. 29697). 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.
March 25, 2021