J.D. Gropp (PSU), A.P. Beardmore (U. Leicester) and A.A. Breeveld (UCL-MSSL) for the Swift team
At 22:02:18 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210323A (trigger=1038247) (Gropp et al. GCN Circ. 29699). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 50° from the Sun (3.0 hours West) and 129° from the 72%-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.
Malesani et al. (GCN Circ. 29703) reported the position from NOT for the optical afterglow of this GRB. Minaev and Pozanenko (GCN Circ. 29711) determined a redshift of 0.42. 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. 29721),
the BAT ground-calculated position is RA, Dec = 317.945, 25.352 deg which is RA(J2000) = 2
The BAT lightcurve (Figure 1) showed complex structure with a duration less than 2 s.
The time-averaged spectrum from T-0.09 to T+1.26 s is best fit by a simple power-law model.
The power law index of the time-averaged spectrum is 1.46 ± 0.17.
The fluence in the 15-150 keV band is 2.4 ± 0.3 x 1
The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/1038247/BA/.
Analysis of the initial XRT data was reported by Beardmore et al. (GCN Circ. 29705). We have analysed 15 ks of XRT data for GRB 210323A, from 88 s to 81.2 ks after the BAT trigger. The data comprise 25 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 29704).
The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=1.69 (+0.17, -0.12). At T+719 s the decay flattens to an α of 0.46 (+0.09, -0.34) before breaking again at T+12.4 ks to a final decay with index α=3.0 (+1.7, -0.5).
A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.72 (+0.20, -0.19). The best-fitting absorption column is 2.9 (+1.0, -0.9) x 1
A summary of the PC-mode spectrum is thus:
Total column: 2.9 (+1.0, -0.9) x 1
Galactic foreground: 1.2 x 1
Excess significance: 3.2 σ
Photon index: 1.72 (+0.20, -0.19)
The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01038247.
The Swift/UVOT began settled observations of the field of GRB 210323A 89 s after the BAT trigger
(Breeveld and Gropp GCN Circ. 29712).
The optical afterglow detected by Malesani et al. (GCN Circ. 29703) and Pozanenko et al. (GCN Circ. 29708) and consistent with the XRT position (Evans et al. GCN Circ. 29704), is not 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 |
|---|---|---|---|---|
| 2 |
+25°22'09.5" | 2.2" | XRT-final | UKSSDC |
| 2 |
+25°22'09.5" | 1.6" | XRT-enhanced | Evans et al. GCN Circ. 29704 |
| 2 |
+25°21'08.7" | 1.4' | BAT-refined | Barthelmy et al. GCN Circ. 29721 |
| Band | Authors | GCN Circ. | Subject | Observatory | Notes |
|---|---|---|---|---|---|
| Optical | Fu et al. | 29700 | Nanshan/NEXT optical upper limit | Xinjiang Astro. Obs. | upper limits |
| Optical | Lipunov et al. | 29702 | Swift GRB 210323A: Global MASTER-Net observations report |
MASTER | |
| Optical | Malesani et al. | 29703 | NOT optical afterglow candidate | NOT | detection |
| Optical | Pozanenko et al. | 29708 | Mondy optical observations, counterpart detection |
Mondy | detection |
| Optical | Pankov et al. | 29710 | Kitab optical upper limit | upper limits | |
| Optical | de Ugarte Postigo et al. | 29717 | Short GRB 210323A: GTC Observations | GTC | |
| Optical | Rastinejad et al. | 29720 | Gemini-North Optical Source Detection | Gemini | detection |
| Optical | Tiurina et al. | 29724 | MASTER optical observation | MASTER | |
| Gamma-ray | Hamburg and Meegan | 29709 | Fermi GBM detection | Fermi GBM | Fluence=1.2±0.1x1 (8 |
| Gamma-ray | Frederiks et al. | 29713 | Konus-Wind detection of GRB 210323A (short) |
Konus-Wind | Duration=~0.45 seconds Fluence=1.57(-0.25,+0.50)x1 |
| Gamma-ray | Zhang et al. | 29726 | Insight-HXMT/HE detection | Insight-HXMT | |
| Other | Minaev and Pozanenko | 29711 | Classification and redshift estimation | redshift | |
| Other | Malesani et al. | 30000 | Short GRB 210323A: likely host galaxy detection |
| Filter | Exp(s) | Mag | ||
|---|---|---|---|---|
| whit |
89 | 239 | 147 | >20.1 |
| 302 | 552 | 246 | >19.9 | |
| white | 89 | 4948 | 569 | >20.5 |
| v | 632 | 1423 | 97 | >17.7 |
| b | 558 | 1522 | 97 | >19.3 |
| u | 302 | 1497 | 324 | >19.3 |
| w1 | 681 | 1473 | 97 | >18.5 |
| m2 | 828 | 848 | 19 | >18.8 |
| w2 | 607 | 5034 | 136 | >19.4 |
Table 3. UVOT observations reported by Breeveld and Gropp (GCN Circ. 29712). 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 13, 2021