Swift Observations of GRB 211129A

J.D. Gropp (PSU), J.A. Kennea (PSU) and A. Belles (PSU) for the Swift team

1. Introduction

At 09:51:06 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 211129A (trigger=1085430) (Gropp et al. GCN Circ. 31136). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 60° from the Sun (1.9 hours East) and 90° from the 31%-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.

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.

2. BAT Observations and Analysis

As reported by Palmer et al. (GCN Circ. 31149), the BAT ground-calculated position is RA, Dec = 274.565, 31.781 deg which is RA(J2000) = 18h18m15.6s Dec(J2000) = +31°46'51.5" with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment). The partial coding was 83%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T-2 s and ends at ~T+120 s. The three main peaks occur at ~T+1 s, ~T+29 s, and ~T+91 s, respectively. T90 (15-350 keV) is 113.01 ± 9.14 s (estimated error including systematics).

The time-averaged spectrum from T-1.80 to T+124.11 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.75 ± 0.13. The fluence in the 15-150 keV band is 2.3 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 63% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.99 s in the 15-150 keV band is 1.5 ± 0.2 ph cm-2 s-1. All the quoted errors are at the 90% confidence level.

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/1085430/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Kennea et al. (GCN Circ. 31148). We have analysed 15 ks of XRT data for GRB 211129A, from 62 s to 137.8 ks after the BAT trigger. The data comprise 325 s in Windowed Timing (WT) mode (the first 9 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 31139).

The late-time light curve (Figure 2) (from T0+4.5 ks) can be modelled with a power-law decay with a decay index of α=1.17 ± 0.05.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.63 ± 0.05. The best-fitting absorption column is 4.8 ± 0.3 x 1021 cm-2, in excess of the Galactic value of 8.8 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.30 (+0.16, -0.15) and a best-fitting absorption column of 5.9 (+0.9, -0.8) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10-11 (7.5 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 5.9 (+0.9, -0.8) x 1021 cm-2
Galactic foreground: 8.8 x 1020 cm-2
Excess significance: 10.1 σ
Photon index: 2.30 (+0.16, -0.15)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01085430.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 211129A 84 s after the BAT trigger (Belles and Gropp GCN Circ. 31147). No optical afterglow consistent with the XRT position (Goad et al. GCN Circ. 31139) 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 EB-V of 0.081 mag. in the direction of the GRB (Schlegel et al. 1998).

BAT light curve

Figure 1. The BAT mask-weighted light curve in the four individual and total energy bands. The units are counts s-1 illuminated-detector-1. The vertical green dash-dotted lines show the T50 interval, the vertical black dashed lines show the T90 interval, and vertical blue (orange) solid lines show the start (stop) of slews.

XRT light curve

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
18h18m19.26s +31°47'20.1" 2.0" XRT-final UKSSDC
18h18m19.27s +31°47'19.9" 2.1" XRT-enhanced Goad et al. GCN Circ. 31139
18h18m15.6s +31°46'51.5" 1.5' BAT-refined Palmer et al. GCN Circ. 31149

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Yamaguchi et al. 31137 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Zhu et al. 31140 Nanshan/NEXT optical upper limit Xinjiang Astro. Obs. upper limits
Optical Lipunov et al. 31141 Swift GRB 211129A: Global MASTER-Net
observations report 		MASTER
Gamma-ray Bissaldi 31142 Fermi GBM observation Fermi GBM Epeak=220±90 keV
T90=111 seconds
Fluence=4.3±0.4x10-6erg cm-2
(brighter than 53% of long GRBs)

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

Filter Tstart(s) Tstop(s) Exp(s) Mag
whiteFC 84 234 147 >20.7
uFC 297 547 246 >20.4
white 84 769 186 >20.9
v 626 818 39 >18.8
b 552 744 39 >19.6
u 297 719 265 >20.5
w1 675 695 19 >19.6
m2 651 831 28 >19.2
w2 602 794 39 >20.1

Table 3. UVOT observations reported by Belles and Gropp (GCN Circ. 31147). 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.

December 1, 2021