Swift Observations of GRB 211024B

J.D. Gropp (PSU), P.A. Evans (U. Leicester) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 22:20:36 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 211024B (trigger=1081073) (Gropp et al. GCN Circ. 30980). Swift slewed immediately to the burst, but immediately slewed away due to a conflicting observation. At the time of the trigger, the initial BAT position was 65° from the Sun (3.6 hours West) and 69° from the 84%-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.

Zhu et al. (GCN Circ. 30984) reported the position from Xinjiang Astro. Obs. for the optical afterglow of this GRB. Kuin and Gropp (GCN Circ. 31005) reported the detection with UVOT of an optical afterglow. de Ugarte Postigo et al. (GCN Circ. 31800) determined a redshift of 1.1137 from VLT. 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 Lien et al. (GCN Circ. 30989), the BAT ground-calculated position is RA, Dec = 154.708, 24.566 deg which is RA(J2000) = 10h18m49.9s Dec(J2000) = +24°33'57.3" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows three episodes. The first episode starts from T-60 s, peaks at T+10 s and ends at T+100 s. The second, and weakest, episode runs from T+250 to T+400 s and the third episode starts around T+500 s, peaks at T+580 s and ends at T+620 s. The burst came into the BAT field of view around T-100 s during a pre-planned slew. T90 (15-350 keV) is 603.5 ± 20.7 s (estimated error including systematics).

The time-averaged spectrum from T-56.37 to T+613.08 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.63 ± 0.09. The fluence in the 15-150 keV band is 6.8 ± 0.3 x 10-6 erg cm-2. This fluence is larger than that of 87% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+5.17 s in the 15-150 keV band is 0.9 ± 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/1081073/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 30994). We have analysed 71 ks of XRT data for GRB 211024B, from 105 s to 3803.6 ks after the BAT trigger. The data comprise 6 s in Windowed Timing (WT) mode (taken while Swift was slewing), with the remainder in Photon Counting (PC) mode.

The late-time light curve (Figure 2) (from T0+5.6 ks) can be modelled with a power-law decay with a decay index of α=2.80 (+0.18, -0.15).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.14 (+0.08, -0.07). The best-fitting absorption column is 9.2 ± 1.7 x 1020 cm-2, in excess of the Galactic value of 2.1 x 1020 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.0 x 10-11 (3.9 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 9.2 ± 1.7 x 1020 cm-2
Galactic foreground: 2.1 x 1020 cm-2
Excess significance: 7.1 σ
Photon index: 2.14 (+0.08, -0.07)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01081073.

4. UVOT Observations and Analysis

UVOT results are not available.

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
10h18m51.04s +24°34'05.7" 1.9" XRT-final UKSSDC
10h18m51.04s +24°34'05.7" 1.9" XRT-refined Evans et al. GCN Circ. 30994
10h18m49.9s +24°33'57.3" 1.0' BAT-refined Lien et al. GCN Circ. 30989

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 30981 Swift GRB 211024B: Global MASTER-Net
observations report 		MASTER
Optical Zhu et al. 30984 Nanshan/NEXT optical afterglow detection Xinjiang Astro. Obs. detection
Optical Kann et al. 30995 CAHA 2.2m Detection; ultra-long GRB? CAHA detection
Optical Jelinek et al. 31028 Optical afterglow observation by
Ondrejov D50		 D50 detection
Optical de Ugarte Postigo et al. 31800 Host galaxy redshift from X-shooter/VLT VLT redshift

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
v 6237 6437 197 19.2 ± 0.4
v 12395 24343 1665 20.4 ± 0.4
b 5826 5927 197 20.0 ± 0.4
u 5621 5821 197 18.8 ± 0.2
u 6852 7052 197 19.3 ± 0.3
w1 6647 6847 197 19.2 ± 0.3
m2 6442 6642 197 19.3 ± 0.3
w2 6032 6232 197 20.1 ± 0.5
w2 7260 7360 98 19.5 ± 0.5

Table 3. UVOT observations reported by Kuin and Gropp (GCN Circ. 31005). 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 29, 2022