Swift Observations of GRB 210824A

K.L. Page (U. Leicester), V. D'Elia (SSDC and INAF-OAR) and S.J. LaPorte (PSU) for the Swift team

1. Introduction

At 04:10:01 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210824A (trigger=1070157) (Page et al. GCN Circ. 30700). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 77° from the Sun (5.3 hours East) and 123° from the 97%-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 Markwardt et al. (GCN Circ. 30722), the BAT ground-calculated position is RA, Dec = 232.115, 11.169 deg which is RA(J2000) = 15^h28^m27.5^s Dec(J2000) = +11°10'09.9" with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment). The partial coding was 60%.

The mask-weighted light curve (Figure 1) shows a single broad pulse with some sub-structure. It begins around T-15 s, peaks at the trigger time and decays to background by T+40 s. We note that the light curve has a gap from T+110 s to T+240 s due to the burst going out of the field of view (for a pre-planned slew that started before the trigger had been fully processed) and then coming back into FOV when the pre-planned slew ended and the automated target slew had started. The burst appears to be over before this time. T₉₀ (15-350 keV) is 37.55 ± 4.79 s (estimated error including systematics).

The time-averaged spectrum from T-13.70 to T+30.94 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.18 ± 0.11. The fluence in the 15-150 keV band is 1.7 ± 0.1 x 10^-6 erg cm^-2. This fluence is larger than that of 55% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+1.02 s in the 15-150 keV band is 1.3 ± 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/1070157/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Elia et al. (GCN Circ. 30704). We have analysed 19 ks of XRT data for GRB 210824A, from 291 s to 206.8 ks after the BAT trigger. The data comprise 8 s in Windowed Timing (WT) mode (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. 30702).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.91 ± 0.08.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.3 ± 0.4. The best-fitting absorption column is 6.7 (+3.0, -2.3) x 10²¹ cm^-2, in excess of the Galactic value of 3.3 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 3.7 x 10^-11 (8.2 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 6.7 (+3.0, -2.3) x 10²¹ cm^-2
Galactic foreground: 3.3 x 10²⁰ cm^-2
Excess significance: 4.6 σ
Photon index: 2.3 ± 0.4

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01070157.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210824A 311 s after the BAT trigger (LaPorte and Page GCN Circ. 30730). No optical afterglow consistent with the XRT position (Goad et al. GCN Circ. 30702) 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 E_B-V of 0.037 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

15^h28^m25.41^s +11°09'12.7" 2.2" XRT-final UKSSDC

15^h28^m25.40^s +11°09'12.6" 2.4" XRT-enhanced Goad et al. GCN Circ. 30702

15^h28^m27.5^s +11°10'09.9" 1.2' BAT-refined Markwardt et al. GCN Circ. 30722

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Liu et al. 30705 Nanshan/NEXT optical upper limit Xinjiang Astro. Obs. upper limits

Optical Lipunov et al. 30706 Fermi GRB 210824A: Global MASTER-Net
observations report MASTER

Optical Hu et al. 30708 BOOTES-5/JGT optical upper limit BOOTES upper limits

Optical Lipunov et al. 30709 Swift GRB 210824A: Global MASTER-Net
observations report MASTER

Optical Niwano et al. 30718 MITSuME Akeno optical upper limits MITSuME Akeno upper limits

Gamma-ray Fermi 30699 Fermi GBM Final Real-time Localization Fermi GBM

Gamma-ray Lesage 30707 Fermi GBM Observation Fermi GBM E_peak=552±286 keV
T₉₀=33 seconds
Fluence=5.3±0.6x10^-6erg cm^-2
(58^th percentile for long GRBs)

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

Filter T_start(s) T_stop(s) Exp(s) Mag

white_FC 311 461 147 >20.8

white 311 4350 364 >20.9

v 467 4760 236 >19.4

b 565 4145 235 >20.1

u 541 5220 83 >19.6

w1 516 5170 236 >20.4

m2 4765 4965 197 >20.3

w2 4356 4555 197 >19.9

RA (J2000)	Dec (J2000)	Error	Note	Reference
15^h28^m25.41^s	+11°09'12.7"	2.2"	XRT-final	UKSSDC
15^h28^m25.40^s	+11°09'12.6"	2.4"	XRT-enhanced	Goad et al. GCN Circ. 30702
15^h28^m27.5^s	+11°10'09.9"	1.2'	BAT-refined	Markwardt et al. GCN Circ. 30722

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Liu et al.	30705	Nanshan/NEXT optical upper limit	Xinjiang Astro. Obs.	upper limits
Optical	Lipunov et al.	30706	Fermi GRB 210824A: Global MASTER-Net observations report	MASTER
Optical	Hu et al.	30708	BOOTES-5/JGT optical upper limit	BOOTES	upper limits
Optical	Lipunov et al.	30709	Swift GRB 210824A: Global MASTER-Net observations report	MASTER
Optical	Niwano et al.	30718	MITSuME Akeno optical upper limits	MITSuME Akeno	upper limits
Gamma-ray	Fermi	30699	Fermi GBM Final Real-time Localization	Fermi GBM
Gamma-ray	Lesage	30707	Fermi GBM Observation	Fermi GBM	E_peak=552±286 keV T₉₀=33 seconds Fluence=5.3±0.6x10^-6erg cm^-2 (58^th percentile for long GRBs)

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white_FC	311	461	147	>20.8
white	311	4350	364	>20.9
v	467	4760	236	>19.4
b	565	4145	235	>20.1
u	541	5220	83	>19.6
w1	516	5170	236	>20.4
m2	4765	4965	197	>20.3
w2	4356	4555	197	>19.9

Table 3. UVOT observations reported by LaPorte and Page (GCN Circ. 30730). 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.

August 28, 2021