Swift Observations of GRB 200829A

M.H. Siegel (PSU), J.D. Gropp (PSU) and N.P.M. Kuin (MSSL-UCL) for the Swift team

1. Introduction

At 13:59:34 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200829A (trigger=993768) (Siegel et al. GCN Circ. 28307). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 82° from the Sun (6.2 hours East) and 101° from the 87%-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 et al. (GCN Circ. 28307) reported the discovery with UVOT of an optical afterglow. Oates et al. (GCN Circ. 28338) determined a redshift. 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. 28325), the BAT ground-calculated position is RA, Dec = 251.136, 72.363 deg which is RA(J2000) = 16^h44^m32.7^s Dec(J2000) = +72°21'45.6" with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment). The partial coding was 88%.

The mask weighted light curve (Figure 1) shows a peak at ~ T-60 s followed by an extended emission. The duration of the initial bright peak is ~15 s. We note that Swift was slewing from T-150 s to T s. The overall structure starts at ~T-75 s and lasts till ~T+200 s. T₉₀ (15-350 keV) is 13.04 ± 2.91 s (estimated error including systematics).

The time-averaged spectrum from T-74 to T+204 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 0.83 ± 0.03. The fluence in the 15-150 keV band is 5.1 ± 0.1 x 10^-5 erg cm^-2. This fluence is larger than that of 99.5% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-60.07 s in the 15-150 keV band is 98.7 ± 1.6 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/993768/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Gropp et al. (GCN Circ. 28317). We have analysed 54 ks of XRT data for GRB 200829A, from 135 s to 957.9 ks after the BAT trigger. The data comprise 1.7 ks in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 28313).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=0.61 (+0.10, -0.23). At T+257 s the decay steepens to an α of 1.30 (+3.53, -0.29). The light curve breaks again at T+328 s to a decay with α=0.880 ± 0.019, and again at T+1148 s s to α=1.218 (+0.029, -0.028), before a final break at T+9682 s s after which the decay index is 1.71 ± 0.04.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.700 ± 0.014. The best-fitting absorption column is 9.9 ± 0.4 x 10²⁰ cm^-2, in excess of the Galactic value of 4.7 x 10²⁰ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.88 ± 0.07 and a best-fitting absorption column of 1.23 (+0.21, -0.20) 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 (4.4 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.23 (+0.21, -0.20) x 10²¹ cm^-2
Galactic foreground: 4.7 x 10²⁰ cm^-2
Excess significance: 6.3 σ
Photon index: 1.88 ± 0.07

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00993768.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200829A 138 s after the BAT trigger (Kuin and Siegel GCN Circ. 28311). 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 0.04 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

16^h44^m49.05^s +72°19'45.3" 0.61" UVOT-initial Siegel et al. GCN Circ. 28307

16^h44^m49.42^s +72°19'45.2" 1.4" XRT-final UKSSDC

16^h44^m49.45^s +72°19'45.2" 1.7" XRT-enhanced Goad et al. GCN Circ. 28313

16^h44^m32.7^s +72°21'45.6" 1.8' BAT-refined Palmer et al. GCN Circ. 28325

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Pozanenko et al. 28308 TSHAO optical observations Zeiss-1000

Optical Lipunov et al. 28309 Swift GRB 200829A: Global MASTER-Net
observations report MASTER

Optical Lipunov et al. 28315 Global MASTER-Net OT detection MASTER detection

Optical Vinko et al. 28316 optical afterglow detection Konkoly detection

Optical Hentunen and Nissinen 28318 THO optical observatios Taurus Hill Obs. detection

Optical Moskvitin and Aitov 28322 SAO RAS optical observations SAO RAS detection

Optical Zhu et al. 28324 Nanshan/NEXT optical observations Xinjiang Astro. Obs. detection

Optical Moskvitin and Aitov 28328 further SAO RAS optical observations SAO RAS detection

Optical Pankov et al. 28329 Kitab optical upper limit upper limits

Optical Zhu et al. 28330 Continued Nanshan/NEXT optical
observations Xinjiang Astro. Obs.

Optical Izzo 28331 Liverpool Telescope observations Liverpool Telescope

Optical Volnova et al. 28333 Koshka Zeiss-1000 optical observations Zeiss-1000 detection

Optical Pozanenko et al. 28359 optical observations, a light curve in
R-filter Tien Shan Astro. Obs. light curve

Optical De Pasquale et al. 28404 T100 observations T100

Gamma-ray Ursi et al. 28314 AGILE/MCAL detection AGILE

Gamma-ray Ridnaia et al. 28323 Konus-Wind detection of GRB 200829A (a
clone of GRB 200826B?) Konus-Wind E_peak=336 (-11,+11) keV
Duration=~39 seconds
Fluence=3.02(-0.07,+0.07)x10^-4erg cm^-2

Gamma-ray Lesage and Meegan 28326 Fermi GBM detection Fermi GBM E_peak=336.8±4.3 keV
T₉₀=6.9 seconds
Fluence=2.069±0.009x10^-4erg cm^-2
(99.6 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 138 288 147 14.29 ± 0.02

white 576 596 19 15.43 ± 0.04

white 1527 1547 19 16.38 ± 0.06

v 626 646 19 15.50 ± 0.10

b 552 572 19 15.68 ± 0.07

u 296 546 246 14.54 ± 0.03

u 700 720 19 15.12 ± 0.07

w1 675 696 19 15.19 ± 0.12

m2 1080 1100 19 16.25 ± 0.25

w2 1032 1398 39 17.64 ± 0.30

RA (J2000)	Dec (J2000)	Error	Note	Reference
16^h44^m49.05^s	+72°19'45.3"	0.61"	UVOT-initial	Siegel et al. GCN Circ. 28307
16^h44^m49.42^s	+72°19'45.2"	1.4"	XRT-final	UKSSDC
16^h44^m49.45^s	+72°19'45.2"	1.7"	XRT-enhanced	Goad et al. GCN Circ. 28313
16^h44^m32.7^s	+72°21'45.6"	1.8'	BAT-refined	Palmer et al. GCN Circ. 28325

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Pozanenko et al.	28308	TSHAO optical observations	Zeiss-1000
Optical	Lipunov et al.	28309	Swift GRB 200829A: Global MASTER-Net observations report	MASTER
Optical	Lipunov et al.	28315	Global MASTER-Net OT detection	MASTER	detection
Optical	Vinko et al.	28316	optical afterglow detection	Konkoly	detection
Optical	Hentunen and Nissinen	28318	THO optical observatios	Taurus Hill Obs.	detection
Optical	Moskvitin and Aitov	28322	SAO RAS optical observations	SAO RAS	detection
Optical	Zhu et al.	28324	Nanshan/NEXT optical observations	Xinjiang Astro. Obs.	detection
Optical	Moskvitin and Aitov	28328	further SAO RAS optical observations	SAO RAS	detection
Optical	Pankov et al.	28329	Kitab optical upper limit		upper limits
Optical	Zhu et al.	28330	Continued Nanshan/NEXT optical observations	Xinjiang Astro. Obs.
Optical	Izzo	28331	Liverpool Telescope observations	Liverpool Telescope
Optical	Volnova et al.	28333	Koshka Zeiss-1000 optical observations	Zeiss-1000	detection
Optical	Pozanenko et al.	28359	optical observations, a light curve in R-filter	Tien Shan Astro. Obs.	light curve
Optical	De Pasquale et al.	28404	T100 observations	T100
Gamma-ray	Ursi et al.	28314	AGILE/MCAL detection	AGILE
Gamma-ray	Ridnaia et al.	28323	Konus-Wind detection of GRB 200829A (a clone of GRB 200826B?)	Konus-Wind	E_peak=336 (-11,+11) keV Duration=~39 seconds Fluence=3.02(-0.07,+0.07)x10^-4erg cm^-2
Gamma-ray	Lesage and Meegan	28326	Fermi GBM detection	Fermi GBM	E_peak=336.8±4.3 keV T₉₀=6.9 seconds Fluence=2.069±0.009x10^-4erg cm^-2 (99.6 percentile for long GRBs)

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white	138	288	147	14.29 ± 0.02
white	576	596	19	15.43 ± 0.04
white	1527	1547	19	16.38 ± 0.06
v	626	646	19	15.50 ± 0.10
b	552	572	19	15.68 ± 0.07
u	296	546	246	14.54 ± 0.03
u	700	720	19	15.12 ± 0.07
w1	675	696	19	15.19 ± 0.12
m2	1080	1100	19	16.25 ± 0.25
w2	1032	1398	39	17.64 ± 0.30

Table 3. UVOT observations reported by Kuin and Siegel (GCN Circ. 28311). 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.

September 10, 2020