Swift Observations of GRB 190821A

A. D'Ai (INAF-IASFPA), J.D. Gropp (PSU) and K.K. Simpson (PSU) for the Swift team

1. Introduction

At 17:10:03 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190821A (trigger=921722) (D'Ai et al. GCN Circ. 25427). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 105° from the Sun (6.7 hours East) and 139° from the 69%-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 Barthelmy et al. (GCN Circ. 25464), the BAT ground-calculated position is RA, Dec = 250.070, -34.027 deg which is RA(J2000) = 16h40m16.9s Dec(J2000) = -34°01'35.5" with an uncertainty of 1.3 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-3 s and ends at ~T+60 s. The highest peak occurs at ~T+30 s. T90 (15-350 keV) is 57.1 ± 2.6 s (estimated error including systematics).

The time-averaged spectrum from T-3.3 to T+59.5 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.03 ± 0.10. The fluence in the 15-150 keV band is 2.4 ± 0.1 x 10-6 erg cm-2. This fluence is larger than that of 65% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+29.98 s in the 15-150 keV band is 2.8 ± 0.3 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/921722/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Gropp et al. (GCN Circ. 25444). We have analysed 6.7 ks of XRT data for GRB 190821A, from 84 s to 33.9 ks after the BAT trigger. The data comprise 290 s 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. 25436).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=7.83 (+0.18, -0.87). At T+140 s the decay flattens to an α of -0.4 ± 0.5 before breaking again at T+1216 s to a final decay with index α=1.19 (+0.11, -0.10).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.71 (+0.17, -0.12). The best-fitting absorption column is 3.54 (+0.96, -0.20) x 1021 cm-2, consistent with the Galactic value of 3.3 x 1021 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.5 x 10-11 (6.0 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 3.54 (+0.96, -0.20) x 1021 cm-2
Galactic foreground: 3.3 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 1.71 (+0.17, -0.12)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00921722.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190821A 89 s after the BAT trigger (Simpson and D'Ai GCN Circ. 25450). No optical afterglow consistent with the XRT position (Goad et al. GCN Circ. 25436) 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 0.53 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
16h40m13.15s -34°00'25.5" 1.5" XRT-final UKSSDC
16h40m13.15s -34°00'25.6" 1.6" XRT-enhanced Goad et al. GCN Circ. 25436
16h40m16.9s -34°01'35.5" 1.3' BAT-refined Barthelmy et al. GCN Circ. 25464

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 25429 Swift GRB190821.72 Global MASTER-Net
observations report 		MASTER
Optical Klotz et al. 25431 T60 Les Makes optical detection of the
afterglow		 detection
Optical Lipunov et al. 25433 MASTER do not see any afterglow
candidate 		MASTER detection
Optical Balakin et al. 25448 MASTER VWFC syncronous optical
observations		 MASTER
Gamma-ray Mailyan et al. 25435 Fermi GBM observation Fermi GBM Epeak=29±6 keV
T90=60 seconds
Fluence=2.54±0.17x10-6erg cm-2
(36th percentile for 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 89 239 147 >19.1
white 89 5595 344 >19.5
v 5806 6006 197 >18.3
b 5191 5390 197 >19.9
u 303 11409 336 >19.6
w1 9373 9551 175 >18.9
m2 6011 6164 151 >19.7
w2 5602 5801 197 >20.2

Table 3. UVOT observations reported by Simpson and D'Ai (GCN Circ. 25450). 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 25, 2019