Swift Observations of GRB 190824A

V. D'Elia (SSDC), A. Tohuvavohu (PSU) and K.K. Simpson (PSU) for the Swift team

1. Introduction

At 14:46:39 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190824A (trigger=922107) (D'Elia et al. GCN Circ. 25463). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 78° from the Sun (4.2 hours East) and 142° from the 40%-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 Cummings et al. (GCN Circ. 25482), the BAT ground-calculated position is RA, Dec = 215.323, -41.900 deg which is RA(J2000) = 14h21m17.6s Dec(J2000) = -41°53'59.5" with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 28%.

The mask-weighted light curve (Figure 1) shows three main peaks, each with underlying substructure. The first peak is from ~T-80 s to T-60 s. The second, largest, peak is from ~T-10 s to T+50 s, and the third peak is from ~T+85 s to T+110 s. T90 (15-350 keV) is 169.4 ± 5.9 s (estimated error including systematics).

The time-averaged spectrum from T-71.9 to T+114.6 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.62 ± 0.08. The fluence in the 15-150 keV band is 8.5 ± 0.4 x 10-6 erg cm-2. This fluence is larger than that of 90.8% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.12 s in the 15-150 keV band is 5.0 ± 0.5 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/922107/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Tohuvavohu et al. (GCN Circ. 25470). We have analysed 10 ks of XRT data for GRB 190824A, from 524 s to 126.1 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 25466).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=0.45 (+0.08, -0.31), followed by a break at T+7014 s to an α of 0.99 (+0.12, -0.10).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.21 (+0.22, -0.21). The best-fitting absorption column is 1.29 (+0.24, -0.22) x 1022 cm-2, in excess of the Galactic value of 1.0 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 4.7 x 10-11 (1.1 x 10-10) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.29 (+0.24, -0.22) x 1022 cm-2
Galactic foreground: 1.0 x 1021 cm-2
Excess significance: 9.0 σ
Photon index: 2.21 (+0.22, -0.21)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00922107.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190824A 568 s after the BAT trigger (Simpson and D'Elia GCN Circ. 25479). No optical afterglow consistent with the XRT position (Osborne et al., GCN Circ. 25466) 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 0.12 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
14h21m13.19s -41°53'37.6" 1.4" XRT-final UKSSDC
14h21m13.18s -41°53'37.8" 1.5" XRT-enhanced Osborne et al. GCN Circ. 25466
14h21m17.6s -41°53'59.5" 1.7' BAT-refined Cummings et al. GCN Circ. 25482

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Gamma-ray Fermi 25462 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Fletcher and Meegan 25469 Fermi GBM observation Fermi GBM Epeak=247±55 keV
T90=109 seconds
Fluence=1.4±0.1x10-5erg cm-2
(82nd 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 568 718 147 >19.9
white 568 1721 389 >19.9
v 1058 6747 196 >17.8
b 799 1697 97 >18.9
u 774 1672 97 >19.3
w1 750 1647 97 >18.4
m2 725 1622 97 >19.6
w2 849 1742 74 >18.7

Table 3. UVOT observations reported by Simpson and D'Elia (GCN Circ. 25479). 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 27, 2019