Swift Observations of GRB 190422A

M.J. Moss (George Washington University), D.N. Burrows (PSU) and K.K. Simpson (PSU) for the Swift team

1. Introduction

At 22:55:29 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190422A (trigger=899979) (Moss et al. GCN Circ. 24147). Swift did not slew until ~T+8 min due to an observing constraint. At the time of the trigger, the initial BAT position was 128° from the Sun (10.1 hours East) and 65° from the 86%-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 Sakamoto et al. (GCN Circ. 24157), the BAT ground-calculated position is RA, Dec = 182.126, -60.167 deg which is RA(J2000) = 12h08m30.3s Dec(J2000) = -60°10'02.6" with an uncertainty of 2.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 82%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure. The emission started before the source entered the BAT FOV, and lasted until ~T+360 s. The main peak occurred at ~ T+175 s. We cannot determine T90 since the available data do not cover the entire burst emission.

The time-averaged spectrum from T+6.39 to T+359.48 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.76 ± 0.08. The fluence in the 15-150 keV band is 6.5 ± 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+174.47 s in the 15-150 keV band is 2.6 ± 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/899979/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Burrows et al. (GCN Circ. 24155). We have analysed 12 ks of XRT data for GRB 190422A, from 536 s to 65.4 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 24151).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=3.8 (+1.1, -1.0). At T+839 s the decay flattens to an α of -0.05 (+0.08, -0.57) before breaking again at T+9484 s to a final decay with index α=0.92 ± 0.10.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.05 (+0.15, -0.14). The best-fitting absorption column is 1.69 (+0.21, -0.19) x 1022 cm-2, in excess of the Galactic value of 8.6 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 5.5 x 10-11 (1.2 x 10-10) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.69 (+0.21, -0.19) x 1022 cm-2
Galactic foreground: 8.6 x 1021 cm-2
Excess significance: 7.0 σ
Photon index: 2.05 (+0.15, -0.14)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00899979.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190422A 532 s after the BAT trigger (Simpson and Moss GCN Circ. 24159). No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 24151) 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 1.58 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
12h08m08.46s -60°13'27.6" 1.4" XRT-final UKSSDC
12h08m08.28s -60°13'27.3" 1.8" XRT-enhanced Evans et al. GCN Circ. 24151
12h08m30.3s -60°10'02.6" 2.4' BAT-refined Sakamoto et al. GCN Circ. 24157

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 24148 Fermi GRB190422.67 Global MASTER-Net
observations report 		MASTER
Optical Lipunov et al. 24150 Swift GRB190422.96 Global MASTER-Net
observations report 		MASTER
Gamma-ray Hamburg et al. 24161 Fermi GBM detection Fermi GBM Epeak=58±7 keV
T90=212 seconds
Fluence=4.8±0.3x10-6erg cm-2
(56th 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 532 682 147 >18.8
white 532 7906 647 >18.8
v 689 6879 352 >17.2
b 788 7700 352 >18.3
u 763 7495 352 >18.9
w1 738 7289 352 >19.1
m2 713 7084 352 >19.1
w2 838 7995 438 >19.6

Table 3. UVOT observations reported by Simpson and Moss (GCN Circ. 24159). 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.

April 24, 2019