Swift Observations of GRB 190220A

K.L. Page (U. Leicester) and V. D'Elia (ASDC) for the Swift team

1. Introduction

At 02:59:34 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190220A (trigger=889785) (Page et al. GCN Circ. 23908). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 66° from the Sun (0.2 hours West) and 115° from the 100%-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.

Page et al. (GCN Circ. 23908) reported the discovery with UVOT of an optical afterglow. 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. 23914), the BAT ground-calculated position is RA, Dec = 330.128, -76.669 deg which is RA(J2000) = 22h00m30.7s Dec(J2000) = -76°40'10.1" with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 46%.

The mask-weighted light curve (Figure 1) shows a single-pulse structure that starts at ~T0, peaks at ~T+2 s, and ends at ~T+12 s. T90 (15-350 keV) is 10.0 ± 2.0 s (estimated error including systematics).

The time-averaged spectrum from T-0.12 to T+11.88 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.79 ± 0.77, and Epeak of 40.3 ± 6.7 keV (χ2 60.03 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 8.9 ± 1.0 x 10-7 erg cm-2 and the 1-s peak flux measured from T+1.88 s in the 15-150 keV band is 2.6 ± 0.4 ph cm-2 s-1. This fluence is larger than that of 36% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). A fit to a simple power law gives a photon index of 2.03 ± 0.13 (χ2 70.39 for 57 d.o.f.). 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/889785/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Elia et al. (GCN Circ. 23917). We have analysed 10 ks of XRT data for GRB 190220A, from 68 s to 120.7 ks after the BAT trigger. The data comprise 67 s in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode.

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=2.4 (+0.3, -0.4), followed by a break at T+177 s to an α of 1.08 ± 0.08.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.69 (+0.25, -0.23). The best-fitting absorption column is 3.9 (+1.5, -1.2) x 1021 cm-2, in excess of the Galactic value of 1.1 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 (6.2 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 3.9 (+1.5, -1.2) x 1021 cm-2
Galactic foreground: 1.1 x 1021 cm-2
Excess significance: 3.7 σ
Photon index: 1.69 (+0.25, -0.23)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00889785.

4. UVOT Observations and Analysis

UVOT results are not available.

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
22h00m39.14s -76°40'50.4" 0.66" UVOT-initial Page et al. GCN Circ. 23908
22h00m39.30s -76°40'52.5" 1.5" XRT-final UKSSDC
22h00m39.30s -76°40'52.5" 1.5" XRT-enhanced Evans et al. GCN Circ. 23912
22h00m30.7s -76°40'10.1" 1.3' BAT-refined Barthelmy et al. GCN Circ. 23914

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Table 2. No GCN Circulars from other observatories were found.

February 22, 2019