Swift Observations of GRB 190320A

J.D. Gropp (PSU), K.L. Page (U. Leicester) and F.E. Marshall (NASA/GSFC) for the Swift team

1. Introduction

At 01:14:36 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190320A (trigger=893808) (Gropp et al. GCN Circ. 23973). Swift did not slew immediately due to an observing constraint. At the time of the trigger, the initial BAT position was 109° from the Sun (7.9 hours East) and 71° from the 99%-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.

Lipunov et al. (GCN Circ. 23979) reported the position from MASTER for the optical afterglow of this GRB. 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 Krimm et al. (GCN Circ. 23980), the BAT ground-calculated position is RA, Dec = 117.824, -45.876 deg which is RA(J2000) = 07h51m17.8s Dec(J2000) = -45°52'33.3" with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment). The partial coding was 29%.

The mask-weighted light curve (Figure 1) shows a single episode with some superimposed structure from roughly T-25 to T+40 s. The spacecraft slewed away from the burst position at around T+500 s. T90 (15-350 keV) is 64.0 ± 12.1 s (estimated error including systematics).

The time-averaged spectrum from T-27.20 to T+44.80 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.20 ± 0.20. The fluence in the 15-150 keV band is 2.7 ± 0.3 x 10-6 erg cm-2. This fluence is larger than that of 69% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.20 s in the 15-150 keV band is 1.3 ± 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/893808/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Page et al. (GCN Circ. 23981). We have analysed 17 ks of XRT data for GRB 190320A, from 2.8 ks to 176.1 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. 23977).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.21 (+0.12, -0.11).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.13 (+0.32, -0.25). The best-fitting absorption column is 3.6 (+1.5, -0.5) x 1021 cm-2, consistent with the Galactic value of 3.1 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 3.5 x 10-11 (5.8 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 3.6 (+1.5, -0.5) x 1021 cm-2
Galactic foreground: 3.1 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 2.13 (+0.32, -0.25)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00893808.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190320A 2834 s after the BAT trigger (Marshall and Gropp GCN Circ. 23982). No optical afterglow consistent with the XRT position (Lipunov et al. GCN Circ. 23976) 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 0.23 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
07h51m23.66s -45°53'16.1" 1.5" XRT-final UKSSDC
07h51m23.55s -45°53'16.3" 4.5" XRT-enhanced Osborne et al. GCN Circ. 23977
07h51m17.8s -45°52'33.3" 1.9' BAT-refined Krimm et al. GCN Circ. 23980

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 23975 SWIFT GRB190320.05 Global MASTER-Net
observations report		 MASTER
Optical Lipunov et al. 23976 MASTER OT detection MASTER detection
Optical Lipunov et al. 23979 MASTER GRB counterpart decay MASTER detection
Gamma-ray von Kienlin et al. 23978 Fermi GBM observation Fermi GBM Epeak=314±35 keV
T90=43 seconds
Fluence=6.7±0.4x10-6erg cm-2
(65th 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 2834 2984 147 >20.0
white 2834 16112 1213 >21.4
v 2992 21862 1645 >20.4
b 3812 15216 1082 >20.8
u 3607 3807 197 >19.6
w1 3402 3602 197 >19.5
m2 3196 22200 524 >19.8
w2 4223 20949 1776 >20.1

Table 3. UVOT observations reported by Marshall and Gropp (GCN Circ. 23982). 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.

March 22, 2019