Swift Observations of GRB 190103B

A.Y. Lien (GSFC/UMBC), B. Sbarufatti (INAF-OAB/PSU) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 21:03:51 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190103B (trigger=881709) (Lien et al. GCN Circ. 23602). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 89° from the Sun (4.8 hours West) and 70° from the 5%-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 Markwardt et al. (GCN Circ. 23606), the BAT ground-calculated position is RA, Dec = 212.561, 35.254 deg which is RA(J2000) = 14h10m14.6s Dec(J2000) = +35°15'13.4" with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 6%.

The mask-weighted light curve (Figure 1) shows several overlapping pulses that starts at ~T-10 s and ends at ~T+16 s. The main peak occurs at ~T0. T90 (15-350 keV) is 19.5 ± 2.7 s (estimated error including systematics).

The time-averaged spectrum from T-10.43 to T+16.18 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.99 ± 0.29, and Epeak of 85.0 ± 18.6 keV (χ2 58.41 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 1.2 ± 0.1 x 10-5 erg cm-2 and the 1-s peak flux measured from T+0.60 s in the 15-150 keV band is 16.3 ± 1.5 ph cm-2 s-1. This fluence is larger than that of 93.7% 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 1.63 ± 0.06 (χ2 74.91 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/881709/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Sbarufatti et al. (GCN Circ. 23604). We have analysed 32 ks of XRT data for GRB 190103B, from 99 s to 498.7 ks after the BAT trigger. The data comprise 437 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 23603).

The late-time light curve (Figure 2) (from T0+4.2 ks) can be modelled with a power-law decay with a decay index of α=1.21 ± 0.04.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.38 ± 0.06. The best-fitting absorption column is 4.27 (+0.28, -0.27) x 1021 cm-2, in excess of the Galactic value of 1.4 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.93 (+0.10, -0.09) and a best-fitting absorption column of 4.1 ± 0.5 x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.0 x 10-11 (6.1 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 4.1 ± 0.5 x 1021 cm-2
Galactic foreground: 1.4 x 1020 cm-2
Excess significance: 13.7 σ
Photon index: 1.93 (+0.10, -0.09)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00881709.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190103B 103 s after the BAT trigger (Breeveld and Lien GCN Circ. 23684). No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 23603) 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 0.01 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
14h10m16.57s +35°15'39.7" 1.4" XRT-final UKSSDC
14h10m16.63s +35°15'39.6" 1.7" XRT-enhanced Evans et al. GCN Circ. 23603
14h10m14.6s +35°15'13.4" 1.3' BAT-refined Markwardt et al. GCN Circ. 23606

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Quadri 23607 Bassano Bresciano observatory upper
limit 		Bassano Bresciano Obs. upper limits
Gamma-ray Tsvetkova et al. 23612 Konus-Wind observation Konus-Wind Epeak=142 (-9,+11) keV
Duration=~27 seconds
Fluence=1.96(-0.08,+0.08)x10-5erg cm-2
Gamma-ray Tezuka et al. 23631 CALET Gamma-Ray Burst Monitor detection CALET

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 103 253 147 >20.5
uFC 316 565 246 >19.5
white 103 23827 1860 >21.8
v 646 40393 2202 >20.6
b 571 46547 2987 >21.5
u 316 46101 3922 >21.3
w1 4776 45188 3715 >21.2
m2 4570 41036 2792 >21.1
w2 4161 39480 1904 >21.0

Table 3. UVOT observations reported by Breeveld and Lien (GCN Circ. 23684). 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.

January 15, 2019