Swift Observations of GRB 190114B

M.H. Siegel (PSU) and A. D'Ai (INAF-IASFPA) for the Swift team

1. Introduction

At 19:31:12 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190114B (trigger=883818) (Siegel et al. GCN Circ. 23686). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 125° from the Sun (8.1 hours West) and 137° from the 55%-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.

Pozanenko et al. (GCN Circ. 23696) reported the position from Mondy 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 Cummings et al. (GCN Circ. 23721), the BAT ground-calculated position is RA, Dec = 174.868, 18.882 deg which is RA(J2000) = 11h39m28.3s Dec(J2000) = +18°52'53.5" with an uncertainty of 2.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 63%.

The mask-weighted light curve (Figure 1) shows a weak pulse that starts at ~T0 and ends at ~T+30 s. The possible precursor seen at ~T-75 s (Siegel et al., GCN Circ. 23686) turns out to be purely instrumental and is not related to this GRB. T90 (15-350 keV) is 26.5 ± 6.7 s (estimated error including systematics).

The time-averaged spectrum from T-1.2 to T+27.9 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.98 ± 0.28. The fluence in the 15-150 keV band is 5.1 ± 1.0 x 10-7 erg cm-2. This fluence is larger than that of 21% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+3.62 s in the 15-150 keV band is 0.5 ± 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/883818/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Ai et al. (GCN Circ. 23730). We have analysed 5.9 ks of XRT data for GRB 190114B, from 165 s to 173.9 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 23691).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=2.1 ± 0.4.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.99 (+0.34, -0.14). The best-fitting absorption column is consistent with the Galactic value of 1.7 x 1020 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.2 x 10-11 (3.4 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.7 ± 4.8 x 1020 cm-2
Galactic foreground: 1.7 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 1.99 (+0.34, -0.14)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00883818.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190114B 158 s after the BAT trigger (Siegel GCN Circ. 23720). No optical afterglow consistent with the enhanced XRT position (Goad et al. GCN Circ. 23691) or the position of the faint optical counterpart (Fernandez-Garcia et al., GCN Circ. 23694; Pozanenko et al., GCN Circ. 23696, Selsing et al. GCN Circ. 23713) 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 0.02 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
11h39m29.81s +18°53'26.9" 2.3" XRT-final UKSSDC
11h39m29.81s +18°53'26.9" 2.3" XRT-enhanced Goad et al. GCN Circ. 23691
11h39m28.3s +18°52'53.5" 2.3' BAT-refined Cummings et al. GCN Circ. 23721

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Fernandez-Garcia et al. 23694 BOOTES-4/MET optical afterglow detection BOOTES-4 detection
Optical Pozanenko et al. 23696 Mondy optical afterglow detection Mondy detection
Optical Xin et al. 23703 GWAC-F60B upper limit Xinglong Obs. upper limits
Optical Itoh et al. 23705 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Selsing et al. 23713 NOT optical counterpart NOT detection
Optical Volnova et al. 23722 optical afterglow observations in Mondy
and Abastumani observatories		 Abastumani detection
Optical Blazek et al. 23723 Liverpool Telescope near-IR observations Liverpool Telescope detection
Optical Volnova et al. 23743 continued optical observations in Mondy Mondy marginal detection

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

Filter Tstart(s) Tstop(s) Exp(s) Mag
uFC 158 408 246 >20.2
v 464 1976 175 >18.5
b 414 2075 214 >19.9
u 158 2050 568 >20.4
w1 513 2025 194 >19.7
m2 786 2000 117 >18.9
w2 440 2100 194 >19.8

Table 3. UVOT observations reported by Siegel (GCN Circ. 23720). 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 17, 2019