Swift Observations of GRB 190211A

F.E. Marshall (NASA/GSFC) and P.A. Evans (U. Leicester) for the Swift team

1. Introduction

At 16:54:44 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190211A (trigger=888648) (Marshall et al. GCN Circ. 23883). Swift slewed after a short delay to the burst. At the time of the trigger, the initial BAT position was 127° from the Sun (8.5 hours West) and 125° from the 37%-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.

Xu et al. (GCN Circ. 23884) reported the position for the optical afterglow of this GRB. Marshall (GCN Circ. 23894) reported the detection 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 Stamatikos et al. (GCN Circ. 23887), the BAT ground-calculated position is RA, Dec = 196.652, 41.976 deg which is RA(J2000) = 13h06m36.5s Dec(J2000) = +41°58'32.1" with an uncertainty of 2.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 26%.

The mask-weighted light curve (Figure 1) shows a a pair of peaks, the first from T-12 to T-5 s, and the second from T-4 to T+2 s. T90 (15-350 keV) is 12.48 ± 1.46 s (estimated error including systematics).

The time-averaged spectrum from T-11.49 to T+1.66 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.51 ± 0.33. The fluence in the 15-150 keV band is 6.5 ± 1.2 x 10-7 erg cm-2. This fluence is larger than that of 27% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.56 s in the 15-150 keV band is 1.9 ± 0.4 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/888648/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 23888). We have analysed 22 ks of XRT data for GRB 190211A, from 205 s to 271.2 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode.

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=0.13 (+0.25, -0.18), followed by a break at T+1099 s to an α of 1.16 (+0.07, -0.04).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.93 (+0.11, -0.10). The best-fitting absorption column is 6.9 (+2.6, -2.4) x 1020 cm-2, in excess of 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 3.4 x 10-11 (3.9 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 6.9 (+2.6, -2.4) x 1020 cm-2
Galactic foreground: 1.7 x 1020 cm-2
Excess significance: 3.6 σ
Photon index: 1.93 (+0.11, -0.10)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00888648.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190211A 203 s after the BAT trigger (Marshall GCN Circ. 23894). A source consistent with the optical position (Xu et al. GCN Circ. 23884) and just outside the 90% confidence XRT position (Evans et al. GCN Circ. 23888) is detected in the only UVOT exposure. 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
13h06m38.45s +41°58'04.9" 0.53" UVOT-refined Marshall GCN Circ. 23894
13h06m38.10s +41°58'01.5" 3.5" XRT-final UKSSDC
13h06m38.10s +41°58'01.5" 3.5" XRT-refined Evans et al. GCN Circ. 23888
13h06m36.5s +41°58'32.1" 2.3' BAT-refined Stamatikos et al. GCN Circ. 23887

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Xu et al. 23884 NEXT-0.6m optical afterglow detection Xinjiang Astro. Obs. detection
Optical Lipunov et al. 23885 MASTER Global Robotic Net optical
observation 		MASTER
Optical Hu et al. 23886 BOOTES-4/MET optical afterglow
confirmation		 BOOTES-4 detection
Optical Heintz et al. 23890 NOT optical observations NOT detection
Optical Oeda et al. 23891 MITSuME Akeno optical observation MITSuME Akeno
Optical Paek et al. 23893 DOAO optical observation DOAO detection
Optical Stecklum and Klose 23895 Tautenburg observations Tautenburg detection
Optical Kumar et al. 23896 GROWTH-India detection of optical
afterglow		 GROWTH-India 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
white 203 351 146 19.73 ± 0.11

Table 3. UVOT observation reported by Marshall (GCN Circ. 23894). The start and stop time of the exposure are given in seconds since the BAT trigger. The preliminary detection is given. No correction has been made for extinction in the Milky Way.

February 15, 2019