Swift Observations of GRB 190109B

B. Sbarufatti (PSU), P.A. Evans (U. Leicester) and K.K. Simpson (PSU) for the Swift team

1. Introduction

At 11:56:09 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190109B (trigger=882809) (Sbarufatti et al. GCN Circ. 23648). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 125° from the Sun (8.4 hours East) and 102° from the 10%-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 Stamatikos et al. (GCN Circ. 23659), the BAT ground-calculated position is RA, Dec = 55.563, 63.592 deg, which is RA(J2000) = 03h42m15.1s Dec(J2000) = +63°35'31.7" with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 60%.

The mask-weighted light curve (Figure 1) shows a couple overlapping peaks starting at ~T-4s, peaking at ~T+1 s, and a shallow tail out to ~T+40 s. T90 (15-350 keV) is 5.92 ± 0.78 s (estimated error including systematics).

The time-averaged spectrum from T-4.24 to T+3.31 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.58 ± 0.16. The fluence in the 15-150 keV band is 4.8 ± 0.5 x 10-7 erg cm-2. This fluence is larger than that of 19% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.12 s in the 15-150 keV band is 1.7 ± 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/882809/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 23662). We have analysed 15 ks of XRT data for GRB 190109B, from 96 s to 109.2 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 23652).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.01 ± 0.05.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.07 (+0.27, -0.24). The best-fitting absorption column is 5.5 (+1.7, -0.9) x 1021 cm-2, consistent with the Galactic value of 4.6 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 4.0 x 10-11 (7.0 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 5.5 (+1.7, -0.9) x 1021 cm-2
Galactic foreground: 4.6 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 2.07 (+0.27, -0.24)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00882809.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190109B 91 s after the BAT trigger (Simpson and Sbarufatti GCN Circ. 23664). No optical afterglow consistent with the XRT position (Beardmore et al. GCN Circ. 23652) 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 0.72 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
03h41m56.94s +63°35'31.3" 1.5" XRT-final UKSSDC
03h41m56.94s +63°35'31.3" 1.8" XRT-enhanced Beardmore et al. GCN Circ. 23652
03h42m15.1s +63°35'31.7" 1.4' BAT-refined Stamatikos et al. GCN Circ. 23659

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Belkin et al. 23654 Mondy optical upper limit Mondy upper limits
Optical Li et al. 23655 BOOTES-5/JGT optical observations BOOTES
Optical Troja et al. 23656 COATLI Optical Observations COATLI
Optical Pozanenko 23657 Correction to the GCN Circ. 23654 /
GRB 190109B: Mondy optical upper limit		 Mondy upper limits
Optical Oeda et al. 23711 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Gamma-ray Fletcher and Meegan 23666 Fermi GBM observation Fermi GBM Epeak=92±14 keV
T90=8 seconds
Fluence=6.76±0.67x10-7erg cm-2
(8th 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 91 241 147 >20.7
uFC 303 553 246 >20.7
white 91 6461 727 >22.1
v 634 6860 441 >20.4
b 559 6256 432 >20.9
u 303 6050 470 >21.0
w1 683 5845 236 >20.2
m2 5440 5640 197 >20.8
w2 5030 6666 393 >20.2

Table 3. UVOT observations reported by Simpson and Sbarufatti (GCN Circ. 23664). 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 16, 2019