Swift Observations of GRB 190109A

J.D. Gropp (PSU), A.P. Beardmore (U. Leicester) and K.K. Simpson (PSU) for the Swift team

1. Introduction

At 05:12:41 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190109A (trigger=882747) (Gropp et al. GCN Circ. 23642). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 114° from the Sun (6.9 hours East) and 83° from the 9%-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 Sakamoto et al. (GCN Circ. 23653), the BAT ground-calculated position is RA, Dec = 33.213, 38.111 deg, which is RA(J2000) = 02h12m51.2s Dec(J2000) = +38°06'39.5" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 28%.

The mask-weighted light curve (Figure 1) shows several overlapping peaks starting at ~T+1 s, peaking at ~T+40 s, and returning to baseline at ~T+200 s. T90 (15-350 keV) is 115.0 ± 33.5 s (estimated error including systematics).

The time-averaged spectrum from T+5.86 to T+205.36 s (with a 1.03 s gap in the data at T+111.97 s) is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.36 ± 0.47, and Epeak of 49.7 ± 4.0 keV (χ2 46.05 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 5.5 ± 0.3 x 10-6 erg cm-2 and the 1-s peak flux measured from T+38.82 s in the 15-150 keV band is 1.9 ± 0.3 ph cm-2 s-1. This fluence is larger than that of 84% 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 1.86 ± 0.08 (χ2 87.76 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/882747/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Beardmore et al. (GCN Circ. 23645). We have analysed 19 ks of XRT data for GRB 190109A, from 175 s to 184.7 ks after the BAT trigger. The data comprise 291 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 23643).

The late-time light curve (Figure 2) (from T0+4.8 ks) can be modelled with a power-law decay with a decay index of α=1.34 (+0.12, -0.11).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.87 ± 0.04. The best-fitting absorption column is 9.5 (+1.3, -1.2) x 1020 cm-2, in excess of the Galactic value of 5.1 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.84 (+0.15, -0.10) and a best-fitting absorption column of 5.4 (+3.9, -0.3) x 1020 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 x 10-11 (3.9 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 5.4 (+3.9, -0.3) x 1020 cm-2
Galactic foreground: 5.1 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 1.84 (+0.15, -0.10)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00882747.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190109A 181 s after the BAT trigger (Simpson and Gropp GCN Circ. 23663). No optical afterglow consistent with the XRT position (Osborne et al. GCN Circ. 23644) 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 0.05 mag. in the direction of the GRB (Schlegel et al. 1998).

The Swift/UVOT began settled observations of the field of GRB 190109A 181 s after the BAT trigger ( GCN Circ. 23673). No optical afterglow consistent with the XRT position (Osborne et al. GCN Circ. 23644) 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 0.05 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
02h12m48.56s +38°06'29.7" 1.4" XRT-final UKSSDC
02h12m48.52s +38°06'30.7" 1.8" XRT-enhanced Osborne et al. GCN Circ. 23644
02h12m51.2s +38°06'39.5" 1.0' BAT-refined Sakamoto et al. GCN Circ. 23653

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Oeda et al. 23649 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Gamma-ray von Kienlin 23658 Fermi GBM observation Fermi GBM Epeak=46.8±2.2 keV
T90=120 seconds
Fluence=7.6±0.4x10-6erg cm-2
(68th 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 181 331 147 >21.60
white 181 12997 1291 >20.27
v 670 6867 490 >16.8
b 596 18739 1727 >18.6
u 340 24226 2551 >17.5
w1 720 23954 2223 >17.4
m2 695 23048 1357 >18.1
w2 646 6662 491 >17.9

Table 3. UVOT observations reported by (GCN Circ. 23673). 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 12, 2019