Swift Observations of GRB 190204A

A.Y. Lien (GSFC/UMBC), J.A. Kennea (PSU) and P. Kuin (UCL/MSSL) for the Swift team

1. Introduction

At 05:46:01 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190204A (trigger=887579 and 887580) (Lien et al. GCN Circ. 23852). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 77° from the Sun (2.3 hours East) and 82° from the 0%-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.

Lien et al. (GCN Circ. 23852) reported the discovery 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

Analysis of the BAT data was reported by Sakamoto et al. (GCN Circ. 23872). In this circular, we adopted the trigger time from the first trigger (#887579), 2019-02-04 05:46:01 UT, to be consistent with the T0 quoted in other circulars. The BAT ground-calculated position is RA, Dec = 351.491, 54.884 deg which is RA(J2000) = 23h25m57.8s Dec(J2000) = +54°53'02.4" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 48%.

The mask-weighted light curve (Figure 1) shows several overlapping pulses that starts at ~T-7 s and ends at ~T+55 s. The main peak occurs at ~T+5 s. T90 (15-350 keV) is 26.4 ± 7.5 s (estimated error including systematics).

The time-averaged spectrum from T-6.93 to T+53.82 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.30 ± 0.04. The fluence in the 15-150 keV band is 1.3 ± 0.03 x 10-5 erg cm-2. This fluence is larger than that of 94.2% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+4.55 s in the 15-150 keV band is 30.5 ± 0.9 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/887580/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Kennea et al. (GCN Circ. 23857). We have analysed 6.4 ks of XRT data for GRB 190204A, from 81 s to 13.0 ks after the BAT trigger. The data comprise 926 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode.

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=1.36 ± 0.16.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.740 (+0.022, -0.018). The best-fitting absorption column is consistent with the Galactic value of 4.4 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.83 (+0.12, -0.11) and a best-fitting absorption column of 5.4 (+0.8, -0.7) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.6 x 10-11 (6.8 x 10-11) erg cm-2 count-1.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190204A 84 s after the BAT trigger (Kuin and Lien GCN Circ. 23863). The updated UVOT position is: 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 0.38 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.

RA (J2000) Dec (J2000) Error Note Reference
23h25m54.58s +54°52'35.1" 0.4" UVOT-refined Kuin and Lien GCN Circ. 23863
23h25m54.77s +54°52'34.9" 1.5" XRT-enhanced Osborne et al. GCN Circ. 23862
23h25m57.8s +54°53'02.4" 1.0' BAT-refined Sakamoto et al. GCN Circ. 23872

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Watson et al. 23861 COATLI Optical Detection of the Fading
Afterglow 		COATLI detection
Optical Volnova et al. 23867 Mondy optical upper limit Mondy upper limits
Optical Moskvitin 23868 SAO RAS optical observations SAO RAS detection
X-ray Oeda et al. 23854 MAXI/GSC detection MAXI detection
Gamma-ray Khanam et al. 23856 AstroSat CZTI detection CZTI T90=11.9 seconds
Gamma-ray Kozlova et al. 23869 Konus-Wind observation Konus-Wind Epeak=308 (-30,+33) keV
Duration=~49 seconds
Fluence=5.31(-0.51,+0.53)x10-5erg cm-2
Gamma-ray Ito et al. 23877 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
white 84 233 147 16.84 ± 0.07
white 576 596 20 18.66 ± 0.20
v 626 1247 78 18.30 ± 0.23
b 551 1507 85 19.29 ± 0.21
u 296 546 246 17.85 ± 0.10
w1 676 1470 97 >18.3
m2 651 1444 97 >19.5
w2 602 1395 97 >18.7

Table 3. UVOT observations reported by Kuin and Lien (GCN Circ. 23863). The start and stop times of the exposures are given in seconds since the BAT trigger. The preliminary detections and 3-σ upper limits are given. No correction has been made for extinction in the Milky Way.

February 7, 2019