Swift Observations of GRB 190203A

A.Y. Lien (GSFC/UMBC), A.P. Beardmore (U. Leicester) and P. Kuin (UCL/MSSL) for the Swift team

1. Introduction

At 15:44:08 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190203A (trigger=887512) (Lien et al. GCN Circ. 23845). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 91° from the Sun (4.4 hours East) and 101° from the 1%-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. 23845) 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

As reported by Palmer et al. (GCN Circ. 23873), the BAT ground-calculated position is RA, Dec = 22.603, 55.736 deg which is RA(J2000) = 01^h30^m24.7^s Dec(J2000) = +55°44'10.5" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 21%.

The mask-weighted light curve (Figure 1) shows several overlapping pulses that starts ~ T-60 s and ends at T+40 s. Note that there is a large data gap from T-117.652 s to T-60.784 s, so it is possible that there is additional burst emission during this interval. T₉₀ (15-350 keV) is 96 ± 16 s (estimated error including systematics).

The time-averaged spectrum from T-60.784 to T+ 43.216 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.06 ± 0.06. The fluence in the 15-150 keV band is 1.5 ± 0.05 x 10^-5 erg cm^-2. This fluence is larger than that of 95.6% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+2.72 s in the 15-150 keV band is 3.1 ± 0.6 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/887512/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Beardmore et al. (GCN Circ. 23858). We have analysed 19 ks of XRT data for GRB 190203A, from 103 s to 228.9 ks after the BAT trigger. The data comprise 669 s in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 23848).

The late-time light curve (Figure 2) (from T0+4.1 ks) can be modelled with a power-law decay with a decay index of α=1.76 ± 0.06.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.69 ± 0.04. The best-fitting absorption column is 7.2 ± 0.5 x 10²¹ cm^-2, in excess of the Galactic value of 4.0 x 10²¹ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.63 (+0.21, -0.20) and a best-fitting absorption column of 7.0 (+2.0, -1.7) x 10²¹ cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.4 x 10^-11 (7.6 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 7.0 (+2.0, -1.7) x 10²¹ cm^-2
Galactic foreground: 4.0 x 10²¹ cm^-2
Excess significance: 3.0 σ
Photon index: 1.63 (+0.21, -0.20)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00887512.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190203A 121 s after the BAT trigger (Kuin and Lien GCN Circ. 23865). A source consistent with the XRT position (Goad et al. GCN Circ. 23848) 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 E_B-V of 0.64 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

01^h30^m23.21^s +55°43'20.6" 0.46" UVOT-refined Kuin and Lien GCN Circ. 23865

01^h30^m23.25^s +55°43'20.4" 1.4" XRT-final UKSSDC

01^h30^m23.18^s +55°43'19.9" 2.7" XRT-enhanced Goad et al. GCN Circ. 23848

01^h30^m24.7^s +55°44'10.5" 1.0' BAT-refined Palmer et al. GCN Circ. 23873

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Novichonok et al. 23846 Koshka Zeiss-1000 optical upper limit Zeiss-1000 upper limits

Optical Moskvitin et al. 23847 SAO RAS optical observations, afterglow
fading SAO RAS detection

Optical Lipunov et al. 23850 MASTER OT detection MASTER detection

Optical Izzo et al. 23860 Liverpool Telescope near-IR observations Liverpool Telescope

Optical Watson et al. 23864 COATLI Optical Observations COATLI

Optical Volnova et al. 23878 Mondy optical observations Mondy detection

Gamma-ray Frederiks et al. 23859 Konus-Wind observation Konus-Wind E_peak=314 (-22,+25) keV

Gamma-ray Khanam et al. 23866 AstroSat CZTI detection CZTI T₉₀=46.45 seconds

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

Filter T_start(s) T_stop(s) Exp(s) Mag

white 121 271 147 18.66 ± 0.06

v 609 5462 225 >19.2

b 535 4846 236 >20.5

u 279 5967 551 >21.0

w1 658 5872 413 >20.2

m2 634 5666 413 >21.0

w2 585 5257 236 >20.2

RA (J2000)	Dec (J2000)	Error	Note	Reference
01^h30^m23.21^s	+55°43'20.6"	0.46"	UVOT-refined	Kuin and Lien GCN Circ. 23865
01^h30^m23.25^s	+55°43'20.4"	1.4"	XRT-final	UKSSDC
01^h30^m23.18^s	+55°43'19.9"	2.7"	XRT-enhanced	Goad et al. GCN Circ. 23848
01^h30^m24.7^s	+55°44'10.5"	1.0'	BAT-refined	Palmer et al. GCN Circ. 23873

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Novichonok et al.	23846	Koshka Zeiss-1000 optical upper limit	Zeiss-1000	upper limits
Optical	Moskvitin et al.	23847	SAO RAS optical observations, afterglow fading	SAO RAS	detection
Optical	Lipunov et al.	23850	MASTER OT detection	MASTER	detection
Optical	Izzo et al.	23860	Liverpool Telescope near-IR observations	Liverpool Telescope
Optical	Watson et al.	23864	COATLI Optical Observations	COATLI
Optical	Volnova et al.	23878	Mondy optical observations	Mondy	detection
Gamma-ray	Frederiks et al.	23859	Konus-Wind observation	Konus-Wind	E_peak=314 (-22,+25) keV
Gamma-ray	Khanam et al.	23866	AstroSat CZTI detection	CZTI	T₉₀=46.45 seconds

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white	121	271	147	18.66 ± 0.06
v	609	5462	225	>19.2
b	535	4846	236	>20.5
u	279	5967	551	>21.0
w1	658	5872	413	>20.2
m2	634	5666	413	>21.0
w2	585	5257	236	>20.2

Table 3. UVOT observations reported by Kuin and Lien (GCN Circ. 23865). 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