Swift Observations of GRB 220403B

N.J. Klingler (GSFC/UMBC/CRESSTII) and A.P. Beardmore (U. Leicester) for the Swift team

1. Introduction

At 20:42:42 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220403B (trigger=1101053) (Klingler et al. GCN Circ. 31820). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 85° from the Sun (11.8 hours East) and 75° from the 7%-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.

Klingler et al. (GCN Circ. 31820) 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 Lien et al. (GCN Circ. 31834), the BAT ground-calculated position is RA, Dec = 190.974, 89.180 deg which is RA(J2000) = 12^h43^m53.9^s Dec(J2000) = +89°10'49.0" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 60%.

The mask-weighted light curve (Figure 1) shows a bright peak structure that starts at T-19 s, peaks at T-1 s and ends at T+40 s. T₉₀ (15-350 keV) is 27.0 ± 3.4 s (estimated error including systematics).

The time-averaged spectrum from T-14.42 to T+27.96 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.94 ± 0.06. The fluence in the 15-150 keV band is 3.7 ± 0.1 x 10^-6 erg cm^-2. This fluence is larger than that of 76% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+1.06 s in the 15-150 keV band is 2.7 ± 0.3 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/1101053/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Beardmore et al. (GCN Circ. 31827). We have analysed 21 ks of XRT data for GRB 220403B, from 86 s to 264.0 ks after the BAT trigger. The data comprise 50 s in Windowed Timing (WT) mode (the first 9 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode.

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=4.4 ± 0.4. At T+236 s the decay flattens to an α of 0.26 ± 0.07 before breaking again at T+9375 s to a final decay with index α=1.16 (+0.16, -0.10).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.90 (+0.16, -0.15). The best-fitting absorption column is 1.7 ± 0.5 x 10²¹ cm^-2, consistent with the Galactic value of 1.2 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 3.6 x 10^-11 (4.7 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.7 ± 0.5 x 10²¹ cm^-2
Galactic foreground: 1.2 x 10²¹ cm^-2
Excess significance: <1.6 σ
Photon index: 1.90 (+0.16, -0.15)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01101053.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 220403B 105 s after the BAT trigger (Klingler GCN Circ. 31876). A source consistent with the XRT position 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 0.208 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

12^h45^m53.46^s +89°11'06.5" 0.52" UVOT-refined Klingler GCN Circ. 31876

12^h45^m54.35^s +89°11'04.7" 3.5" XRT-final UKSSDC

12^h45^m53.62^s +89°11'04.6" 3.5" XRT-refined Beardmore et al. GCN Circ. 31827

12^h43^m53.9^s +89°10'49.0" 1.0' BAT-refined Lien et al. GCN Circ. 31834

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Jelinek et al. 31821 optical detection by D50 D50 detection

Optical Zhu et al. 31823 Nanshan/NEXT optical afterglow
power-law decay Xinjiang Astro. Obs. detection

Optical Lipunov et al. 31825 Swift GRB 220403B: Global MASTER-Net
observations report MASTER

Optical Rossi 31826 LBT observations LBT detection

Optical 31830 NUTTelA-TAO / BSTI Early Optical Limits
(Preliminary) NUTTelA upper limits

Optical Grossan et al. 31830 NUTTelA-TAO / BSTI Early Optical Limits
(Preliminary) NUTTelA upper limits

Optical Pankov et al. 31837 Kitab optical observations detection

Optical Hu et al. 31841 1.5m OSN optical observation Obs.de Sierra Nevada

Optical Pankov et al. 31844 Mondy optical upper limit Mondy upper limits

Optical Ror et al. 31869 1.3m DFOT Optical upper limit DFOT upper limits

Optical Song et al. 31883 GRANDMA observations GRANDMA detection

Gamma-ray Hamburg and Hristov 31829 Fermi GBM detection Fermi GBM E_peak=79±3 keV
T₉₀=28 seconds
Fluence=5.2±0.1x10^-6erg cm^-2
(brighter than 58% of long GRBs)

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 105 255 147 19.86 ± 0.12

v 647 1074 58 >18.6

b 573 766 39 >19.3

u 318 741 265 19.93 ± 0.22

w1 697 716 19 >18.9

w2 797 1049 39 >19.9

RA (J2000)	Dec (J2000)	Error	Note	Reference
12^h45^m53.46^s	+89°11'06.5"	0.52"	UVOT-refined	Klingler GCN Circ. 31876
12^h45^m54.35^s	+89°11'04.7"	3.5"	XRT-final	UKSSDC
12^h45^m53.62^s	+89°11'04.6"	3.5"	XRT-refined	Beardmore et al. GCN Circ. 31827
12^h43^m53.9^s	+89°10'49.0"	1.0'	BAT-refined	Lien et al. GCN Circ. 31834

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Jelinek et al.	31821	optical detection by D50	D50	detection
Optical	Zhu et al.	31823	Nanshan/NEXT optical afterglow power-law decay	Xinjiang Astro. Obs.	detection
Optical	Lipunov et al.	31825	Swift GRB 220403B: Global MASTER-Net observations report	MASTER
Optical	Rossi	31826	LBT observations	LBT	detection
Optical		31830	NUTTelA-TAO / BSTI Early Optical Limits (Preliminary)	NUTTelA	upper limits
Optical	Grossan et al.	31830	NUTTelA-TAO / BSTI Early Optical Limits (Preliminary)	NUTTelA	upper limits
Optical	Pankov et al.	31837	Kitab optical observations		detection
Optical	Hu et al.	31841	1.5m OSN optical observation	Obs.de Sierra Nevada
Optical	Pankov et al.	31844	Mondy optical upper limit	Mondy	upper limits
Optical	Ror et al.	31869	1.3m DFOT Optical upper limit	DFOT	upper limits
Optical	Song et al.	31883	GRANDMA observations	GRANDMA	detection
Gamma-ray	Hamburg and Hristov	31829	Fermi GBM detection	Fermi GBM	E_peak=79±3 keV T₉₀=28 seconds Fluence=5.2±0.1x10^-6erg cm^-2 (brighter than 58% of long GRBs)

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white	105	255	147	19.86 ± 0.12
v	647	1074	58	>18.6
b	573	766	39	>19.3
u	318	741	265	19.93 ± 0.22
w1	697	716	19	>18.9
w2	797	1049	39	>19.9

Table 3. UVOT observations reported by Klingler (GCN Circ. 31876). 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.

April 13, 2022