Swift Observations of GRB 220618A

E. Troja (NASA/GSFC/UMCP), M. Perri (SSDC and INAF-OAR) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 09:10:29 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220618A (trigger=1110821) (Troja et al. GCN Circ. 32214). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 47° from the Sun (3.2 hours West) and 97° from the 79%-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

Analysis of the BAT data was reported by Palmer and Barthelmy (GCN Circ. 32239).

The BAT light curve (Figure 1) showed a single peak structure with a duration of about 40 s.

The time-averaged spectrum from -15.264 to 128.736 s is best fit by a power law

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/1110821/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Perri et al. (GCN Circ. 32221). We have analysed 28 ks of XRT data for GRB 220618A, from 131 s to 274.4 ks after the BAT trigger. The data comprise 235 s in Windowed Timing (WT) mode (the first 10 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 Osborne et al. (GCN Circ. 32216).

The late-time light curve (Figure 2) (from T0+3.9 ks) can be modelled with a power-law decay with a decay index of α=0.61 ± 0.19.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.62 ± 0.11. The best-fitting absorption column is 5.9 (+0.6, -0.5) x 10²¹ cm^-2, in excess of the Galactic value of 4.7 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 3.0 x 10^-11 (8.4 x 10^-11) erg cm^-2 count^-1.

A summary of the WT-mode spectrum is thus:
Total column: 5.9 (+0.6, -0.5) x 10²¹ cm^-2
Galactic foreground: 4.7 x 10²¹ cm^-2
Excess significance: 3.7 σ
Photon index: 2.62 ± 0.11

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01110821.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 220618A 151 s after the BAT trigger (Kuin and Troja GCN Circ. 32218). No optical afterglow 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 0.391 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

02^h35^m56.64^s +55°12'55.7" 2.0" XRT-final UKSSDC

02^h35^m56.65^s +55°12'56.4" 2.2" XRT-enhanced Osborne et al. GCN Circ. 32216

02^h35^m56.6^s +55°13'05.0" 2.0' BAT-refined Palmer and Barthelmy GCN Circ. 32239

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Lipunov et al. 32213 Swift GRB220618.38: Global MASTER-Net
observations report MASTER

Optical Hu et al. 32219 BOOTES-5/JGT early optical upper limit BOOTES upper limits

Optical Koyanagi et al. 32238 iTelescope T-19 upper limit iTelescope upper limits

Gamma-ray Poolakkil and Meegan 32241 Fermi GBM detection Fermi GBM E_peak=126±8 keV
T₉₀=44 seconds
Fluence=3.6±0.2x10^-6erg cm^-2
(brighter than 48% 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_FC 151 301 147 >19.8

u_FC 309 559 246 >18.9

white 151 609 167 >19.8

v 641 649 8 >17.4

b 565 585 19 >17.9

u 309 559 246 >18.9

w2 617 637 19 >18.4

RA (J2000)	Dec (J2000)	Error	Note	Reference
02^h35^m56.64^s	+55°12'55.7"	2.0"	XRT-final	UKSSDC
02^h35^m56.65^s	+55°12'56.4"	2.2"	XRT-enhanced	Osborne et al. GCN Circ. 32216
02^h35^m56.6^s	+55°13'05.0"	2.0'	BAT-refined	Palmer and Barthelmy GCN Circ. 32239

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Lipunov et al.	32213	Swift GRB220618.38: Global MASTER-Net observations report	MASTER
Optical	Hu et al.	32219	BOOTES-5/JGT early optical upper limit	BOOTES	upper limits
Optical	Koyanagi et al.	32238	iTelescope T-19 upper limit	iTelescope	upper limits
Gamma-ray	Poolakkil and Meegan	32241	Fermi GBM detection	Fermi GBM	E_peak=126±8 keV T₉₀=44 seconds Fluence=3.6±0.2x10^-6erg cm^-2 (brighter than 48% of long GRBs)

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white_FC	151	301	147	>19.8
u_FC	309	559	246	>18.9
white	151	609	167	>19.8
v	641	649	8	>17.4
b	565	585	19	>17.9
u	309	559	246	>18.9
w2	617	637	19	>18.4

Table 3. UVOT observations reported by Kuin and Troja (GCN Circ. 32218). 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.

June 22, 2022