Swift Observations of GRB 210618A

M.G. Bernardini (INAF-OAB), J.P. Osborne (U. Leicester) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 01:43:37 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210618A (trigger=1056426) (Bernardini et al. GCN Circ. 30250). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 105° from the Sun (9.9 hours East) and 63° from the 49%-illuminated Moon. Table 1 contains the best reported positions from Swift.

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 Laha et al. (GCN Circ. 30254), the BAT ground-calculated position is RA, Dec = 235.763, 46.072 deg which is RA(J2000) = 15h43m03.2s Dec(J2000) = +46°04'17.5" with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows a fairly complex structure between roughly T-1.5 and T+0.5 seconds. The spacecraft slewed away from the burst location starting at around T+320 seconds. T90 (15-350 keV) is 2.13 ± 0.44 s (estimated error including systematics).

The time-averaged spectrum from T-1.42 to T+0.96 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.18 ± 0.28. The fluence in the 15-150 keV band is 1.8 ± 0.3 x 10-7 erg cm-2. This fluence is larger than that of 5.8% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.50 s in the 15-150 keV band is 1.1 ± 0.2 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/1056426/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Osborne et al. (GCN Circ. 30312).

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210618A 3728 s after the BAT trigger (Kuin and Bernardini GCN Circ. 30256). No optical afterglow consistent with the XRT candidates source 1, 2 or 4 (Osborne et al. , GCN Circ. 30252) 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 0.015 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
15h43m04.82s +46°04'09.5" 9.0" XRT-final UKSSDC
15h43m04.75s +46°04'08.6" 9.3" XRT Osborne et al. GCN Circ. 30312
15h43m03.2s +46°04'17.5" 2.0' BAT-refined Laha et al. GCN Circ. 30254

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 30249 Swift GRB210618.07: Global MASTER-Net
observations report 		MASTER
Optical Lopez et al. 30251 DDOTI upper limit for the optical
afterglow		 Deca-Degree Optical Imager upper limits
Optical Hu et al. 30255 BOOTES-2/TELMA optical upper limit BOOTES-2 upper limits
Optical Hu et al. 30322 1.5m OSN optical upper limit Obs.de Sierra Nevada upper limits
Gamma-ray Fermi 30248 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Wood and Meegan 30253 Fermi GBM observation Fermi GBM Epeak=440±140 keV
T90=3.6 seconds
Fluence=1.2±0.2x10-6erg cm-2
(18th 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 3728 3878 147 >20.4
white 3728 4496 344 >21.1
v 4707 4843 133 >19.2
b 4092 4291 197 >20.5
u 3886 4086 197 >20.0
w2 4502 4702 197 >20.1

Table 3. UVOT observations reported by Kuin and Bernardini (GCN Circ. 30256). 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 26, 2021