Swift Observations of GRB 220412A

N.J. Klingler (GSFC/UMBC/CRESSTII), A. D'Ai (INAF-IASFPA) and A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 06:36:50 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220412A (trigger=1102281) (Klingler et al. GCN Circ. 31881). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 97° from the Sun (6.9 hours East) and 27° from the 78%-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.

Watson et al. (GCN Circ. 31882) reported the position from RATIR for the optical afterglow of this GRB. 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 Parsotan and Barthelmy (GCN Circ. 31899).

The BAT light curve (Figure 1) showed a complex

The time-averaged spectrum from T-11.07 to T+38.00 s is best fit by a simple

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/1102281/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Ai et al. (GCN Circ. 31888). We have analysed 10 ks of XRT data for GRB 220412A, from 106 s to 73.6 ks after the BAT trigger. The data comprise 158 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 enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 31887).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=2.08 (+0.08, -0.07), followed by a break at T+1318 s to an α of 0.98 (+0.13, -0.34).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.05 (+0.18, -0.11). The best-fitting absorption column is 3.86 (+3.45, -0.16) x 1020 cm-2, consistent with the Galactic value of 3.7 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.70 (+0.22, -0.18) and a best-fitting absorption column of 5.8 (+5.9, -2.1) x 1020 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.9 x 10-11 (4.2 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 5.8 (+5.9, -2.1) x 1020 cm-2
Galactic foreground: 3.7 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 1.70 (+0.22, -0.18)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01102281.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 220412A 127 s after the BAT trigger (Breeveld and Klingler GCN Circ. 31885). No optical afterglow consistent with the RATIR position (Watson et al., GCN Circ. 31882) 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 0.0373 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
08h15m52.35s +30°20'34.5" 2.1" XRT-final UKSSDC
08h15m52.34s +30°20'33.3" 3.7" XRT-enhanced Evans et al. GCN Circ. 31887
08h15m54.0s +30°20'08.5" 1.9' BAT-refined Parsotan and Barthelmy GCN Circ. 31899

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 31880 Swift GRB220412.28: Global MASTER-Net
observations report 		MASTER
Optical Watson et al. 31882 RATIR Afterglow Candidate RATIR detection
Optical Murata et al. 31891 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Watson et al. 31898 RATIR Observations of the Fading
Afterglow 		RATIR light curve
Optical Hu et al. 31901 BOOTES-network optical upper limits BOOTES upper limits
Optical Watson et al. 31902 Continued RATIR Observations and
Steepening of the Light Curve 		RATIR light curve
Optical 31903 GRANDMA observations GRANDMA
Optical Noonan et al. 32326 VIRT Optical Upper Limit Virgin Island Robotic Telescope upper limits

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 127 276 147 >20.5
uFC 285 479 191 >19.6
white 566 758 34 >19.7
b 542 734 34 >18.97
v 616 1059 53 >18.18
uvw1 665 857 34 >18.25
uvw2 592 1039 58 >18.64
uvm2 813 833 20 >17.37

Table 3. UVOT observations reported by Breeveld and Klingler (GCN Circ. 31885). 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.

July 6, 2022