Swift Observations of GRB 230216A

N.J. Klingler (GSFC/UMBC/CRESSTII), E. Ambrosi (INAF-IASFPA) and N. Klingler (GSFC/UMBC/CRESTII) for the Swift team

1. Introduction

At 14:48:34 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 230216A (trigger=1154815) (Klingler et al. GCN Circ. 33328). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 139° from the Sun (9.6 hours East) and 141° from the 19%-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.

Gendre et al. (GCN Circ. 33329) reported the position from Zadko 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

As reported by Stamatikos et al. (GCN Circ. 33336), the BAT ground-calculated position is RA, Dec = 113.966, -7.993 deg which is RA(J2000) = 07h35m51.9s Dec(J2000) = -07°59'36.2" with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 85%.

The mask-weighted light curve (Figure 1) shows a trio of peaks at approximately T-80 s, T-20 s and T+0 s, with some possible substructure and other, smaller peaks. T90 (15-350 keV) is 91.20 ± 7.31 s (estimated error including systematics).

The time-averaged spectrum from T-81.07 to T+16.40 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.90 ± 0.23. The fluence in the 15-150 keV band is 1.3 ± 0.2 x 10-6 erg cm-2. The 1-s peak photon flux measured from T-0.13 s in the 15-150 keV band is 1.2 ± 0.3 ph cm-2 s-1. This fluence is larger than that of 47% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). 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/1154815/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Ambrosi et al. (GCN Circ. 33335). We have analysed 23 ks of XRT data for GRB 230216A, from 84 s to 635.7 ks after the BAT trigger. The data comprise 8 s in Windowed Timing (WT) mode (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. 33331).

The late-time light curve (Figure 2) (from T0+4.5 ks) can be modelled with a power-law decay with a decay index of α=1.83 (+0.13, -0.11).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.01 (+0.19, -0.09). The best-fitting absorption column is consistent with the Galactic value of 3.2 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 3.7 x 10-11 (5.6 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 3.2 ± 0.7 x 1021 cm-2
Galactic foreground: 3.2 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 2.01 (+0.19, -0.09)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01154815.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 230216A 106 s after the BAT trigger (Klingler GCN Circ. 33369). No optical afterglow consistent with the optical position (Gendre et al. GCN Circ. 33329) 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 0.198 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
07h35m50.17s -08°00'41.6" 2.0" XRT-final UKSSDC
07h35m50.17s -08°00'40.8" 2.3" XRT-enhanced Osborne et al. GCN Circ. 33331
07h35m51.9s -07°59'36.2" 2.1' BAT-refined Stamatikos et al. GCN Circ. 33336

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Gendre et al. 33329 Zadko observatory - Gingin optical
observations		 Zadko
Optical M. Odeh 33330 AKO Upper Limit Al-Khatim Obs. upper limits
Optical Lipunov et al. 33332 Swift GRB 230216A: Global MASTER-Net
observations report 		MASTER
Optical Kang et al. 33340 SVOM/C-GFT optical upper limit C-GFT upper limits
Optical Pankov et al. 33373 Mondy Observations Mondy detection

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 106 256 147 >16.9
uFC 264 514 246 >16.9
white 106 1188 353 >16.8
v 594 1065 58 >15.9
b 520 1164 58 >16.5
u 264 1139 304 >16.9
w1 643 1114 58 >17.1
m2 618 1090 58 >17.6
w2 569 1213 78 >17.7

Table 3. UVOT observations reported by Klingler (GCN Circ. 33369). 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.

April 29, 2023