Swift Observations of GRB 200216B

P. D'Avanzo (INAF-OAB) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 13:32:33 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200216B (trigger=956824) (D'Avanzo et al. GCN Circ. 27102). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 167° from the Sun (11.3 hours West) and 90° from the 43%-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.

Levan et al. (GCN Circ. 27114) reported the position from VLT 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 Krimm et al. (GCN Circ. 27117), the BAT ground-calculated position is RA, Dec = 160.452, 19.468 deg which is RA(J2000) = 10h41m48.4s Dec(J2000) = +19°28'05.5" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 84%.

The mask-weighted light curve (Figure 1) shows a single bright pulse followed by some extended emission. The main pulse has a FRED shape, starting at T-2 s, peaking around T+2 s and decaying by T+18 s. Lower-level extended emission with some weak pulses lasts until around T+100 s. T90 (15-350 keV) is 81.3 ± 13.0 s (estimated error including systematics).

The time-averaged spectrum from -0.72 to +100.33 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.81 ± 0.52, and Epeak of 47.6 ± 6.6 keV (χ2 48.83 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 1.6 ± 0.1 x 10-6 erg cm-2 and the 1-s peak flux measured from T+2.80 s in the 15-150 keV band is 1.5 ± 0.2 ph cm-2 s-1. This fluence is larger than that of 53% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). A fit to a simple power law gives a photon index of 1.90 ± 0.10 (χ2 65.47 for 57 d.o.f.). 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/956824/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Avanzo and Evans (GCN Circ. 27120).

We have analysed 1.6 ks of XRT data for GRB 200216B, from 105 s to 1.7 ks after the BAT trigger. The data comprise 90 s in Windowed Timing (WT) mode (the first 8 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 Goad et al. (GCN Circ. 27106).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.91 (+0.07, -0.06).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.18 (+0.20, -0.19). The best-fitting absorption column is 7.3 (+2.3, -1.9) x 1021 cm-2, in excess of the Galactic value of 2.0 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.63 (+0.27, -0.26) and a best-fitting absorption column of 9.2 (+2.9, -2.5) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.8 x 10-11 (8.5 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 9.2 (+2.9, -2.5) x 1021 cm-2
Galactic foreground: 2.0 x 1020 cm-2
Excess significance: 6.0 σ
Photon index: 1.63 (+0.27, -0.26)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00956824.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200216B 124 s after the BAT trigger (Kuin and D'Avanzo GCN Circ. 27108). No optical afterglow consistent with the XRT position (Goad et al. GCN Circ. 27106) 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 0.02 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
10h41m44.32s +19°28'30.6" 1.7" XRT-final UKSSDC
10h41m44.32s +19°28'30.6" 1.7" XRT-enhanced Goad et al. GCN Circ. 27106
10h41m48.4s +19°28'05.5" 1.0' BAT-refined Krimm et al. GCN Circ. 27117

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Mao et al. 27103 GMG upper limit Gao-Mei-Gu upper limits
Optical Xu et al. 27104 Nanshan/NEXT early optical upper limit
and possible host galaxy		 Xinjiang Astro. Obs. upper limits
Optical de Ugarte Postigo et al. 27110 z-band limit from NOT NOT upper limits
Optical Levan et al. 27114 possible IR counterpart from VLT VLT possible detection
Optical Pozanenko et al. 27122 Mondy and AbAO optical upper limits Abastumani Astro. Obs. upper limits
Optical Belkin et al. 27128 second epoch optical upper limit at
Mondy observatory		 Mondy upper limits
Gamma-ray Fermi 27101 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Malacaria et al. 27109 Fermi GBM observation Fermi GBM Epeak=69.5±4.9 keV
T90=12.0 seconds

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 124 274 147 >21.0
white 124 1535 334 >21.5
w1 1095 1635 39 >20.2

Table 3. UVOT observations reported by Kuin and D'Avanzo (GCN Circ. 27108). 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.

February 20, 2020