Swift Observations of GRB 210217A

K.K. Simpson (PSU) and D.N. Burrows (PSU) for the Swift team

1. Introduction

At 23:25:42 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210217A (trigger=1033264) (Simpson et al. GCN Circ. 29521). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 114° from the Sun (8.4 hours East) and 68° from the 33%-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.

Simpson et al. (GCN Circ. 29521) reported the discovery with UVOT of an optical afterglow. 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 Sakamoto et al. (GCN Circ. 29534), the BAT ground-calculated position is RA, Dec = 97.578, 68.715 deg which is RA(J2000) = 06h30m18.6s Dec(J2000) = +68°42'55.4" with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 30%.

The mask-weighted light curve (Figure 1) shows a single-peaked FRED-like structure that starts and peaks at ~T0, and ends at ~T+5 s. T90 (15-350 keV) is 4.22 ± 1.15 s (estimated error including systematics).

The time-averaged spectrum from T-0.65 to T+4.89 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.01 ± 0.16. The fluence in the 15-150 keV band is 6.8 ± 0.7 x 10-7 erg cm-2. This fluence is larger than that of 29% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.13 s in the 15-150 keV band is 5.1 ± 0.5 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/1033264/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Burrows et al. (GCN Circ. 29538). We have analysed 13 ks of XRT data for GRB 210217A, from 109 s to 210.9 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 29525).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=0.3 (+0.4, -0.5), followed by a break at T+1790 s to an α of 1.25 (+0.13, -0.12).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.97 (+0.23, -0.22). The best-fitting absorption column is 2.5 (+0.9, -0.8) x 1021 cm-2, in excess of the Galactic value of 8.7 x 1020 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 (5.1 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.5 (+0.9, -0.8) x 1021 cm-2
Galactic foreground: 8.7 x 1020 cm-2
Excess significance: 3.3 σ
Photon index: 1.97 (+0.23, -0.22)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01033264.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210217A 103 s after the BAT trigger (Simpson GCN Circ. 29555). A source consistent with the XRT position *** POSITION REFERENCE is GCN_Notice *** 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 0.102 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
06h30m19.90s +68°43'30.6" 0.46" UVOT-refined Simpson GCN Circ. 29555
06h30m20.28s +68°43'31.0" 1.5" XRT-final UKSSDC
06h30m20.42s +68°43'31.3" 1.9" XRT-enhanced Evans et al. GCN Circ. 29525
06h30m18.6s +68°42'55.4" 1.4' BAT-refined Sakamoto et al. GCN Circ. 29534

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Zhu et al. 29523 NEXT early optical afterglow
observations		 Xinjiang Astro. Obs. detection
Optical Zheng and Filippenko 29533 KAIT Optical Observations KAIT marginal detection
Optical Shrestha et al. 29535 Liverpool Telescope First Hour
Observations		 Liverpool Telescope light curve
Optical Kumar et al. 29539 1.3m DFOT optical detection Nainital detection
Optical Pankov et al. 29577 Mondy optical observations Mondy light curve
Optical Dimple et al. 29591 3.6m DOT optical observations DOT detection
Gamma-ray Fletcher 29536 Fermi GBM Sub-Threshold Detection Fermi GBM Epeak=230 keV

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

Filter Tstart(s) Tstop(s) Exp(s) Mag
white 103 253 147 18.80 ± 0.06
u 315 508 190 18.64 ± 0.11

Table 3. UVOT observations reported by Simpson (GCN Circ. 29555). The start and stop times of the exposures are given in seconds since the BAT trigger. The preliminary detections and 3-σ upper limits are given. No correction has been made for extinction in the Milky Way.

March 2, 2021