Swift Observations of GRB 210218A

B. Sbarufatti (PSU), J.P. Osborne (U. Leicester) and A. Belles (PSU) for the Swift team

1. Introduction

At 13:35:19 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210218A (trigger=1033328) (Sbarufatti et al. GCN Circ. 29529). Swift did not slew immediately due to an observing constraint. At the time of the trigger, the initial BAT position was 85° from the Sun (5.0 hours East) and 21° from the 39%-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 Stamatikos et al. (GCN Circ. 29537), the BAT ground-calculated position is RA, Dec = 47.318, 35.322 deg which is RA(J2000) = 03h09m16.3s Dec(J2000) = +35°19'18.9" with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 57%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T-15 s and peaks at ~T0. The burst emission lasted until the burst went out of the BAT FOV at T+287 s. Using the available data, the lower limit of T90 is ~ 240 s.

The time-averaged spectrum from T-14.27 to T+266.26 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.77 ± 0.49, and Epeak of 91.3 ± 48.2 keV (χ2 50.53 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 4.3 ± 0.3 x 10-6 erg cm-2 and the 1-s peak flux measured from T+0.09 s in the 15-150 keV band is 1.0 ± 0.2 ph cm-2 s-1. This fluence is larger than that of 79% 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 1.46 ± 0.11 (χ2 57.43 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/1033328/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Sbarufatti et al. (GCN Circ. 29573).

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations (Belles and Sbarufatti GCN Circ. 29556) of the field of GRB 210218A approximately 170 ks after the BAT trigger (B. Sbarufatti et al., GCN Circ. 29529) due to a Moon constraint. 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 0.19 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
03h09m17.67s +35°19'26.5" 4.8" XRT-final UKSSDC
03h09m17.67s +35°19'26.5" 4.80" XRT ?
03h09m16.3s +35°19'18.9" 1.4' BAT-refined Stamatikos et al. GCN Circ. 29537

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Zhu et al. 29530 NEXT optical upper limit Xinjiang Astro. Obs. upper limits
Optical Xin et al. 29531 ： GWAC-F60A optical upper limit Xinglong Obs. upper limits
Optical Ogawa et al. 29532 MITSuME Akeno optical upper limits MITSuME Akeno 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
wh 170031 188263 1665 >21.65

Table 3. UVOT observation reported by Belles and Sbarufatti (GCN Circ. 29556). The start and stop time of the exposure are given in seconds since the BAT trigger. The preliminary 3-σ upper limit is given. No correction has been made for extinction in the Milky Way.

February 27, 2021