Swift Observations of GRB 210226A

A.P. Beardmore (U. Leicester), M. Capalbi (INAF-IASFPA) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 04:43:57 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210226A (trigger=1034721) (Beardmore et al. GCN Circ. 29568). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 124° from the Sun (9.7 hours East) and 43° from the 98%-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.

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 Barthelmy et al. (GCN Circ. 29579), the BAT ground-calculated position is RA, Dec = 124.153, 57.585 deg which is RA(J2000) = 08h16m36.7s Dec(J2000) = +57°35'06.9" with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment). The partial coding was 47%.

The BAT light curve (Figure 1) showed a complex structure with a duration of about ~30 s. T90 (15-350 keV) is 20.80 ± 1.68 s (estimated error including systematics).

The time-averaged spectrum from T-1.92 to T+20.95 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.44 ± 0.15. The fluence in the 15-150 keV band is 1.2 ± 0.1 x 10-6 erg cm-2. This fluence is larger than that of 45% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.02 s in the 15-150 keV band is 2.8 ± 0.3 ph cm-2 s-1. All the quoted errors are at the 90% confidence level.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Capalbi et al. (GCN Circ. 29575). We have analysed 19 ks of XRT data for GRB 210226A, from 101 s to 194.4 ks after the BAT trigger. The data comprise 12 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 29571).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=1.0 (+0.3, -0.4). At T+422 s the decay flattens to an α of -0.3 (+0.4, -1.1) before breaking again at T+1000 s to a final decay with index α=1.12 (+0.07, -0.06).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.97 (+0.18, -0.17). The best-fitting absorption column is 1.9 ± 0.6 x 1021 cm-2, in excess of the Galactic value of 5.2 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 3.5 x 10-11 (4.7 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.9 ± 0.6 x 1021 cm-2
Galactic foreground: 5.2 x 1020 cm-2
Excess significance: 3.9 σ
Photon index: 1.97 (+0.18, -0.17)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01034721.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210226A 102 s after the BAT trigger (Kuin and Beardmore GCN Circ. 29590). No optical afterglow consistent with the XRT position 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 0.049 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
08h16m27.67s +57°35'02.2" 1.5" XRT-final UKSSDC
08h16m27.67s +57°35'02.2" 1.5" XRT-enhanced Goad et al. GCN Circ. 29571
08h16m36.7s +57°35'06.9" 1.5' BAT-refined Barthelmy et al. GCN Circ. 29579

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Jelinek et al. 29570 Ondrejov BART/SBT optical limit Small Binocular Tel. upper limits
Optical 29572 Burke-Gaffney Observatory optical upper
limit		 Burke-Gaffney Obs. upper limits
Optical Lipunov et al. 29574 Swift GRB 210226A: Global MASTER-Net
observations report 		MASTER
Optical Zheng and Filippenko 29576 KAIT Optical Upper Limit KAIT upper limits
Optical Kumar et al. 29656 GIT optical upper limit GROWTH-India upper limits
Gamma-ray Wood and Meegan 29578 Fermi GBM detection Fermi GBM Epeak=170±29 keV
T90=26 seconds
Fluence=2.7±0.2x10-6erg cm-2
(38th percentile for long GRBs)

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 102 251 147 >20.9
uFC 314 563 246 >20.4
white 102 1192 353 >21.4
v 643 1236 72 >19.1
b 569 1167 58 >19.9
u 314 5121 463 >20.5
w1 692 4935 236 >20.4
m2 4530 4730 197 >20.0

Table 3. UVOT observations reported by Kuin and Beardmore (GCN Circ. 29590). 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.

March 16, 2021