Swift Observations of GRB 210420B

M.J. Moss (GWU), J.P. Osborne (U. Leicester) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

Moss et al. (GCN Circ. 29844) reported the initial Swift results. BAT triggered on GRB 210420B (trigger 1044382) at some time before 18:50 UT. 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.

Lipunov et al. (GCN Circ. 29845) reported the position from MASTER for the optical afterglow of this GRB. Breeveld and Moss (GCN Circ. 29862) reported the detection with UVOT of an optical afterglow. de Ugarte Postigo et al. (GCN Circ. 29853) determined a redshift of 1.400 from GTC. 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. 29879), the BAT ground-calculated position is RA, Dec = 254.313, 42.558 deg which is RA(J2000) = 16h57m15.1s Dec(J2000) = +42°33'28.1" with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 77%.

The BAT light curve (Figure 1) showed a complex structure with a duration of about ~160 s. T90 (15-350 keV) is 158.8 ± 29.5 s (estimated error including systematics).

The time-averaged spectrum from T-21.30 to T+160.09 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.49 ± 0.19. The fluence in the 15-150 keV band is 1.5 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 52% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+21.02 s in the 15-150 keV band is 0.5 ± 0.2 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 Osborne et al. (GCN Circ. 29876). We have analysed 18 ks of XRT data for GRB 210420B, from 141 s to 351.9 ks after the BAT trigger. The data comprise 177 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode.

The late-time light curve (Figure 2) (from T0+12.7 ks) can be modelled with a power-law decay with a decay index of α=1.60 (+0.32, -0.22).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.72 (+0.08, -0.07). The best-fitting absorption column is 9 (+11, -9) x 1020 cm-2, at a redshift of 1.4, in addition to the Galactic value of 2.1 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.61 (+0.08, -0.07) and a best-fitting absorption column consistent with the Galactic value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.0 x 10-11 (4.2 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 2.1 x 1020 cm-2
Intrinsic column: 2.1 (+6.0, -0.0) x 1020 cm-2 at z=1.4
Photon index: 1.61 (+0.08, -0.07)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01044382.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations (Breeveld and Moss GCN Circ. 29862) of the field of GRB 210420B 147s after the BAT trigger (Moss et al., GCN Circ. 29844 and updated in Lien et al., GCN Circ. 29854). A fading source consistent with the optical counterpart reported by Lipunov et al. GCN Circ. 29845, Oksanen GCN Circ. 29846, Hu et al. GCN Circ. 29847, Tianrui et al. GCN Circ. 29848, Leonini et al. GCN Circ. 29851, de Ugarte Postigo et al. GCN Circ. 29852 and Watson et al. GCN Circ. 29858, 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 0.022 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
16h57m18.03s +42°34'12.4" 0.42" UVOT ?
16h57m17.90s +42°34'12.7" 3.1" XRT-final UKSSDC
16h57m17.45s +42°34'10.6" 3.5" XRT-refined Osborne et al. GCN Circ. 29876
16h57m15.1s +42°33'28.1" 2.1' BAT-refined Stamatikos et al. GCN Circ. 29879

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 29845 MASTER optical counterpart detection MASTER detection
Optical Oksanen 29846 Optical observations detection
Optical Hu et al. 29847 BOOTES-4/MET optical afterglow detection BOOTES-4 detection
Optical Sun et al. 29848 AST3-3 YaoAn Optical Observation Antarctic Survey Tel.
Optical Leonini et al. 29851 Montarrenti Observatory optical
afterglow detection		 Montarrenti detection
Optical de Ugarte Postigo et al. 29852 Multiband photometry from NOT NOT detection
Optical de Ugarte Postigo et al. 29853 Redshift from GTC GTC redshift
Optical Watson et al. 29858 DDOTI Detection of the Afterglow Deca-Degree Optical Imager detection
Optical Zheng and Filippenko 29861 KAIT Optical Observations KAIT detection
Optical Stecklum et al. 29863 Tautenburg observations Tautenburg detection
Optical Butler et al. 29864 RATIR Optical and NIR Observations RATIR
Optical Jelinek et al. 29866 Ondrejov D50 optical afterglow detection D50 detection
Optical 29867 Correction to DDOTI Magnitude Deca-Degree Optical Imager
Optical Leonini et al. 29872 Montarrenti Observatory follow-up
observations		 Montarrenti
Optical Vinko et al. 29883 follow-up photometry of the optical
afterglow at Konkoly		 Konkoly detection
Optical 29884 Correction to AST3-3 YaoAn magnitude Antarctic Survey Tel.
Optical Guelbenzu et al. 29890 continued Tautenburg observations Tautenburg light curve
Optical Pankov et al. 29894 Zeiss-1000 of Koshka optical
observations		 Koshka Obs. marginal detection
Optical Ibrahimov et al. 29895 follow-up photometry of optical
afterglow at INASAN/Simeiz		 marginal detection
Optical Belkin et al. 29901 Assy optical observations Assy-Turgen Obs. marginal detection
Other Lien et al. 29854 update on the BAT trigger time

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
u_fc 147 391 240 16.2 ± 0.04
u 18793 19317 512 18.6 ± 0.1
v 447 467 20 16.7 ± 0.2
b 397 417 19 17.1 ± 0.1
w1 496 516 20 16.5 ± 0.2
m2 620 1121 39 17.2 ± 0.2
w2 1201 1371 39 17.6 ± 0.2

Table 3. UVOT observations reported by Breeveld and Moss (GCN Circ. 29862). 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.

April 26, 2021