Swift Observations of GRB 201209A

J.D. Gropp (PSU) and K.K. Simpson (PSU) for the Swift team

1. Introduction

At 05:44:52 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 201209A (trigger=1011980) (Gropp et al. GCN Circ. 29005). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 123° from the Sun (8.5 hours East) and 162° from the 37%-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.

Zhu et al. (GCN Circ. 29013) reported the position from Xinglong 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 Stamatikos et al. (GCN Circ. 29021), the BAT ground-calculated position is RA, Dec = 23.096, -1.762 deg which is RA(J2000) = 01h32m23.0s Dec(J2000) = -01°45'43.8" with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 11%.

The mask-weighted light curve (Figure 1) shows some early emission starting at ~T-20 s, and two bright, symmetrical peaks centered at T+0 and T+7 s. The burst has faded to background by T+30 s. T90 (15-350 keV) is 48.0 ± 16.0 s (estimated error including systematics).

The time-averaged spectrum from T-31.66 to T+32.34 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.56 ± 0.06. The fluence in the 15-150 keV band is 1.4 ± 0.1 x 10-5 erg cm-2. This fluence is larger than that of 94.6% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+7.84 s in the 15-150 keV band is 12.2 ± 1.1 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/1011980/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Gropp et al. (GCN Circ. 29014). We have analysed 25 ks of XRT data for GRB 201209A, from 163 s to 975.3 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 29009).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=2.9 (+0.7, -0.8). At T+217 s the decay flattens to an α of -0.07 (+0.10, -0.15). The light curve breaks again at T+3258 s to a decay with α=0.90 ± 0.05, before a final break at T+98.2 ks s after which the decay index is 1.60 (+0.19, -0.27).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.98 ± 0.08. The best-fitting absorption column is 1.61 (+0.25, -0.24) x 1021 cm-2, in excess of the Galactic value of 3.6 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 (4.3 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.61 (+0.25, -0.24) x 1021 cm-2
Galactic foreground: 3.6 x 1020 cm-2
Excess significance: 8.5 σ
Photon index: 1.98 ± 0.08

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01011980.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 201209A 121 s after the BAT trigger (Simpson and Gropp GCN Circ. 29026). No optical afterglow consistent with the XRT position (Goad et al. GCN Circ. 29009) 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 0.04 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
01h32m21.77s -01°45'16.9" 1.4" XRT-final UKSSDC
01h32m21.77s -01°45'16.8" 1.7" XRT-enhanced Goad et al. GCN Circ. 29009
01h32m23.0s -01°45'43.8" 1.1' BAT-refined Stamatikos et al. GCN Circ. 29021

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 29004 Swift GRB201209.24: Global MASTER-Net
observations report 		MASTER
Optical Jelinek et al. 29006 FRAM-Auger optical limit FRAM upper limits
Optical V 29007 KAIT Optical Upper Limit KAIT upper limits
Optical Zhu et al. 29013 Xinglong-2.16m likely optical afterglow
detection		 Xinglong detection
Optical Pankov et al. 29094 Mondy optical observations, afterglow
confirmation		 Mondy detection
Gamma-ray Nadella et al. 29011 AstroSat CZTI detection CZTI
Gamma-ray Zhang et al. 29050 Insight-HXMT/HE detection Insight-HXMT T90=9.99 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 121 271 147 >20.1
uFC 334 584 246 >20.3
white 121 1706 412 >20.9
v 663 1583 117 >19.2
b 590 1682 117 >19.5
u 334 1657 343 >20.7
w1 712 1632 97 >19.8
m2 688 1260 39 >19.2
w2 1018 1211 39 >19.4

Table 3. UVOT observations reported by Simpson and Gropp (GCN Circ. 29026). 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.

December 21, 2020