Swift Observations of GRB 201223A

J.D. Gropp (PSU) and M. Perri (SSDC and INAF-OAR) for the Swift team

1. Introduction

At 17:58:26 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 201223A (trigger=1014316) (Gropp et al. GCN Circ. 29158). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 127° from the Sun (9.3 hours West) and 93° from the 67%-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.

Gropp et al. (GCN Circ. 29158) reported the detection 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 Laha et al. (GCN Circ. 29163), the BAT ground-calculated position is RA, Dec = 132.735, 71.176 deg which is RA(J2000) = 08h50m56.3s Dec(J2000) = +71°10'34.9" with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 53%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T-25 s and ends at ~T+8 s. The main peak occurs at ~T0. T90 (15-350 keV) is 29.0 ± 6.7 s (estimated error including systematics).

The time-averaged spectrum from T-25.18 to T+7.82 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.35 ± 0.13. The fluence in the 15-150 keV band is 1.3 ± 0.1 x 10-6 erg cm-2. This fluence is larger than that of 47% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.18 s in the 15-150 keV band is 1.5 ± 0.2 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/1014316/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Perri et al. (GCN Circ. 29171). We have analysed 19 ks of XRT data for GRB 201223A, from 81 s to 504.6 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. 29170).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=0.42 ± 0.16, followed by a break at T+2679 s to an α of 1.54 (+0.18, -0.17).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.87 (+0.18, -0.13). The best-fitting absorption column is 4.1 (+4.0, -0.9) x 1020 cm-2, consistent with the Galactic value of 3.1 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 (2.8 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 4.1 (+4.0, -0.9) x 1020 cm-2
Galactic foreground: 3.1 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 1.87 (+0.18, -0.13)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01014316.

4. UVOT Observations and Analysis

Analysis of the UVOT data was reported by Gropp (GCN Circ. 29177). 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
08h51m09.51s +71°10'47.4" 0.42" UVOT-refined Gropp GCN Circ. 29177
08h51m09.61s +71°10'45.3" 1.5" XRT-final UKSSDC
08h51m09.64s +71°10'45.2" 1.8" XRT-enhanced Goad et al. GCN Circ. 29170
08h50m56.3s +71°10'34.9" 1.4' BAT-refined Laha et al. GCN Circ. 29163

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 29156 Swift GRB201223.75: Global MASTER-Net
observations report
MASTER
Optical Lipunov et al. 29157 Swift GRB201223.75: Global MASTER-Net
OT detection
MASTER detection
Optical Zhu et al. 29159 Nanshan/NEXT early optical afterglow
detection
Xinjiang Astro. Obs. detection
Optical Hu et al. 29160 BOOTES-4/MET early optical observation BOOTES-4 detection
Optical Hosokawa et al. 29164 MITSuME Akeno optical observation MITSuME Akeno detection
Optical 29165 iTelescope optical afterglow
observations
iTelescope
Optical Xin et al. 29166 GWAC-F60A optical afterglow observations Xinglong Obs.
Optical Belkin et al. 29167 Mondy and Kitab optical observations Mondy detection
Optical Kumar et al. 29168 GROWTH-India Telescope (GIT)
photometric follow-up.
GROWTH-India detection
Optical Xu et al. 29169 NOT optical upper limit and NEXT
powerlaw decay
NOT upper limits
Optical Gupta et al. 29173 1.3m DFOT and 1.04m ST optical
observations
Devasthal Opt.Tel. detection
Optical Pankov et al. 29174 AbAO optical observations Abastumani Astro. Obs. detection
Optical Kumar et al. 29176 HCT optical upper limit Himalayan Chandra Telescope upper limits
Optical Nakamura et al. 29180 AROMA-N Optical Observation AROMA-N
Optical Xin et al. 29201 SVOM-GWAC detected the optical
counterpart during prompt emission
phase
Xinglong Obs. detection
Gamma-ray Wood 29161 Fermi GBM Detection Fermi GBM Epeak=86±12 keV
T90=33 seconds
Fluence=2.1±0.3x10-6erg cm-2
(30th 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 79 229 147 16.06±0.03
white 571 1019 186 18.04±0.08
white 5921 6121 196 >20.17
white 12121 12630 497 20.09±0.30
v 621 1069 58 >17.44
b 547 1167 58 17.97±0.25
b 11208 12115 885 >20.38
uFC 292 541 246 16.75 ± 0.04
u 694 1142 38 >17.58
w1 670 1118 58 >17.65
m2 645 1093 58 >17.87
w2 596 1045 58 >17.81

Table 3. UVOT observations reported by Gropp (GCN Circ. 29177). 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.

December 30, 2020