Swift Observations of GRB 211211A

A. D'Ai (INAF-IASFPA), J.P. Osborne (U. Leicester) and A. Belles (PSU) for the Swift team

1. Introduction

At 13:09:59 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 211211A (trigger=1088940) (D'Ai et al. GCN Circ. 31202). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 68° from the Sun (3.1 hours West) and 141° from the 55%-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.

Zheng and Filippenko (GCN Circ. 31203) reported the position from KAIT for the optical afterglow of this GRB. Belles and D'Ai (GCN Circ. 31222) reported the detection with UVOT of an optical afterglow. Malesani et al. (GCN Circ. 31221) determined a redshift of 0.076 from NOT, and Minaev and Pozanenko (GCN Circ. 31230) determined a redshift of 0.080. 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. 31209), the BAT ground-calculated position is RA, Dec = 212.272, 27.884 deg which is RA(J2000) = 14h09m05.2s Dec(J2000) = +27°53'03.8" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 21%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T0 and ends at ~T+140 s. The main peak occurs at ~T+7 s. T90 (15-350 keV) is 51.37 ± 0.80 s (estimated error including systematics).

The time-averaged spectrum from T0 to T+142.2 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.56 ± 0.02. The fluence in the 15-150 keV band is 1.5 ± 0.02 x 10-4 erg cm-2. This fluence is larger than that of 100.0% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+6.86 s in the 15-150 keV band is 155.0 ± 3.1 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/1088940/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Osborne et al. (GCN Circ. 31212). We have analysed 23 ks of XRT data for GRB 211211A, from 69 s to 286.2 ks after the BAT trigger. The data comprise 214 s in Windowed Timing (WT) mode (the first 9 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 31205).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=1.99 ± 0.07. At T+177 s the decay steepens to an α of 4.60 (+0.16, -0.18). The light curve breaks again at T+282 s to a decay with α=1.106 (+0.025, -0.052), before a final break at T+42.1 ks s after which the decay index is 3.0 (+0.5, -0.4).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.80 ± 0.03. The best-fitting absorption column is 4.5 ± 0.8 x 1020 cm-2, in excess of the Galactic value of 1.8 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.50 (+0.12, -0.06) 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 4.2 x 10-11 (4.3 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.8 (+2.5, -0.0) x 1020 cm-2
Galactic foreground: 1.8 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 1.50 (+0.12, -0.06)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01088940.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 211211A 92 s after the BAT trigger (Belles and D'Ai GCN Circ. 31222). A fading source consistent with the XRT position (Beardmore et al. GCN Circ. 31205) and optical position (Zheng & Filippenko, GCN Circ. 31203) 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 0.018 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
14h09m10.08s +27°53'18.8" 1.9" XRT-final UKSSDC
14h09m10.09s +27°53'18.8" 2.1" XRT-enhanced Beardmore et al. GCN Circ. 31205
14h09m05.2s +27°53'03.8" 1.0' BAT-refined Stamatikos et al. GCN Circ. 31209

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Zheng and Filippenko 31203 KAIT Optical Afterglow Candidate KAIT detection
Optical Lipunov et al. 31206 Fermi GRB 211211A: Global MASTER-Net
observations report 		MASTER
Optical Jiang et al. 31213 Nanshan/NEXT optical observations Xinjiang Astro. Obs. detection
Optical Strausbaugh and Cucchiara 31214 LCO Optical Observations LCO detection
Optical Lipunov et al. 31216 Swift GRB 211211A: Global MASTER-Net
observations report 		MASTER
Optical Ito et al. 31217 MITSuME Akeno optical observation MITSuME Akeno marginal detection
Optical de Ugarte Postigo et al. 31218 Afterglow detection from CAFOS/2.2m CAHA CAHA detection
Optical Malesani et al. 31221 NOT optical spectroscopy NOT spectroscopy
Optical Kumar et al. 31227 HCT and GIT optical follow up
observations		 GROWTH-India Tel.
Optical de Ugarte Postigo et al. 31228 Further observations from CAFOS/2.2m
CAHA 		CAHA
Optical de Ugarte Postigo et al. 31229 Very fast decay observed from
CAFOS/2.2mCAHA		 CAHA light curve
Optical Mao et al. 31232 GMG upper limit Gao-Mei-Gu upper limits
Optical Pankov et al. 31233 AbAO optical observations Abastumani Astro. Obs. marginal detection
Optical Moskvitin et al. 31234 SAO RAS optical observations SAO RAS marginal detection
Optical Levan et al. 31235 Gemini K-band detection Gemini detection
Optical D'Avanzo et al. 31242 TNG NIR observations TNG
Optical Rastinejad et al. 31264 MMT/MMIRS Observations Indicate Fading
of K-band Source		 MMT
Optical Gupta et al. 31299 observations with the 3.6m Devasthal
Optical Telescope		 Devasthal Opt.Tel.
Gamma-ray Fermi 31201 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Mangan et al. 31210 Fermi GBM observation Fermi GBM Epeak=646.8±7.8 keV
Fluence=5.4±0.01x10-4erg cm-2
Gamma-ray Tamura et al. 31226 CALET Gamma-Ray Burst Monitor detection CALET
Gamma-ray Minaev and Pozanenko 31230 redshift estimation and
SPI-ACS/INTEGRAL detection		 INTEGRAL
Gamma-ray Zhang et al. 31236 Insight-HXMT/HE detection Insight-HXMT
Other Malesani 31223 incorrect Eiso value in GCN Circ. 31221

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
whFC 92 242 147 18.93±0.22
wh 3906 4106 196 18.33±0.09
wh 68215 68397 179 >19.74
b 3701 3901 196 18.97±0.28
b 16837 17437 587 19.27±0.28
b 67303 68210 885 19.70±0.24
v 4317 4517 196 >18.33
u 3496 5012 275 18.26±0.16
u 15924 16831 885 18.44±0.11
u 57177 74247 362 >18.89
w1 4727 4927 196 17.46±0.16
m2 4522 4721 196 17.58±0.19
m2 21678 22153 467 18.18±0.18
w2 4112 4312 196 17.30±0.15
w2 61756 62656 885 18.94±0.24

Table 3. UVOT observations reported by Belles and D'Ai (GCN Circ. 31222). 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 25, 2021