Swift Observations of GRB 201229A

B. Sbarufatti (PSU), A. D'Ai (INAF-IASFPA) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 10:59:30 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 201229A (trigger=1015088) (Sbarufatti et al. GCN Circ. 29192). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 94° from the Sun (4.5 hours West) and 90° from the 100%-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.

Sbarufatti et al. (GCN Circ. 29192) reported the discovery 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 Lien et al. (GCN Circ. 29199), the BAT ground-calculated position is RA, Dec = 210.692, 48.209 deg which is RA(J2000) = 14h02m46.1s Dec(J2000) = +48°12'33.0" with an uncertainty of 2.6 arcmin, (radius, sys+stat, 90% containment). The partial coding was 26%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T-45 s and ends at ~T+15 s. There is a weak pulse occurring at ~T-40 s, followed by the main peak at ~T0. T90 (15-350 keV) is 53.3 ± 4.6 s (estimated error including systematics).

The time-averaged spectrum from T-44.43 to T+13.62 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.40 ± 0.29. The fluence in the 15-150 keV band is 1.3 ± 0.2 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+0.41 s in the 15-150 keV band is 1.7 ± 0.4 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/1015088/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Ai et al. (GCN Circ. 29200). We have analysed 18 ks of XRT data for GRB 201229A, from 115 s to 218.1 ks after the BAT trigger. The data comprise 22 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 29193).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=2.2 (+0.6, -0.5). At T+272 s the decay flattens to an α of 0.38 (+0.07, -0.09) before breaking again at T+12.7 ks to a final decay with index α=0.92 (+0.09, -0.08).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.80 ± 0.12. The best-fitting absorption column is 1.3 ± 0.4 x 1021 cm-2, in excess of the Galactic value of 1.8 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 3.8 x 10-11 (3.9 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.3 ± 0.4 x 1021 cm-2
Galactic foreground: 1.8 x 1020 cm-2
Excess significance: 5.2 σ
Photon index: 1.80 ± 0.12

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01015088.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 201229A 117 s after the BAT trigger (Siegel and Sbarufatti GCN Circ. 29213). A source consistent with the XRT position (Osborne et al. GCN Circ. 29193) is detected in the initial UVOT exposures, consistent with the optical transient reported in the initial circular and confirmed by SAO (Moskvitin et al., GCN Circ. 29203) and AbAO (Pankov et al., GCN Circ. 29205). 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 0.01 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
14h02m45.40s +48°11'53.3" 0.44" UVOT-refined Siegel and Sbarufatti GCN Circ. 29213
14h02m45.24s +48°11'52.0" 1.4" XRT-final UKSSDC
14h02m45.23s +48°11'51.9" 1.7" XRT-enhanced Osborne et al. GCN Circ. 29193
14h02m46.1s +48°12'33.0" 2.6' BAT-refined Lien et al. GCN Circ. 29199

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Fu et al. 29203 Nanshan/NEXT optical upper limit Xinjiang Astro. Obs. upper limits
Optical Moskvitin and Spiridonova 29204 SAO RAS optical observations SAO RAS detection
Optical Pankov et al. 29205 AbAO optical afterglow detection, TSHAO
optical upper limit		 Abastumani Astro. Obs. upper limits

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
white (fc) 117 266 147 18.22 ± 0.05
white 555 749 38 18.50±0.13
white 879 1029 147 19.17±0.13
white 1182 1548 58 19.42±0.34
white 23280 24102 801 20.88±0.22
v 605 1597 116 >18.48
v 17146 34853 1217 >20.39
b 531 1523 97 >19.33
b 22367 23274 885 >21.24
u (fc) 275 525 245 18.13±0.09
u 680 5086 107 18.57±0.25
u 12362 29756 1220 20.54±0.34
uvw1 654 5071 254 19.04±0.25
uvw1 28021 28921 885 >20.45
uvw2 16239 34408 1771 20.27±0.21

Table 3. UVOT observations reported by Siegel and Sbarufatti (GCN Circ. 29213). 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.

January 1, 2021