Swift Observations of GRB 201029A

S. Dichiara (NASA/GSFC/UMCP), M. Perri (ASDC) and F.E. Marshall (NASA/GSFC) for the Swift team

1. Introduction

At 20:19:50 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 201029A (trigger=1003002) (Dichiara et al. GCN Circ. 28803). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 60° from the Sun (1.0 hours East) and 125° from the 97%-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.

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

Analysis of the BAT data was reported by Laha et al. (GCN Circ. 28813). Dichiara et al., GCN Circ. 28803). The BAT ground-calculated position is RA, Dec = 229.607, 44.473 deg which is RA(J2000) = 15h18m25.7s Dec(J2000) = +44°28'21.7" with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 96%.

The BAT light curve (Figure 1) showed a complex structure with a duration of about 150 s. T90 (15-350 keV) is 120.55 ± 9.54 s (estimated error including systematics).

The time-averaged spectrum from T-2.19 to T+133.11 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.09 ± 0.11. The fluence in the 15-150 keV band is 2.2 ± 0.1 x 10-6 erg cm-2. This fluence is larger than that of 62% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+7.06 s in the 15-150 keV band is 0.6 ± 0.1 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/1003002/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Perri et al. (GCN Circ. 28808). We have analysed 14 ks of XRT data for GRB 201029A, from 128 s to 103.8 ks after the BAT trigger. The data comprise 764 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 28806).

The late-time light curve (Figure 2) (from T0+6.9 ks) can be modelled with a power-law decay with a decay index of α=2.2 (+1.2, -0.9).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.65 ± 0.05. The best-fitting absorption column is 5.0 ± 0.3 x 1021 cm-2, in excess of the Galactic value of 2.1 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.01 ± 0.11 and a best-fitting absorption column of 2.7 ± 0.4 x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10-11 (5.3 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.7 ± 0.4 x 1021 cm-2
Galactic foreground: 2.1 x 1020 cm-2
Excess significance: 9.8 σ
Photon index: 2.01 ± 0.11

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01003002.

4. UVOT Observations and Analysis

Analysis of the UVOT data was reported by Marshall and Dichiara (GCN Circ. 28810). 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 0.02 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
15h18m19.65s +44°27'34.2" 1.4" XRT-final UKSSDC
15h18m19.58s +44°27'33.4" 1.7" XRT-enhanced Beardmore et al. GCN Circ. 28806
15h18m25.7s +44°28'21.7" 2.0' BAT-refined Laha et al. GCN Circ. 28813

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Jelinek and Strobl 28805 FRAM-ORM optical limit FRAM upper limits
Optical Hu et al. 28807 BOOTES-1 optical upper limit BOOTES-1 upper limits
Optical Lipunov et al. 28811 Swift GRB 201029A: Global MASTER-Net
observations report 		MASTER
Gamma-ray Veres and Meegan 28809 Fermi GBM detection Fermi GBM Epeak=647±230 keV
T90=78 seconds
Fluence=1.59±0.14x10-5erg cm-2
(84th 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 131 281 147 >20.6
uFC 289 539 246 >19.9
white 131 1708 412 >21.4
v 619 1759 127 >19.2
b 545 1858 136 >19.9
u 289 1833 362 >20.0
w1 842 1808 97 >19.5

Table 3. UVOT observations reported by Marshall and Dichiara (GCN Circ. 28810). 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.

October 31, 2020