Swift Observations of GRB 201006A

J.D. Gropp (PSU) and F.E. Marshall (NASA/GSFC) for the Swift team

1. Introduction

At 01:17:52 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 201006A (trigger=998907) (Gropp et al. GCN Circ. 28560). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 111° from the Sun (8.7 hours West) and 47° from the 85%-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

As reported by Barthelmy et al. (GCN Circ. 28567), the BAT ground-calculated position is RA, Dec = 61.917, 65.148 deg which is RA(J2000) = 04h07m40.1s Dec(J2000) = +65°08'53.3" with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 33%.

The mask-weighted light curve (Figure 1) shows a pulse that starts and peaks at ~T0, and ends at ~T+0.5 s. T90 (15-350 keV) is 0.49 ± 0.09 s (estimated error including systematics).

The time-averaged spectrum from T-0.01 to T+0.54 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.42 ± 0.24. The fluence in the 15-150 keV band is 1.3 ± 0.2 x 10-7 erg cm-2. This fluence is larger than that of 79% of the short GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.23 s in the 15-150 keV band is 2.2 ± 0.3 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/998907/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Gropp et al. (GCN Circ. 28568). We have analysed 12 ks of XRT data for GRB 201006A, from 91 s to 316.1 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. 28562).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.99 (+0.14, -0.13).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.1 ± 0.6. The best-fitting absorption column is 1.3 (+0.7, -0.5) x 1022 cm-2, in excess of the Galactic value of 4.5 x 1021 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.9 x 10-11 (1.1 x 10-10) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.3 (+0.7, -0.5) x 1022 cm-2
Galactic foreground: 4.5 x 1021 cm-2
Excess significance: 2.5 σ
Photon index: 2.1 ± 0.6

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00998907.

4. UVOT Observations and Analysis

Analysis of the UVOT data was reported by Marshall and Gropp (GCN Circ. 28565). 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 1.27 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
04h07m34.20s +65°09'52.4" 1.9" XRT-final UKSSDC
04h07m34.25s +65°09'52.6" 2.1" XRT-enhanced Goad et al. GCN Circ. 28562
04h07m40.1s +65°08'53.3" 2.1' BAT-refined Barthelmy et al. GCN Circ. 28567

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 28559 Swift GRB201006.05: Global MASTER-Net
observations report 		MASTER
Optical Ito et al. 28571 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Kumar et al. 28573 GROWTH-India Telescope optical upper
limit		 GROWTH-India upper limits
X-ray Escorial et al. 28598 Chandra upper limit on the X-ray
afterglow		 Chandra upper limits
Gamma-ray Mereghetti et al. 28561 INTEGRAL sub-threshold detection INTEGRAL
Gamma-ray Hamburg and Meegan 28564 Fermi GBM detection Fermi GBM Epeak=130±26 keV
T90=1.7 seconds
Fluence=3.33±0.38x10-7erg cm-2
(51st percentile for short GRBs)
Other 28563 is likely a short burst
Other Dichiara et al. 28572 Lowell Discovery Telescope upper limits 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
whiteFC 88 238 147 >21.1
uFC 300 549 246 >20.2
white 88 1539 385 >21.0
v 629 1423 97 >18.9
b 555 1521 97 >19.8
u 300 722 265 >20.3
w1 678 1472 97 >18.7
w2 1205 1225 19 >17.3

Table 3. UVOT observations reported by Marshall and Gropp (GCN Circ. 28565). 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 12, 2020