Swift Observations of GRB 181020A

M.J. Moss (George Washington University), K.L. Page (U. Leicester) and J.D. Gropp (PSU) for the Swift team

1. Introduction

At 19:00:33 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 181020A (trigger=867987) (Moss et al. GCN Circ. 23349). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 121° from the Sun (11.3 hours East) and 45° 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.

Moss et al. (GCN Circ. 23349) reported the discovery with UVOT of an optical afterglow. Fynbo et al. (GCN Circ. 23356) determined a redshift of 2.938 from VLT. 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 Sakamoto et al. (GCN Circ. 23357), the BAT ground-calculated position is RA, Dec = 13.987, -47.372 deg which is RA(J2000) = 00^h55^m56.9^s Dec(J2000) = -47°22'18.1" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 94%.

The mask-weighted light curve (Figure 1) shows a large pulse that starts at ~T-1 s, peaks at ~T+8 s and ends at ~T+30 s. There is a second, much weaker, pulse that occurs at ~T+200 s and lasts until ~T+270 s. T₉₀ (15-350 keV) is 238.0 ± 11.6 s (estimated error including systematics).

The time-averaged spectrum from T-1.45 to T+271.20 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.25 ± 0.06. The fluence in the 15-150 keV band is 8.1 ± 0.3 x 10^-6 erg cm^-2. This fluence is larger than that of 89% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+7.26 s in the 15-150 keV band is 7.7 ± 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/867987/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Page et al. (GCN Circ. 23353). We have analysed 24 ks of XRT data for GRB 181020A, from 48 s to 299.0 ks after the BAT trigger. The data comprise 1.1 ks in Windowed Timing (WT) mode (the first 6 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 Goad et al. (GCN Circ. 23351).

The late-time light curve (Figure 2) (from T0+4.8 ks) can be modelled with an initial power-law decay with an index of α=0.02 (+0.14, -0.15), followed by a break at T+14.2 ks to an α of 2.45 (+0.11, -0.10).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.862 ± 0.018. The best-fitting absorption column is 3.7 ± 0.6 x 10²¹ cm^-2, at a redshift of 2.938, in addition to the Galactic value of 1.6 x 10²⁰ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.94 (+0.07, -0.06) and a best-fitting absorption column of 4.3 ± 2.1 x 10²¹ cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.4 x 10^-11 (3.7 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.6 x 10²⁰ cm^-2
Intrinsic column: 4.3 ± 2.1 x 10²¹ cm^-2 at z=2.938
Photon index: 1.94 (+0.07, -0.06)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00867987.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 181020A 122 s after the BAT trigger (Gropp et al. GCN Circ. 23360). A fading source consistent with the XRT enhanced position (Goad et al. GCN Circ. 23351) 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 E_B-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

00^h55^m55.77^s -47°22'51.9" 0.45" UVOT-refined Gropp et al. GCN Circ. 23360

00^h55^m55.78^s -47°22'50.4" 1.4" XRT-final UKSSDC

00^h55^m55.80^s -47°22'49.2" 1.7" XRT-enhanced Goad et al. GCN Circ. 23351

00^h55^m56.9^s -47°22'18.1" 1.0' BAT-refined Sakamoto et al. GCN Circ. 23357

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Fynbo et al. 23356 VLT/X-shooter spectroscopy and redshift VLT spectroscopy

Optical Cucchiara et al. 23358 LCO Cerro Tololo observations LCO detection

Optical Lipunov et al. 23359 Global MASTER Net OT observations MASTER

Optical Melandri et al. 23362 REM detection REM detection

Gamma-ray Axelsson 23350 Fermi-LAT detection Fermi LAT E_max=930 MeV

Gamma-ray Veres 23352 Fermi GBM detection Fermi GBM E_peak=367±17 keV
T₉₀=15 seconds
Fluence=3.07±0.04x10^-5erg cm^-2
(91^st percentile for long GRBs)

Gamma-ray Tsvetkova et al. 23363 Konus-Wind observation Konus-Wind E_peak=371 (-52,+57) keV
Duration=~26 seconds
Fluence=4.17(-0.54,+0.58)x10^-5erg cm^-2

Gamma-ray Sharma et al. 23436 AstroSat CZTI detection CZTI T₉₀=19.4 seconds

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

Filter T_start(s) T_stop(s) Exp(s) Mag

b 352 1027 97

m2 427 745 58 >18.06

u_fc 96 6797 768

u 96 346 245

v 21 720 67

w1 452 769 58 >18.20

w2 379 1053 97 >18.56

RA (J2000)	Dec (J2000)	Error	Note	Reference
00^h55^m55.77^s	-47°22'51.9"	0.45"	UVOT-refined	Gropp et al. GCN Circ. 23360
00^h55^m55.78^s	-47°22'50.4"	1.4"	XRT-final	UKSSDC
00^h55^m55.80^s	-47°22'49.2"	1.7"	XRT-enhanced	Goad et al. GCN Circ. 23351
00^h55^m56.9^s	-47°22'18.1"	1.0'	BAT-refined	Sakamoto et al. GCN Circ. 23357

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Fynbo et al.	23356	VLT/X-shooter spectroscopy and redshift	VLT	spectroscopy
Optical	Cucchiara et al.	23358	LCO Cerro Tololo observations	LCO	detection
Optical	Lipunov et al.	23359	Global MASTER Net OT observations	MASTER
Optical	Melandri et al.	23362	REM detection	REM	detection
Gamma-ray	Axelsson	23350	Fermi-LAT detection	Fermi LAT	E_max=930 MeV
Gamma-ray	Veres	23352	Fermi GBM detection	Fermi GBM	E_peak=367±17 keV T₉₀=15 seconds Fluence=3.07±0.04x10^-5erg cm^-2 (91^st percentile for long GRBs)
Gamma-ray	Tsvetkova et al.	23363	Konus-Wind observation	Konus-Wind	E_peak=371 (-52,+57) keV Duration=~26 seconds Fluence=4.17(-0.54,+0.58)x10^-5erg cm^-2
Gamma-ray	Sharma et al.	23436	AstroSat CZTI detection	CZTI	T₉₀=19.4 seconds

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
b	352	1027	97
m2	427	745	58	>18.06
u_fc	96	6797	768
u	96	346	245
v	21	720	67
w1	452	769	58	>18.20
w2	379	1053	97	>18.56

Table 3. UVOT observations reported by Gropp et al. (GCN Circ. 23360). 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.

November 24, 2018