Swift Observations of GRB 180720C

S.J. LaPorte (PSU), A. Melandri (INAF-OAB) and S.R. Oates (U. Warwick) for the Swift team

1. Introduction

At 22:23:57 UT, the Swift Burst Alert Telescope (BAT) triggered and located a possible GRB 180720C or Galactic transient (trigger=848932) (LaPorte et al. GCN Circ. 22982). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 148° from the Sun (9.7 hours East) and 46° from the 62%-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

BAT refined results are not available.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Melandri et al. (GCN Circ. 23003). We have analysed 10.0 ks of XRT data for GRB 180720C, from 105 s to 79.8 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 22987).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.61 ± 0.06.

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, -0.5). The best-fitting absorption column is 2.8 (+1.2, -1.0) x 10²² cm^-2, in excess of the Galactic value of 6.8 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 6.7 x 10^-11 (1.7 x 10^-10) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.8 (+1.2, -1.0) x 10²² cm^-2
Galactic foreground: 6.8 x 10²¹ cm^-2
Excess significance: 3.7 σ
Photon index: 2.1 (+0.6, -0.5)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00848932.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180720C 100 s after the BAT trigger (Oates and LaPorte GCN Circ. 22992). No optical afterglow consistent with the XRT position (Evans et al., GCN Circ. 22987) 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 2.11 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

17^h42^m32.56^s -26°37'44.9" 1.8" XRT-final UKSSDC

17^h42^m32.54^s -26°37'44.5" 1.9" XRT-enhanced Evans et al. GCN Circ. 22987

17^h42^m30^s -26°35'01" 3' BAT-initial LaPorte et al. GCN Circ. 22982

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Guidorzi et al. 22991 LT observations Liverpool Telescope

Optical Gorbovskoy et al. 23009 MASTER optical observation MASTER

Optical del Pulgar et al. 23016 BOOTES-2/TELMA optical observations BOOTES-2

Gamma-ray Roberts and Meegan 22990 Fermi GBM observation Fermi GBM E_peak=583±161 keV
T₉₀=23 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

white_FC 100 250 147 >20.3

white 100 5490 541 >21.0

u_FC 312 425 111 >19.2

u 312 11199 1193 >20.6

v 4263 5901 393 >19.1

b 3646 11902 1072 >20.8

uvw1 4674 10285 1133 >20.3

uvm2 4469 6106 393 >19.7

uvw2 4058 5696 393 >19.8

RA (J2000)	Dec (J2000)	Error	Note	Reference
17^h42^m32.56^s	-26°37'44.9"	1.8"	XRT-final	UKSSDC
17^h42^m32.54^s	-26°37'44.5"	1.9"	XRT-enhanced	Evans et al. GCN Circ. 22987
17^h42^m30^s	-26°35'01"	3'	BAT-initial	LaPorte et al. GCN Circ. 22982

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Guidorzi et al.	22991	LT observations	Liverpool Telescope
Optical	Gorbovskoy et al.	23009	MASTER optical observation	MASTER
Optical	del Pulgar et al.	23016	BOOTES-2/TELMA optical observations	BOOTES-2
Gamma-ray	Roberts and Meegan	22990	Fermi GBM observation	Fermi GBM	E_peak=583±161 keV T₉₀=23 seconds

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white_FC	100	250	147	>20.3
white	100	5490	541	>21.0
u_FC	312	425	111	>19.2
u	312	11199	1193	>20.6
v	4263	5901	393	>19.1
b	3646	11902	1072	>20.8
uvw1	4674	10285	1133	>20.3
uvm2	4469	6106	393	>19.7
uvw2	4058	5696	393	>19.8

Table 3. UVOT observations reported by Oates and LaPorte (GCN Circ. 22992). 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.

July 23, 2018