Swift Observations of GRB 180818A

F.E. Marshall (NASA/GSFC), V. D'Elia (ASDC) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 03:12:04 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180818A (trigger=853824) (Marshall et al. GCN Circ. 23146). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 65° from the Sun (1.5 hours East) and 62° from the 48%-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 D'Elia et al. (GCN Circ. 23148). We have analysed 16 ks of XRT data for GRB 180818A, from 80 s to 81.3 ks after the BAT trigger. The data comprise 952 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. 23147).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=6.0 (+0.8, -0.6), followed by a break at T+168 s to an α of 0.69 ± 0.07.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.9 ± 0.4. The best-fitting absorption column is 8.9 (+2.9, -2.4) x 10²¹ cm^-2, in excess of the Galactic value of 1.1 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 3.1 x 10^-11 (1.3 x 10^-10) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 8.9 (+2.9, -2.4) x 10²¹ cm^-2
Galactic foreground: 1.1 x 10²¹ cm^-2
Excess significance: 5.3 σ
Photon index: 2.9 ± 0.4

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00853824.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180818A 83 s after the BAT trigger (Kuin and Marshall GCN Circ. 23150). No optical afterglow consistent with the XRT position (Osborne et al. GCN Circ. 23147) 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 0.11 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

11^h18^m32.76^s -48°20'40.3" 1.5" XRT-final UKSSDC

11^h18^m32.83^s -48°20'39.9" 1.9" XRT-enhanced Osborne et al. GCN Circ. 23147

11^h18^m27^s -48°20'39" 3' BAT-initial Marshall et al. GCN Circ. 23146

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Table 2. No GCN Circulars from other observatories were found.

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

white_FC 83 232 147 >20.7

u_FC 241 490 246 >20.1

white 83 7151 785 >21.9

v 570 11683 1198 >19.8

b 496 6946 510 >20.8

u 241 17153 1326 >20.9

w1 621 13097 985 >20.1

m2 596 12588 1199 >20.5

w2 546 7357 510 >20.7

RA (J2000)	Dec (J2000)	Error	Note	Reference
11^h18^m32.76^s	-48°20'40.3"	1.5"	XRT-final	UKSSDC
11^h18^m32.83^s	-48°20'39.9"	1.9"	XRT-enhanced	Osborne et al. GCN Circ. 23147
11^h18^m27^s	-48°20'39"	3'	BAT-initial	Marshall et al. GCN Circ. 23146

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white_FC	83	232	147	>20.7
u_FC	241	490	246	>20.1
white	83	7151	785	>21.9
v	570	11683	1198	>19.8
b	496	6946	510	>20.8
u	241	17153	1326	>20.9
w1	621	13097	985	>20.1
m2	596	12588	1199	>20.5
w2	546	7357	510	>20.7

Table 3. UVOT observations reported by Kuin and Marshall (GCN Circ. 23150). 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.

August 20, 2018