Swift Observations of GRB 180805B

P. D'Avanzo (INAF-OAB), S.J. LaPorte (PSU) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 13:02:36 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180805B (trigger=851855) (D'Avanzo et al. GCN Circ. 23076). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 113° from the Sun (7.3 hours West) and 40° from the 42%-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 Sakamoto et al. (GCN Circ. 23082), the BAT ground-calculated position is RA, Dec = 25.877, -17.475 deg which is RA(J2000) = 01^h43^m30.6^s Dec(J2000) = -17°28'29.3" with an uncertainty of 3.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 67%.

The mask-weighted light curve (Figure 1) show a short pulse that starts from ~T-0.1 s, peaks at ~T+0.1 s, and ends at ~T+0.7 s. The short pulse is followed by some weak emission that lasts until ~ T+130 s. T₉₀ (15-350 keV) is 122.5 ± 18.3 s (estimated error including systematics).

The time-averaged spectrum from T-0.10 to T+127.46 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.13 ± 0.29. The fluence in the 15-150 keV band is 9.6 ± 1.7 x 10^-7 erg cm^-2. This fluence is larger than that of 38% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.11 s in the 15-150 keV band is 1.7 ± 0.2 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/851855/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by LaPorte et al. (GCN Circ. 23083). We have analysed 11 ks of XRT data for GRB 180805B, from 66 s to 145.6 ks after the BAT trigger. The data comprise 197 s in Windowed Timing (WT) mode (the first 9 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 Evans et al. (GCN Circ. 23077).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=0.64 (+0.16, -0.15), followed by a break at T+165 s to an α of 2.71 (+0.12, -0.10).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 0.93 (+0.09, -0.06). The best-fitting absorption column is 2.3 (+4.7, -0.7) x 10²⁰ cm^-2, consistent with the Galactic value of 1.6 x 10²⁰ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.90 (+0.25, -0.23) and a best-fitting absorption column of 7.3 (+6.6, -5.5) x 10²⁰ cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 x 10^-11 (4.0 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 7.3 (+6.6, -5.5) x 10²⁰ cm^-2
Galactic foreground: 1.6 x 10²⁰ cm^-2
Excess significance: 1.7 σ
Photon index: 1.90 (+0.25, -0.23)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00851855.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180805B 85 s after the BAT trigger (Kuin and D'Avanzo GCN Circ. 23080). No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 23077) 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 0.02 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

01^h43^m07.61^s -17°29'36.5" 1.5" XRT-final UKSSDC

01^h43^m07.61^s -17°29'36.4" 1.8" XRT-enhanced Evans et al. GCN Circ. 23077

01^h43^m30.6^s -17°28'29.3" 3.1' BAT-refined Sakamoto et al. GCN Circ. 23082

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Lipunov et al. 23085 MASTER optical late inspection short
GRB with extended emission MASTER

Optical Malesani et al. 23086 VLT optical and near-infrared
observations VLT

Optical Ramsay et al. 23087 GOTO optical limits Gravitational-wave Optical Transient Observer upper limits

Optical Hu et al. 23088 1.5m OSN I-band observation Obs.de Sierra Nevada

Optical Watson et al. 23089 COATLI Optical Observations COATLI

Optical Higgins et al. 23090 ESO/NTT optical observations

Optical Malesani and Crouzet 23108 Teide LCO optical observations LCO

Gamma-ray Hamburg et al. 23078 Fermi GBM detection Fermi GBM E_peak=346±75 keV
T₉₀=1 seconds
Fluence=5.9±0.7x10^-7erg cm^-2
(68^th percentile for short GRBs)

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 85 235 147 >20.8

u_FC 297 546 246 >20.2

white 85 1544 392 >21.3

v 626 1421 78 >18.6

b 552 1519 97 >20.0

u 297 546 246 >20.2

w1 675 1470 97 >18.8

m2 1078 1097 19 >17.8

w2 774 1569 78 >19.0

RA (J2000)	Dec (J2000)	Error	Note	Reference
01^h43^m07.61^s	-17°29'36.5"	1.5"	XRT-final	UKSSDC
01^h43^m07.61^s	-17°29'36.4"	1.8"	XRT-enhanced	Evans et al. GCN Circ. 23077
01^h43^m30.6^s	-17°28'29.3"	3.1'	BAT-refined	Sakamoto et al. GCN Circ. 23082

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Lipunov et al.	23085	MASTER optical late inspection short GRB with extended emission	MASTER
Optical	Malesani et al.	23086	VLT optical and near-infrared observations	VLT
Optical	Ramsay et al.	23087	GOTO optical limits	Gravitational-wave Optical Transient Observer	upper limits
Optical	Hu et al.	23088	1.5m OSN I-band observation	Obs.de Sierra Nevada
Optical	Watson et al.	23089	COATLI Optical Observations	COATLI
Optical	Higgins et al.	23090	ESO/NTT optical observations
Optical	Malesani and Crouzet	23108	Teide LCO optical observations	LCO
Gamma-ray	Hamburg et al.	23078	Fermi GBM detection	Fermi GBM	E_peak=346±75 keV T₉₀=1 seconds Fluence=5.9±0.7x10^-7erg cm^-2 (68^th percentile for short GRBs)

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white_FC	85	235	147	>20.8
u_FC	297	546	246	>20.2
white	85	1544	392	>21.3
v	626	1421	78	>18.6
b	552	1519	97	>20.0
u	297	546	246	>20.2
w1	675	1470	97	>18.8
m2	1078	1097	19	>17.8
w2	774	1569	78	>19.0

Table 3. UVOT observations reported by Kuin and D'Avanzo (GCN Circ. 23080). 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 10, 2018