Swift Observations of GRB 181110A

P.A. Evans (U. Leicester), B. Sbarufatti (INAF-OAB/PSU) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 08:43:31 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 181110A (trigger=871316) (Evans et al. GCN Circ. 23413). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 70° from the Sun (5.1 hours East) and 41° from the 8%-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.

Evans et al. (GCN Circ. 23413) reported the discovery with UVOT of an optical afterglow. Perley et al. (GCN Circ. 23421) determined a redshift of 1.505 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 Lien et al. (GCN Circ. 23420), the BAT ground-calculated position is RA, Dec = 302.330, -36.878 deg which is RA(J2000) = 20^h09^m19.2^s Dec(J2000) = -36°52'41.4" with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T-110 s and ends at ~T+130 s. The main peak occurs at ~T+25 s. T₉₀ (15-350 keV) is 138.4 ± 10.9 s (estimated error including systematics).

The time-averaged spectrum from T-109.3 to T+129.8 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.02 ± 0.05. The fluence in the 15-150 keV band is 9.9 ± 0.3 x 10^-6 erg cm^-2. This fluence is larger than that of 92.5% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+25.03 s in the 15-150 keV band is 3.7 ± 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/871316/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Sbarufatti et al. (GCN Circ. 23418). We have analysed 22 ks of XRT data for GRB 181110A, from 55 s to 116.5 ks after the BAT trigger. The data comprise 260 s in Windowed Timing (WT) mode (the first 8 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. 23417).

The late-time light curve (Figure 2) (from T0+5.2 ks) can be modelled with a power-law decay with a decay index of α=1.84 (+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 1.88 ± 0.03. The best-fitting absorption column is 6.4 (+6.4, -6.1) x 10²⁰ cm^-2, at a redshift of 1.505, in addition to the Galactic value of 7.7 x 10²⁰ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.77 (+0.10, -0.06) and a best-fitting absorption column consistent with the Galactic value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.7 x 10^-11 (4.2 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 7.7 x 10²⁰ cm^-2
Intrinsic column: 7.7 (+16.4, -0.0) x 10²⁰ cm^-2 at z=1.505
Photon index: 1.77 (+0.10, -0.06)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00871316.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 181110A 72 s after the BAT trigger (Kuin and Evans GCN Circ. 23419). The GRB was detected in ground photometry (Yamanaka et al., GCN Circ. 23414; Murata et al., GCN Circ. 23415; Denisenko et al. GCN Circ. 23416). 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 0.07 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

20^h09^m16.27^s -36°53'47.9" 0.46" UVOT-refined Kuin and Evans GCN Circ. 23419

20^h09^m16.32^s -36°53'48.2" 1.4" XRT-final UKSSDC

20^h09^m16.32^s -36°53'47.9" 1.4" XRT-enhanced Goad et al. GCN Circ. 23417

20^h09^m19.2^s -36°52'41.4" 1.4' BAT-refined Lien et al. GCN Circ. 23420

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Yamanaka et al. 23414 Kanata 1.5m optical observation Kanata

Optical Murata et al. 23415 MITSuME Akeno optical observation MITSuME Akeno

Optical Denisenko et al. 23416 optical observations from iTelescope SSO iTelescope detection

Optical Perley et al. 23421 VLT/X-shooter redshift VLT redshift

Optical Bolmer and Schady 23422 GROND observations GROND detection

Optical Belkin et al. 23423 optical observations in Chilescope
observatory detection

Gamma-ray Frederiks et al. 23424 Konus-Wind observation Konus-Wind E_peak=48 (-27,+14) keV

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 72 222 147 18.52 ± 0.07

v 614 634 19 16.09 ± 0.13

b 540 559 20 16.98 ± 0.11

u 284 534 246 16.66 ± 0.04

w1 663 683 19 15.65 ± 0.13

m2 1068 1088 20 14.66 ± 0.11

w2 1018 1038 20 15.74 ± 0.14

RA (J2000)	Dec (J2000)	Error	Note	Reference
20^h09^m16.27^s	-36°53'47.9"	0.46"	UVOT-refined	Kuin and Evans GCN Circ. 23419
20^h09^m16.32^s	-36°53'48.2"	1.4"	XRT-final	UKSSDC
20^h09^m16.32^s	-36°53'47.9"	1.4"	XRT-enhanced	Goad et al. GCN Circ. 23417
20^h09^m19.2^s	-36°52'41.4"	1.4'	BAT-refined	Lien et al. GCN Circ. 23420

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Yamanaka et al.	23414	Kanata 1.5m optical observation	Kanata
Optical	Murata et al.	23415	MITSuME Akeno optical observation	MITSuME Akeno
Optical	Denisenko et al.	23416	optical observations from iTelescope SSO	iTelescope	detection
Optical	Perley et al.	23421	VLT/X-shooter redshift	VLT	redshift
Optical	Bolmer and Schady	23422	GROND observations	GROND	detection
Optical	Belkin et al.	23423	optical observations in Chilescope observatory		detection
Gamma-ray	Frederiks et al.	23424	Konus-Wind observation	Konus-Wind	E_peak=48 (-27,+14) keV

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white	72	222	147	18.52 ± 0.07
v	614	634	19	16.09 ± 0.13
b	540	559	20	16.98 ± 0.11
u	284	534	246	16.66 ± 0.04
w1	663	683	19	15.65 ± 0.13
m2	1068	1088	20	14.66 ± 0.11
w2	1018	1038	20	15.74 ± 0.14

Table 3. UVOT observations reported by Kuin and Evans (GCN Circ. 23419). 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 13, 2018