Swift Observations of GRB 180111A

A.Y. Lien (GSFC/UMBC), S.J. LaPorte (PSU) and J.D. Gropp (PSU) for the Swift team

1. Introduction

At 16:42:06 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180111A (trigger=804692) (Lien et al. GCN Circ. 22318). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 140° from the Sun (9.5 hours West) and 94° from the 24%-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.

Lien et al. (GCN Circ. 22318) reported the discovery with UVOT of an optical afterglow. 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 Stamatikos et al. (GCN Circ. 22324), the BAT ground-calculated position is RA, Dec = 149.782, 48.254 deg which is RA(J2000) = 09h59m07.7s Dec(J2000) = +48°15'14.6" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 70%.

The mask-weighted light curve (Figure 1) shows several overlapping pulses that starts at ~ T0 and ends at ~ T+70 s. The main peak occurs at ~ T+ 14 s. T90 (15-350 keV) is 50.6 ± 1.9 s (estimated error including systematics).

The time-averaged spectrum from T-0.30 to T+68.59 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.30 ± 0.03. The fluence in the 15-150 keV band is 1.02 ± 0.02 x 10-5 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+13.89 s in the 15-150 keV band is 4.9 ± 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/804692/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by LaPorte et al. (GCN Circ. 22322). We have analysed 12 ks of XRT data for GRB 180111A, from 76 s to 74.4 ks after the BAT trigger. The data comprise 117 s in Windowed Timing (WT) mode (the first 10 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. 22319).

The late-time light curve (Figure 2) (from T0+4.0 ks) can be modelled with a power-law decay with a decay index of α=1.42 (+0.28, -0.23).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.48 ± 0.09. The best-fitting absorption column is 4.0 (+3.0, -2.6) x 1020 cm-2, in excess of the Galactic value of 1.1 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.68 (+0.20, -0.13) and a best-fitting absorption column of 1.6 (+4.9, -0.5) x 1020 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.8 x 10-11 (3.9 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.6 (+4.9, -0.5) x 1020 cm-2
Galactic foreground: 1.1 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 1.68 (+0.20, -0.13)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00804692.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180111A 98 s after the BAT trigger (Gropp and Lien GCN Circ. 22327). A fading source consistent with the XRT position (Evans et al. GCN Circ. 22319) 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 EB-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
09h59m07.0s +48°16'03.3" UVOT-initial Lien et al. GCN Circ. 22318
09h59m07.12s +48°16'02.7" 1.5" XRT-final UKSSDC
09h59m07.14s +48°16'03.7" 1.8" XRT-enhanced Evans et al. GCN Circ. 22319
09h59m07.7s +48°15'14.6" 1.0' BAT-refined Stamatikos et al. GCN Circ. 22324

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Xin et al. 22320 Xinglong TNT possible optical detection TNT possible detection
Optical Butler et al. 22323 RATIR Optical and NIR Observations RATIR
Optical Mazaeva et al. 22331 AbAO optical observations Abastumani Astro. Obs. detection
Optical Kitaoka and Sakamoto 22352 AROMA-N optical observation AROMA-N
Gamma-ray Verrecchia et al. 22321 AGILE/MCAL detection AGILE
Gamma-ray Asaoka et al. 22333 CALET Gamma-Ray Burst Monitor detection CALET
Gamma-ray Svinkin et al. 22336 Konus-Wind observation Konus-Wind Epeak=894 (-95,+113) keV
Duration=~61 seconds
Fluence=8.44(-0.57,+0.64)x10-5erg cm-2
Gamma-ray Li et al. 22341 Insight-HXMT/HE observation Insight-HXMT T90=19.94 seconds

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
white 84 233 147 19.95± 0.21
white 86 6202 552 20.37± 0.16
v 64 5181 206 >19.23
b 4365 6002 393 >20.60
u 296 5795 639 20.25± 0.35
w1 5391 5591 196 >19.50
m2 650 670 19 >17.15
w2 4776 6407 393 19.60± 0.27

Table 3. UVOT observations reported by Gropp and Lien (GCN Circ. 22327). 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.

January 17, 2018