Swift Observations of GRB 180818B

F.E. Marshall (NASA/GSFC), A. D'Ai (INAF-IASFPA) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 12:30:18 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180818B (trigger=853882) (Marshall et al. GCN Circ. 23149). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 46° from the Sun (2.9 hours West) and 132° from the 52%-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 Cummings et al. (GCN Circ. 23155), the BAT ground-calculated position is RA, Dec = 104.227, 39.310 deg which is RA(J2000) = 06h56m54.5s Dec(J2000) = +39°18'35.1" with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 89%.

The mask-weighted light curve (Figure 1) shows two pulses. The first pulse starts at at ~T-100 s, peaks at ~T-80 s, and ends at ~T-50 s. The second pulse starts at ~T+10 s, peaks at ~T+30 s, and ends at ~T+60 s. T90 (15-350 keV) is 134.4 ± 9.8 s (estimated error including systematics). However, note that the burst came into the BAT FOV at ~T-128 s, and thus there might be additional burst emission beforehand.

The time-averaged spectrum from T-93.58 to T+62.74 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.77 ± 0.12. The fluence in the 15-150 keV band is 2.3 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 63% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-82.11 s in the 15-150 keV band is 1.2 ± 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/853882/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Ai et al. (GCN Circ. 23156). We have analysed 10 ks of XRT data for GRB 180818B, from 210 s to 69.0 ks after the BAT trigger. The data comprise 205 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 23151).

The late-time light curve (Figure 2) (from T0+4.5 ks) can be modelled with a power-law decay with a decay index of α=0.93 (+0.07, -0.06).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 3.47 (+0.12, -0.11). The best-fitting absorption column is 2.89 (+0.25, -0.23) x 1021 cm-2, in excess of the Galactic value of 1.3 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.77 (+0.07, -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 3.8 x 10-11 (4.5 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.27 (+0.11, -0.00) x 1021 cm-2
Galactic foreground: 1.3 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 1.77 (+0.07, -0.06)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00853882.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180818B 212 s after the BAT trigger (Kuin and Marshall GCN Circ. 23152). No optical afterglow consistent with the XRT position (Beardmore et al. GCN Circ. 23151) 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 0.09 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
06h56m53.89s +39°18'56.4" 1.5" XRT-final UKSSDC
06h56m53.93s +39°18'56.6" 1.7" XRT-enhanced Beardmore et al. GCN Circ. 23151
06h56m54.5s +39°18'35.1" 1.7' BAT-refined Cummings et al. GCN Circ. 23155

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Hu et al. 23161 1.5m OSN optical afterglow candidate Obs.de Sierra Nevada detection
Optical Mazaeva et al. 23177 TSHAO optical upper limit Zeiss-1000 upper limits
Gamma-ray Veres et al. 23153 Fermi GBM observations Fermi GBM Epeak=78±9 keV
T90=169 seconds
Fluence=4.7±0.7x10-7erg cm-2
(4.8 percentile for long GRBs)

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
whiteFC 212 362 147 >19.9
white 212 11082 1482 >21.1
v 369 12690 1148 >19.8
b 467 6919 471 >19.9
u 442 6714 469 >19.6
w1 418 6509 468 >19.9
m2 393 6304 471 >20.5
w2 517 11988 1160 >20.3

Table 3. UVOT observations reported by Kuin and Marshall (GCN Circ. 23152). 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 27, 2018