Swift Observations of GRB 181103A

A. Tohuvavohu (PSU), J.A. Kennea (PSU) and S.J. LaPorte (PSU) for the Swift team

1. Introduction

At 04:22:30 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 181103A (trigger=870201) (Tohuvavohu et al. GCN Circ. 23402). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 70° from the Sun (1.4 hours West) and 49° from the 23%-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.

LaPorte and Tohuvavohu (GCN Circ. 23407) reported the detection 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 Krimm et al. (GCN Circ. 23405), the BAT ground-calculated position is RA, Dec = 197.093, 52.220 deg which is RA(J2000) = 13h08m22.3s Dec(J2000) = +52°13'11.8" with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 78%.

The mask-weighted light curve (Figure 1) s shows a single FRED-shaped peak from T0-1 s, peaking at T0, and decaying to background by T0+10 s. T90 (15-350 keV) is 8.38 ± 2.41 s (estimated error including systematics).

The time-averaged spectrum from T-0.89 to T+10.18 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.90 ± 0.27. The fluence in the 15-150 keV band is 2.8 ± 0.5 x 10-7 erg cm-2. This fluence is larger than that of 11% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.46 s in the 15-150 keV band is 0.8 ± 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/870201/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Kennea et al. (GCN Circ. 23404). We have analysed 20 ks of XRT data for GRB 181103A, from 76 s to 235.8 ks after the BAT trigger. The data comprise 8 s in Windowed Timing (WT) mode (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. 23403).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=0.36 (+0.08, -0.06), followed by a break at T+7798 s to an α of 0.82 (+0.35, -0.08).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.20 (+0.20, -0.19). The best-fitting absorption column is 6.3 (+1.3, -1.1) x 1021 cm-2, in excess of the Galactic value of 1.3 x 1020 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.9 x 10-11 (7.6 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 6.3 (+1.3, -1.1) x 1021 cm-2
Galactic foreground: 1.3 x 1020 cm-2
Excess significance: 8.9 σ
Photon index: 2.20 (+0.20, -0.19)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00870201.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 181103A 92 s after the BAT trigger (LaPorte and Tohuvavohu GCN Circ. 23407). A fading source is detected in the initial UVOT exposures at a position consistent with the XRT position (Evans et al., GCN Circ. 23403). 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 0.03 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
13h08m27.78s +52°13'32.5" 1.6" XRT-final UKSSDC
13h08m27.81s +52°13'33.1" 1.6" XRT-enhanced Evans et al. GCN Circ. 23403
13h08m22.3s +52°13'11.8" 2.0' BAT-refined Krimm et al. GCN Circ. 23405

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Liu et al. 23406 Nanshan-1m optical upper limit Xinjiang Astro. Obs. upper limits
Optical Murata et al. 23408 MITSuME Okayama optical upper limits MITSuME Okayama upper limits
Optical Volnova et al. 23410 AbAO optical upper limit Abastumani Astro. Obs. upper limits

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
b 534 898 58 >19.08
uvm2 632 1126 57 >18.19
u 279 874 530 19.58± 0.25
v 608 1103 58 >18.21
uvw1 657 849 38 >18.16
uvw2 584 1079 58 >18.36
whiteFC 121 271 294 20.47± 0.29
white 559 6763 579 >21.12

Table 3. UVOT observations reported by LaPorte and Tohuvavohu (GCN Circ. 23407). 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 6, 2018