Swift Observations of GRB 180904A

A. D'Ai (INAF-IASFPA), K.L. Page (U. Leicester) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 21:28:32 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180904A (trigger=859282) (D'Ai et al. GCN Circ. 23194). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 99° from the Sun (7.4 hours East) and 112° from the 30%-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 Palmer et al. (GCN Circ. 23203), the BAT ground-calculated position is RA, Dec = 274.260, 46.640 deg which is RA(J2000) = 18h17m02.4s Dec(J2000) = +46°38'23.3" with an uncertainty of 2.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 56%.

The mask-weighted light curve (Figure 1) shows a weak structure that starts at ~T0 and ends at ~T+6 s. T90 (15-350 keV) is 5.40 ± 0.89 s (estimated error including systematics).

The time-averaged spectrum from T+0.28 to T+6.18 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.02 ± 0.30. The fluence in the 15-150 keV band is 2.4 ± 0.4 x 10-7 erg cm-2. This fluence is larger than that of 9.5% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+5.14 s in the 15-150 keV band is 0.5 ± 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/859282/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Page et al. (GCN Circ. 23200). We have analysed 8.5 ks of XRT data for GRB 180904A, from 129 s to 51.9 ks after the BAT trigger. The data comprise 91 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 23196).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=0.9 ± 0.6. At T+179 s the decay steepens to an α of 7.84 (+0.16, -1.21) before breaking again at T+247 s to a final decay with index α=1.20 (+0.14, -0.13).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.61 (+0.24, -0.22). The best-fitting absorption column is 4.6 (+1.0, -0.9) x 1021 cm-2, in excess of the Galactic value of 3.4 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.19 (+0.26, -0.24) and a best-fitting absorption column of 4.3 (+1.3, -1.1) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10-11 (6.4 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 4.3 (+1.3, -1.1) x 1021 cm-2
Galactic foreground: 3.4 x 1020 cm-2
Excess significance: 6.0 σ
Photon index: 2.19 (+0.26, -0.24)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00859282.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180904A 131 s after the BAT trigger (Siegel and D'Ai GCN Circ. 23205). No optical afterglow consistent with the XRT position (Osborne et al. GCN Circ. 23196) is detected in the initial UVOT exposures, consistent with the non-detections of Malesani et al. (GCN Circ. 23195), Izzy et al. (GCN Circ. 23197), Moskvitin et al. (GCN Circ. 23198) and Guidorzi et al. (GCN Circ. 23199). 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 0.04 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
18h16m59.96s +46°37'47.3" 1.5" XRT-final UKSSDC
18h16m59.95s +46°37'46.9" 1.8" XRT-enhanced Osborne et al. GCN Circ. 23196
18h17m02.4s +46°38'23.3" 2.4' BAT-refined Palmer et al. GCN Circ. 23203

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Malesani et al. 23195 NOT optical observations, candidate
host galaxy		 NOT
Optical Izzo et al. 23197 OSN observations Obs.de Sierra Nevada
Optical Moskvitin et al. 23198 SAO RAS optical observations SAO RAS
Optical Guidorzi et al. 23199 LT early observations Liverpool Telescope
Optical Butler et al. 23204 RATIR Optical and NIR Observations RATIR upper limit
Optical Volnova et al. 23206 AbAO optical upper limit Abastumani Astro. Obs. upper limits
Optical del Pulgar et al. 23209 BOOTES-2/TELMA early observations BOOTES-2

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 131 281 147 >21.1
uFC 289 539 246 >20.1
white 131 589 167 >21.1
v 620 640 19 >20.1
b 545 565 19 >19.4
u 289 539 246 >20.1
w1 670 690 19 >18.1
w2 595 615 19 >18.6

Table 3. UVOT observations reported by Siegel and D'Ai (GCN Circ. 23205). 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.

September 6, 2018