Swift Observations of GRB 180905A

V. D'Elia (SSDC), A.P. Beardmore (U. Leicester) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 13:57:46 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180905A (trigger=859421) (D'Elia et al. GCN Circ. 23201). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 74° from the Sun (4.9 hours West) and 30° 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.

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 Sakamoto et al. (GCN Circ. 23213), the BAT ground-calculated position is RA, Dec = 91.038, -4.592 deg which is RA(J2000) = 06h04m09.2s Dec(J2000) = -04°35'31.9" with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 29%.

The mask-weighted light curve (Figure 1) shows a weak long pulse that starts at ~T-60 s and ends at ~T+150 s. Note that the burst came into the BAT field of view at T-84 s, and BAT entered the SAA after ~ T+250 s. Therefore, there might be additional burst emission before and after the available event data range. T90 (15-350 keV) is 165.3 ± 28.9 s (estimated error including systematics).

The time-averaged spectrum from T-58.9 to T+146.7 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.60 ± 0.12. The fluence in the 15-150 keV band is 4.2 ± 0.3 x 10-6 erg cm-2. This fluence is larger than that of 78% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-2.09 s in the 15-150 keV band is 1.0 ± 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/859421/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Beardmore et al. (GCN Circ. 23210).

We have analysed 9.0 ks of XRT data for GRB 180905A, from 152 s to 35.0 ks after the BAT trigger. The data comprise 135 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. 23207).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.556 (+0.029, -0.027).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.45 (+0.11, -0.10). The best-fitting absorption column is 5.7 (+1.1, -1.0) x 1021 cm-2, in excess of the Galactic value of 3.5 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.68 (+0.28, -0.26) and a best-fitting absorption column of 5.5 (+2.2, -1.8) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.0 x 10-11 (6.9 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 5.5 (+2.2, -1.8) x 1021 cm-2
Galactic foreground: 3.5 x 1021 cm-2
Excess significance: 1.8 σ
Photon index: 1.68 (+0.28, -0.26)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00859421.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180905A 154 s after the BAT trigger (Siegel and D'Elia GCN Circ. 23215). No optical afterglow consistent with the XRT position is detected in the initial UVOT exposures, as confirmed by Bootes (Hu et al., GCN Circ. 23208), MITSUME (Niwano et al., GCN Circ. 23211) and RATIR (Watson et al., GCN Circ. 23212). 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 0.60 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
06h04m15.04s -04°34'12.5" 1.5" XRT-final UKSSDC
06h04m15.04s -04°34'12.6" 1.8" XRT-enhanced Beardmore et al. GCN Circ. 23207
06h04m09.2s -04°35'31.9" 1.7' BAT-refined Sakamoto et al. GCN Circ. 23213

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Hu et al. 23208 BOOTES-3/YA optical observations BOOTES-3
Optical Niwano et al. 23211 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Watson et al. 23212 RATIR Optical and NIR Observations RATIR 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
whiteFC 154 289 133 >20.4
white 154 6385 373 >21.1
v 5316 5515 197 >19.1
b 4700 6335 393 >20.5
u 4495 6130 393 >20.3
w1 5726 5925 197 >19.6
m2 5520 5720 197 >19.8
w2 5111 5311 197 >19.2

Table 3. UVOT observations reported by Siegel and D'Elia (GCN Circ. 23215). 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 10, 2018