Swift Observations of GRB 180809B

M.J. Moss (George Washington University), Z. Liu (NAOC / U. Leicester) and S. Sebzda (PSU) for the Swift team

1. Introduction

At 20:28:48 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180809B (trigger=852553) (Moss et al. GCN Circ. 23105). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 161° from the Sun (10.7 hours East) and 174° from the 4%-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.

Moss et al. (GCN Circ. 23105) 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. 23123), the BAT ground-calculated position is RA, Dec = 299.712, -15.293 deg which is RA(J2000) = 19h58m50.8s Dec(J2000) = -15°17'34.0" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 63%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T-36 s and ends at ~T+80 s, followed by another pulse that starts at ~T+200 s and ends at ~T+250 s. The total burst emission ends at ~T+300 s. T90 (15-350 keV) is 233.2 ± 1.1 s (estimated error including systematics).

The time-averaged spectrum from T-36.1 to T+302.3 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.26 ± 0.02. The fluence in the 15-150 keV band is 7.3 ± 0.1 x 10-5 erg cm-2. This fluence is larger than that of 99.5% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+35.10 s in the 15-150 keV band is 26.3 ± 0.6 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/852553/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Liu et al. (GCN Circ. 23125). We have analysed 20 ks of XRT data for GRB 180809B, from 60 s to 529.4 ks after the BAT trigger. The data comprise 2.0 ks in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode.

The late-time light curve (Figure 2) (from T0+4.7 ks) can be modelled with an initial power-law decay with an index of α=1.24 ± 0.07, followed by a break at T+54.7 ks to an α of 2.2 ± 0.3.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.610 ± 0.017. The best-fitting absorption column is 2.23 ± 0.08 x 1021 cm-2, in excess of the Galactic value of 1.2 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.67 ± 0.10 and a best-fitting absorption column of 2.6 (+0.5, -0.4) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.5 x 10-11 (5.7 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.6 (+0.5, -0.4) x 1021 cm-2
Galactic foreground: 1.2 x 1021 cm-2
Excess significance: 5.3 σ
Photon index: 1.67 ± 0.10

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00852553.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180809B 82 s after the BAT trigger (Sebzda and Moss GCN Circ. 23120). A fading source consistent with the XRT position 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 0.16 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
19h58m47.96s -15°17'57.1" 0.48" UVOT-refined Sebzda and Moss GCN Circ. 23120
19h58m47.98s -15°17'56.3" 1.4" XRT-final UKSSDC
19h58m47.99s -15°17'56.1" 1.4" XRT-refined Liu et al. GCN Circ. 23125
19h58m50.8s -15°17'34.0" 1.0' BAT-refined Stamatikos et al. GCN Circ. 23123

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 23109 MASTER OT observation MASTER
Optical Watson et al. 23110 RATIR Optical Observations and
Confirmation of Rapid Fading 		RATIR detection
Optical Schweyer and Schady 23111 GROND Detection of the NIR Afterglow GROND detection
Optical Guidorzi et al. 23112 LCO Sutherland observations LCO detection
Optical Tiurina et al. 23113 MASTER Triple Independent Automatic OT
Detection		 MASTER detection
Optical Japelj et al. 23114 VLT optical upper limits VLT upper limits
Optical Mazaeva et al. 23124 CrAO optical observations CrAO detection
Optical Kilic et al. 23127 TUG T60 observations TUG detection
Optical Kostrzewa-Rutkowska et al. 23133 ePESSTO NTT NIR observations PESSTO detection
Gamma-ray Cherry et al. 23116 CALET Gamma-Ray Burst Monitor detection CALET
Gamma-ray Svinkin et al. 23128 Konus-Wind observation Konus-Wind Epeak=275 (-30,+25) keV
Fluence=3.07(-0.30,+0.36)x10-4erg cm-2
Gamma-ray Sharma et al. 23144 AstroSat CZTI detection CZTI T90=44.1 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 81 231 293 18.99± 0.07
white 573 6926 441 >21.55
v 623 7289 444 >19.8
b 549 6721 294 >20.9
u 294 6516 520 >21.0
w1 672 13027 915 >20.7
m2 648 11053 906 >20.9
w2 599 16967 1262 >21.3

Table 3. UVOT observations reported by Sebzda and Moss (GCN Circ. 23120). 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.

August 18, 2018