Swift Observations of GRB 180705A

M. Stamatikos (OSU/NASA/GSFC), A. Melandri (INAF-OAB) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 15:41:14 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180705A (trigger=846299) (Stamatikos et al. GCN Circ. 22902). Swift did not slew due to an observing constraint. At the time of the trigger, the initial BAT position was 111° from the Sun (7.2 hours East) and 151° from the 57%-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. 22907), the BAT ground-calculated position is RA, Dec = 212.605, -11.640 deg which is RA(J2000) = 14h10m25.1s Dec(J2000) = -11°38'23.7" with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 36%.

The mask-weighted light curve (Figure 1) shows some weak emission that starts at ~T0 and ends at ~T+130 s. However, the burst went out of the BAT FOV from T+158 s to T+204 s due to a preplanned slew, and thus there might be additional emission when the burst was out of the FOV. The burst came back in the BAT FOV shortly during the slew period (from T0+205 s to T0+262 s), though no emission are found during this time. T90 (15-350 keV) is 123.2 ± 16.6 s (estimated error including systematics).

The time-averaged spectrum from T+0.26 to T+133.70 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.63 ± 0.22. The fluence in the 15-150 keV band is 2.0 ± 0.3 x 10-6 erg cm-2. This fluence is larger than that of 58% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+3.60 s in the 15-150 keV band is 0.7 ± 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/846299/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Melandri et al. (GCN Circ. 22920).

We have analysed 5.7 ks of XRT data for GRB 180705A, from 3.4 ks to 56.6 ks after the BAT trigger. The data comprise 1.1 ks in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 22906).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.12 (+0.17, -0.13).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.99 (+0.32, -0.30). The best-fitting absorption column is 3.9 (+1.8, -1.5) x 1021 cm-2, in excess of the Galactic value of 5.8 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 (6.0 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 3.9 (+1.8, -1.5) x 1021 cm-2
Galactic foreground: 5.8 x 1020 cm-2
Excess significance: 3.7 σ
Photon index: 1.99 (+0.32, -0.30)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00846299.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180705A 3368 s after the BAT trigger (Breeveld et al. GCN Circ. 22912). No optical afterglow consistent with the enhanced XRT position (Goad et al. GCN Circ. 22906) 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 0.06 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
14h10m27.15s -11°38'50.4" 1.5" XRT-final UKSSDC
14h10m27.17s -11°38'50.8" 1.8" XRT-enhanced Goad et al. GCN Circ. 22906
14h10m25.1s -11°38'23.7" 2.1' BAT-refined Palmer et al. GCN Circ. 22907

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 22904 MASTER optical observation MASTER
Optical Watson et al. 22911 RATIR Optical Observations RATIR upper limits
Optical Mazaeva et al. 22913 TSHAO optical upper limit Zeiss-1000 upper limits
Optical Bikmaev et al. 22929 RTT150 optical upper limit RTT150 upper limits
Optical Kann et al. 22941 OSN observations Obs.de Sierra Nevada detection

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 3368 3485 115 >20.2
white 3368 5327 312 >20.8
v 4103 5738 393 >18.9
b 4923 11649 311 >20.5
u 4718 11527 1082 >21.0
w1 4513 16889 483 >20.5
m2 4308 15205 647 >20.1
w2 5333 5533 197 >20.4

Table 3. UVOT observations reported by Breeveld et al. (GCN Circ. 22912). 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.

July 12, 2018