Swift Observations of GRB 180721A

B. Sbarufatti (PSU), A.P. Beardmore (U. Leicester) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 11:06:48 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180721A (trigger=849018) (Sbarufatti et al. GCN Circ. 22994). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 127° from the Sun (8.9 hours West) and 121° from the 67%-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 Krimm et al. (GCN Circ. 23001), the BAT ground-calculated position is RA, Dec = 347.708, 4.871 deg which is RA(J2000) = 23h10m49.9s Dec(J2000) = +04°52'16.1" with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows a single-pulse structure that starts at ~T-1 s, peaks at ~T+2 s, and ends at ~T+10 s. The main pulse is followed by some weak emission that last till ~T+50 s. T90 (15-350 keV) is 47.6 ± 3.0 s (estimated error including systematics).

The time-averaged spectrum from T-0.39 to T+48.92 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.25 ± 0.22. The fluence in the 15-150 keV band is 5.9 ± 0.9 x 10-7 erg cm-2. This fluence is larger than that of 24% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+1.82 s in the 15-150 keV band is 0.9 ± 0.1 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/849018/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Beardmore et al. (GCN Circ. 22999). We have analysed 14 ks of XRT data for GRB 180721A, from 63 s to 138.8 ks after the BAT trigger. The data comprise 17 s 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. 22997).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.95 (+0.08, -0.07).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.14 (+0.24, -0.23). The best-fitting absorption column is 1.4 (+0.7, -0.6) x 1021 cm-2, in excess of the Galactic value of 6.5 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 3.2 x 10-11 (4.3 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.4 (+0.7, -0.6) x 1021 cm-2
Galactic foreground: 6.5 x 1020 cm-2
Excess significance: 2.1 σ
Photon index: 2.14 (+0.24, -0.23)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00849018.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180721A 120 s after the BAT trigger (Kuin and Sbarufatti GCN Circ. 23014). No optical afterglow consistent with the XRT position (Goad et al. GCN Circ. 22997) 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 0.07 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
23h10m50.61s +04°51'23.1" 2.2" XRT-final UKSSDC
23h10m50.80s +04°51'24.4" 2.8" XRT-enhanced Goad et al. GCN Circ. 22997
23h10m49.9s +04°52'16.1" 1.8' BAT-refined Krimm et al. GCN Circ. 23001

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Guidorzi et al. 22995 LCO Haleakala observations LCO
Optical Carrasco et al. 23018 BOOTES-5/JGT optical observations BOOTES
Optical Oeda et al. 23022 MITSuME Akeno optical upper limits MITSuME Akeno 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
uFC 120 370 246 >20.1
v 425 1693 156 >19.1
b 375 1642 175 >19.9
u 120 1617 529 >20.1
w1 474 1742 156 >19.6
m2 1697 1717 19 >18.3
w2 401 1668 156 >20.5

Table 3. UVOT observations reported by Kuin and Sbarufatti (GCN Circ. 23014). 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 25, 2018