Swift Observations of GRB 180620B

P.A. Evans (U. Leicester), A. Melandri (INAF-OAB) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 15:50:37 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180620B (trigger=843211) (Evans et al. GCN Circ. 22807). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 110° from the Sun (6.1 hours West) and 125° from the 53%-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.

Evans et al. (GCN Circ. 22807) reported the discovery with UVOT of an optical afterglow. Schweyer and Schady (GCN Circ. 22819) reported the position from GROND for the optical afterglow of this GRB. Izzo et al. (GCN Circ. 22823) determined a redshift of 1.1175 from VLT. 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 Ukwatta et al. (GCN Circ. 22821), the BAT ground-calculated position is RA, Dec = 357.514, -57.954 deg which is RA(J2000) = 23h50m03.3s Dec(J2000) = -57°57'13.5" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 78%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T-160 s and ends at ~T+160 s. The three main peaks occur at ~T0, ~T+40 s, and ~T+100 s, respectively. Note that because the event data are only available from ~T-180 s, there might be additional burst emission before the available event data range. T90 (15-350 keV) is 198.8 ± 27.4 s (estimated error including systematics).

The time-averaged spectrum from T-159.376 to T+162.428 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.30 ± 0.20, and Epeak of 123.9 ± 68.3 keV (χ2 58.09 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 1.0 ± 0.03 x 10-5 erg cm-2 and the 1-s peak flux measured from T-0.33 s in the 15-150 keV band is 3.6 ± 0.2 ph cm-2 s-1. This fluence is larger than that of 92.5% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). A fit to a simple power law gives a photon index of 1.60 ± 0.05 (χ2 65.76 for 57 d.o.f.). 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/843211/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Melandri et al. (GCN Circ. 22817). We have analysed 25 ks of XRT data for GRB 180620B, from 89 s to 436.0 ks after the BAT trigger. The data comprise 237 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. 22814).

The late-time light curve (Figure 2) (from T0+4.3 ks) can be modelled with an initial power-law decay with an index of α=0.58 (+0.11, -0.08), followed by a break at T+39.1 ks to an α of 1.05 ± 0.09.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.25 ± 0.04. The best-fitting absorption column is 6.3 (+0.5, -0.4) x 1021 cm-2, at a redshift of 1.1175, in addition to the Galactic value of 1.4 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.93 ± 0.09 and a best-fitting absorption column of 4.4 (+1.0, -0.9) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 x 10-11 (4.4 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.4 x 1020 cm-2
Intrinsic column: 4.4 (+1.0, -0.9) x 1021 cm-2 at z=1.1175
Photon index: 1.93 ± 0.09

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00843211.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180620B 92 s after the BAT trigger (Breeveld et al. GCN Circ. 22827). A source consistent with the enhanced XRT position (Beardmore et al., GCN Circ. 22814) and the Grond position (Schweyer and Schady, GCN Circ. 22819), and also detected by Izzo et al. (GCN Circ. 22823) is detected in the initial UVOT exposures in all filters except uvw2. 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 0.01 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
23h50m05.10s -57°57'44.1" 0.75" UVOT-initial Evans et al. GCN Circ. 22807
23h50m05.10s -57°57'44.8" 1.4" XRT-final UKSSDC
23h50m05.09s -57°57'44.8" 1.4" XRT-enhanced Beardmore et al. GCN Circ. 22814
23h50m03.3s -57°57'13.5" 1.0' BAT-refined Ukwatta et al. GCN Circ. 22821

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Schweyer and Schady 22819 GROND observations GROND detection
Optical Izzo et al. 22823 VLT/X-shooter redshift VLT redshift
Gamma-ray Mailyan 22813 Fermi GBM detection Fermi GBM Epeak=149±11 keV
T90=46.7 seconds
Fluence=7.7±0.04x10-6erg cm-2
(68th percentile for long GRBs)
Gamma-ray Svinkin et al. 22825 Konus-Wind observation Konus-Wind Epeak=118 (-16,+23) keV
Duration=~260 seconds
Fluence=1.19(-0.12,+0.14)x10-5erg cm-2
Gamma-ray Sharma et al. 22844 AstroSat CZTI detection CZTI T90=22.3 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 92 242 147 18.06 ± 0.08
v 634 826 39 17.41 ± 0.22
b 559 579 20 17.56 ± 0.18
u 304 554 246 17.40 ± 0.09
uvw1 9950 18219 1394 20.17 ± 0.2
uvm2 6563 16382 804 20.6 ± 0.3
uvw2 4256 4443 183 > 19.7

Table 3. UVOT observations reported by Breeveld et al. (GCN Circ. 22827). 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.

June 26, 2018