Swift Observations of GRB 180630A

P. D'Avanzo (INAF-OAB), P.A. Evans (U. Leicester) and S.J. LaPorte (PSU) for the Swift team

1. Introduction

At 11:11:51 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180630A (trigger=845443) (D'Avanzo et al. GCN Circ. 22872). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 112° from the Sun (3.5 hours West) and 71° from the 95%-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.

D'Avanzo et al. (GCN Circ. 22872) 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 Lien et al. (GCN Circ. 22877), the BAT ground-calculated position is RA, Dec = 48.885, -87.476 deg which is RA(J2000) = 03h15m32.5s Dec(J2000) = -87°28'33.4" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 59%.

The mask-weighted light curve (Figure 1) shows a double-peaked structure. The first pulse starts at T0, peaks at T+4 s and ends at T+15 s. The second weak pulse starts at T+15 s and ends at T+30 s. T90 (15-350 keV) is 18.85 ± 6.45 s (estimated error including systematics).

The time-averaged spectrum from T-0.36 to T+30.02 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.07 ± 0.10. The fluence in the 15-150 keV band is 1.8 ± 0.1 x 10-6 erg cm-2. This fluence is larger than that of 56% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+3.65 s in the 15-150 keV band is 6.6 ± 0.4 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/845443/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 22876). We have analysed 9.1 ks of XRT data for GRB 180630A, from 87 s to 121.5 ks after the BAT trigger. The data comprise 69 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. 22873).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=3.0 (+1.1, -0.5). At T+146 s the decay flattens to an α of 0.48 (+0.12, -0.26) before breaking again at T+2147 s to a final decay with index α=0.99 (+0.08, -0.10).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.91 (+0.21, -0.17). The best-fitting absorption column is consistent with the Galactic value of 1.3 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.66 ± 0.12 and a best-fitting absorption column of 1.7 (+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 4.2 x 10-11 (5.1 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.7 (+0.5, -0.4) x 1021 cm-2
Galactic foreground: 1.3 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 1.66 ± 0.12

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00845443.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180630A 88 s after the BAT trigger (LaPorte and D'Avanzo GCN Circ. 22881). A fading source consistent with the XRT position (Beardmore et al. GCN Circ. 22873) 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 0.13 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
03h15m55.74s -87°28'42.3" 0.43" UVOT-refined LaPorte and D'Avanzo GCN Circ. 22881
03h15m55.47s -87°28'43.5" 1.4" XRT-final UKSSDC
03h15m55.39s -87°28'43.7" 1.7" XRT-enhanced Beardmore et al. GCN Circ. 22873
03h15m32.5s -87°28'33.4" 1.0' BAT-refined Lien et al. GCN Circ. 22877

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Gamma-ray Hamburg et al. 22875 Fermi GBM detection Fermi GBM Epeak=84±8 keV
T90=12 seconds
Fluence=2.5±0.1x10-6erg cm-2
(35th percentile for long GRBs)

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 88 237 148 17.49 ± 0.04
white 6384 6584 196 19.50 ± 0.11
v 629 1597 117 18.56 ± 0.20
b 555 748 39 18.76 ± 0.19
u 300 549 246 17.76 ± 0.06
w1 678 7392 510 19.42 ± 0.17
m2 654 7187 452 20.16 ± 0.33
w2 605 6777 255 >19.9

Table 3. UVOT observations reported by LaPorte and D'Avanzo (GCN Circ. 22881). 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.

July 2, 2018