Swift Observations of GRB 180624A

S.L. Gibson (U. Leicester), B. Sbarufatti (INAF-OAB/PSU) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 13:49:40 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180624A (trigger=844192) (Gibson et al. GCN Circ. 22832). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 132° from the Sun (9.0 hours West) and 87° from the 88%-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.

Rossi et al. (GCN Circ. 22845) reported the position from LBT for the optical afterglow of this GRB. Rossi et al. (GCN Circ. 22845) determined a redshift of 2.86 from LBT. 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 Cummings et al. (GCN Circ. 22848), the BAT ground-calculated position is RA, Dec = 318.096, -2.356 deg which is RA(J2000) = 21h12m22.9s Dec(J2000) = -02°21'21.0" with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 47%.

The mask-weighted light curve (Figure 1) shows a multi-peaked light curve that starts at ~T-60 s and ends at ~T+540 s. The largest peak occurs at ~T+7 s. The burst went out of the BAT FOV at ~T+720 s. T90 (15-350 keV) is 486.4 ± 38.5 s (estimated error including systematics).

The time-averaged spectrum from T-60.61 to T+541.80 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.91 ± 0.10. The fluence in the 15-150 keV band is 5.8 ± 0.3 x 10-6 erg cm-2. This fluence is larger than that of 85% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+6.02 s in the 15-150 keV band is 1.4 ± 0.2 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/844192/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Sbarufatti et al. (GCN Circ. 22837). We have analysed 18 ks of XRT data for GRB 180624A, from 100 s to 269.2 ks after the BAT trigger. The data comprise 569 s in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 22835).

The late-time light curve (Figure 2) (from T0+3.9 ks) can be modelled with a power-law decay with a decay index of α=0.95 ± 0.07.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.427 (+0.022, -0.021). The best-fitting absorption column is 1.28 (+0.15, -0.14) x 1022 cm-2, at a redshift of 2.855, in addition to the Galactic value of 4.6 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.07 (+0.16, -0.15) and a best-fitting absorption column of 9.2 (+6.0, -5.4) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.2 x 10-11 (4.1 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 4.6 x 1020 cm-2
Intrinsic column: 9.2 (+6.0, -5.4) x 1021 cm-2 at z=2.855
Photon index: 2.07 (+0.16, -0.15)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00844192.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180624A 118 s after the BAT trigger (Kuin and Gibson GCN Circ. 22836). No optical afterglow 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 0.05 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
21h12m23.39s -02°20'17.0" 1.4" XRT-final UKSSDC
21h12m23.40s -02°20'17.7" 1.5" XRT-enhanced Goad et al. GCN Circ. 22835
21h12m22.9s -02°21'21.0" 1.3' BAT-refined Cummings et al. GCN Circ. 22848

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Itoh et al. 22838 MITSuME Okayama optical upper limits MITSuME Okayama upper limits
Optical Guidorzi et al. 22839 LCO Siding Springs afterglow candidate LCO detection
Optical Schady 22840 GROND observations GROND detection
Optical Rossi et al. 22845 LBT/MODS redshift measurement LBT redshift
Optical Watson et al. 22846 RATIR Optical Observations RATIR
Optical Guidorzi et al. 22858 LCO Haleakala observations LCO
Optical Selsing et al. 22867 NOT optical counterpart NOT 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 118 268 147 >20.7
uFC 276 526 246 >19.4
white 118 6151 541 >21.3
v 4927 6395 229 >20.0
b 4312 5947 393 >21.2
u 276 5742 639 >20.6
w1 3902 5537 393 >20.2
m2 5132 5332 197 >19.7
w2 4723 6357 393 >20.8

Table 3. UVOT observations reported by Kuin and Gibson (GCN Circ. 22836). 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.

June 28, 2018