Swift Observations of GRB 180102A

K.L. Page (U. Leicester), A. D'Ai (INAF-IASFPA) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 15:49:44 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180102A (trigger=802999) (Page et al. GCN Circ. 22292). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 106° from the Sun (5.3 hours West) and 74° from the 100%-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 Sakamoto et al. (GCN Circ. 22300), the BAT ground-calculated position is RA, Dec = 203.078, 62.161 deg which is RA(J2000) = 13h32m18.8s Dec(J2000) = +62°09'38.1" with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment). The partial coding was 95%.

The mask-weighted light curve (Figure 1) shows two overlapping pulses that starts at ~ T-8 s and ends at ~ T+6 s. The main peak occurs at ~T+1 s. T90 (15-350 keV) is 10.8 ± 1.7 s (estimated error including systematics).

The time-averaged spectrum from T-7.62 to T+6.30 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.01 ± 0.18. The fluence in the 15-150 keV band is 3.5 ± 0.4 x 10-7 erg cm-2. This fluence is larger than that of 15% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.80 s in the 15-150 keV band is 1.0 ± 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/802999/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Ai et al. (GCN Circ. 22296). We have analysed 16 ks of XRT data for GRB 180102A, from 82 s to 189.5 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 22293).

The late-time light curve (Figure 2) (from T0+5.3 ks) can be modelled with a power-law decay with a decay index of α=0.89 (+0.20, -0.17).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.30 (+0.32, -0.29). The best-fitting absorption column is 4.1 (+1.4, -1.2) x 1021 cm-2, in excess of the Galactic value of 1.9 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 3.3 x 10-11 (6.3 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 4.1 (+1.4, -1.2) x 1021 cm-2
Galactic foreground: 1.9 x 1020 cm-2
Excess significance: 5.3 σ
Photon index: 2.30 (+0.32, -0.29)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00802999.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180102A 74 s after the BAT trigger (Breeveld and Page GCN Circ. 22299). No optical afterglow consistent with the XRT position (Beardmore et al. GCN Circ. 22294) 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 0.01 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
13h32m16.07s +62°10'19.6" 1.5" XRT-final UKSSDC
13h32m15.98s +62°10'18.4" 2.0" XRT-enhanced Beardmore et al. GCN Circ. 22294
13h32m18.8s +62°09'38.1" 1.5' BAT-refined Sakamoto et al. GCN Circ. 22300

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 22297 MASTER-Net robotic report MASTER upper limits
Optical Malesani et al. 22298 NOT optical observations NOT
Optical Mazaeva et al. 22301 TSHAO optical upper limit Zeiss-1000 upper limits
Optical Kitaoka and Sakamoto 22303 AROMA-N optical observation AROMA-N
Gamma-ray Veres 22295 Fermi GBM detection Fermi GBM Epeak=54.9±6.7 keV
T90=16 seconds
Fluence=4.31±0.45x10-7erg cm-2
(3.9 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
whiteFC 74 224 147 >20.8
uFC 286 535 246 >19.9
white 74 7508 824 >21.3
v 616 6484 352 >19.3
b 541 7303 529 >20.3
u 286 7098 717 >20.4
w1 665 6893 529 >20.1
m2 640 6688 350 >19.7
w2 591 7692 528 >20.1

Table 3. UVOT observations reported by Breeveld and Page (GCN Circ. 22299). 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.

January 6, 2018