Swift Observations of GRB 190123A

A. D'Ai (INAF-IASFPA), A. Tohuvavohu (PSU) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 12:19:13 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190123A (trigger=885314) (D'Ai et al. GCN Circ. 23769). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 116° from the Sun (7.2 hours East) and 91° from the 92%-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 Lien et al. (GCN Circ. 23780), the BAT ground-calculated position is RA, Dec = 53.175, 45.505 deg, which is RA(J2000) = 03h32m42.0s Dec(J2000) = +45°30'16.3" with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 64%.

The mask-weighted light curve (Figure 1) shows a single FRED-shaped peak from about T-5 to T+20 s, with a peak around T+3 s. Observations were discontinued due to an observing constraint at ~T+325 s. T90 (15-350 keV) is 24.4 ± 4.3 s (estimated error including systematics).

The time-averaged spectrum from T-1.22 to T+28.06 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.13 ± 0.23. The fluence in the 15-150 keV band is 8.4 ± 1.1 x 10-7 erg cm-2. This fluence is larger than that of 34% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+4.82 s in the 15-150 keV band is 1.0 ± 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/885314/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Tohuvavohu et al. (GCN Circ. 23775). We have analysed 13 ks of XRT data for GRB 190123A, from 123 s to 80.3 ks after the BAT trigger. The data comprise 41 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. 23771).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.28 (+0.06, -0.05).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.8 (+0.5, -0.4). The best-fitting absorption column is 2.3 (+0.9, -0.7) x 1022 cm-2, in excess of the Galactic value of 4.4 x 1021 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 7.0 x 10-11 (1.3 x 10-10) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.3 (+0.9, -0.7) x 1022 cm-2
Galactic foreground: 4.4 x 1021 cm-2
Excess significance: 4.3 σ
Photon index: 1.8 (+0.5, -0.4)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00885314.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190123A 124 s after the BAT trigger (Siegel and D'Ai GCN Circ. 23784). No optical afterglow consistent with the XRT position (Beardmore et al. GCN Circ. 23771) 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 0.44 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
03h32m52.93s +45°31'31.1" 1.6" XRT-final UKSSDC
03h32m52.94s +45°31'31.1" 2.3" XRT-enhanced Beardmore et al. GCN Circ. 23771
03h32m42.0s +45°30'16.3" 1.7' BAT-refined Lien et al. GCN Circ. 23780

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 23770 MASTER Global Robotic Net optical
observation		 MASTER
Optical Hu et al. 23772 BOOTES-4/MET optical limit BOOTES-4 upper limits
Optical Steeghs et al. 23774 GOTO optical limit Gravitational-wave Optical Transient Observer upper limits
Optical Mao et al. 23777 GMG observational limit Gao-Mei-Gu upper limits
Optical Shiraishi et al. 23778 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Watson et al. 23781 COATLI Optical Observations COATLI
Optical Nakamura and Sakamoto 23786 AROMA-N Optical Observation AROMA-N
Optical Xu and Zhu 23788 NEXT-0.6m optical limit Xinjiang Astro. Obs. upper limits
Optical Volnova et al. 23789 IKI-GRB-FuN optical upper limit Zeiss-1000 upper limits
Gamma-ray Bissaldi 23773 Fermi GBM observation Fermi GBM Epeak=310±100 keV
T90=17 seconds
Fluence=2.07±0.04x10-6erg cm-2
(30th 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 124 250 123 >20.3
white 124 10248 1258 >22.0
v 3470 11721 947 >20.3
b 4290 5926 393 >21.1
u 4085 5721 393 >21.0
w1 3880 5516 393 >20.1
m2 3675 5310 393 >19.9
w2 4701 11151 1079 >21.1

Table 3. UVOT observations reported by Siegel and D'Ai (GCN Circ. 23784). 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 25, 2019