Swift Observations of GRB 190106A

E. Sonbas (Adiyaman Univ.), K.L. Page (U. Leicester) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 13:34:44 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190106A (trigger=882252) (Sonbas et al. GCN Circ. 23615). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 110° from the Sun (6.9 hours East) and 105° from the 0%-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.

Sonbas et al. (GCN Circ. 23615) reported the detection with UVOT of an optical afterglow. Zhu et al. (GCN Circ. 23623) determined a redshift of 0.896 from Xinglong, Xu et al. (GCN Circ. 23629) determined a redshift of 1.86 from Xinglong, and Schady et al. (GCN Circ. 23632) determined a redshift of 1.859 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 Palmer et al. (GCN Circ. 23625), the BAT ground-calculated position is RA, Dec = 29.877, 23.839 deg, which is RA(J2000) = 01h59m30.6s Dec(J2000) = +23°50'20.8" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 65%.

The mask-weighted light curve (Figure 1) shows it started at ~T-10s, with a pair of overlapping FRED-like peaks at ~T+2 and ~T+10 s, then a long decay and two more peaks at ~T+57 and ~T+76 s, and returning to baseline at ~T+125 s. T90 (15-350 keV) is 76.8 ± 2.4 s (estimated error including systematics).

The time-averaged spectrum from T-0.17 to T+99.72 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.34 ± 0.21, and Epeak of 120 ± 83 keV (χ2 37.25 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 6.0 ± 0.2 x 10-6 erg cm-2 and the 1-s peak flux measured from T+75.28 s in the 15-150 keV band is 5.5 ± 0.3 ph cm-2 s-1. This fluence is larger than that of 86% 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 1.64 ± 0.05 (χ2 43.44 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/882252/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Page et al. (GCN Circ. 23633). We have analysed 35 ks of XRT data for GRB 190106A (Yurkov et al. GCN Circ. 23614), from 71 s to 878.9 ks after the BAT trigger. The data comprise 217 s in Windowed Timing (WT) mode (the first 9 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. 23619).

The late-time light curve (Figure 2) (from T0+11.7 ks) can be modelled with an initial power-law decay with an index of α=1.01 (+0.12, -0.07), followed by a break at T+69.2 ks to an α of 1.49 (+0.14, -0.13).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.06 (+0.06, -0.05). The best-fitting absorption column is 1.9 ± 1.1 x 1021 cm-2, at a redshift of 1.86, in addition to the Galactic value of 1.0 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.95 (+0.09, -0.06) and a best-fitting absorption column consistent with the Galactic value. 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: 1.0 x 1021 cm-2
Intrinsic column: 1.0 (+1.6, -0.0) x 1021 cm-2 at z=1.86
Photon index: 1.95 (+0.09, -0.06)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00882252.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190106A 90 s after the BAT trigger (Kuin and Sonbas GCN Circ. 23626). A source consistent with the XRT position (Goad et al. GCN Circ. 23619) 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 0.09 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
01h59m31.16s +23°50'43.9" 0.42" UVOT-refined Kuin and Sonbas GCN Circ. 23626
01h59m31.23s +23°50'43.8" 1.4" XRT-final UKSSDC
01h59m31.21s +23°50'43.6" 1.8" XRT-enhanced Goad et al. GCN Circ. 23619
01h59m30.6s +23°50'20.8" 1.0' BAT-refined Palmer et al. GCN Circ. 23625

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Yurkov et al. 23614 SWIFT GRB190106.57 Global MASTER-Net
OT detection		 MASTER detection
Optical Lipunov et al. 23616 MASTER Amur OT detection MASTER detection
Optical Itoh et al. 23617 MITSuME Akeno optical observation MITSuME Akeno
Optical Mao et al. 23618 GMG detection Gao-Mei-Gu detection
Optical Reva et al. 23620 TSHAO optical observations Zeiss-1000
Optical Xin et al. 23622 GWAC-F60B optical detection Xinglong Obs. detection
Optical Zhu et al. 23623 Xinglong-2.16m spectroscopy Xinglong spectroscopy
Optical Quadri 23624 Bassano Bresciano Observatory optical
detection		 Bassano Bresciano Obs. detection
Optical Hu et al. 23627 BOOTES-4/MET optical observations BOOTES-4
Optical Noschese et al. 23628 OASDG optical observations S.Di Giacomo Obs.
Optical Xu et al. 23629 update of the Xinglong-2.16m
spectroscopy		 Xinglong spectroscopy
Optical Mao et al. 23630 GMG spectral results Gao-Mei-Gu
Optical Schady et al. 23632 VLT/X-shooter redshift confirmation VLT redshift
Optical Mao et al. 23634 GMG photometry in the 2nd night Gao-Mei-Gu
Optical Chakrabarti et al. 23635 IERCOO/ICSP optical observation
Optical Zhu et al. 23636 NEXT-0.6m and Xinglong-2.16m
photometric single powerlaw decay		 Xinglong light curve
Optical Reva et al. 23638 continued TSHAO optical observations Zeiss-1000
Optical Belkin et al. 23640 Mondy optical observations Mondy light curve
Optical Izzo et al. 23660 Liverpool Telescope observations Liverpool Telescope detection
Optical Belkin et al. 23661 optical observations, possible jet
break detection		 Mondy possible detection
Optical Blazek et al. 23665 OSN Afterglow Observations Obs.de Sierra Nevada detection
Optical Mao et al. 23668 GMG continued observation Gao-Mei-Gu
Optical Dichiara et al. 23744 Discovery Channel Telescope observations Discovery Channel
Gamma-ray Tsvetkova et al. 23637 Konus-Wind observation Konus-Wind Epeak=171 (-42,+90) keV
Duration=~79 seconds
Fluence=1.12(-0.17,+0.25)x10-5erg cm-2

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 90 240 147 16.64 ± 0.03
v 633 653 20 16.54 ± 0.16
b 559 579 20 16.70 ± 0.09
u 304 553 246 15.97 ± 0.03
w1 682 702 20 17.36 ± 0.29

Table 3. UVOT observations reported by Kuin and Sonbas (GCN Circ. 23626). 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.

January 17, 2019