Swift Observations of GRB 190202A

A.Y. Lien (GSFC/UMBC) and P. Kuin (UCL/MSSL) for the Swift team

1. Introduction

At 01:39:30 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190202A (trigger=887217) (Lien et al. GCN Circ. 23830). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 149° from the Sun (9.9 hours West) and 118° from the 7%-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.

Lien et al. (GCN Circ. 23830) 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 Markwardt et al. (GCN Circ. 23836), the BAT ground-calculated position is RA, Dec = 166.506, 9.393 deg which is RA(J2000) = 11h06m01.4s Dec(J2000) = +09°23'33.9" with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment). The partial coding was 4%.

The mask-weighted light curve (Figure 1) shows two overlapping pulses that start at ~T0 and end at ~T+25 s. The two peaks occur at ~T0 and ~T+5 s, respectively, Note that the burst location came into the BAT field of view at ~T-69 s, and Swift went into SAA at ~T+210 s. T90 (15-350 keV) is 19.4 ± 6.9 s (estimated error including systematics).

The time-averaged spectrum from T-0.90 to T+26.52 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.32 ± 0.13. The fluence in the 15-150 keV band is 6.0 ± 0.5 x 10-6 erg cm-2. This fluence is larger than that of 86% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+4.68 s in the 15-150 keV band is 7.8 ± 1.5 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/887217/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Lien (GCN Circ. 23840). We have analysed 32 ks of XRT data for GRB 190202A, from 152 s to 500.7 ks after the BAT trigger. The data comprise 823 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 Evans et al. (GCN Circ. 23832).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=0.594 (+0.021, -0.085), followed by a break at T+1874 s to an α of 1.453 (+0.025, -0.027).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.88 ± 0.05. The best-fitting absorption column is consistent with the Galactic value of 2.4 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.82 ± 0.12 and a best-fitting absorption column of 6.8 (+3.1, -2.8) x 1020 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10-11 (4.1 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 6.8 (+3.1, -2.8) x 1020 cm-2
Galactic foreground: 2.4 x 1020 cm-2
Excess significance: 2.5 σ
Photon index: 1.82 ± 0.12

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00887217.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190202A 173 s after the BAT trigger (Kuin and Lien GCN Circ. 23834). The source was also observed in R (Novichonok et al., GCN Circ. 23831). 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 0.02 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
11h06m00.63s +09°22'36.7" 0.42" UVOT-refined Kuin and Lien GCN Circ. 23834
11h06m00.59s +09°22'36.7" 1.5" XRT-final UKSSDC
11h06m00.56s +09°22'36.8" 1.7" XRT-enhanced Evans et al. GCN Circ. 23832
11h06m01.4s +09°23'33.9" 1.9' BAT-refined Markwardt et al. GCN Circ. 23836

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Novichonok et al. 23831 Koshka Zeiss-1000 optical observations Zeiss-1000 detection
Optical Steeghs et al. 23833 GOTO optical observations Gravitational-wave Optical Transient Observer detection
Optical Cucchiara 23839 LCO-CTIO optical observations LCO detection
Optical Paek et al. 23841 McDonald 0.8-m telescope optical
observation		 McDonald 0.8 detection
Optical Malesani et al. 23842 NOT optical observations NOT
Optical Belkin et al. 23843 TSHAO optical observations Zeiss-1000
Optical Moskvitin and Spiridonova 23844 SAO RAS optical observations SAO RAS detection
Optical Kumar et al. 23853 R and I band observation from HCT Himalayan Chandra Telescope detection
Optical Moskvitin et al. 23870 SAO RAS further optical observations SAO RAS detection
Optical Xu et al. 23871 Xinglong-2.16m optical observations Xinglong
Optical Kumar et al. 23874 GROWTH-India detection of optical
afterglow		 GROWTH-India detection
Optical Sokolov et al. 23881 Terskol 2-m optical observations Terskol detection
Gamma-ray Verrecchia et al. 23835 AGILE ratemeters detection AGILE
Gamma-ray Veres and Meegan 23838 Fermi GBM observation Fermi GBM Epeak=287±18 keV
T90=18.4 seconds
Fluence=1.81±0.03x10-5erg cm-2
(85th percentile for long GRBs)
Gamma-ray Sharma et al. 23875 AstroSat CZTI detection CZTI T90=14.8 seconds

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 173 299 124 16.02 ± 0.03
v 1613 2153 78 17.94 ± 0.18
b 1711 1903 39 18.38 ± 0.16
u 1686 2054 58 18.20 ± 0.17
w1 1662 2376 97 >18.5

Table 3. UVOT observations reported by Kuin and Lien (GCN Circ. 23834). 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.

February 8, 2019