Swift Observations of GRB 190719C

E. Ambrosi (INAF-IASFPA), A.P. Beardmore (U. Leicester) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 14:58:34 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190719C (trigger=915381) (Ambrosi et al. GCN Circ. 25106). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 114° from the Sun (8.1 hours East) and 93° from the 93%-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.

Heintz et al. (GCN Circ. 25144) reported the position for the optical afterglow of this GRB. Rossi et al. (GCN Circ. 25252) determined a redshift of 2.469 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 Sakamoto et al. (GCN Circ. 25129), the BAT ground-calculated position is RA, Dec = 240.208, 13.005 deg which is RA(J2000) = 16h00m49.8s Dec(J2000) = +13°00'18.4" with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T0 and ends at ~T+200 s. The main peak occurs at ~T0. T90 (15-350 keV) is 185.7 ± 9.7 s (estimated error including systematics).

The time-averaged spectrum from T-0.09 to T+201.96 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.64 ± 0.09. The fluence in the 15-150 keV band is 5.1 ± 0.3 x 10-6 erg cm-2. This fluence is larger than that of 82% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.27 s in the 15-150 keV band is 5.5 ± 0.3 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/915381/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Beardmore et al. (GCN Circ. 25125). We have analysed 22 ks of XRT data for GRB 190719C, from 54 s to 844.0 ks after the BAT trigger. The data comprise 353 s in Windowed Timing (WT) mode (the first 5 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode.

The late-time light curve (Figure 2) (from T0+45.7 ks) can be modelled with a power-law decay with a decay index of α=1.44 (+0.09, -0.08).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.42 (+0.04, -0.03). The best-fitting absorption column is 2.26 (+0.19, -0.18) x 1021 cm-2, in excess of the Galactic value of 4.7 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.54 (+0.29, -0.25) and a best-fitting absorption column of 9.0 (+9.8, -4.3) x 1020 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.5 x 10-11 (5.0 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 9.0 (+9.8, -4.3) x 1020 cm-2
Galactic foreground: 4.7 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 1.54 (+0.29, -0.25)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00915381.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190719C 71 s after the BAT trigger (Kuin and Ambrosi GCN Circ. 25128). 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 0.04 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
16h00m49.65s +13°00'00.0" 3.6" XRT-final UKSSDC
16h00m49.65s +13°00'00.0" 3.6" XRT-refined Beardmore et al. GCN Circ. 25125
16h00m49.8s +13°00'18.4" 1.2' BAT-refined Sakamoto et al. GCN Circ. 25129

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Malesani et al. 25110 NOT candidate optical afterglow and host NOT
Optical Heintz et al. 25144 optical afterglow confirmation NOT detection
Optical Rossi et al. 25252 VLT/X-shooter spectroscopic redshift of
the host galaxy		 VLT redshift
Gamma-ray Fermi 25104 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Poolakkil and Meegan 25130 Fermi GBM detection Fermi GBM Epeak=81±9 keV
T90=175 seconds
Fluence=1.155±0.082x10-6erg cm-2
(17th 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 71 221 147 >20.6
uFC 283 533 246 >20.6
white 71 756 186 >20.8
v 613 807 39 >18.7
b 538 732 39 >19.9
u 283 706 265 >20.5
w1 662 856 39 >19.2

Table 3. UVOT observations reported by Kuin and Ambrosi (GCN Circ. 25128). 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.

August 2, 2019