Swift Observations of GRB 190519A

T.N. Ukwatta (LANL), A. D'Ai (INAF-IASFPA) and J.D. Gropp (PSU) for the Swift team

1. Introduction

At 07:25:39 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190519A (trigger=904394) (Ukwatta et al. GCN Circ. 24595). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 81° from the Sun (3.9 hours East) and 105° 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 Lien et al. (GCN Circ. 24606), the BAT ground-calculated position is RA, Dec = 114.758, -38.813 deg which is RA(J2000) = 07h39m01.9s Dec(J2000) = -38°48'46.6" with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 8%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T-60 s and ends at ~T+15 s. The main peak occurs at ~T0. T90 (15-350 keV) is 45.58 ± 2.86 s (estimated error including systematics).

The time-averaged spectrum from T-56.97 to T+14.79 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.32 ± 0.24, and Epeak of 90.0 ± 35.0 keV (χ2 76.36 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 1.8 ± 0.1 x 10-5 erg cm-2 and the 1-s peak flux measured from T-0.21 s in the 15-150 keV band is 20.6 ± 1.7 ph cm-2 s-1. This fluence is larger than that of 96.4% 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 1.72 ± 0.06 (χ2 85.04 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/904394/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Ai et al. (GCN Circ. 24608). We have analysed 16 ks of XRT data for GRB 190519A, from 91 s to 212.8 ks after the BAT trigger. The data comprise 100 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. 24597).

The late-time light curve (Figure 2) (from T0+13.7 ks) can be modelled with a power-law decay with a decay index of α=1.40 ± 0.12.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.34 (+0.20, -0.18). The best-fitting absorption column is 6.5 (+1.2, -1.1) x 1021 cm-2, in excess of the Galactic value of 4.3 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.77 (+0.12, -0.11) and a best-fitting absorption column of 6.6 (+1.0, -0.9) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.0 x 10-11 (7.4 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 6.6 (+1.0, -0.9) x 1021 cm-2
Galactic foreground: 4.3 x 1021 cm-2
Excess significance: 4.1 σ
Photon index: 1.77 (+0.12, -0.11)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00904394.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190519A 150 s after the BAT trigger (Gropp and Ukwatta GCN Circ. 24612). No optical afterglow consistent with the enhanced XRT position (Ukwatta et al. GCN Circ. 24597) 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 0.42 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
07h39m04.24s -38°48'19.6" 1.6" XRT-final UKSSDC
07h39m04.22s -38°48'19.4" 1.6" XRT-enhanced Beardmore et al. GCN Circ. 24597
07h39m01.9s -38°48'46.6" 1.7' BAT-refined Lien et al. GCN Circ. 24606

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Gamma-ray Fermi 24594 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray von Kienlin 24596 Fermi GBM observation Fermi GBM Epeak=112±3 keV
T90=46 seconds
Fluence=3.16±0.04x10-5erg cm-2
(91st percentile for long GRBs)
Gamma-ray Yamaoka et al. 24611 CALET Gamma-Ray Burst Monitor detection CALET
Gamma-ray Ghumatkar et al. 24613 AstroSat CZTI detection CZTI T90=45.2 seconds
Gamma-ray Tsvetkova et al. 24652 Konus-Wind observation Konus-Wind Epeak=125 (-6,+7) keV
Fluence=3.20(-0.16,+0.18)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
uFC 150 400 246 >18.9
v 456 2571 272 >18.7
b 406 2520 272 >19.7
u 150 2495 646 >19.5
w1 505 2610 262 >19.5
m2 481 2595 272 >19.1
w2 432 2546 272 >19.2

Table 3. UVOT observations reported by Gropp and Ukwatta (GCN Circ. 24612). 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.

May 23, 2019