Swift Observations of GRB 190511A

M.G. Bernardini (INAF-OAB), K.L. Page (U. Leicester) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 07:14:48 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190511A (trigger=903158) (Bernardini et al. GCN Circ. 24472). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 86° from the Sun (5.3 hours East) and 40° from the 42%-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.

Malesani et al. (GCN Circ. 24492) reported the position from NOT for the optical afterglow of this GRB. Kuin et al. (GCN Circ. 24494) reported the detection 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 Barthelmy et al. (GCN Circ. 24502), the BAT ground-calculated position is RA, Dec = 126.443, -20.252 deg which is RA(J2000) = 08h25m46.2s Dec(J2000) = -20°15'05.6" with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment). The partial coding was 9%.

The mask-weighted light curve (Figure 1) shows a complex, multi-peaked structure, starting at about T-25 s, peaking at T+0 s and declining to background by T+10 s. The spacecraft slewed away from the burst position by T+475 s. T90 (15-350 keV) is 27.7 ± 2.4 s (estimated error including systematics).

The time-averaged spectrum from T-24.27 to T+6.88 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.19 ± 0.10. The fluence in the 15-150 keV band is 7.4 ± 0.5 x 10-6 erg cm-2. This fluence is larger than that of 88% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.36 s in the 15-150 keV band is 6.2 ± 1.2 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/903158/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Page et al. (GCN Circ. 24498). We have analysed 19 ks of XRT data for GRB 190511A, from 129 s to 366.2 ks after the BAT trigger. The data comprise 175 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 24481).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=1.15 (+0.04, -0.03), followed by a break at T+23.2 ks to an α of 2.2 (+0.5, -0.3).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.07 (+0.08, -0.07). The best-fitting absorption column is consistent with the Galactic value of 1.1 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.12 (+0.24, -0.23) and a best-fitting absorption column of 1.6 (+0.7, -0.5) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.2 x 10-11 (4.5 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.6 (+0.7, -0.5) x 1021 cm-2
Galactic foreground: 1.1 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 2.12 (+0.24, -0.23)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00903158.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190511A 131 s after the BAT trigger (Kuin et al. GCN Circ. 24494). A source consistent with the XRT position (Goad et al. GCN Circ. 24481) 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
08h25m46.44s -20°15'33.6" 0.42" UVOT-refined Kuin et al. GCN Circ. 24494
08h25m46.61s -20°15'34.1" 1.6" XRT-final UKSSDC
08h25m46.62s -20°15'33.7" 2.0" XRT-enhanced Goad et al. GCN Circ. 24481
08h25m46.2s -20°15'05.6" 1.5' BAT-refined Barthelmy et al. GCN Circ. 24502

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Malesani et al. 24492 NOT optical afterglow NOT detection
Gamma-ray Fermi 24471 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray von Kienlin 24482 Fermi GBM observation Fermi GBM Epeak=147.8±7.2 keV
T90=28 seconds
Fluence=2.21±0.05x10-5erg cm-2
(88th percentile for long GRBs)
Gamma-ray Axelsson et al. 24483 Fermi-LAT detection Fermi LAT
Gamma-ray Kozlova et al. 24534 Konus-Wind observation Konus-Wind Epeak=185 (-15,+18) keV
Duration=~31.8 seconds
Fluence=1.93(-0.11,+0.12)x10-5erg cm-2
Other Alberto 24499 en

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 131 281 147 16.0 ± 0.1
b 5684 5884 197 21.6 ± 0.3

Table 3. UVOT observations reported by Kuin et al. (GCN Circ. 24494). 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.

May 18, 2019