Swift Observations of GRB 190613B

A. D'Ai (INAF-IASFPA), K.L. Page (U. Leicester) and F.E. Marshall (NASA/GSFC) for the Swift team

1. Introduction

At 10:47:02 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190613B (trigger=908329) (D'Ai et al. GCN Circ. 24807). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 133° from the Sun (9.1 hours West) and 92° from the 83%-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.

D'Ai et al. (GCN Circ. 24807) 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 Ukwatta et al. (GCN Circ. 24820), the BAT ground-calculated position is RA, Dec = 305.434, -4.655 deg which is RA(J2000) = 20h21m44.2s Dec(J2000) = -04°39'17.0" with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment). The partial coding was 18%.

The mask-weighted light curve (Figure 1) shows a main pulse structure that starts at ~T-3 s, peaks at ~T0, and ends at ~T+2 s. In addition, there are some weak emissions that lasts until ~T+175 s. T90 (15-350 keV) is 160.84 ± 16.83 s (estimated error including systematics).

The time-averaged spectrum from T-2.85 to T+175.42 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.84 ± 0.24. The fluence in the 15-150 keV band is 2.3 ± 0.3 x 10-6 erg cm-2. This fluence is larger than that of 63% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.01 s in the 15-150 keV band is 5.7 ± 0.7 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/908329/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Page et al. (GCN Circ. 24823). We have analysed 26 ks of XRT data for GRB 190613B, from 103 s to 822.3 ks after the BAT trigger. The data comprise 352 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. 24812).

The late-time light curve (Figure 2) (from T0+5.3 ks) can be modelled with an initial power-law decay with an index of α=0.50 ± 0.10, followed by a break at T+44.1 ks to an α of 1.19 (+0.16, -0.15).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.93 ± 0.05. The best-fitting absorption column is 3.26 (+0.24, -0.23) x 1021 cm-2, in excess of the Galactic value of 6.0 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.91 ± 0.11 and a best-fitting absorption column of 2.6 (+0.5, -0.4) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.8 x 10-11 (5.2 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.6 (+0.5, -0.4) x 1021 cm-2
Galactic foreground: 6.0 x 1020 cm-2
Excess significance: 7.8 σ
Photon index: 1.91 ± 0.11

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00908329.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190613B 106 s after the BAT trigger (Marshall and D'Ai GCN Circ. 24817). A source consistent with the enhanced XRT position (Beardmore et al. GCN Circ. 24812) 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 0.07 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
20h21m43.66s -04°38'48.0" 0.43" UVOT-refined Marshall and D'Ai GCN Circ. 24817
20h21m43.67s -04°38'49.6" 1.4" XRT-final UKSSDC
20h21m43.69s -04°38'48.7" 1.7" XRT-enhanced Beardmore et al. GCN Circ. 24812
20h21m44.2s -04°39'17.0" 1.9' BAT-refined Ukwatta et al. GCN Circ. 24820

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Zheng and Filippenko 24818 KAIT Optical Afterglow Confirmation KAIT detection
Optical de Ugarte Postigo et al. 24825 Optical observations from NOT NOT detection
Optical Hu et al. 24826 BOOTES-5/JGT early optical observations BOOTES detection
Optical Schady and Bolmer 24831 GROND Detection of the Optical/NIR
Afterglow		 GROND detection
Optical Yang et al. 24832 Optical observation from Asiago Schmidt detection
Optical Cunningham and Neill 24836 SEDM Observations Spectral Energy Distribution Machine at Palomar
Optical Belkin et al. 24841 Kitab observatory optical upper limit upper limits
Optical Jelinek et al. 25081 FRAM-Auger afterglow detection FRAM detection
Radio Smith et al. 24848 JCMT SCUBA-2 sub-mm observations SCUBA
Gamma-ray Fermi 24806 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Burgess et al. 24808 BALROG localization (Fermi Trigger
582115625 / GRB 190613449)		 BALROG
Gamma-ray Bissaldi et al. 24814 Fermi GBM observation Fermi GBM Epeak=334±35 keV
T90=5 seconds
Fluence=4.4±0.2x10-6erg cm-2
(54th percentile for long GRBs)
Gamma-ray Luo et al. 24829 Insight-HXMT/HE detection Insight-HXMT T90=3.59 seconds
Gamma-ray Svinkin et al. 24844 Konus-Wind observation Konus-Wind Epeak=277 (-107,+331) keV
Duration=~4 seconds
Fluence=3.48(-0.79,+1.29)x10-6erg cm-2
Gamma-ray Ghumatkar et al. 24846 AstroSat CZTI detection CZTI T90=11.75 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 106 256 147 17.84 ± 0.04
v 648 667 19 >18.0
b 574 594 19 >18.7
u 318 568 246 18.86 ± 0.12
w1 697 717 19 >18.6
m2 672 692 19 >18.4
w2 624 643 19 >18.8

Table 3. UVOT observations reported by Marshall and D'Ai (GCN Circ. 24817). 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.

July 18, 2019