Swift Observations of GRB 190613A

E. Ambrosi (INAF-IASFPA), D.N. Burrows (PSU) and F.E. Marshall (NASA/GSFC) for the Swift team

1. Introduction

At 04:07:18 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190613A (trigger=908288) (Ambrosi et al. GCN Circ. 24798). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 73° from the Sun (6.8 hours East) and 76° from the 81%-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.

Ambrosi et al. (GCN Circ. 24798) 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 Stamatikos et al. (GCN Circ. 24819), the BAT ground-calculated position is RA, Dec = 182.552, 67.244 deg which is RA(J2000) = 12^h10^m12.5^s Dec(J2000) = +67°14'39.7" with an uncertainty of 2.2 arcmin, (radius, sys+stat, 90% containment). The partial coding was 9%.

The mask-weighted light curve (Figure 1) shows a weak pulse that starts at ~T₀, peaks at ~T+7 s, and ends at ~T+20 s. T₉₀ (15-350 keV) is 17.6 ± 3.1 s (estimated error including systematics).

The time-averaged spectrum from T+0.6 to T+20.3 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.22 ± 0.26. The fluence in the 15-150 keV band is 1.7 ± 0.3 x 10^-6 erg cm^-2. This fluence is larger than that of 55% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+6.88 s in the 15-150 keV band is 1.5 ± 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/908288/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Burrows et al. (GCN Circ. 24810). We have analysed 16 ks of XRT data for GRB 190613A, from 128 s to 229.8 ks after the BAT trigger. The data comprise 127 s in Windowed Timing (WT) mode (the first 6 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 Osborne et al. (GCN Circ. 24809).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=2.00 ± 0.27. At T+243 s the decay steepens to an α of 8.0 (+0.0, -1.0) before breaking again at T+322 s to a final decay with index α=1.26 (+0.13, -0.11).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.01 ± 0.12. The best-fitting absorption column is 5.2 (+2.8, -2.6) x 10²⁰ cm^-2, in excess of the Galactic value of 1.5 x 10²⁰ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.78 (+0.21, -0.15) and a best-fitting absorption column of 2.5 (+4.7, -1.0) 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 (3.8 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.5 (+4.7, -1.0) x 10²⁰ cm^-2
Galactic foreground: 1.5 x 10²⁰ cm^-2
Excess significance: <1.6 σ
Photon index: 1.78 (+0.21, -0.15)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00908288.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190613A 145 s after the BAT trigger (Marshall and Ambrosi GCN Circ. 24815). A source consistent with the enhanced XRT position (Osborne et al. GCN Circ. 24809) 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 E_B-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

12^h10^m07.01^s +67°14'07.0" 0.43" UVOT-refined Marshall and Ambrosi GCN Circ. 24815

12^h10^m07.01^s +67°14'07.2" 1.5" XRT-final UKSSDC

12^h10^m07.11^s +67°14'07.3" 1.5" XRT-enhanced Osborne et al. GCN Circ. 24809

12^h10^m12.5^s +67°14'39.7" 2.2' BAT-refined Stamatikos et al. GCN Circ. 24819

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Zheng and Filippenko 24801 KAIT Optical Afterglow Confirmation KAIT detection

Optical Lipunov et al. 24802 Swift GRB190613.17 Global MASTER-Net
observations report MASTER

Optical Lipunov et al. 24803 MASTER OT detection MASTER detection

Optical Jelinek et al. 24822 FRAM-ORM afterglow detection FRAM detection

Optical Belkin et al. 24838 Kitab and AbAO optical upper limit Abastumani Astro. Obs. upper limits

Optical Cunningham and Neill 24850 SEDM Observations Spectral Energy Distribution Machine at Palomar

Gamma-ray Fermi 24799 Fermi GBM Final Real-time Localization Fermi GBM

Gamma-ray Burgess et al. 24800 BALROG localization (Fermi Trigger
582091643 / GRB 190613172) BALROG

Gamma-ray Poolakkil and Meegan 24816 Fermi GBM detection Fermi GBM E_peak=108±5 keV
T₉₀=18 seconds
Fluence=3.092±0.129x10^-6erg cm^-2
(43^rd percentile for long GRBs)

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

Filter T_start(s) T_stop(s) Exp(s) Mag

white 145 295 147 17.88 ± 0.04

v 634 1061 58 18.63 ± 0.29

b 560 753 39 18.93 ± 0.21

u 303 553 246 18.88 ± 0.11

w1 685 11118 1121 >20.1

m2 5323 6859 296 >20.8

w2 4913 6548 393 >21.5

RA (J2000)	Dec (J2000)	Error	Note	Reference
12^h10^m07.01^s	+67°14'07.0"	0.43"	UVOT-refined	Marshall and Ambrosi GCN Circ. 24815
12^h10^m07.01^s	+67°14'07.2"	1.5"	XRT-final	UKSSDC
12^h10^m07.11^s	+67°14'07.3"	1.5"	XRT-enhanced	Osborne et al. GCN Circ. 24809
12^h10^m12.5^s	+67°14'39.7"	2.2'	BAT-refined	Stamatikos et al. GCN Circ. 24819

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Zheng and Filippenko	24801	KAIT Optical Afterglow Confirmation	KAIT	detection
Optical	Lipunov et al.	24802	Swift GRB190613.17 Global MASTER-Net observations report	MASTER
Optical	Lipunov et al.	24803	MASTER OT detection	MASTER	detection
Optical	Jelinek et al.	24822	FRAM-ORM afterglow detection	FRAM	detection
Optical	Belkin et al.	24838	Kitab and AbAO optical upper limit	Abastumani Astro. Obs.	upper limits
Optical	Cunningham and Neill	24850	SEDM Observations	Spectral Energy Distribution Machine at Palomar
Gamma-ray	Fermi	24799	Fermi GBM Final Real-time Localization	Fermi GBM
Gamma-ray	Burgess et al.	24800	BALROG localization (Fermi Trigger 582091643 / GRB 190613172)	BALROG
Gamma-ray	Poolakkil and Meegan	24816	Fermi GBM detection	Fermi GBM	E_peak=108±5 keV T₉₀=18 seconds Fluence=3.092±0.129x10^-6erg cm^-2 (43^rd percentile for long GRBs)

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white	145	295	147	17.88 ± 0.04
v	634	1061	58	18.63 ± 0.29
b	560	753	39	18.93 ± 0.21
u	303	553	246	18.88 ± 0.11
w1	685	11118	1121	>20.1
m2	5323	6859	296	>20.8
w2	4913	6548	393	>21.5

Table 3. UVOT observations reported by Marshall and Ambrosi (GCN Circ. 24815). 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.

June 20, 2019