Swift Observations of GRB 180613A

A. Tohuvavohu (PSU), K.L. Page (U. Leicester) and S. Sebzda (PSU) for the Swift team

1. Introduction

At 15:36:18 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180613A (trigger=840853) (Tohuvavohu et al. GCN Circ. 22769). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 134° from the Sun (8.7 hours East) and 134° from the 0%-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 Markwardt et al. (GCN Circ. 22776), the BAT ground-calculated position is RA, Dec = 211.545, -43.082 deg which is RA(J2000) = 14^h06^m10.9^s Dec(J2000) = -43°04'56.5" with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment). The partial coding was 96%.

The mask-weighted light curve (Figure 1) shows some weak emission that starts at ~T+25 s and ends at ~T+80 s. T₉₀ (15-350 keV) is 50.8 ± 8.4 s (estimated error including systematics).

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/840853/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Page et al. (GCN Circ. 22774). We have analysed 15 ks of XRT data for GRB 180613A, from 155 s to 93.1 ks after the BAT trigger. The data comprise 95 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. 22771).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=2.07 (+0.07, -0.06).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.03 (+0.17, -0.16). The best-fitting absorption column is 1.5 (+1.1, -0.9) x 10²¹ cm^-2, consistent with the Galactic value of 6.5 x 10²⁰ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.48 (+0.24, -0.22) and a best-fitting absorption column of 2.5 (+1.5, -1.2) 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 (5.9 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.5 (+1.5, -1.2) x 10²¹ cm^-2
Galactic foreground: 6.5 x 10²⁰ cm^-2
Excess significance: 2.7 σ
Photon index: 1.48 (+0.24, -0.22)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00840853.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180613A 156 s after the BAT trigger (Sebzda and Tohuvavohu GCN Circ. 22779). No optical afterglow consistent with the XRT position (Beardmore et al. GCN Circ. 22773) 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 0.09 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

14^h06^m04.96^s -43°04'19.4" 4.7" XRT-final UKSSDC

14^h06^m04.94^s -43°04'20.5" 4.9" XRT-enhanced Beardmore et al. GCN Circ. 22773

14^h06^m10.9^s -43°04'56.5" 1.8' BAT-refined Markwardt et al. GCN Circ. 22776

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Thoene et al. 22775 Possible IR afterglow HAWKI

Optical Schady et al. 22777 GROND observations GROND detection

Optical Schady 22778 GROND observations: correction GROND

Optical Thoene et al. 22820 Afterglow confirmation and added
photometry VLT detection

Optical Cucchiara et al. 22841 LCO Afterglow detection LCO detection

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

u_FC 156 405 246 >20.4

v 461 24234 1434 >20.9

b 411 13230 1022 >99.0

u 156 18973 2109 >21.8

w1 510 18472 2212 >21.2

m2 486 24474 1627 >21.0

w2 1036 23321 1318 >20.1

RA (J2000)	Dec (J2000)	Error	Note	Reference
14^h06^m04.96^s	-43°04'19.4"	4.7"	XRT-final	UKSSDC
14^h06^m04.94^s	-43°04'20.5"	4.9"	XRT-enhanced	Beardmore et al. GCN Circ. 22773
14^h06^m10.9^s	-43°04'56.5"	1.8'	BAT-refined	Markwardt et al. GCN Circ. 22776

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Thoene et al.	22775	Possible IR afterglow	HAWKI
Optical	Schady et al.	22777	GROND observations	GROND	detection
Optical	Schady	22778	GROND observations: correction	GROND
Optical	Thoene et al.	22820	Afterglow confirmation and added photometry	VLT	detection
Optical	Cucchiara et al.	22841	LCO Afterglow detection	LCO	detection

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
u_FC	156	405	246	>20.4
v	461	24234	1434	>20.9
b	411	13230	1022	>99.0
u	156	18973	2109	>21.8
w1	510	18472	2212	>21.2
m2	486	24474	1627	>21.0
w2	1036	23321	1318	>20.1

Table 3. UVOT observations reported by Sebzda and Tohuvavohu (GCN Circ. 22779). 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.

June 26, 2018