Swift Observations of GRB 180626A

P.A. Evans (U. Leicester), D.N. Burrows (PSU) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 08:21:16 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180626A (trigger=844615) (Evans et al. GCN Circ. 22850). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 131° from the Sun (9.9 hours East) and 34° from the 97%-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.

Evans et al. (GCN Circ. 22850) 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 Krimm et al. (GCN Circ. 22862), the BAT ground-calculated position is RA, Dec = 243.569, 14.759 deg which is RA(J2000) = 16^h14^m16.6^s Dec(J2000) = +14°45'32.8" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 35%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure. The first peak is from approximately T-10 to T+5 s, with a second overlapping peak from T+5 to T+20 s, followed by a weaker peak from T+20 to T+30 s. T₉₀ (15-350 keV) is 30.07 ± 1.42 s (estimated error including systematics).

The time-averaged spectrum from T-10.01 to T+25.59 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.64 ± 0.27, and E_peak of 47.7 ± 12.2 keV (χ² 40.18 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 4.8 ± 0.2 x 10^-6 erg cm^-2 and the 1-s peak flux measured from T-3.69 s in the 15-150 keV band is 4.1 ± 0.4 ph cm^-2 s^-1. This fluence is larger than that of 82% 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 2.01 ± 0.06 (χ² 46.26 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/844615/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Burrows et al. (GCN Circ. 22860). We have analysed 24 ks of XRT data for GRB 180626A, from 129 s to 288.0 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 22852).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=0.39 ± 0.04, followed by a break at T+9120 s to an α of 1.23 (+0.08, -0.07).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.03 (+0.11, -0.10). The best-fitting absorption column is 2.3 ± 0.4 x 10²¹ cm^-2, in excess of the Galactic value of 3.5 x 10²⁰ cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 x 10^-11 (5.0 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.3 ± 0.4 x 10²¹ cm^-2
Galactic foreground: 3.5 x 10²⁰ cm^-2
Excess significance: 8.9 σ
Photon index: 2.03 (+0.11, -0.10)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00844615.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180626A 123 s after the BAT trigger (Breeveld and Evans GCN Circ. 22857). A fading source consistent with the XRT position (Osborne et al. GCN Circ. 22852) and also seen by Martone et al. (GCN Circ. 22854) and Troja et al. (GCN Circ. 22855) 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 0.04 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

16^h14^m18.12^s +14°45'25.5" 0.81" UVOT-initial Evans et al. GCN Circ. 22850

16^h14^m18.20^s +14°45'25.5" 1.4" XRT-final UKSSDC

16^h14^m18.25^s +14°45'24.9" 1.8" XRT-enhanced Osborne et al. GCN Circ. 22852

16^h14^m16.6^s +14°45'32.8" 1.0' BAT-refined Krimm et al. GCN Circ. 22862

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Elenin et al. 22853 ISON-MN optical observations ISON-NM

Optical Martone et al. 22854 LCO Haleakala observations LCO detection

Optical Troja et al. 22855 RATIR Optical Detection RATIR detection

Optical Watson et al. 22859 COATLI Optical Detection COATLI detection

Optical Zheng and Filippenko 22861 KAIT Optical Upper Limit KAIT upper limits

Optical Watson et al. 22865 RATIR Optical Observations RATIR

Optical Mazaeva et al. 22870 TSHAO optical upper limit Zeiss-1000 upper limits

Gamma-ray Gotz et al. 22851 A long GRB detected by INTEGRAL INTEGRAL Fluence=2x10^-6erg cm^-2

Gamma-ray Kozlova et al. 22869 Konus-Wind observation Konus-Wind Fluence=7.21(-0.79,+0.97)x10^-6erg cm^-2

Other Coughlin et al. 22871 Zwicky Transient Facility Follow-Up of
a Fermi Short GRB (Trigger 548793993)

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_FC 123 273 147 20.57 ± 0.22

white 633 1033 190 >21.4

v 665 6473 452 >20.2

b 591 5858 246 >20.4

u 336 6936 529 >20.7

w1 714 6882 432 >21.0

m2 5043 6677 393 >20.1

w2 641 6268 432 >21.3

RA (J2000)	Dec (J2000)	Error	Note	Reference
16^h14^m18.12^s	+14°45'25.5"	0.81"	UVOT-initial	Evans et al. GCN Circ. 22850
16^h14^m18.20^s	+14°45'25.5"	1.4"	XRT-final	UKSSDC
16^h14^m18.25^s	+14°45'24.9"	1.8"	XRT-enhanced	Osborne et al. GCN Circ. 22852
16^h14^m16.6^s	+14°45'32.8"	1.0'	BAT-refined	Krimm et al. GCN Circ. 22862

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Elenin et al.	22853	ISON-MN optical observations	ISON-NM
Optical	Martone et al.	22854	LCO Haleakala observations	LCO	detection
Optical	Troja et al.	22855	RATIR Optical Detection	RATIR	detection
Optical	Watson et al.	22859	COATLI Optical Detection	COATLI	detection
Optical	Zheng and Filippenko	22861	KAIT Optical Upper Limit	KAIT	upper limits
Optical	Watson et al.	22865	RATIR Optical Observations	RATIR
Optical	Mazaeva et al.	22870	TSHAO optical upper limit	Zeiss-1000	upper limits
Gamma-ray	Gotz et al.	22851	A long GRB detected by INTEGRAL	INTEGRAL	Fluence=2x10^-6erg cm^-2
Gamma-ray	Kozlova et al.	22869	Konus-Wind observation	Konus-Wind	Fluence=7.21(-0.79,+0.97)x10^-6erg cm^-2
Other	Coughlin et al.	22871	Zwicky Transient Facility Follow-Up of a Fermi Short GRB (Trigger 548793993)

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white_FC	123	273	147	20.57 ± 0.22
white	633	1033	190	>21.4
v	665	6473	452	>20.2
b	591	5858	246	>20.4
u	336	6936	529	>20.7
w1	714	6882	432	>21.0
m2	5043	6677	393	>20.1
w2	641	6268	432	>21.3

Table 3. UVOT observations reported by Breeveld and Evans (GCN Circ. 22857). 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 30, 2018