Swift Observations of GRB 180504A

A.Y. Lien (GSFC/UMBC), V. D'Elia (ASDC) and J.D. Gropp (PSU) for the Swift team

1. Introduction

At 19:46:13 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180504A (trigger=830822) (Lien et al. GCN Circ. 22682). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 76° from the Sun (4.7 hours West) and 50° from the 79%-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

Analysis of the BAT data was reported by Stamatikos et al. (GCN Circ. 22690). The partial coding was 61%.

The mask-weighted light curve (Figure 1) shows a triangular shaped profile, beginning at ~T-60 s, peaking at ~T+20 s, and decaying by T+80 s. T₉₀ (15-350 keV) is 88.3 ± 44.4 s (estimated error including systematics).

The time-averaged spectrum from T-25.03 to T+113.85 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.50 ± 0.13. The fluence in the 15-150 keV band is 2.2 ± 0.2 x 10^-6erg cm^-2. This fluence is larger than that of 62% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+16.80 s in the 15-150 keV band is 0.8 ± 0.2 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/830822/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Elia et al. (GCN Circ. 22685). We have analysed 23 ks of XRT data for GRB 180504A, from 98 s to 196.9 ks after the BAT trigger. The data comprise 163 s in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode.

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=1.46 (+0.16, -0.17). At T+222 s the decay steepens to an α of 2.81 (+0.20, -0.14) before breaking again at T+1081 s to a final decay with index α=0.82 (+0.11, -0.12).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.83 ± 0.11. The best-fitting absorption column is 7.5 (+3.2, -2.9) x 10²⁰ cm^-2, in excess of the Galactic value of 3.5 x 10²⁰ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.94 (+0.22, -0.21) and a best-fitting absorption column of 1.7 (+0.7, -0.6) x 10²¹ cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10^-11 (4.7 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.7 (+0.7, -0.6) x 10²¹ cm^-2
Galactic foreground: 3.5 x 10²⁰ cm^-2
Excess significance: 3.5 σ
Photon index: 1.94 (+0.22, -0.21)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00830822.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180504A 118 s after the BAT trigger (Gropp et al. GCN Circ. 22691). No optical afterglow consistent with the enhanced XRT position (Evans GCN Circ. 22683) 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

22^h04^m34.52^s -14°39'31.3" 1.5" XRT-final UKSSDC

22^h04^m34.61^s -14°39'30.0" 2.0" XRT-enhanced Evans GCN Circ. 22683

22^h04^m32^s -14°39'17" 3' BAT-initial Lien et al. GCN Circ. 22682

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Malesani and Losada 22684 NOT afterglow candidate NOT detection

Optical Clark et al. 22687 ePESSTO NTT NIR observations PESSTO 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

b 5159 6793 116 >19.04

uvm2 5978 7606 136 >18.81

u_FC 302 6588 688 >19.53

u 705 1869 381 >19.47

v 631 651 136 >18.20

uvw1 6183 6383 136 >18.58

uvw2 5569 7204 136 >18.87

white_FC 143 6998 557 >20.67

white 582 1746 393 >20.46

RA (J2000)	Dec (J2000)	Error	Note	Reference
22^h04^m34.52^s	-14°39'31.3"	1.5"	XRT-final	UKSSDC
22^h04^m34.61^s	-14°39'30.0"	2.0"	XRT-enhanced	Evans GCN Circ. 22683
22^h04^m32^s	-14°39'17"	3'	BAT-initial	Lien et al. GCN Circ. 22682

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Malesani and Losada	22684	NOT afterglow candidate	NOT	detection
Optical	Clark et al.	22687	ePESSTO NTT NIR observations	PESSTO	detection

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
b	5159	6793	116	>19.04
uvm2	5978	7606	136	>18.81
u_FC	302	6588	688	>19.53
u	705	1869	381	>19.47
v	631	651	136	>18.20
uvw1	6183	6383	136	>18.58
uvw2	5569	7204	136	>18.87
white_FC	143	6998	557	>20.67
white	582	1746	393	>20.46

Table 3. UVOT observations reported by Gropp et al. (GCN Circ. 22691). 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.

May 8, 2018