Swift Observations of GRB 181228A

M.J. Moss (George Washington University) and S.J. LaPorte (PSU) for the Swift team

1. Introduction

At 02:53:17 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 181228A (trigger=879909) (Moss et al. GCN Circ. 23570). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 135° from the Sun (8.9 hours East) and 119° from the 64%-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 Lien et al. (GCN Circ. 23583), the BAT ground-calculated position is RA, Dec = 49.831, 13.212 deg which is RA(J2000) = 03^h19^m19.5^s Dec(J2000) = +13°12'44.0" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 30%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure. The main structure starts at ~T-4 s and ends at ~T+20 s, with the highest peak occurs at ~ T+13 s. After the main pulse, there is another much weaker pulse that starts at ~ T+200 s and ends at ~T+340 s. T₉₀ (15-350 keV) is 257.3 ± 102.5 s (estimated error including systematics).

The time-averaged spectrum from T-4.0 to T+337.9 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.45 ± 0.06. The fluence in the 15-150 keV band is 1.0 ± 0.03 x 10^-5 erg cm^-2. This fluence is larger than that of 92.5% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+13.08 s in the 15-150 keV band is 9.5 ± 0.5 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/879909/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by LaPorte et al. (GCN Circ. 23584). We have analysed 12 ks of XRT data for GRB 181228A, from 73 s to 397.2 ks after the BAT trigger. The data comprise 417 s in Windowed Timing (WT) mode (the first 7 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 Beardmore et al. (GCN Circ. 23574).

The late-time light curve (Figure 2) (from T0+12.9 ks) can be modelled with a power-law decay with a decay index of α=1.16 ± 0.19.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.60 ± 0.05. The best-fitting absorption column is 3.30 (+0.28, -0.27) x 10²¹ cm^-2, in excess of the Galactic value of 2.9 x 10²¹ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.80 (+0.20, -0.19) and a best-fitting absorption column of 5.3 (+1.4, -1.2) x 10²¹ cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.6 x 10^-11 (6.8 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 5.3 (+1.4, -1.2) x 10²¹ cm^-2
Galactic foreground: 2.9 x 10²¹ cm^-2
Excess significance: 3.2 σ
Photon index: 1.80 (+0.20, -0.19)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00879909.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 181228A 93 s after the BAT trigger (LaPorte and Moss GCN Circ. 23581). No optical afterglow consistent with the XRT position 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 0.51 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

03^h19^m14.94^s +13°12'07.4" 1.4" XRT-final UKSSDC

03^h19^m14.91^s +13°12'07.5" 1.8" XRT-enhanced Beardmore et al. GCN Circ. 23574

03^h19^m19.5^s +13°12'44.0" 1.0' BAT-refined Lien et al. GCN Circ. 23583

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Zheng and Filippenko 23571 KAIT Optical Upper Limit KAIT upper limits

Optical D'Avanzo et al. 23575 REM NIR detection REM detection

Optical Lipunov et al. 23576 Global MASTER Net OT detection MASTER detection

Optical Volnova et al. 23580 TSHAO optical upper limit Zeiss-1000 upper limits

Optical Zhu et al. 23588 Xinglong-2.16m optical upper limit Xinglong upper limits

Optical Oeda et al. 23590 MITSuME Akeno optical upper limits MITSuME Akeno upper limits

Optical Xu et al. 23593 possible NOT optical detection NOT possible detection

Optical Lipunov et al. 23594 Global MASTER Net OT is the same REM
IR transient MASTER

Optical Belkin et al. 23596 Mondy and AbAO optical upper limits Abastumani Astro. Obs. upper limits

Optical Jelinek et al. 23669 FRAM-ORM optical limit FRAM upper limits

Gamma-ray Bissaldi and Veres 23579 Fermi GBM observation Fermi GBM E_peak=182±8 keV
T₉₀=18 seconds
Fluence=2.56±0.05x10^-5erg cm^-2
(90^th 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_FC 93 243 147 >20.4

u_FC 305 555 246 >20.6

white 93 1354 373 >21.0

v 634 1404 97 >19.1

b 561 1330 78 >20.5

u 305 1305 304 >20.5

w1 684 1280 39 >18.4

RA (J2000)	Dec (J2000)	Error	Note	Reference
03^h19^m14.94^s	+13°12'07.4"	1.4"	XRT-final	UKSSDC
03^h19^m14.91^s	+13°12'07.5"	1.8"	XRT-enhanced	Beardmore et al. GCN Circ. 23574
03^h19^m19.5^s	+13°12'44.0"	1.0'	BAT-refined	Lien et al. GCN Circ. 23583

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Zheng and Filippenko	23571	KAIT Optical Upper Limit	KAIT	upper limits
Optical	D'Avanzo et al.	23575	REM NIR detection	REM	detection
Optical	Lipunov et al.	23576	Global MASTER Net OT detection	MASTER	detection
Optical	Volnova et al.	23580	TSHAO optical upper limit	Zeiss-1000	upper limits
Optical	Zhu et al.	23588	Xinglong-2.16m optical upper limit	Xinglong	upper limits
Optical	Oeda et al.	23590	MITSuME Akeno optical upper limits	MITSuME Akeno	upper limits
Optical	Xu et al.	23593	possible NOT optical detection	NOT	possible detection
Optical	Lipunov et al.	23594	Global MASTER Net OT is the same REM IR transient	MASTER
Optical	Belkin et al.	23596	Mondy and AbAO optical upper limits	Abastumani Astro. Obs.	upper limits
Optical	Jelinek et al.	23669	FRAM-ORM optical limit	FRAM	upper limits
Gamma-ray	Bissaldi and Veres	23579	Fermi GBM observation	Fermi GBM	E_peak=182±8 keV T₉₀=18 seconds Fluence=2.56±0.05x10^-5erg cm^-2 (90^th percentile for long GRBs)

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white_FC	93	243	147	>20.4
u_FC	305	555	246	>20.6
white	93	1354	373	>21.0
v	634	1404	97	>19.1
b	561	1330	78	>20.5
u	305	1305	304	>20.5
w1	684	1280	39	>18.4

Table 3. UVOT observations reported by LaPorte and Moss (GCN Circ. 23581). 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.

January 11, 2019