Swift Observations of GRB 180329B

J.L. Racusin (NASA/GSFC), A. Melandri (INAF-OAB) and S.W.K. Emery (UCL-MSSL) for the Swift team

1. Introduction

At 14:08:23 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180329B (trigger=819490) (Racusin et al. GCN Circ. 22558). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 78° from the Sun (5.0 hours East) and 87° from the 95%-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.

Racusin et al. (GCN Circ. 22558) reported the discovery with UVOT of an optical afterglow. Perley et al. (GCN Circ. 22562) reported the position from NOT for the optical afterglow of this GRB. Izzo et al. (GCN Circ. 22567) determined a redshift of 1.998 from VLT. 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 Palmer et al. (GCN Circ. 22566), the BAT ground-calculated position is RA, Dec = 82.905, -23.694 deg which is RA(J2000) = 05^h31^m37.2^s Dec(J2000) = -23°41'39.8" with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment). The partial coding was 15%.

The mask-weighted light curve (Figure 1) shows a triple-peaked structure, with the first peak running from T-10 to T+40 s, and the second peak from T+130 to T+160 s. This is followed by a much weaker, softer peak from T+160 to T+280 s. T₉₀ (15-350 keV) is 210.0 ± 44.0 s (estimated error including systematics).

The time-averaged spectrum from T-9.91 to T+230.39 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.97 ± 0.56, and E_peak of 48.6 ± 9.1 keV (χ² 54.64 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 3.3 ± 0.3 x 10^-6 erg cm^-2 and the 1-s peak flux measured from T+1.56 s in the 15-150 keV band is 1.4 ± 0.4 ph cm^-2 s^-1. This fluence is larger than that of 73% 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 1.93 ± 0.12 (χ² 65.32 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/819490/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Melandri et al. (GCN Circ. 22560). We have analysed 21 ks of XRT data for GRB 180329B, from 109 s to 190.3 ks after the BAT trigger. The data comprise 260 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. 22559).

The late-time light curve (Figure 2) (from T0+4.6 ks) can be modelled with a power-law decay with a decay index of α=1.29 ± 0.06.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.52 ± 0.03. The best-fitting absorption column is 2.9 (+0.9, -0.8) x 10²¹ cm^-2, at a redshift of 1.998, in addition to the Galactic value of 2.5 x 10²⁰ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.87 ± 0.09 and a best-fitting absorption column of 2.6 (+1.8, -1.7) x 10²¹ cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 x 10^-11 (4.0 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 2.5 x 10²⁰ cm^-2
Intrinsic column: 2.6 (+1.8, -1.7) x 10²¹ cm^-2 at z=1.998
Photon index: 1.87 ± 0.09

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00819490.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180329B 113 s after the BAT trigger (Emery and Racusin GCN Circ. 22565). A fading source consistent with the XRT position (Beardmore et al. GCN Circ. 22559) also detected by the NOT (Perley et al. GCN Circ. 22562) and RATIR (Watson et al. GCN Circ. 22564) 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

05^h31^m36.91^s -23°41'25.8" 0.43" UVOT-refined Emery and Racusin GCN Circ. 22565

05^h31^m36.91^s -23°41'27.4" 1.4" XRT-final UKSSDC

05^h31^m36.90^s -23°41'27.1" 1.7" XRT-enhanced Beardmore et al. GCN Circ. 22559

05^h31^m37.2^s -23°41'39.8" 1.8' BAT-refined Palmer et al. GCN Circ. 22566

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Perley et al. 22562 NOT optical afterglow detection NOT detection

Optical Watson et al. 22564 RATIR Optical Observations RATIR

Optical Izzo et al. 22567 VLT/X-shooter spectroscopic observations VLT

Optical Guidorzi et al. 22571 LCO FTS observations FTS 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

white 113 263 147 17.69 ± 0.04

white 550 6892 599 17.64 ± 0.03

v 601 5867 274 17.25 ± 0.07

b 526 6687 452 17.94 ± 0.05

u 271 6482 697 17.00 ± 0.04

u 16039 39219 1961 18.74 ± 0.07

w1 650 6277 452 18.72 ± 0.14

m2 626 6073 274 >19.8

w2 577 7017 390 >20.2

RA (J2000)	Dec (J2000)	Error	Note	Reference
05^h31^m36.91^s	-23°41'25.8"	0.43"	UVOT-refined	Emery and Racusin GCN Circ. 22565
05^h31^m36.91^s	-23°41'27.4"	1.4"	XRT-final	UKSSDC
05^h31^m36.90^s	-23°41'27.1"	1.7"	XRT-enhanced	Beardmore et al. GCN Circ. 22559
05^h31^m37.2^s	-23°41'39.8"	1.8'	BAT-refined	Palmer et al. GCN Circ. 22566

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Perley et al.	22562	NOT optical afterglow detection	NOT	detection
Optical	Watson et al.	22564	RATIR Optical Observations	RATIR
Optical	Izzo et al.	22567	VLT/X-shooter spectroscopic observations	VLT
Optical	Guidorzi et al.	22571	LCO FTS observations	FTS	detection

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white	113	263	147	17.69 ± 0.04
white	550	6892	599	17.64 ± 0.03
v	601	5867	274	17.25 ± 0.07
b	526	6687	452	17.94 ± 0.05
u	271	6482	697	17.00 ± 0.04
u	16039	39219	1961	18.74 ± 0.07
w1	650	6277	452	18.72 ± 0.14
m2	626	6073	274	>19.8
w2	577	7017	390	>20.2

Table 3. UVOT observations reported by Emery and Racusin (GCN Circ. 22565). 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.

April 1, 2018