Swift Observations of GRB 180331B

A.Y. Lien (GSFC/UMBC), P. D'Avanzo (INAF-OAB) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 15:38:39 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180331B (trigger=820513) (Lien et al. GCN Circ. 22573). Swift executed a delayed slewed due to an observing constraint. At the time of the trigger, the initial BAT position was 114° from the Sun (11.5 hours West) and 63° from the 100%-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 Stamatikos et al. (GCN Circ. 22576), the BAT ground-calculated position is RA, Dec = 196.781, 61.735 deg which is RA(J2000) = 13^h07^m07.6^s Dec(J2000) = +61°44'05.9" with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 30%.

The mask-weighted light curve (Figure 1) shows a double-peaked structure that starts at ~T-20 s and ends at ~T+140 s. The two peaks occur at ~T-20 s and ~T+1 s, respectively. T₉₀ (15-350 keV) is 147.0 ± 33.1 s (estimated error including systematics).

The time-averaged spectrum from T-22.03 to T+142.97 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.48 ± 0.22. The fluence in the 15-150 keV band is 1.9 ± 0.2 x 10^-6 erg cm^-2. This fluence is larger than that of 57% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.97 s in the 15-150 keV band is 2.8 ± 0.3 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/820513/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Avanzo et al. (GCN Circ. 22578). We have analysed 12 ks of XRT data for GRB 180331B, from 671 s to 99.2 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 22575).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=0.25 (+0.06, -0.12), followed by a break at T+16.2 ks to an α of 1.00 (+0.15, -0.22).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.85 (+0.13, -0.12). The best-fitting absorption column is 9.8 (+3.4, -3.1) x 10²⁰ cm^-2, in excess of the Galactic value of 1.6 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 3.6 x 10^-11 (4.2 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 9.8 (+3.4, -3.1) x 10²⁰ cm^-2
Galactic foreground: 1.6 x 10²⁰ cm^-2
Excess significance: 4.3 σ
Photon index: 1.85 (+0.13, -0.12)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00820513.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180331B 668 s after the BAT trigger (Kuin and Lien GCN Circ. 22579). No optical afterglow consistent with the XRT position Goad er al., (GCN Circ. 22575) is detected in the initial UVOT exposures, consistent with the report of Volnova et al. (GCN Circ. 22577). 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 0.01 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

13^h07^m07.96^s +61°43'35.2" 1.4" XRT-final UKSSDC

13^h07^m08.01^s +61°43'34.5" 1.8" XRT-enhanced Goad et al. GCN Circ. 22575

13^h07^m07.6^s +61°44'05.9" 2.0' BAT-refined Stamatikos et al. GCN Circ. 22576

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Volnova et al. 22577 AbAO optical observations Abastumani Astro. Obs.

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 668 818 147 >21.2

white 668 2066 412 >21.8

v 1056 2115 136 >19.2

b 1156 2041 117 >20.3

u 1130 2016 117 >20.3

w1 849 1992 78 >20.2

RA (J2000)	Dec (J2000)	Error	Note	Reference
13^h07^m07.96^s	+61°43'35.2"	1.4"	XRT-final	UKSSDC
13^h07^m08.01^s	+61°43'34.5"	1.8"	XRT-enhanced	Goad et al. GCN Circ. 22575
13^h07^m07.6^s	+61°44'05.9"	2.0'	BAT-refined	Stamatikos et al. GCN Circ. 22576

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Volnova et al.	22577	AbAO optical observations	Abastumani Astro. Obs.

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white_FC	668	818	147	>21.2
white	668	2066	412	>21.8
v	1056	2115	136	>19.2
b	1156	2041	117	>20.3
u	1130	2016	117	>20.3
w1	849	1992	78	>20.2

Table 3. UVOT observations reported by Kuin and Lien (GCN Circ. 22579). 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.

April 2, 2018