Swift Observations of GRB 180331A

C.B. Markwardt (GSFC), P.A. Evans (U. Leicester) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 04:14:55 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180331A (trigger=820347) (Markwardt et al. GCN Circ. 22568). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 57° from the Sun (3.8 hours East) and 120° 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 Sakamoto et al. (GCN Circ. 22574), the BAT ground-calculated position is RA, Dec = 66.005, 13.386 deg which is RA(J2000) = 04^h24^m01.1^s Dec(J2000) = +13°23'09.2" with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment). The partial coding was 81%.

The mask-weighted light curve (Figure 1) shows a weak pulse that starts at ~T₀, peaks at ~T+9 s, and ends at ~T+50 s. T₉₀ (15-350 keV) is 48.0 ± 22.6 s (estimated error including systematics).

The time-averaged spectrum from T+0.58 to T+48.58 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.11 ± 1.04, and E_peak of 66.1 ± 28.3 keV (χ² 57.70 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 6.1 ± 1.0 x 10^-7 erg cm^-2 and the 1-s peak flux measured from T+8.08 s in the 15-150 keV band is 0.5 ± 0.1 ph cm^-2 s^-1. This fluence is larger than that of 25% 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 1.46 ± 0.19 (χ² 64.59 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/820347/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans and Markwardt (GCN Circ. 22572). We have analysed 10 ks of XRT data for GRB 180331A, from 115 s to 93.2 ks after the BAT trigger. The data comprise 70 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 22569).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=0.67 (+0.22, -0.21), followed by a break at T+349 s to an α of 3.04 (+0.23, -0.17).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.34 (+0.21, -0.20). The best-fitting absorption column is 4.8 (+1.7, -1.4) x 10²¹ cm^-2, in excess of the Galactic value of 2.9 x 10²¹ cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 6.0 x 10^-11 (7.3 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 4.8 (+1.7, -1.4) x 10²¹ cm^-2
Galactic foreground: 2.9 x 10²¹ cm^-2
Excess significance: 2.2 σ
Photon index: 1.34 (+0.21, -0.20)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00820347.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180331A 116 s after the BAT trigger (Kuin and Markwardt GCN Circ. 22570). No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 22569) 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 0.60 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

04^h24^m05.92^s +13°23'55.6" 1.5" XRT-final UKSSDC

04^h24^m05.92^s +13°23'56.1" 1.8" XRT-enhanced Evans et al. GCN Circ. 22569

04^h24^m01.1^s +13°23'09.2" 1.9' BAT-refined Sakamoto et al. GCN Circ. 22574

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Table 2. No GCN Circulars from other observatories were found.

Filter T_start(s) T_stop(s) Exp(s) Mag

white_FC 116 266 147 >20.2

u_FC 274 524 246 >19.9

white 116 1373 371 >20.4

v 604 1250 78 >18.6

b 530 1350 78 >19.4

u 274 524 246 >19.9

RA (J2000)	Dec (J2000)	Error	Note	Reference
04^h24^m05.92^s	+13°23'55.6"	1.5"	XRT-final	UKSSDC
04^h24^m05.92^s	+13°23'56.1"	1.8"	XRT-enhanced	Evans et al. GCN Circ. 22569
04^h24^m01.1^s	+13°23'09.2"	1.9'	BAT-refined	Sakamoto et al. GCN Circ. 22574

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white_FC	116	266	147	>20.2
u_FC	274	524	246	>19.9
white	116	1373	371	>20.4
v	604	1250	78	>18.6
b	530	1350	78	>19.4
u	274	524	246	>19.9

Table 3. UVOT observations reported by Kuin and Markwardt (GCN Circ. 22570). 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