Swift Observations of GRB 180324A

E. Sonbas (Adiyaman Univ.), J.P. Osborne (U. Leicester) and S.J. Laporte (PSU) for the Swift team

1. Introduction

At 04:37:09 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180324A (trigger=817345) (Sonbas et al. GCN Circ. 22524). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 80° from the Sun (4.9 hours East) and 38° from the 45%-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 Krimm et al. (GCN Circ. 22533), the BAT ground-calculated position is RA, Dec = 76.581, 56.744 deg which is RA(J2000) = 05h06m19.4s Dec(J2000) = +56°44'36.8" with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 78%.

The mask-weighted light curve (Figure 1) shows a small precursor at ~T-5 s, followed by the main pulse that starts at ~T-0.2, peaks at ~T+0.3 s, and ends at ~T+1 s. There is also some weak emission that lasts till ~T+4 s. T90 (15-350 keV) is 7.2 ± 1.3 s (estimated error including systematics).

The time-averaged spectrum from T-5.4 to T+3.7 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.45 ± 0.14. The fluence in the 15-150 keV band is 5.6 ± 0.5 x 10-7 erg cm-2. This fluence is larger than that of 23% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.13 s in the 15-150 keV band is 4.4 ± 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/817345/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Osborne et al. (GCN Circ. 22530). We have analysed 22 ks of XRT data for GRB 180324A, from 80 s to 202.4 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. 22525).

The late-time light curve (Figure 2) (from T0+5.7 ks) can be modelled with an initial power-law decay with an index of α=1.14 (+0.18, -0.31), followed by a break at T+19.7 ks to an α of 3.4 (+2.6, -0.9).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.36 (+0.13, -0.12). The best-fitting absorption column is 8.9 (+1.6, -1.4) x 1021 cm-2, in excess of the Galactic value of 3.8 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 6.7 x 10-11 (8.7 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 8.9 (+1.6, -1.4) x 1021 cm-2
Galactic foreground: 3.8 x 1021 cm-2
Excess significance: 5.8 σ
Photon index: 1.36 (+0.13, -0.12)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00817345.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180324A 72 s after the BAT trigger (Laporte and Sonbas GCN Circ. 22531). No optical afterglow consistent with the XRT position (Goad et al. GCN Circ. 22529) or RATIR position (Butler et al. GCN Circ. 22528) 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 EB-V of 0.90 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
05h06m06.55s +56°42'50.1" 1.4" XRT-final UKSSDC
05h06m06.37s +56°42'51.5" 1.7" XRT-enhanced Goad et al. GCN Circ. 22529
05h06m19.4s +56°44'36.8" 1.4' BAT-refined Krimm et al. GCN Circ. 22533

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Zheng and Filippenko 22527 KAIT Optical Upper Limit KAIT upper limits
Optical Butler et al. 22528 RATIR Optical Observations and Possible
Afterglow Detection		 RATIR possible detection
Gamma-ray Frederiks et al. 22547 Konus-Wind observation Konus-Wind Epeak=154 (-38,+77) keV

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

Filter Tstart(s) Tstop(s) Exp(s) Mag
whiteFC 72 221 147 >19.8
uFC 284 534 246 >19.3
white 72 12383 1552 >21.3
v 614 13894 1172 >19.5
b 539 8158 549 >20.9
u 284 7992 814 >20.3
w1 664 7787 471 >20.2
w2 6970 13290 1082 >21.5

Table 3. UVOT observations reported by Laporte and Sonbas (GCN Circ. 22531). 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.

March 27, 2018