Swift Observations of GRB 180404B

S.J. LaPorte (PSU), D.N. Burrows (PSU) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 02:11:44 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180404B (trigger=821902) (LaPorte et al. GCN Circ. 22590). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 66° from the Sun (2.7 hours East) and 115° from the 86%-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.

LaPorte et al. (GCN Circ. 22590) reported the discovery with UVOT of an optical afterglow. 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 Cummings et al. (GCN Circ. 22601), the BAT ground-calculated position is RA, Dec = 53.390, -50.213 deg which is RA(J2000) = 03h33m33.6s Dec(J2000) = -50°12'47.4" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 84%.

The mask-weighted light curve (Figure 1) shows a major pulse that starts at ~ T-6 s and peaks at ~ T+ 10 s. Another weaker pulse arises at the end of the major pulse at ~ T+40 s. The second pulse peaks at ~ T+50 s and lasts until ~ T+200 s. T90 (15-350 keV) is 111.5 ± 7.1 s (estimated error including systematics).

The time-averaged spectrum from T-5.91 to T+195.64 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.35 ± 0.04. The fluence in the 15-150 keV band is 1.20 ± 0.02 x 10-5 erg cm-2. This fluence is larger than that of 93.7% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+8.88 s in the 15-150 keV band is 5.2 ± 0.2 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/821902/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Burrows et al. (GCN Circ. 22598). We have analysed 24 ks of XRT data for GRB 180404B, from 63 s to 327.6 ks after the BAT trigger. The data comprise 349 s in Windowed Timing (WT) mode (the first 5 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. 22594).

The late-time light curve (Figure 2) (from T0+5.2 ks) can be modelled with a power-law decay with a decay index of α=1.27 (+0.09, -0.08).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.61 ± 0.04. The best-fitting absorption column is 3.00 (+0.25, -0.23) x 1021 cm-2, in excess of the Galactic value of 1.1 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.06 (+0.14, -0.13) and a best-fitting absorption column of 4.0 (+0.7, -0.6) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.8 x 10-11 (6.0 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 4.0 (+0.7, -0.6) x 1021 cm-2
Galactic foreground: 1.1 x 1020 cm-2
Excess significance: 10.4 σ
Photon index: 2.06 (+0.14, -0.13)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00821902.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180404B 82 s after the BAT trigger (Siegel and LaPorte GCN Circ. 22611). A fading source consistent with the XRT position (Beardmore et al. GCN Circ. 22594) 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 0.01 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
03h33m34.23s -50°12'54.2" 0.46" UVOT-refined Siegel and LaPorte GCN Circ. 22611
03h33m34.31s -50°12'55.6" 1.4" XRT-final UKSSDC
03h33m34.24s -50°12'55.8" 2.9" XRT-enhanced Beardmore et al. GCN Circ. 22594
03h33m33.6s -50°12'47.4" 1.0' BAT-refined Cummings et al. GCN Circ. 22601

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 22596 MASTER-OAFA OT observation MASTER
Gamma-ray von Kienlin 22597 Fermi GBM observation Fermi GBM Epeak=209±6 keV
T90=80 seconds
Fluence=3.3±0.4x10-6erg cm-2
(44th percentile for long GRBs)
Gamma-ray Svinkin et al. 22609 Konus-Wind observation Konus-Wind Epeak=205 (-17,+21) keV
Duration=~29 seconds
Fluence=2.60(-0.16,+0.18)x10-5erg cm-2
Gamma-ray Sharma et al. 22645 AstroSat CZTI detection CZTI T90=44 seconds

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
white (fc) 82 231 147 18.66 ± 0.09
white 82 1720 411 19.39 ± 0.09
white 6421 6620 196 >20.7
v 624 7031 510 >19.5
b 550 7661 322 >20.3
u (fc) 295 545 245 19.49 ± 0.31
u 295 1670 343 19.70 ± 0.29
u 6011 7648 393 >20.1
w1 674 7442 510 >20.6
m2 1600 7236 413 >20.5
w2 600 6826 452 >20.2

Table 3. UVOT observations reported by Siegel and LaPorte (GCN Circ. 22611). 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 17, 2018