Swift Observations of GRB 180404A

J.D. Gropp (PSU), A. Tohuvavohu (PSU) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 00:45:35 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180404A (trigger=821881) (Gropp et al. GCN Circ. 22589). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 78° from the Sun (4.7 hours East) and 122° from the 87%-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.

Gropp et al. (GCN Circ. 22589) reported the discovery with UVOT of an optical afterglow. Selsing et al. (GCN Circ. 22591) reported the position from VLT for the optical afterglow of this GRB. Selsing et al. (GCN Circ. 22591) determined a redshift of 1.000 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 Barthelmy et al. (GCN Circ. 22599), the BAT ground-calculated position is RA, Dec = 83.540, -37.168 deg which is RA(J2000) = 05h34m09.5s Dec(J2000) = -37°10'05.0" with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 82%.

The mask-weighted light curve (Figure 1) shows a weak emission that starts at T-25 s, followed by a bright peak which peaks at T+3 s and ends at T+30 s. T90 (15-350 keV) is 35.2 ± 5.3 s (estimated error including systematics).

The time-averaged spectrum from T-20.7 to T+24.2 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.95 ± 0.12. The fluence in the 15-150 keV band is 1.3 ± 0.1 x 10-6 erg cm-2. This fluence is larger than that of 47% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+1.99 s in the 15-150 keV band is 1.3 ± 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/821881/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Tohuvavohu et al. (GCN Circ. 22602). We have analysed 8.4 ks of XRT data for GRB 180404A, from 90 s to 166.7 ks after the BAT trigger. The data comprise 26 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 22592).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=4.0 (+0.7, -0.6). At T+248 s the decay flattens to an α of 0.24 (+0.19, -0.16) before breaking again at T+16.7 ks to a final decay with index α=1.4 (+0.7, -0.5).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.62 (+0.28, -0.20). The best-fitting absorption column is consistent with the Galactic value of 3.1 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 4.1 x 10-11 (4.3 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 3.1 x 1020 cm-2
Intrinsic column: 3 ± 13 x 1020 cm-2 at z=1
Photon index: 1.62 (+0.28, -0.20)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00821881.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180404A 90 s after the BAT trigger (Siegel and Gropp GCN Circ. 22605). A source consistent with the XRT position (Osborne et al. GCN Circ. 22592) 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 0.04 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
05h34m11.64s -37°10'03.9" 0.50" UVOT-refined Siegel and Gropp GCN Circ. 22605
05h34m11.74s -37°10'04.8" 1.9" XRT-final UKSSDC
05h34m11.77s -37°10'05.1" 2.0" XRT-enhanced Osborne et al. GCN Circ. 22592
05h34m09.5s -37°10'05.0" 1.1' BAT-refined Barthelmy et al. GCN Circ. 22599

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Selsing et al. 22591 VLT/X-shooter redshift VLT redshift
Optical Lipunov et al. 22593 MASTER-OAFA OT observation MASTER upper limits
Optical Guidorzi et al. 22603 LCO FTS observations FTS marginal detection

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) 90 240 147 19.47 ± 0.09
white 90 770 180 19.34 +/ -0.10
white 45830 52026 1028 >21.66
v 632 5179 99 >18.9
b 558 750 39 >19.4
u (fc) 302 552 246 18.95 ± 0.12
u 303 726 265 18.95 +/ -0.14
w1 682 701 19 >17.8
m2 656 676 19 >17.4
w2 607 5094 216 19.45 ± 0.28

Table 3. UVOT observations reported by Siegel and Gropp (GCN Circ. 22605). 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 6, 2018