Swift Observations of GRB 180402A

A.Y. Lien (GSFC/UMBC), V. D'Elia (ASDC) and S.W.K. Emery (UCL-MSSL) for the Swift team

1. Introduction

At 09:44:59 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180402A (trigger=821103) (Lien et al. GCN Circ. 22580). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 120° from the Sun (8.0 hours West) and 37° from the 96%-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 Ukwatta et al. (GCN Circ. 22584), the BAT ground-calculated position is RA, Dec = 251.923, -14.928 deg which is RA(J2000) = 16h47m41.6s Dec(J2000) = -14°55'41.6" with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 37%.

The mask-weighted light curve (Figure 1) shows a single short pulse that starts at ~T0, peaks at ~0.1 s, and ends at ~0.3 s T90 (15-350 keV) is 0.18 ± 0.03 s (estimated error including systematics). The ground analysis confirmed that the additional variation seen in the raw light curve are indeed from MAXI J1820+070 (Lien et al., GCN Circ. 22580).

The time-averaged spectrum from T+0.05 to T+0.26 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 0.52 ± 0.30. The fluence in the 15-150 keV band is 1.4 ± 0.2 x 10-7 erg cm-2. This fluence is larger than that of 79% of the short GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.34 s in the 15-150 keV band is 1.8 ± 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/821103/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Elia et al. (GCN Circ. 22586).

We have analysed 5.4 ks of XRT data for GRB 180402A, from 83 s to 18.3 ks after the BAT trigger. The data comprise 24 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode.

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.94 ± 0.10.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.46 (+0.28, -0.26). The best-fitting absorption column is 4.6 (+2.2, -1.7) x 1021 cm-2, consistent with 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 5.6 x 10-11 (7.0 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 4.6 (+2.2, -1.7) x 1021 cm-2
Galactic foreground: 2.9 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 1.46 (+0.28, -0.26)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00821103.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180402A 140 s after the BAT trigger (Emery and Lien GCN Circ. 22587). No optical afterglow consistent with the XRT position (D'Elia et al., GCN Circ. 22586) 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 0.77 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
16h47m43.80s -14°58'12.5" 3.6" XRT-final UKSSDC
16h47m44.06s -14°58'06.9" 2.1" XRT-refined D'Elia et al. GCN Circ. 22586
16h47m41.6s -14°55'41.6" 2.0' BAT-refined Ukwatta et al. GCN Circ. 22584

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Guidorzi et al. 22581 LCO McDonald observations LCO
Optical Butler et al. 22582 RATIR Upper Limits RATIR upper limits
Optical Zheng and Filippenko 22583 KAIT Optical Upper Limit KAIT upper limits
Optical Mazaeva et al. 22617 ISON-NM early optical limits ISON-NM upper limits
Gamma-ray Malacaria 22585 Fermi GBM detection Fermi GBM Epeak=123.80±44.00 keV
Fluence=8.69±0.62x10-7erg cm-2
Gamma-ray Svinkin et al. 22588 Konus-Wind observation Konus-Wind Epeak=703 (-161,+276) keV
Fluence=2.14(-0.33,+0.53)x10-6erg cm-2
Gamma-ray Malacaria 22600 Fermi GBM refined spectral analysis Fermi GBM Epeak=1319±161 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 4328 4478 147 >20.8
uFC 140 390 246 >19.7
white 4328 6805 407 >20.6
v 423 5916 286 >19.2
b 5101 6736 393 >20.2
u 140 6531 845 >20.3
w1 4691 6326 393 >20.4
w2 5512 5712 197 >19.6

Table 3. UVOT observations reported by Emery and Lien (GCN Circ. 22587). 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 9, 2018