Swift Observations of GRB 180418A

V. D'Elia (ASDC), Z. Liu (NAOC / U. Leicester) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 06:44:06 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180418A (trigger=826428) (D'Elia et al. GCN Circ. 22646). Swift could not immediately slew to the burst due to an observing constraint. At the time of the trigger, the initial BAT position was 130° from the Sun (9.6 hours East) and 105° from the 6%-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.

Zheng and Filippenko (GCN Circ. 22647) reported the position from KAIT for the optical afterglow of this GRB. Siegel and D'Elia (GCN Circ. 22665) reported the detection with UVOT of an optical afterglow. Fong et al. (GCN Circ. 22659) determined a redshift of 0.5 from Gemini. 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 Palmer et al. (GCN Circ. 22658), the BAT ground-calculated position is RA, Dec = 170.132, 24.925 deg which is RA(J2000) = 11h20m31.6s Dec(J2000) = +24°55'28.9" with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows a single FRED-like pulse that starts at ~T0, peaks at ~T+0.4 s, and ends at ~T+3.5 s. T90 (15-350 keV) is 2.29 ± 0.83 s (estimated error including systematics).

The time-averaged spectrum from T+0.10 to T+3.50 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.44 ± 0.12. The fluence in the 15-150 keV band is 3.2 ± 0.2 x 10-7 erg cm-2. This fluence is larger than that of 13% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.23 s in the 15-150 keV band is 3.0 ± 0.2 ph cm-2 s-1. All the quoted errors are at the 90% confidence level.

Using a 16-ms binned light curve (Figure 1) , the lag analysis finds a lag of 0.1000 ± 0.026 s for the 100-350 keV to 25-50 keV band, and 0.088 ± 0.026 s for the 50-100 keV to 15-25 keV band. These values are also intermediate between canonical short and long GRBs.

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/826428/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Liu et al. (GCN Circ. 22655). We have analysed 41 ks of XRT data for GRB 180418A, from 3.1 ks to 1852.2 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. 22650).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.88 (+0.08, -0.07).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.02 (+0.28, -0.26). The best-fitting absorption column is 8.0 (+7.1, -5.8) x 1020 cm-2, in excess of the Galactic value of 1.1 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 3.2 x 10-11 (3.8 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 8.0 (+7.1, -5.8) x 1020 cm-2
Galactic foreground: 1.1 x 1020 cm-2
Excess significance: 2.0 σ
Photon index: 2.02 (+0.28, -0.26)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00826428.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180418A 3086 s after the BAT trigger (Siegel and D'Elia GCN Circ. 22665). A fading source consistent with the XRT position (Goad et al. GCN Circ. 22650) is detected in the initial UVOT exposures, confirming the previously reported optical afterglows (Zheng & Filippenko, GCN Circ. 22647; Guidorzi et al., GCN Circ. 22648; Troja et al., GCN Circ. 22652; Fong et al., GCN Circ. 22659; Malesani et al., GCN Circ. 22660; Schady et al., GCN Circ. 22662; Misra et al., GCN Circ. 22663). 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 0.02 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
11h20m29.21s +24°55'59.2" 0.49" UVOT-refined Siegel and D'Elia GCN Circ. 22665
11h20m29.16s +24°55'59.1" 1.6" XRT-final UKSSDC
11h20m29.17s +24°55'59.1" 1.8" XRT-enhanced Goad et al. GCN Circ. 22650
11h20m31.6s +24°55'28.9" 1.2' BAT-refined Palmer et al. GCN Circ. 22658

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Zheng and Filippenko 22647 KAIT Optical Afterglow Candidate KAIT detection
Optical Guidorzi et al. 22648 LCO FTN afterglow confirmation FTN detection
Optical Troja et al. 22652 RATIR Optical Observations RATIR
Optical Sota et al. 22657 1.5m OSN optical observations Obs.de Sierra Nevada
Optical Fong et al. 22659 Gemini-North imaging and spectroscopy Gemini spectroscopy
Optical Malesani et al. 22660 NOT optical observations NOT
Optical Xin et al. 22661 Xinglong TNT optical upper limit TNT upper limits
Optical 22662 GROND observations GROND detection
Optical Misra et al. 22663 Imaging of the optical afterglow with
the 1.3 m DFOT and 3.6 m DOT in ARIES,
India		 Devasthal Opt.Tel. detection
Optical Troja et al. 22664 Continued RATIR Optical Observations RATIR detection
Optical Schady and Chen 22666 Further GROND observations GROND detection
Optical Choi et al. 22668 SQUEAN observations SQUEAN
Optical Horiuchi et al. 22670 MITSuME Ishigakijima Optical Observation MITSuME Ishigakijima detection
Optical Klotz et al. 22671 TAROT La Silla observatory early
optical observations		 TAROT
Radio Bright et al. 22697 15 GHz upper limits from AMI AMI upper limits
Gamma-ray Bissaldi and Veres 22656 Fermi GBM observation Fermi GBM T90=2.5 seconds
Fluence=7.6±0.4x10-7erg cm-2
(9th percentile for long GRBs)

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) 3087 3236 147 19.19± 0.11
white 3087 4470 344 19.45± 0.09
white 49806 72093 4850 >22.62
v 3243 4835 347 >19.53
b 4064 4264 196 >20.19
b 15510 16114 590 >20.82
u (fc) 3859 4059 196 19.35± 0.24
u 14598 31858 894 >20.73
uvw1 3655 3855 196 19.19± 0.28
uvw1 9720 27635 1206 20.02± 0.21
uvm2 3450 3649 196 18.93± 0.27
uvm2 8814 26975 2165 20.65± 0.26
uvw2 4476 4676 196 >19.69

Table 3. UVOT observations reported by Siegel and D'Elia (GCN Circ. 22665). 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.

May 11, 2018