Swift Observations of GRB 180703A

S.L. Gibson (U. Leicester) and S.W.K. Emery (UCL-MSSL) for the Swift team

1. Introduction

Racusin et al. (GCN Circ. 22883) reported the GRB detection. At 21:01:40.50 on July, 03, 2018 Fermi-LAT detected high-energy emission from GRB 180703A, which was also detected by Fermi-GBM (trigger 552344505 / 180703876). Table 1 contains the best reported positions from Swift.

Guidorzi et al. (GCN Circ. 22892) reported the position from LCO for the optical afterglow of this GRB. Izzo et al. (GCN Circ. 23889) determined a redshift of 0.6678 from VLT. Table 2 is a summary of GCN Circulars about this GRB from observatories other than Swift.

2. BAT Observations and Analysis

BAT did not observe this burst.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Gibson et al. (GCN Circ. 22931). The source is fading with α >0.8.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations (Emery and Sonbas GCN Circ. 22898) of the field of GRB 180703A 17269 s after the Fermi/LAT trigger (Racusin et al. GCN Circ. 22883). No optical afterglow consistent with the XRT position of the uncatalogued X-ray source, source 1 (Gibson et al. GCN Circ. 22891) or the position from LCO Sutherland (Guidorzi et al. GCN Circ. 22892) 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 0.02 mag. in the direction of the GRB (Schlegel et al. 1998).

Figure 1. The BAT light curve is not available.

RA (J2000) Dec (J2000) Error Note Reference
00h25m52.79s -67°10'48.7" 4.8" XRT Gibson et al. GCN Circ. 22931

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Guidorzi et al. 22892 LCO Sutherland possible optical
candidate		 LCO possible detection
Optical Lipunov et al. 22926 MASTER optical observation MASTER
Optical Guidorzi et al. 22944 optical afterglow confirmation FTN detection
Optical Izzo et al. 23889 VLT/MUSE host galaxy redshift
measurement		 VLT redshift
Gamma-ray Racusin et al. 22883 Fermi-LAT detection Fermi LAT
Gamma-ray Poolakkil and Meegan 22896 Fermi GBM detection Fermi GBM Epeak=360±33 keV
T90=20.2 seconds
Fluence=2.001±0.043x10-5erg cm-2
(87th percentile for long GRBs)
Gamma-ray Sharma et al. 22900 AstroSat CZTI detection CZTI T90=20.9 seconds
Gamma-ray Hurley et al. 22925 IPN Triangulation IPN
Gamma-ray Frederiks et al. 22927 Konus-Wind observation Konus-Wind Epeak=367 (-136,+196) 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
b 17353 28862 47 >19.2
u 17323 28834 116 >19.5
uvw1 17269 28807 234 >19.8
uvw2 17384 28917 97 >19.2

Table 3. UVOT observations reported by Emery and Sonbas (GCN Circ. 22898). 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.

February 13, 2019