Swift Observations of GRB 180305A

P. D'Avanzo (INAF-OAB) and S.W.K. Emery (MSSL-UCL) for the Swift team

1. Introduction

Axelsson and Bissaldi (GCN Circ. 22457) reported the GRB detection. At 09:26:08.66 UT on March 05, 2018, Fermi-LAT detected high-energy emission from GRB 180305A, which was also detected by Fermi-GBM (trigger 541934773 / 180305393). Table 1 contains the best reported positions from Swift.

Troja et al. (GCN Circ. 22460) reported the position from RATIR for the optical afterglow of this GRB. 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 D'Avanzo et al. (GCN Circ. 22462). Four uncatalogued X-ray sources are detected, of which one ("Source 2") is above the RASS limit and fading with 3-sigma significance, and is therefore likely the GRB afterglow.

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=2.7 (+0.5, -0.8).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.5 ± 0.4. The best-fitting absorption column is 4.7 (+3.2, -2.3) x 1021 cm-2, consistent with the Galactic value of 2.4 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 5.4 x 10-11 (6.9 x 10-11) erg cm-2 count-1.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations (Emery and Siegel GCN Circ. 22464) of the field of GRB 180305A 30621 s after the Fermi/LAT trigger (Axelsson et al., GCN Circ. 22457). No optical afterglow consistent with the enhanced XRT position (D'Avanzo et al., GCN Circ. 22462) or the RATIR and Mondy detections (Troja et al., GCN Circ. 22460; Mazaeva et al., GCN Circ. 22463) 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 0.454 mag. in the direction of the GRB (Schlegel et al. 1998).

Figure 1. The BAT light curve is not available.

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
03h18m28.37s +32°06'35.8" 1.7" XRT-final UKSSDC
03h18m28.37s +32°06'35.8" 1.7" XRT D'Avanzo et al. GCN Circ. 22462

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Troja et al. 22460 RATIR Optical Afterglow Candidate RATIR detection
Optical Mazaeva et al. 22463 Mondy optical observations Mondy detection
Optical Watson et al. 22465 RATIR Optical Observations RATIR
Gamma-ray Axelsson and Bissaldi 22457 Fermi-LAT detection Fermi LAT Emax=9 GeV
Gamma-ray Veres and von Kienlin 22458 Fermi GBM detection Fermi GBM Epeak=334±10 keV
T90=12.5 seconds
Fluence=3.302±0.023x10-5erg cm-2
(91st percentile for long GRBs)
Gamma-ray Svinkin et al. 22461 Konus-Wind observation Konus-Wind Epeak=348 (-27,+30) keV
Duration=~44 seconds
Fluence=8.43(-0.58,+0.59)x10-5erg cm-2
Gamma-ray Penacchioni et al. 22467 CALET Gamma-Ray Burst Monitor detection CALET
Gamma-ray Sharma et al. 22476 AstroSat CZTI detection CZTI T90=8.9 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 30808 52964 961 >21.4
u 30621 52533 2623 >20.7
v 30994 48392 712 >19.5

Table 3. UVOT observations reported by Emery and Siegel (GCN Circ. 22464). 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.

March 14, 2018