Swift Observations of GRB 190531B

P. D'Avanzo (INAF-OAB) and K.K. Simpson (PSU) for the Swift team

1. Introduction

Axelsson et al. (GCN Circ. 24701) reported the GRB detection. Table 1 contains the best reported positions from Swift.

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. 24706).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.0 ± 0.3.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.20 (+0.15, -0.14). The best-fitting absorption column is 9.1 (+1.2, -1.1) x 1021 cm-2, in excess of the Galactic value of 1.8 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 4.2 x 10-11 (9.0 x 10-11) erg cm-2 count-1.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations (Simpson GCN Circ. 24731) of the field of GRB 190531B 18175 s after the LAT trigger (Axelsson et al., GCN Circ. 24701). No optical afterglow consistent with the XRT position (D'Avanzo et al., GCN Circ. 24706) 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 0.01 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
01h37m11.09s -41°57'30.3" 1.6" XRT-final UKSSDC
01h37m11.09s -41°57'30.3" 1.6" XRT D'Avanzo et al. GCN Circ. 24706

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 24707 MASTER optical observation MASTER
Optical Japelj et al. 24711 VLT optical upper limit VLT upper limits
Gamma-ray Fermi 24695 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Axelsson et al. 24701 Fermi-LAT detection Fermi LAT Emax=7.0 GeV
Gamma-ray Veres and Meegan 24705 Fermi GBM observations Fermi GBM Epeak=473±14 keV
T90=38.4 seconds
Fluence=9.91±0.08x10-5erg cm-2
(97.8 percentile for long GRBs)
Gamma-ray Yi et al. 24713 Insight-HXMT/HE detection Insight-HXMT T90=27.82 seconds
Gamma-ray Svinkin et al. 24730 Konus-Wind observation Konus-Wind Epeak=364 (-34,+40) keV
Duration=~110 seconds
Fluence=1.51(-0.15,+0.16)x10-4erg cm-2
Gamma-ray Shimizu et al. 24738 CALET Gamma-Ray Burst Monitor detection CALET

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 18661 24137 365 >20.5
u 18497 29702 551 >21.1
w1 18175 29509 1132 >20.7
m2 19630 25766 972 >20.9
w2 18826 24990 1459 >21.3

Table 3. UVOT observations reported by Simpson (GCN Circ. 24731). 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.

June 5, 2019