Swift Observations of GRB 230405B

K.L. Page (U. Leicester), P.A. Evans (U. Leicester) and S.R. Oates (U. Birmingham) for the Swift team

1. Introduction

At 19:58:03 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 230405B (trigger=1163119) (Page et al. GCN Circ. 33571). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 103° from the Sun (6.8 hours West) and 82° from the 100%-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

BAT refined results are not available.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 33584). We have analysed 11 ks of XRT data for GRB 230405B, from 54 s to 184.2 ks after the BAT trigger. The data comprise 246 s in Windowed Timing (WT) mode (the first 10 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 33572).

The late-time light curve (Figure 2) (from T0+4.7 ks) can be modelled with a power-law decay with a decay index of α=1.20 (+0.10, -0.09).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.10 (+0.12, -0.11). The best-fitting absorption column is 4.0 (+0.6, -0.5) x 1021 cm-2, in excess of the Galactic value of 1.6 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.95 (+0.17, -0.16) and a best-fitting absorption column of 5.4 (+1.1, -1.0) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.2 x 10-11 (6.8 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 5.4 (+1.1, -1.0) x 1021 cm-2
Galactic foreground: 1.6 x 1021 cm-2
Excess significance: 6.2 σ
Photon index: 1.95 (+0.17, -0.16)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01163119.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 230405B 75 s after the BAT trigger (Oates and Page GCN Circ. 33576). No optical afterglow consistent with the XRT position (Beardmore et al. GCN Circ. 33574) 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 0.166 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
18h05m45.74s -47°04'12.1" 2.0" XRT-final UKSSDC
18h05m45.75s -47°04'11.4" 2.2" XRT-enhanced Beardmore et al. GCN Circ. 33574
18h05m52s -47°04'12" 3' BAT-initial Page et al. GCN Circ. 33571

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 33573 Swift GRB 230405B: Global MASTER-Net
observations report 		MASTER
Optical de Wet et al. 33582 MeerLICHT upper limits MeerLICHT upper limits
Gamma-ray Fermi 33570 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Navaneeth et al. 33575 AstroSat CZTI detection CZTI
Gamma-ray Wood and Meegan 33583 Fermi GBM Observation Fermi GBM Epeak=225±11 keV
T90=4.3 seconds
Fluence=1.2±0.2x10-5erg cm-2
(brighter than 79% of long GRBs)
Gamma-ray Casentini et al. 33585 AGILE detection AGILE
Gamma-ray Ridnaia et al. 33586 Konus-Wind detection Konus-Wind Epeak=184 (-20,+23) keV
Duration=~10.7 seconds
Fluence=2.24(-0.23,+0.27)x10-5erg cm-2
Gamma-ray Dafcikova et al. 33590 VZLUSAT-2 detection VZLUSAT

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 75 224 147 >20.5
uFC 287 537 246 >19.7
white 75 1363 373 >20.7
v 618 4860 274 >19.1
b 542 1339 78 >19.0
u 287 1313 304 >19.7
w1 668 1289 78 >18.5
m2 642 5065 274 >19.4
w2 593 1389 97 >18.8

Table 3. UVOT observations reported by Oates and Page (GCN Circ. 33576). 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 8, 2023