Swift Observations of GRB 230409B

A.P. Beardmore (U. Leicester), P. D'Avanzo (INAF-OAB) and N.P. Kuin (UCL/MSSL) for the Swift team

1. Introduction

At 04:56:57 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 230409B (trigger=1163401) (Beardmore et al. GCN Circ. 33592). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 60° from the Sun (3.2 hours West) and 111° from the 90%-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.

Kuin and Beardmore (GCN Circ. 33595) reported the detection with UVOT of an optical afterglow. 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. 33598), the BAT ground-calculated position is RA, Dec = 329.516, 52.832 deg which is RA(J2000) = 21h58m03.8s Dec(J2000) = +52°49'55.1" with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 32%.

The mask-weighted light curve (Figure 1) shows a single-pulse structure that starts at ~T-1 s, peaks at ~T+1 s, and ends at ~T+12 s. The hump at ~50 s to 100 s mentioned in the previous circular (Beardmore et al., GCN Circ. 33592) is due to background fluctuation and is not part of the burst. T90 (15-350 keV) is 9.79 ± 2.59 s (estimated error including systematics).

The time-averaged spectrum from T-0.58 to T+11.78 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.86 ± 0.19. The fluence in the 15-150 keV band is 9.9 ± 1.2 x 10-7 erg cm-2. This fluence is larger than that of 38% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.43 s in the 15-150 keV band is 3.7 ± 0.5 ph cm-2 s-1. All the quoted errors are at the 90% confidence level.

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/1163401/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Avanzo et al. (GCN Circ. 33599). We have analysed 11 ks of XRT data for GRB 230409B, from 338 s to 126.6 ks after the BAT trigger. The data comprise 19 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 33594).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=0.62 ± 0.12, followed by a break at T+4570 s to an α of 1.15 (+0.19, -0.18).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.97 (+0.22, -0.21). The best-fitting absorption column is 1.23 (+0.27, -0.24) x 1022 cm-2, in excess of the Galactic value of 7.6 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 5.2 x 10-11 (9.9 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.23 (+0.27, -0.24) x 1022 cm-2
Galactic foreground: 7.6 x 1021 cm-2
Excess significance: 3.3 σ
Photon index: 1.97 (+0.22, -0.21)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01163401.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 230409B 386 s after the BAT trigger (Kuin and Beardmore GCN Circ. 33595). A weak fading source consistent with the XRT position is detected in the initial UVOT exposures at position RA=329.5158, Dec=52.8321 deg. 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 1.226 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
21h58m03.79s +52°49'55.6" 1.0" UVOT GCN Circ. 33595
21h58m03.55s +52°49'55.7" 2.0" XRT-final UKSSDC
21h58m03.54s +52°49'55.6" 2.2" XRT-enhanced Evans et al. GCN Circ. 33594
21h58m03.8s +52°49'55.1" 1.7' BAT-refined Palmer et al. GCN Circ. 33598

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 33596 Swift GRB 230409B: Global MASTER-Net
observations report 		MASTER
Optical Adami et al. 33607 OHP/T193 optical upper limit Obs.de Haute-Provence upper limits
Optical Belkin et al. 33608 Mondy optical upper limit Mondy upper limits
Optical Gupta et al. 33627 3.6m DOT near-infrared detection,
afterglow confirmation		 DOT detection

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 386 536 147 19.35 ± 0.20
v 542 1931 156 >18.1
b 642 2029 156 >19.4
u 616 2004 156 >19.0
w1 592 1980 156 >18.8
m2 567 1955 156 >19.2
w2 692 1906 136 >20.0

Table 3. UVOT observations reported by Kuin and Beardmore (GCN Circ. 33595). 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.

April 16, 2023