Swift Observations of GRB 220430A

E. Ambrosi (INAF-IASFPA) and P.A. Evans (U. Leicester) for the Swift team

1. Introduction

At 13:53:15 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220430A (trigger=1104692) (Ambrosi et al. GCN Circ. 31972). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 59° from the Sun (4.0 hours East) and 62° from the 0%-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

As reported by Barthelmy et al. (GCN Circ. 31981), the BAT ground-calculated position is RA, Dec = 97.616, 9.547 deg which is RA(J2000) = 06h30m27.9s Dec(J2000) = +09°32'50.1" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows two major pulse emissions. The first one starts at ~T-7 s, peaks at ~T+20 s, and ends at ~T+30 s. It is followed by another larger pulse with several peaks. The largest peak occurs at ~T+42 s. The burst emission ends at ~T+100 s. T90 (15-350 keV) is 43.10 ± 0.57 s (estimated error including systematics).

The time-averaged spectrum from T-6.53 to T+94.36 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.24 ± 0.02. The fluence in the 15-150 keV band is 4.4 ± 0.05 x 10-5 erg cm-2. This fluence is larger than that of 99.3% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+42.00 s in the 15-150 keV band is 29.6 ± 0.6 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/1104692/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 31979). We have analysed 23 ks of XRT data for GRB 220430A, from 39 s to 263.7 ks after the BAT trigger. The data comprise 539 s in Windowed Timing (WT) mode (the first 11 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 Osborne et al. (GCN Circ. 31973).

The late-time light curve (Figure 2) (from T0+4.2 ks) can be modelled with a power-law decay with a decay index of α=1.45 ± 0.06.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.65 ± 0.05. The best-fitting absorption column is 1.05 (+0.07, -0.06) x 1022 cm-2, in excess of the Galactic value of 6.5 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.83 (+0.17, -0.16) and a best-fitting absorption column of 1.01 (+0.19, -0.17) x 1022 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.3 x 10-11 (8.8 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.01 (+0.19, -0.17) x 1022 cm-2
Galactic foreground: 6.5 x 1021 cm-2
Excess significance: 3.6 σ
Photon index: 1.83 (+0.17, -0.16)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01104692.

4. UVOT Observations and Analysis

UVOT results are not available.

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
06h30m26.90s +09°32'51.1" 2.0" XRT-final UKSSDC
06h30m26.92s +09°32'51.1" 2.0" XRT-enhanced Osborne et al. GCN Circ. 31973
06h30m27.9s +09°32'50.1" 1.0' BAT-refined Barthelmy et al. GCN Circ. 31981

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Groot et al. 31974 MeerLICHT optical upper limits MeerLICHT upper limits
Optical Pankov et al. 31976 Mondy and AbAO optical upper limits Abastumani Astro. Obs. upper limits
Optical Dornic et al. 31977 GRANDMA observations GRANDMA
Optical Lipunov et al. 31978 Swift GRB 220430A: Global MASTER-Net
observations report 		MASTER
Optical Hu et al. 31980 CAHA 2.2m telescope optical limit CAHA upper limits
Optical Swain et al. 31997 GIT optical upper limits GROWTH-India Tel. upper limits
Optical Jiang et al. 31998 Nanshan/HMT optical upper limit Xinjiang Astro. Obs. upper limits
Optical Jiang et al. 31999 Nanshan/NEXT optical upper limit Xinjiang Astro. Obs. upper limits
Gamma-ray Ursi et al. 31975 AGILE detection AGILE
Gamma-ray Gopalakrishnan et al. 31987 AstroSat CZTI detection CZTI

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

May 7, 2022