Swift Observations of GRB 230116D

E. Sonbas (Adiyaman Univ.) and D.N. Burrows (PSU) for the Swift team

1. Introduction

At 21:04:43 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 230116D (trigger=1149293) (Sonbas et al. GCN Circ. 33176). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 147° from the Sun (10.7 hours East) and 127° from the 31%-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.

Zhu et al. (GCN Circ. 33177) reported the position from Xinjiang Astro. Obs. for the optical afterglow of this GRB. Moskvitin et al. (GCN Circ. 33187) determined a redshift of 3.81 from SAO RAS. 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 Markwardt et al. (GCN Circ. 33194), the BAT ground-calculated position is RA, Dec = 98.584, 49.831 deg which is RA(J2000) = 06h34m20.1s Dec(J2000) = +49°49'50.3" with an uncertainty of 2.9 arcmin, (radius, sys+stat, 90% containment). The partial coding was 98%.

The BAT light curve (Figure 1) showed a complex structure with a duration of about 40 s. T90 (15-350 keV) is 41.00 ± 11.18 s (estimated error including systematics).

The time-averaged spectrum from T+1.14 to T+53.144 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.38 ± 0.22. The fluence in the 15-150 keV band is 8.12 ± 1.2 x 10-7 erg cm-2. This fluence is larger than that of 33% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+12.14 s in the 15-150 keV band is 0.6 ± 0.2 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/1149293/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Burrows et al. (GCN Circ. 33185). We have analysed 10 ks of XRT data for GRB 230116D, from 113 s to 68.2 ks after the BAT trigger. The data comprise 47 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 Osborne et al. (GCN Circ. 33180).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=1.88 (+0.23, -0.21). At T+615 s the decay flattens to an α of 0.62 (+0.15, -0.20) before breaking again at T+8689 s to a final decay with index α=2.0 (+0.5, -0.4).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.76 (+0.19, -0.11). The best-fitting absorption column is consistent with the Galactic value of 1.6 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 3.9 x 10-11 (4.8 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.6 x 1021 cm-2
Intrinsic column: 1 ± 21 x 1021 cm-2 at z=3.81
Photon index: 1.76 (+0.19, -0.11)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01149293.

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
06h34m28.28s +49°52'22.3" 2.0" XRT-final UKSSDC
06h34m28.13s +49°52'20.6" 2.3" XRT-enhanced Osborne et al. GCN Circ. 33180
06h34m20.1s +49°49'50.3" 2.9' BAT-refined Markwardt et al. GCN Circ. 33194

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Zhu et al. 33177 Nanshan/NEXT optical afterglow detection Xinjiang Astro. Obs. detection
Optical Nazarov et al. 33178 Sintez-Newton/CrAO optical afterglow
detection		 CrAO detection
Optical Moskvitin et al. 33179 SAO RAS observations of OT brightening SAO RAS detection
Optical G. Waratkar 33181 GIT optical follow-up GROWTH-India Tel.
Optical Swain et al. 33182 GIT optical follow-up GROWTH-India Tel. detection
Optical Moskvitin et al. 33187 SAO RAS redshift SAO RAS redshift
Optical Strausbaugh and Cucchiara 33188 LCOGT Optical Upper Limit Las Cumbres Obs. Global Tele.Network upper limits
Optical Belkin et al. 33189 Sintez-Newton/CrAO optical observations CrAO
Optical Belkin et al. 33190 KGO RC-25 optical observations KGO
Optical Belkin et al. 33191 SAO RAS photometry SAO RAS light curve
Optical Moskvitin et al. 33192 SAO RAS further observations SAO RAS detection
Optical Gupta et al. 33198 3.6m DOT optical upper limit DOT upper limits

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

January 20, 2023