Swift Observations of GRB 221027B

A.Y. Lien (U. Tampa), M. Perri (SSDC and INAF-OAR) and A. Belles (PSU) for the Swift team

1. Introduction

At 19:13:38 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 221027B (trigger=1131397) (Lien et al. GCN Circ. 32867). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 62° from the Sun (0.9 hours East) and 72° from the 7%-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 Markwardt et al. (GCN Circ. 32892), the BAT ground-calculated position is RA, Dec = 225.601, 47.451 deg which is RA(J2000) = 15h02m24.2s Dec(J2000) = +47°27'04.4" with an uncertainty of 1.6 arcmin, (radius, sys+stat, 90% containment). The partial coding was 45%.

The mask-weighted light curve (Figure 1) shows a structure with two pulses that start at ~T-6 s and end at ~T+4 s. The first weaker peak occurs at ~T-5 s, followed by the main peak that occurs at ~T0. T90 (15-350 keV) is 8.66 ± 1.10 s (estimated error including systematics).

The time-averaged spectrum from T-5.98 to T+4.20 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.39 ± 0.19. The fluence in the 15-150 keV band is 5.8 ± 0.6 x 10-7 erg cm-2. This fluence is larger than that of 24% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.08 s in the 15-150 keV band is 3.4 ± 0.4 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/1131397/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Perri et al. (GCN Circ. 32870). We have analysed 16 ks of XRT data for GRB 221027B, from 109 s to 103.4 ks after the BAT trigger. The data comprise 8 s in Windowed Timing (WT) mode (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. 32868).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=0.71 (+0.06, -0.10), followed by a break at T+9616 s to an α of 1.41 (+0.22, -0.24).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.95 ± 0.16. The best-fitting absorption column is 1.3 (+0.5, -0.4) x 1021 cm-2, in excess of the Galactic value of 2.9 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 3.5 x 10-11 (4.3 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.3 (+0.5, -0.4) x 1021 cm-2
Galactic foreground: 2.9 x 1020 cm-2
Excess significance: 3.8 σ
Photon index: 1.95 ± 0.16

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01131397.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 221027B 131 s after the BAT trigger (Belles and Lien GCN Circ. 32875). No optical afterglow consistent with the XRT position (Beardmore et al. GCN Circ. 32868) 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 0.033 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
15h02m29.03s +47°26'31.4" 2.2" XRT-final UKSSDC
15h02m29.03s +47°26'31.4" 2.2" XRT-enhanced Beardmore et al. GCN Circ. 32868
15h02m24.2s +47°27'04.4" 1.6' BAT-refined Markwardt et al. GCN Circ. 32892

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Hu et al. 32869 BOOTES-1 optical upper limit BOOTES-1 upper limits
Optical Lipunov et al. 32871 Swift GRB 221027B: Global MASTER-Net
observations report 		MASTER
Optical Jelinek et al. 32873 FRAM-ORM and Ondrejov SBT optical limits FRAM upper limits
Optical Becerra et al. 32880 COATLI Upper Limits COATLI upper limits
Optical Kochiashvili et al. 32905 GRANDMA observations GRANDMA

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 131 281 147 >21.1
uFC 344 594 246 >20.2
white 131 1544 392 >21.5
v 673 1594 117 >19.2
b 599 1520 97 >19.4
u 344 1661 335 >20.4
w1 722 1644 97 >18.8
m2 698 718 19 >17.6
w2 111 841 30 >19.2

Table 3. UVOT observations reported by Belles and Lien (GCN Circ. 32875). 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.

November 4, 2022