Swift Observations of GRB 190926A

A. Melandri (INAF-OAB), B. Sbarufatti (PSU) and S.R. Oates (U.Warwick) for the Swift team

1. Introduction

At 09:52:16 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190926A (trigger=926515) (Melandri et al. GCN Circ. 25848). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 87° from the Sun (5.4 hours West) and 58° from the 8%-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. 25860), the BAT ground-calculated position is RA, Dec = 100.616, 59.531 deg which is RA(J2000) = 06h42m27.9s Dec(J2000) = +59°31'51.2" with an uncertainty of 2.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 50%.

The BAT mask-weighted light curve (Figure 1) shows a complex structure with multiple peaks extending from roughly T-120 s to T+450 s. T90 (15-350 keV) is 368.9 ± 20.9 s (estimated error including systematics).

The time-averaged spectrum from T-0.00 to T+395.46 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.14 ± 0.18. The fluence in the 15-150 keV band is 2.5 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 67% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+52.95 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/926515/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Sbarufatti et al. (GCN Circ. 25851). We have analysed 23 ks of XRT data for GRB 190926A, from 140 s to 270.0 ks after the BAT trigger. The data comprise 685 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 25850).

The late-time light curve (Figure 2) (from T0+4.1 ks) can be modelled with a power-law decay with a decay index of α=0.51 (+0.13, -0.12).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.803 ± 0.023. The best-fitting absorption column is 3.00 ± 0.11 x 1021 cm-2, in excess of the Galactic value of 7.5 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 4.1 x 10-11 (5.5 x 10-11) erg cm-2 count-1.

A summary of the WT-mode spectrum is thus:
Total column: 3.00 ± 0.11 x 1021 cm-2
Galactic foreground: 7.5 x 1020 cm-2
Excess significance: 33.3 σ
Photon index: 1.803 ± 0.023

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00926515.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190926A 144 s after the BAT trigger (Oates and Melandri GCN Circ. 25853). No optical afterglow consistent with the enhanced XRT position (Goad et al. GCN Circ. 25850) 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 0.06 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
06h42m16.94s +59°31'44.9" 2.0" XRT-final UKSSDC
06h42m16.90s +59°31'45.1" 2.0" XRT-enhanced Goad et al. GCN Circ. 25850
06h42m27.9s +59°31'51.2" 2.3' BAT-refined Markwardt et al. GCN Circ. 25860

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 25849 Swift GRB190926.41: Global MASTER-Net
observations report 		MASTER
Optical Zheng and Filippenko 25852 KAIT Optical Upper Limit KAIT upper limits
Optical Adachi et al. 25854 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Kann et al. 25856 Upper Limit from OSN T150 Observation Obs.de Sierra Nevada upper limits
Optical Belkin et al. 25862 AbAO upper optical limit Abastumani Astro. Obs. upper limits
Optical Blazek et al. 25937 Upper limit from CAHA 2.2m observation CAHA upper limits

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 144 294 147 >20.2
uFC 303 553 246 >19.3
white 144 6525 580 >20.9
v 634 11670 1116 >20.0
b 559 6321 432 >20.2
u 303 6115 659 >20.0
w1 685 5910 413 >19.8
m2 659 12294 1022 >20.3
w2 610 10756 1148 >20.6

Table 3. UVOT observations reported by Oates and Melandri (GCN Circ. 25853). 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.

October 4, 2019