Swift Observations of GRB 191016A

J.D. Gropp (PSU), K.L. Page (U. Leicester) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 04:09:00 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 191016A (trigger=929744) (Gropp et al. GCN Circ. 26008). Swift did not immediately slew due to an observing constraint. At the time of the trigger, the initial BAT position was 162° from the Sun (11.3 hours West) and 19° from the 95%-illuminated Moon. Table 1 contains the best reported positions from Swift.

Watson et al. (GCN Circ. 26010) reported the position from COATLI for the optical afterglow of this GRB. Siegel and Gropp (GCN Circ. 26024) 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 Barthelmy et al. (GCN Circ. 26012), the BAT ground-calculated position is RA, Dec = 30.268, 24.508 deg which is RA(J2000) = 02h01m04.4s Dec(J2000) = +24°30'30.4" with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment). The partial coding was 64%.

The mask weighted light curve (Figure 1) shows two overlapping pulses. Overall structure starts at ~ T-40 s and lasts till ~ T+420 s. The two peaks occur at ~ T-10 s and ~ T+35 s, respectively. T90 (15-350 keV) is 219.70 ± 183.35 s (estimated error including systematics).

The time-averaged spectrum from T-42.82 to T+422.61 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.55 ± 0.09. The fluence in the 15-150 keV band is 6.5 ± 0.4 x 10-6 erg cm-2. This fluence is larger than that of 87% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-7.75 s in the 15-150 keV band is 1.5 ± 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/929744/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Page et al. (GCN Circ. 26026).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.8 (+1.5, -0.5).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.9 (+0.5, -0.4). The best-fitting absorption column is 1.4 (+1.7, -0.4) x 1021 cm-2, consistent with the Galactic value of 1.0 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 3.4 x 10-11 (4.3 x 10-11) erg cm-2 count-1.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 191016A 45 ks after the BAT trigger (Siegel and Gropp GCN Circ. 26024). A source is detected consistent with the XRT position and the optical source reported by Watson et al. (GCN Circ. 26010, 26015), Zheng and Filippenko (GCN Circ. 26011), Hu et al. (GCN Circ. 26017), Kim et al. (GCN Circ. 26018) and Toma et al. (GCN Circ. 26019). 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 0.09 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
02h01m04.68s +24°30'35.7" 0.5" UVOT-refined Siegel and Gropp GCN Circ. 26024
02h01m04.64s +24°30'35.6" 1.9" XRT-final UKSSDC
02h01m04.64s +24°30'35.6" 1.9" XRT Page et al. GCN Circ. 26026
02h01m04.4s +24°30'30.4" 1.5' BAT-refined Barthelmy et al. GCN Circ. 26012

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 26009 Swift GRB191016.17: Global MASTER-Net
observations report 		MASTER
Optical Watson et al. 26010 COATLI Optical Observations and
Detection of the Afterglow		 COATLI detection
Optical Zheng and Filippenko 26011 KAIT Optical Afterglow Confirmation KAIT detection
Optical Jelinek and Strobl 26013 Optical limit by Ondrejov D50 D50 upper limits
Optical Watson et al. 26015 RATIR Optical and NIR Observations RATIR detection
Optical Hu et al. 26017 early time BOOTES Network and follow-up
1.5m OSN optical observations		 BOOTES detection
Optical Kim et al. 26018 AZT-20 (Assy observatory) optical
photometry		 detection
Optical Toma et al. 26019 MITSuME Okayama optical observation MITSuME Okayama detection
Optical Melandri et al. 26028 REM optical observations REM detection
Optical Schady and Bolmer 26176 GROND observations GROND 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 45023 46878 1822 21.09±0.17
white 50812 52638 1794 21.21±0.19
white 56420 56938 506 >21.23

Table 3. UVOT observations reported by Siegel and Gropp (GCN Circ. 26024). 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.

November 6, 2019