A. Melandri (INAF-OAB), B. Sbarufatti (PSU) and S.R. Oates (U.Warwick) for the Swift team
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.
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) = 0
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.
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 1
The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/926515/BA/.
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 1
A summary of the WT-mode spectrum is thus:
Total column: 3.00 ± 0.11 x 1
Galactic foreground: 7.5 x 1
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.
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
Figure 1. The BAT
mask-weighted light curve in the four individual and total
energy bands. The units are counts
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 |
---|---|---|---|---|
0 |
+59°31'44.9" | 2.0" | XRT-final | UKSSDC |
0 |
+59°31'45.1" | 2.0" | XRT-enhanced | Goad et al. GCN Circ. 25850 |
0 |
+59°31'51.2" | 2.3' | BAT-refined | Markwardt et al. GCN Circ. 25860 |
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 |
Filter | Exp(s) | Mag | ||
---|---|---|---|---|
whit |
144 | 294 | 147 | >20.2 |
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