Swift Observations of GRB 200906A

K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester) and J.D. Gropp (PSU) for the Swift team

1. Introduction

At 13:11:04 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200906A (trigger=994856) (Page et al. GCN Circ. 28371). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 90° from the Sun (7.1 hours East) and 93° from the 84%-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 Barthelmy et al. (GCN Circ. 28380), the BAT ground-calculated position is RA, Dec = 272.284, 67.887 deg which is RA(J2000) = 18h09m08.2s Dec(J2000) = +67°53'13.4" with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 97%.

The mask-weighted light curve (Figure 1) shows a double-peaked structure. The first pulse starts at ~T-9 s and peaks at ~T+5 s. The second pulse starts at ~T+46 s, peaks at ~T+51 s, and ends at ~T+90 s. T90 (15-350 keV) is 70.90 ± 7.63 s (estimated error including systematics).

The time-averaged spectrum from T-8.70 to T+90.01 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.00 ± 0.09. The fluence in the 15-150 keV band is 2.8 ± 0.1 x 10-6 erg cm-2. This fluence is larger than that of 70% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+50.96 s in the 15-150 keV band is 2.5 ± 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/994856/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Beardmore et al. (GCN Circ. 28375). We have analysed 10 ks of XRT data for GRB 200906A, from 70 s to 103.5 ks after the BAT trigger. The data comprise 92 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 28373).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=5.05 ± 0.17, followed by a break at T+258 s to an α of 0.63 ± 0.04.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.10 ± 0.09. The best-fitting absorption column is 7.2 (+0.7, -0.6) x 1021 cm-2, in excess of the Galactic value of 5.2 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.26 (+0.25, -0.24) and a best-fitting absorption column of 7.0 (+1.7, -1.5) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.9 x 10-11 (8.1 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 7.0 (+1.7, -1.5) x 1021 cm-2
Galactic foreground: 5.2 x 1020 cm-2
Excess significance: 7.3 σ
Photon index: 2.26 (+0.25, -0.24)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00994856.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200906A 76 s after the BAT trigger (Gropp and Page GCN Circ. 28379). No optical afterglow consistent with the XRT position (Beardmore et al. GCN Circ. 28373) 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 0.05 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
18h09m06.09s +67°52'48.3" 1.5" XRT-final UKSSDC
18h09m06.08s +67°52'48.1" 1.9" XRT-enhanced Beardmore et al. GCN Circ. 28373
18h09m08.2s +67°53'13.4" 1.4' BAT-refined Barthelmy et al. GCN Circ. 28380

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Fu et al. 28372 Nanshan/NEXT optical upper limit Xinjiang Astro. Obs. upper limits
Optical Kann et al. 28376 OSN 1.5m Upper Limit Obs.de Sierra Nevada upper limits
Optical Kumar et al. 28377 GROWTH-India Telescope (GIT) optical r
band upper limit		 GROWTH-India upper limits
Optical Rossi 28381 LBT near-infrared upper limit LBT upper limits
Optical de Ugarte Postigo et al. 28382 EMIR/GTC NIR upper limit GTC upper limits
Optical Lipunov et al. 28386 Fermi GRB 200906A: Global MASTER-Net
observations report 		MASTER
Gamma-ray Fermi 28370 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Hamburg et al. 28374 Fermi GBM detection Fermi GBM Epeak=50.5±9.9 keV
T90=67.3 seconds
Fluence=3.9±0.2x10-6erg cm-2
(51st percentile for long GRBs)

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 76 226 147 >21.2
uFC 289 538 246 >20.4
white 76 5054 472 >21.3
v 618 1413 97 >19.4
b 544 4948 274 >20.1
u 289 1487 324 >20.4
w1 668 1462 97 >19.3
m2 643 1438 78 >20.1
w2 594 1389 97 >19.5

Table 3. UVOT observations reported by Gropp and Page (GCN Circ. 28379). 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.

September 8, 2020