Swift Observations of GRB 200901A

K.K. Simpson (PSU), J.A. Kennea (PSU) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 03:47:31 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200901A (trigger=994143) (Simpson et al. GCN Circ. 28335). Swift did not slew immediately due to an observing constraint. At the time of the trigger, the initial BAT position was 102° from the Sun (6.6 hours West) and 76° from the 99%-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.

Breeveld and Simpson (GCN Circ. 28342) 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 Sakamoto et al. (GCN Circ. 28344), the BAT ground-calculated position is RA, Dec = 61.759, -59.901 deg which is RA(J2000) = 04h07m02.3s Dec(J2000) = -59°54'03.4" with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 21%.

The mask-weighted light curve (Figure 1) shows several overlapping pulses that start at ~T-5 s and end at ~T+27 s. The main peak occurs at ~T0. The burst location went out of the BAT field of view at ~T+514 s. T90 (15-350 keV) is 20.37 ± 7.55 s (estimated error including systematics).

The time-averaged spectrum from T-4.43 to T+26.59 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.72 ± 0.11. The fluence in the 15-150 keV band is 2.9 ± 0.2 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-0.01 s in the 15-150 keV band is 7.7 ± 0.6 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/994143/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Kennea et al. (GCN Circ. 28341). We have analysed 19 ks of XRT data for GRB 200901A, from 3.1 ks to 269.0 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode.

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.99 (+0.08, -0.07).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.31 (+0.30, -0.28). The best-fitting absorption column is 2.2 (+0.9, -0.8) x 1021 cm-2, in excess of the Galactic value of 1.5 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 3.0 x 10-11 (4.8 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.2 (+0.9, -0.8) x 1021 cm-2
Galactic foreground: 1.5 x 1020 cm-2
Excess significance: 4.2 σ
Photon index: 2.31 (+0.30, -0.28)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00994143.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200901A 3053 s after the BAT trigger (Breeveld and Simpson GCN Circ. 28342). A possible optical afterglow consistent with the XRT position (Kennea et al., GCN Circ. 28341) is detected in the initial UVOT exposures in the white filter only. 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 0.02 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
04h07m07.97s -59°53'27.0" 1.8" XRT-final UKSSDC
04h07m07.97s -59°53'27.0" 1.8" XRT-refined Kennea et al. GCN Circ. 28341
04h07m02.3s -59°54'03.4" 1.1' BAT-refined Sakamoto et al. GCN Circ. 28344

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 28334 Swift GRB200901.16: Global MASTER-Net
observations report 		MASTER
Optical Lipunov et al. 28348 MASTER OT early detection MASTER detection
Optical Jelinek et al. 28767 FRAM-Auger optical limit FRAM upper limits
Gamma-ray Veres et al. 28340 Fermi GBM observation Fermi GBM Epeak=410±150 keV
T90=10.2 seconds
Fluence=9.4±0.54x10-6erg cm-2
(74th 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 3053 3202 147 20.6 ± 0.3
white 3916 4115 197 20.5 ± 0.2
v 4326 4526 197 >19.1
b 3711 3910 197 >20.4
u 3211 4987 290 >20.0
w1 4736 4936 197 >19.1
m2 4531 4731 197 >19.1
w2 4121 4321 197 >19.3

Table 3. UVOT observations reported by Breeveld and Simpson (GCN Circ. 28342). 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.

October 25, 2020