Swift Observations of GRB 200901B

B. Sbarufatti (PSU), J.P. Osborne (U. Leicester) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 07:09:26 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200901B (trigger=994165) (Sbarufatti et al. GCN Circ. 28337). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 71° from the Sun (3.8 hours East) and 98° 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 Sbarufatti (GCN Circ. 28343) 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 Stamatikos et al. (GCN Circ. 28345), the BAT ground-calculated position is RA, Dec = 217.860, -38.704 deg which is RA(J2000) = 14^h31^m26.5^s Dec(J2000) = -38°42'15.2" with an uncertainty of 3.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 82%.

The mask-weighted light curve (Figure 1) shows some weak emission that starts at ~T₀ and ends at ~T+900 s. The available event data end at T+963 s, and the burst went out of the BAT FOV soon afterwards (~T+1000 s). T₉₀ (15-350 keV) is 705.94 ± 148.71 s (estimated error including systematics).

The time-averaged spectrum from T+32.14 to T+901.32 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.95 ± 0.63, and E_peak of 46.7 ± 8.6 keV (χ² 61.80 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 4.3 ± 0.4 x 10^-6 erg cm^-2 and the 1-s peak flux measured from T+68.62 s in the 15-150 keV band is 0.4 ± 0.2 ph cm^-2 s^-1. This fluence is larger than that of 79% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). A fit to a simple power law gives a photon index of 1.95 ± 0.13 (χ² 71.14 for 57 d.o.f.). 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/994165/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Osborne et al. (GCN Circ. 28347). We have analysed 7.5 ks of XRT data for GRB 200901B, from 11.3 ks to 104.1 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 28339).

The light curve (Figure 2) can be modelled with a series of power-law decays. The light curve initially rises, with an index α=-0.6 (+0.7, -0.8). At T+239 s it breaks to an α of 0.54 (+0.08, -0.07) before breaking again at T+1054 s to a final decay with index α=2.76 (+5.24, -0.20).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.15 ± 0.05. The best-fitting absorption column is 1.66 (+0.27, -0.26) x 10²¹ cm^-2, in excess of the Galactic value of 1.2 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 5.8 x 10^-11 (6.3 x 10^-11) erg cm^-2 count^-1.

A summary of the WT-mode spectrum is thus:
Total column: 1.66 (+0.27, -0.26) x 10²¹ cm^-2
Galactic foreground: 1.2 x 10²¹ cm^-2
Excess significance: 2.9 σ
Photon index: 1.15 ± 0.05

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00994165.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200901B 176 s after the BAT trigger (Breeveld and Sbarufatti GCN Circ. 28343). A source consistent with the XRT positionis detected in the initial UVOT exposures in the u filter only, but many exposures are smeared, so the magnitudes may not be reliable. 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 E_B-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

14^h31^m26.19^s -38°42'41.3" 4.2" XRT-final UKSSDC

14^h31^m26.19^s -38°42'41.3" 4.2" XRT-enhanced Osborne et al. GCN Circ. 28339

14^h31^m26.5^s -38°42'15.2" 3.0' BAT-refined Stamatikos et al. GCN Circ. 28345

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Table 2. No GCN Circulars from other observatories were found.

Filter T_start(s) T_stop(s) Exp(s) Mag

v 482 801 58 >18.1

b 431 750 58 >19.1

u 176 426 245 19.3 ± 0.3

w1 531 851 58 >18.1

m2 506 826 58 >17.9

w2 457 776 58 >18.2

RA (J2000)	Dec (J2000)	Error	Note	Reference
14^h31^m26.19^s	-38°42'41.3"	4.2"	XRT-final	UKSSDC
14^h31^m26.19^s	-38°42'41.3"	4.2"	XRT-enhanced	Osborne et al. GCN Circ. 28339
14^h31^m26.5^s	-38°42'15.2"	3.0'	BAT-refined	Stamatikos et al. GCN Circ. 28345

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
v	482	801	58	>18.1
b	431	750	58	>19.1
u	176	426	245	19.3 ± 0.3
w1	531	851	58	>18.1
m2	506	826	58	>17.9
w2	457	776	58	>18.2

Table 3. UVOT observations reported by Breeveld and Sbarufatti (GCN Circ. 28343). 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.

September 3, 2020