Swift Observations of GRB 200409A

J.D. Gropp (PSU) and D.N. Burrows (PSU) for the Swift team

1. Introduction

At 03:19:59 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200409A (trigger=965484) (Gropp et al. GCN Circ. 27509). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 69° from the Sun (2.8 hours East) and 116° from the 98%-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.

Gropp et al. (GCN Circ. 27509) reported the discovery 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. 27519), the BAT ground-calculated position is RA, Dec = 60.555, -50.243 deg which is RA(J2000) = 04^h02^m13.3^s Dec(J2000) = -50°14'36.4" with an uncertainty of 2.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 75%.

The mask-weighted light curve (Figure 1) shows several overlapping pulses that start at ~T₀ and ends at~T+19 s. The main peak occurs at ~T₀. T₉₀ (15-350 keV) is 17.91 ± 5.20 s (estimated error including systematics).

The time-averaged spectrum from T+0.01 to T+19.01 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.14 ± 0.44. The fluence in the 15-150 keV band is 2.3 ± 0.6 x 10^-7 erg cm^-2. This fluence is larger than that of 8.0% 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 1.2 ± 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/965484/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Burrows et al. (GCN Circ. 27511). We have analysed 18 ks of XRT data for GRB 200409A, from 59 s to 114.6 ks after the BAT trigger. The data comprise 15 s in Windowed Timing (WT) mode (the first 7 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 27510).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=2.7 (+5.3, -0.9). At T+119 s the decay flattens to an α of 0.13 (+0.16, -0.18) before breaking again at T+1482 s to a final decay with index α=1.16 (+0.11, -0.10).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.04 (+0.17, -0.16). The best-fitting absorption column is 4.3 (+3.5, -3.0) x 10²⁰ cm^-2, consistent with the Galactic value of 1.3 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 3.2 x 10^-11 (3.6 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 4.3 (+3.5, -3.0) x 10²⁰ cm^-2
Galactic foreground: 1.3 x 10²⁰ cm^-2
Excess significance: <1.6 σ
Photon index: 2.04 (+0.17, -0.16)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00965484.

4. UVOT Observations and Analysis

UVOT results are not available.

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

04^h02^m08.75^s -50°13'28.4" 0.77" UVOT-initial Gropp et al. GCN Circ. 27509

04^h02^m08.74^s -50°13'29.6" 1.5" XRT-final UKSSDC

04^h02^m08.72^s -50°13'29.9" 1.6" XRT-enhanced Evans et al. GCN Circ. 27510

04^h02^m13.3^s -50°14'36.4" 2.7' BAT-refined Barthelmy et al. GCN Circ. 27519

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Lipunov et al. 27512 Swift GRB 200409A: Global MASTER-Net
observations report MASTER

Radio Dobie et al. 27521 No detection of radio emission 5 hours
post-burst detection

Gamma-ray Burns et al. 27514 Fermi GBM Sub-Threshold Detection Fermi GBM E_peak=. keV

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

RA (J2000)	Dec (J2000)	Error	Note	Reference
04^h02^m08.75^s	-50°13'28.4"	0.77"	UVOT-initial	Gropp et al. GCN Circ. 27509
04^h02^m08.74^s	-50°13'29.6"	1.5"	XRT-final	UKSSDC
04^h02^m08.72^s	-50°13'29.9"	1.6"	XRT-enhanced	Evans et al. GCN Circ. 27510
04^h02^m13.3^s	-50°14'36.4"	2.7'	BAT-refined	Barthelmy et al. GCN Circ. 27519

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Lipunov et al.	27512	Swift GRB 200409A: Global MASTER-Net observations report	MASTER
Radio	Dobie et al.	27521	No detection of radio emission 5 hours post-burst		detection
Gamma-ray	Burns et al.	27514	Fermi GBM Sub-Threshold Detection	Fermi GBM	E_peak=. keV

April 11, 2020