Swift Observations of GRB 200303A

S.B. Cenko (GSFC), P.A. Evans (U. Leicester) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 02:34:57 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200303A (trigger=959431) (Cenko et al. GCN Circ. 27297). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 120° from the Sun (8.8 hours West) and 99° from the 53%-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 Palmer et al. (GCN Circ. 27311), the BAT ground-calculated position is RA, Dec = 212.695, 51.358 deg which is RA(J2000) = 14^h10^m46.9^s Dec(J2000) = +51°21'29.3" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 59%.

The burst entered the BAT field of view around T-100 s during a spacecraft slew. The mask-weighted light curve (Figure 1) shows a single episode with multiple pulses, starting at T-45 s, peaking at the trigger time and returning to near background by T+70 s. There is extended low-level emission out to nearly T+200 s. T₉₀ (15-350 keV) is 94.2 ± 6.4 s (estimated error including systematics).

The time-averaged spectrum from T-36.55 to T+193.26 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.39 ± 0.17, and E_peak of 131.4 ± 75.1 keV (χ² 46.32 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 1.3 ± 0.0 x 10^-5 erg cm^-2 and the 1-s peak flux measured from T+0.34 s in the 15-150 keV band is 5.0 ± 0.3 ph cm^-2 s^-1. This fluence is larger than that of 94.2% 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 1.65 ± 0.04 (χ² 53.75 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/959431/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 27312). We have analysed 11 ks of XRT data for GRB 200303A, from 77 s to 73.7 ks after the BAT trigger. The data comprise 595 s in Windowed Timing (WT) mode (the first 9 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 Goad et al. (GCN Circ. 27300).

The late-time light curve (Figure 2) (from T0+4.8 ks) can be modelled with a power-law decay with a decay index of α=1.36 ± 0.07.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.41 ± 0.05. The best-fitting absorption column is 1.10 (+0.05, -0.04) x 10²² cm^-2, in excess of the Galactic value of 1.6 x 10²⁰ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.38 (+0.15, -0.14) and a best-fitting absorption column of 7.5 (+1.0, -0.9) x 10²¹ cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.7 x 10^-11 (8.8 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 7.5 (+1.0, -0.9) x 10²¹ cm^-2
Galactic foreground: 1.6 x 10²⁰ cm^-2
Excess significance: 13.4 σ
Photon index: 2.38 (+0.15, -0.14)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00959431.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200303A 96 s after the BAT trigger (Breeveld and Cenko GCN Circ. 27308). No optical afterglow consistent with the XRT position (Goad et al. GCN Circ. 27300) 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 E_B-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

14^h10^m52.55^s +51°21'33.5" 1.4" XRT-final UKSSDC

14^h10^m52.55^s +51°21'33.5" 1.7" XRT-enhanced Goad et al. GCN Circ. 27300

14^h10^m46.9^s +51°21'29.3" 1.0' BAT-refined Palmer et al. GCN Circ. 27311

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Lipunov et al. 27296 Swift GRB200303.11: Global MASTER-Net
observations report MASTER

Optical de Ugarte Postigo et al. 27303 Optical limit from CAHA CAHA upper limits

Optical Mong et al. 27304 GOTO observations upper limit Gravitational-wave Optical Transient Observer upper limits

Optical Jelinek et al. 27307 FRAM-ORM optical limit FRAM upper limits

Optical Xu et al. 27309 NOT optical observations NOT

Gamma-ray Fermi 27294 Fermi GBM Final Real-time Localization Fermi GBM

Gamma-ray Asaoka et al. 27301 CALET Gamma-Ray Burst Monitor detection CALET

Gamma-ray Malacaria and Meegan 27314 Fermi GBM observation Fermi GBM E_peak=151±18 keV
T₉₀=82 seconds
Fluence=3.23±0.08x10^-5erg cm^-2
(91^st percentile for long GRBs)

Gamma-ray Tsvetkova et al. 27316 Konus-Wind observation Konus-Wind E_peak=154 (-44,+82) keV
Fluence=3.88(-1.20,+0.80)x10^-5erg cm^-2

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

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

white_FC 96 246 147 >20.7

u_FC 308 558 246 >20.4

white 96 5034 550 >21.5

v 1219 1238 19 >18.4

b 1144 1163 19 >19.4

u 308 558 246 >20.4

RA (J2000)	Dec (J2000)	Error	Note	Reference
14^h10^m52.55^s	+51°21'33.5"	1.4"	XRT-final	UKSSDC
14^h10^m52.55^s	+51°21'33.5"	1.7"	XRT-enhanced	Goad et al. GCN Circ. 27300
14^h10^m46.9^s	+51°21'29.3"	1.0'	BAT-refined	Palmer et al. GCN Circ. 27311

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Lipunov et al.	27296	Swift GRB200303.11: Global MASTER-Net observations report	MASTER
Optical	de Ugarte Postigo et al.	27303	Optical limit from CAHA	CAHA	upper limits
Optical	Mong et al.	27304	GOTO observations upper limit	Gravitational-wave Optical Transient Observer	upper limits
Optical	Jelinek et al.	27307	FRAM-ORM optical limit	FRAM	upper limits
Optical	Xu et al.	27309	NOT optical observations	NOT
Gamma-ray	Fermi	27294	Fermi GBM Final Real-time Localization	Fermi GBM
Gamma-ray	Asaoka et al.	27301	CALET Gamma-Ray Burst Monitor detection	CALET
Gamma-ray	Malacaria and Meegan	27314	Fermi GBM observation	Fermi GBM	E_peak=151±18 keV T₉₀=82 seconds Fluence=3.23±0.08x10^-5erg cm^-2 (91^st percentile for long GRBs)
Gamma-ray	Tsvetkova et al.	27316	Konus-Wind observation	Konus-Wind	E_peak=154 (-44,+82) keV Fluence=3.88(-1.20,+0.80)x10^-5erg cm^-2

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white_FC	96	246	147	>20.7
u_FC	308	558	246	>20.4
white	96	5034	550	>21.5
v	1219	1238	19	>18.4
b	1144	1163	19	>19.4
u	308	558	246	>20.4

Table 3. UVOT observations reported by Breeveld and Cenko (GCN Circ. 27308). 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.

March 5, 2020