Swift Observations of GRB 190427A

A. Tohuvavohu (PSU), S. Campana (INAF-OAB) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 04:34:15 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190427A (trigger=900730) (Tohuvavohu et al. GCN Circ. 24261). Swift's slew will be delayed due to an observing constraint. At the time of the trigger, the initial BAT position was 98° from the Sun (7.6 hours West) and 67° from the 48%-illuminated Moon. Table 1 contains the best reported positions from Swift.

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 Ukwatta et al. (GCN Circ. 24330), the BAT ground-calculated position is RA, Dec = 280.217, 40.304 deg which is RA(J2000) = 18^h40^m52.0^s Dec(J2000) = +40°18'15.6" with an uncertainty of 2.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 68%.

The mask-weighted light curve (Figure 1) shows a double-peaked structure that starts from ~ T₀ and ends at ~T+0.3. The main peak occurs at ~T₀, followed by a weaker peak at ~T+0.2 s. T₉₀ (15-350 keV) is 0.3 ± 0.1 s (estimated error including systematics).

The time-averaged spectrum from T-0.0 to T+0.3 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.17 ± 0.21. The fluence in the 15-150 keV band is 1.3 ± 0.2 x 10^-7 erg cm^-2. This fluence is larger than that of 79% of the short GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.33 s in the 15-150 keV band is 1.6 ± 0.3 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/900730/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Campana et al. (GCN Circ. 24264).

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190427A 3737 s after the BAT trigger (Kuin and Tohuvavohu GCN Circ. 24333). No optical afterglow consistent with the BAT position (Ukwatta et al., GCN Circ. 24330) 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 0.06 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.

RA (J2000) Dec (J2000) Error Note Reference

18^h40^m52.0^s +40°18'15.6" 2.4' BAT-refined Ukwatta et al. GCN Circ. 24330

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Lipunov et al. 24263 Swift GRB190427.19 Global MASTER-Net
observations report MASTER

Optical Izzo et al. 24265 NOT optical observations NOT

Optical Hu et al. 24272 BOOTES-1 and BOOTES-5/JGT early
follow-up observations BOOTES-1

Optical Troja et al. 24282 RATIR Optical and NIR Observations RATIR

Optical Watson et al. 24287 COATLI Optical Observations COATLI

Optical Murata et al. 24288 MITSuME Akeno optical upper limits MITSuME Akeno upper limits

Optical Zhang et al. 24290 Lick/Nickel telescope optical
observations Pan-STARRS

Optical Lipunov et al. 24294 Fermi GRB190427.19 Global MASTER-Net
observations report MASTER

Gamma-ray Malacaria et al. 24293 Fermi GBM detection Fermi GBM E_peak=246.40±78.20 keV
T₉₀=0.37 seconds
Fluence=4.855±0.717x10^-7erg cm^-2
(65^th percentile for short GRBs)

Gamma-ray Cannady et al. 24339 CALET Gamma-Ray Burst Monitor detection CALET

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

u 3737 3936 197 >19.9

RA (J2000)	Dec (J2000)	Error	Note	Reference
18^h40^m52.0^s	+40°18'15.6"	2.4'	BAT-refined	Ukwatta et al. GCN Circ. 24330

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Lipunov et al.	24263	Swift GRB190427.19 Global MASTER-Net observations report	MASTER
Optical	Izzo et al.	24265	NOT optical observations	NOT
Optical	Hu et al.	24272	BOOTES-1 and BOOTES-5/JGT early follow-up observations	BOOTES-1
Optical	Troja et al.	24282	RATIR Optical and NIR Observations	RATIR
Optical	Watson et al.	24287	COATLI Optical Observations	COATLI
Optical	Murata et al.	24288	MITSuME Akeno optical upper limits	MITSuME Akeno	upper limits
Optical	Zhang et al.	24290	Lick/Nickel telescope optical observations	Pan-STARRS
Optical	Lipunov et al.	24294	Fermi GRB190427.19 Global MASTER-Net observations report	MASTER
Gamma-ray	Malacaria et al.	24293	Fermi GBM detection	Fermi GBM	E_peak=246.40±78.20 keV T₉₀=0.37 seconds Fluence=4.855±0.717x10^-7erg cm^-2 (65^th percentile for short GRBs)
Gamma-ray	Cannady et al.	24339	CALET Gamma-Ray Burst Monitor detection	CALET

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
u	3737	3936	197	>19.9

Table 3. UVOT observation reported by Kuin and Tohuvavohu (GCN Circ. 24333). The start and stop time of the exposure are given in seconds since the BAT trigger. The preliminary 3-σ upper limit is given. No correction has been made for extinction in the Milky Way.

April 30, 2019