Swift Observations of GRB 210419C

M.J. Moss (GWU), A.P. Beardmore (U. Leicester) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 23:27:49 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210419C (trigger=1044236) (Moss et al. GCN Circ. 29834). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 88° from the Sun (3.9 hours East) and 93° from the 47%-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 Sakamoto et al. (GCN Circ. 29856), the BAT ground-calculated position is RA, Dec = 212.969, 36.011 deg which is RA(J2000) = 14h11m52.6s Dec(J2000) = +36°00'40.5" with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 72%.

The BAT light curve (Figure 1) showed a complex structure with a duration of about ~70 s. T90 (15-350 keV) is 73.00 ± 16.40 s (estimated error including systematics).

The time-averaged spectrum from T+16.00 to T+102.00 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.50 ± 0.27. The fluence in the 15-150 keV band is 8.1 ± 1.2 x 10-7 erg cm-2. This fluence is larger than that of 33% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+48.00 s in the 15-150 keV band is 0.8 ± 0.2 ph cm-2 s-1. All the quoted errors are at the 90% confidence level.

3. XRT Observations and Analysis

We have analysed 5.2 ks of XRT data for GRB 210419C, from 154 s to 28.7 ks after the BAT trigger. The data comprise 58 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 29839).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=8.00 (+0.00, -0.24), followed by a break at T+266 s to an α of 0.85 (+0.10, -0.09).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 4.2 (+0.4, -0.3). The best-fitting absorption column is 3.8 (+0.7, -0.6) x 1021 cm-2, in excess of the Galactic value of 1.1 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 2.0 x 10-11 (1.7 x 10-10) erg cm-2 count-1.

A summary of the WT-mode spectrum is thus:
Total column: 3.8 (+0.7, -0.6) x 1021 cm-2
Galactic foreground: 1.1 x 1020 cm-2
Excess significance: 9.8 σ
Photon index: 4.2 (+0.4, -0.3)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01044236.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210419C 156 s after the BAT trigger (Siegel and Moss GCN Circ. 29868). No optical afterglow consistent with the XRT position (Beardmore et al. GCN Circ. 29839) 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 EB-V of 0.010 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
14h11m51.01s +36°00'26.5" 2.1" XRT-final UKSSDC
14h11m51.00s +36°00'26.7" 2.4" XRT-enhanced Beardmore et al. GCN Circ. 29839
14h11m52.6s +36°00'40.5" 2.1' BAT-refined Sakamoto et al. GCN Circ. 29856

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Zheng and Filippenko 29840 KAIT Optical Upper Limit KAIT upper limits
Optical Leonini et al. 29842 Montarrenti Observatory Upper Limit Montarrenti upper limits
Optical Butler et al. 29843 RATIR Optical and NIR Upper Limits RATIR upper limits
Optical Murata et al. 29859 MITSuME Akeno optical upper limits MITSuME Akeno upper limits

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 156 305 147 >20.83
white 6954 17801 254 >21.14
v 11428 28719 155 >19.22
b 6749 17713 1081 >21.37
u 313 408 93 >19.55
u 6544 22974 258 >20.20
UVW1 6339 22905 1082 >20.49
UVM2 6134 6334 196 >19.21
UVW2 10522 28626 1771 >20.84

Table 3. UVOT observations reported by Siegel and Moss (GCN Circ. 29868). 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.

April 22, 2021