Swift Observations of GRB 210318A

E. Troja (NASA/GSFC/UMCP), B. Sbarufatti (PSU) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 03:54:31 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210318A (trigger=1037339) (Troja et al. GCN Circ. 29662). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 50° from the Sun (2.4 hours East) and 55° from the 20%-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 Sakamoto et al. (GCN Circ. 29672), the BAT ground-calculated position is RA, Dec = 33.892, -37.755 deg which is RA(J2000) = 02h15m34.0s Dec(J2000) = -37°45'17.7" with an uncertainty of 2.9 arcmin, (radius, sys+stat, 90% containment). The partial coding was 75%.

The mask-weighted light curve (Figure 1) s shows a single broad FRED-shaped peak with some structure. The emission starts at ~T10 s, peaks at T+5 s, and falls to background by T+50 s. T90 (15-350 keV) is 48.00 ± 22.63 s (estimated error including systematics).

The time-averaged spectrum from T-12.37 to T+35.63 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.48 ± 0.33. The fluence in the 15-150 keV band is 6.4 ± 1.3 x 10-7 erg cm-2. This fluence is larger than that of 27% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+11.13 s in the 15-150 keV band is 0.3 ± 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/1037339/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Sbarufatti et al. (GCN Circ. 29671).

We have analysed 3.1 ks of XRT data for GRB 210318A, from 107 s to 45.6 ks after the BAT trigger. The data comprise 9 s in Windowed Timing (WT) mode (taken while Swift was slewing), with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 29670).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.09 (+0.27, -0.29).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.8 (+0.6, -0.5). The best-fitting absorption column is 1.4 (+1.9, -1.2) x 1021 cm-2, consistent with the Galactic value of 1.9 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 3.8 x 10-11 (4.6 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.4 (+1.9, -1.2) x 1021 cm-2
Galactic foreground: 1.9 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 1.8 (+0.6, -0.5)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01037339.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210318A 127 s after the BAT trigger (Siegel and Troja GCN Circ. 29706). No optical afterglow consistent with the XRT position 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 0.017 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
02h15m42.27s -37°46'36.2" 2.0" XRT-final UKSSDC
02h15m42.29s -37°46'36.9" 2.4" XRT-enhanced Osborne et al. GCN Circ. 29670
02h15m34.0s -37°45'17.7" 2.9' BAT-refined Sakamoto et al. GCN Circ. 29672

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 29665 Swift GRB 210318A: Global MASTER-Net
observations report 		MASTER
Optical Lipunov et al. 29669 Swift GRB210318.13: Global MASTER-Net
observations report 		MASTER

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 127 277 147 >20.5
uFC 285 535 246 >19.9
white 127 1188 353 >21.0
v 614 1064 58 >18.6
b 541 1163 58 >19.3
u 285 535 246 >19.9

Table 3. UVOT observations reported by Siegel and Troja (GCN Circ. 29706). 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 25, 2021