Swift Observations of GRB 210318B

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

1. Introduction

At 03:08:33 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210318B (trigger=1037335) (Troja et al. GCN Circ. 29663). Swift slewed immediately to the burst. 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 Stamatikos et al. (GCN Circ. 29666), the BAT ground-calculated position is RA, Dec = 264.228, -35.823 deg which is RA(J2000) = 17h36m54.7s Dec(J2000) = -35°49'21.4" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 95%.

The mask-weighted light curve (Figure 1) shows a multi-peak structure running from ~T-4 s to T+15 s. T90 (15-350 keV) is 14.95 ± 0.50 s (estimated error including systematics).

The time-averaged spectrum from T-11.22 to T+19.42 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.10 ± 0.13, and Epeak of 252.1 ± 191.6 keV (χ2 41.33 for 0 d.o.f.). For this model the total fluence in the 15-150 keV band is 1.0 ± 0.02 x 10-5 erg cm-2 and the 1-s peak flux measured from T+2.70 s in the 15-150 keV band is 14.5 ± 0.4 ph cm-2 s-1. This fluence is larger than that of 92.5% 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 1.31 ± 0.03 (χ2 48.69 for 0 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/1037335/BA/.

3. XRT Observations and Analysis

We have analysed 6.1 ks of XRT data for GRB 210318B, from 52 s to 76.0 ks after the BAT trigger. The data comprise 584 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 light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=0.70 ± 0.03, followed by a break at T+597 s to an α of 1.45 (+0.08, -0.06).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.78 ± 0.09. The best-fitting absorption column is 1.43 ± 0.13 x 1022 cm-2, in excess of the Galactic value of 7.1 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.69 (+0.27, -0.25) and a best-fitting absorption column of 1.6 ± 0.4 x 1022 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 6.5 x 10-11 (1.1 x 10-10) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.6 ± 0.4 x 1022 cm-2
Galactic foreground: 7.1 x 1021 cm-2
Excess significance: 4.0 σ
Photon index: 1.69 (+0.27, -0.25)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01037335.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210318B 70 s after the BAT trigger (Siegel and Troja GCN Circ. 29695). No optical afterglow consistent with the XRT position (Troja et al. GCN Circ. 29663) 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 1.718 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
17h36m55.88s -35°49'19.2" 3.5" XRT-final UKSSDC
17h36m55.88s -35°49'19.2" 3.5" XRT-initial Troja et al. GCN Circ. 29663
17h36m54.7s -35°49'21.4" 1.0' BAT-refined Stamatikos et al. GCN Circ. 29666

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Hosokawa et al. 29675 MITSuME Okayama optical upper limits MITSuME Okayama upper limits
Gamma-ray Svinkin et al. 29664 Konus-Wind detection Konus-Wind Epeak=264 (-32,+33) keV
Duration=~14 seconds
Fluence=3.83(-0.44,+0.50)x10-5erg cm-2
Gamma-ray Malacaria and Meegan 29668 Fermi GBM detection Fermi GBM Epeak=84.5±8.7 keV
T90=24.3 seconds
Fluence=2.537±0.393x10-7erg cm-2
(1.2 percentile for long GRBs)

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 70 219 147 >20.0
uFC 282 532 246 >20.1
white 70 1357 373 >20.9
v 612 1406 97 >18.2
b 538 1500 92 >19.1
u 282 1479 324 >20.1
w1 661 1455 97 >18.8
m2 1066 1430 58 >19.3
w2 588 1382 97 >19.0

Table 3. UVOT observations reported by Siegel and Troja (GCN Circ. 29695). 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 23, 2021