Swift Observations of GRB 210307A

E. Ambrosi (INAF-IASFPA), A. D'Ai (INAF-IASFPA) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 08:42:38 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210307A (trigger=1036139) (Ambrosi et al. GCN Circ. 29610). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 100° from the Sun (8.7 hours East) and 86° from the 36%-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 Markwardt et al. (GCN Circ. 29632), the BAT ground-calculated position is RA, Dec = 119.244, -66.965 deg which is RA(J2000) = 07h56m58.5s Dec(J2000) = -66°57'54.3" with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 79%.

The mask-weighted light curve (Figure 1) shows two isolated peaks. The initial peak starts at T-30 s, peaks at T0 and ends at T+40 s. The second peak starts at T+110 s, peaks at T+118 s and ends at T+150 s. T90 (15-350 keV) is 262 ± 83 s (estimated error including systematics).

The time-averaged spectrum from T-159.61 to T+174.53 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.56 ± 0.16. The fluence in the 15-150 keV band is 2.8 ± 0.3 x 10-6 erg cm-2. This fluence is larger than that of 70% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+1.27 s in the 15-150 keV band is 1.0 ± 0.2 ph cm-2 s-1. All the quoted errors are at the 90% confidence level.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Ai et al. (GCN Circ. 29624). We have analysed 12 ks of XRT data for GRB 210307A, from 97 s to 144.7 ks after the BAT trigger. The data comprise 149 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode.

The late-time light curve (Figure 2) (from T0+11.7 ks) can be modelled with an initial rise, with a power-law index of α=-0.29 (+0.23, -0.24), followed by a break at T+64.1 ks to an α of 2.7 (+1.2, -1.1).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.520 (+0.035, -0.026). The best-fitting absorption column is consistent with the Galactic value of 1.4 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.75 (+0.16, -0.14) and a best-fitting absorption column of 1.75 (+0.61, -0.30) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.0 x 10-11 (4.9 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.75 (+0.61, -0.30) x 1021 cm-2
Galactic foreground: 1.4 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 1.75 (+0.16, -0.14)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01036139.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210307A 100 s after the BAT trigger (Siegel and Ambrosi GCN Circ. 29629). 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 0.138 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
07h56m57.19s -66°58'15.3" 1.5" XRT-final UKSSDC
07h56m57.19s -66°58'15.3" 1.5" XRT-refined D'Ai et al. GCN Circ. 29624
07h56m58.5s -66°57'54.3" 2.1' BAT-refined Markwardt et al. GCN Circ. 29632

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 29609 Swift GRB210307.36: Global MASTER-Net
observations report 		MASTER
Optical Strausbaugh and Cucchiara 29613 LCO Optical Upper Limits LCO 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 100 250 147 >20.9
uFC 313 562 246 >20.6
white 100 786 186 >21.1
v 642 836 39 >19.3
b 568 761 39 >19.9
u 313 736 265 >20.6
w1 692 711 19 >19.6
m2 667 687 19 >18.7
w2 618 812 39 >19.9

Table 3. UVOT observations reported by Siegel and Ambrosi (GCN Circ. 29629). 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 9, 2021