Swift Observations of GRB 210305A

A. D'Ai (INAF-IASFPA), M. Perri (SSDC and INAF-OAR) and A. Belles (PSU) for the Swift team

1. Introduction

At 19:03:04 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210305A (trigger=1035922) (D'Ai et al. GCN Circ. 29595). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 48° from the Sun (1.8 hours West) and 87° from the 53%-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 Krimm et al. (GCN Circ. 29611), the BAT ground-calculated position is RA, Dec = 319.808, 34.548 deg which is RA(J2000) = 21h19m13.9s Dec(J2000) = +34°32'52.8" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 74%.

The BAT light curve (Figure 1) showed a complex structure with a duration of about ~100 s. T90 (15-350 keV) is 68.6 ± 4.4 s (estimated error including systematics).

The time-averaged spectrum from T-21.65 to T+71.76 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.93 ± 0.19, and Epeak of 104.1 ± 19.7 keV (χ2 43.93 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 7.1 ± 0.2 x 10-6 erg cm-2 and the 1-s peak flux measured from T+0.26 s in the 15-150 keV band is 3.4 ± 0.3 ph cm-2 s-1. This fluence is larger than that of 88% 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 1.49 ± 0.04 (χ2 71.71 for 57 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/1035922/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Perri et al. (GCN Circ. 29625).

We have analysed 11 ks of XRT data for GRB 210305A, from 67 s to 178.1 ks after the BAT trigger. The data comprise 184 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 late-time light curve (Figure 2) (from T0+5.3 ks) can be modelled with a power-law decay with a decay index of α=0.91 (+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 1.44 ± 0.08. The best-fitting absorption column is 3.2 (+0.5, -0.4) x 1021 cm-2, in excess of the Galactic value of 2.0 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.84 ± 0.16 and a best-fitting absorption column of 4.0 (+0.9, -0.8) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.3 x 10-11 (6.1 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 4.0 (+0.9, -0.8) x 1021 cm-2
Galactic foreground: 2.0 x 1021 cm-2
Excess significance: 4.1 σ
Photon index: 1.84 ± 0.16

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01035922.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210305A 85 s after the BAT trigger (Belles and D'Ai GCN Circ. 29615). 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 0.155 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
21h19m14.53s +34°33'14.5" 1.4" XRT-final UKSSDC
21h19m14.53s +34°33'14.5" 1.4" XRT-refined Perri et al. GCN Circ. 29625
21h19m13.9s +34°32'52.8" 1.0' BAT-refined Krimm et al. GCN Circ. 29611

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 29596 Swift GRB 210305A: Global MASTER-Net
observations report 		MASTER
Gamma-ray Veres et al. 29628 Fermi GBM observation Fermi GBM Epeak=98±3 keV
T90=61.7 seconds
Fluence=9.4±0.2x10-6erg cm-2
(74th percentile for long GRBs)
Gamma-ray Ridnaia et al. 29641 Konus-Wind detection Konus-Wind Epeak=84 (-26,+25) keV
Duration=~78 seconds
Fluence=1.63(-0.31,+0.43)x10-5erg cm-2

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 86 235 147 >20.19
uFC 298 548 245 >19.54
white 577 1721 264 >20.51
v 627 1771 134 >18.55
b 553 1696 116 >19.37
u 700 1672 97 >18.86
w1 676 1818 134 >18.70
m2 651 1795 136 >18.63
w2 603 1746 136 >18.60

Table 3. UVOT observations reported by Belles and D'Ai (GCN Circ. 29615). 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 11, 2021