Swift Observations of GRB 210905A

E. Sonbas (Adiyaman Univ.), K.L. Page (U. Leicester) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 00:12:41 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210905A (trigger=1071993) (Sonbas et al. GCN Circ. 30765). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 132° from the Sun (9.7 hours East) and 154° from the 5%-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.

Strausbaugh and Cucchiara (GCN Circ. 30769) reported the position from LCO for the optical afterglow of this GRB. Tanvir et al. (GCN Circ. 30771) determined a redshift of 6.318 from VLT. 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

BAT refined results are not available.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Page et al. (GCN Circ. 30776). We have analysed 106 ks of XRT data for GRB 210905A, from 82 s to 3784.5 ks after the BAT trigger. The data comprise 219 s in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 30768).

The late-time light curve (Figure 2) (from T0+3.8 ks) can be modelled with an initial power-law decay with an index of α=0.1 (+0.4, -0.6), followed by a break at T+9440 s to an α of 1.05 (+0.05, -0.04).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.42 (+0.05, -0.04). The best-fitting absorption column is 4.5 (+1.9, -1.8) x 1022 cm-2, at a redshift of 6.318, in addition to the Galactic value of 3.4 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.86 (+0.13, -0.12) and a best-fitting absorption column of 2.8 (+3.7, -2.8) x 1022 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 x 10-11 (3.9 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 3.4 x 1020 cm-2
Intrinsic column: 2.8 (+3.7, -2.8) x 1022 cm-2 at z=6.318
Photon index: 1.86 (+0.13, -0.12)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01071993.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210905A 157 s after the BAT trigger (Siegel and Sonbas GCN Circ. 30785). No optical afterglow consistent with the XRT position (Beardmore et al. GCN Circ. 30768) 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 0.034 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
20h36m11.64s -44°26'24.3" 1.5" XRT-final UKSSDC
20h36m11.58s -44°26'22.4" 2.6" XRT-enhanced Beardmore et al. GCN Circ. 30768
20h36m15s -44°25'50" 3' BAT-initial Sonbas et al. GCN Circ. 30765

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 30766 Swift GRB 210905A: Global MASTER-Net
observations report 		MASTER
Optical Strausbaugh and Cucchiara 30769 LCO Optical Detection LCO detection
Optical Strausbaugh and Cucchiara 30770 Continued LCO Optical Observations LCO
Optical Tanvir et al. 30771 VLT/X-shooter spectroscopic redshift VLT redshift
Optical D'Avanzo et al. 30772 REM NIR afterglow detection REM detection
Optical Cooke et al. 30773 DECam Detection Dark Energy Camera detection
Optical Guelbenzu et al. 30781 GROND observations GROND detection
Radio Laskar et al. 30783 ALMA detection ALMA detection
Radio Laskar et al. 30784 ATCA detection ATCA detection
X-ray Laskar et al. 31127 Chandra observations Chandra detection
Gamma-ray Veres and Roberts 30779 Fermi GBM Sub-Threshold Detection Fermi GBM Epeak=230 keV
Gamma-ray Frederiks et al. 30780 Konus-Wind detection Konus-Wind Epeak=203 (-58,+111) keV
Gamma-ray Prasad et al. 30782 AstroSat CZTI detection CZTI

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

Filter Tstart(s) Tstop(s) Exp(s) Mag
uFC 157 407 246 >20.1
v 4193 4393 197 >19.3
b 3782 3982 197 >21.4
w1 4603 4803 197 >20.1
m2 4398 4597 197 >19.5
w2 3988 4188 197 >19.4

Table 3. UVOT observations reported by Siegel and Sonbas (GCN Circ. 30785). 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.

November 26, 2021