Swift Observations of GRB 210104B

S. Laha (GSFC/UMBC/CRESST), J.D. Gropp (PSU) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 21:10:10 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210104B (trigger=1015919) (Laha et al. GCN Circ. 29243). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 133° from the Sun (8.5 hours East) and 111° from the 67%-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. 29266), the BAT ground-calculated position is RA, Dec = 53.561, 37.951 deg which is RA(J2000) = 03h34m14.7s Dec(J2000) = +37°57'05.1" with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 59%.

The mask-weighted light curve (Figure 1) shows a double peaks structure that starts at ~T0 and ends at ~T+23 s. The two peaks occurs at ~T0 and ~T+11 s, respectively. T90 (15-350 keV) is 19.94 ± 2.33 s (estimated error including systematics).

The time-averaged spectrum from T-0.15 to T+22.76 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.83 ± 0.14. The fluence in the 15-150 keV band is 1.2 ± 0.1 x 10-6 erg cm-2. This fluence is larger than that of 45% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+10.83 s in the 15-150 keV band is 1.9 ± 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/1015919/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Gropp et al. (GCN Circ. 29256). We have analysed 9.0 ks of XRT data for GRB 210104B, from 100 s to 108.9 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 29253).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.62 ± 0.04.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.29 (+0.27, -0.25). The best-fitting absorption column is 1.13 (+0.26, -0.22) x 1022 cm-2, in excess of the Galactic value of 2.7 x 1021 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.3 x 10-11 (1.2 x 10-10) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.13 (+0.26, -0.22) x 1022 cm-2
Galactic foreground: 2.7 x 1021 cm-2
Excess significance: 6.4 σ
Photon index: 2.29 (+0.27, -0.25)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01015919.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210104B 84 s after the BAT trigger (Breeveld and Laha GCN Circ. 29259). No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 29253) 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 0.79 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
03h34m12.86s +37°55'49.6" 1.5" XRT-final UKSSDC
03h34m12.71s +37°55'49.8" 1.9" XRT-enhanced Evans et al. GCN Circ. 29253
03h34m14.7s +37°57'05.1" 1.7' BAT-refined Sakamoto et al. GCN Circ. 29266

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Hu et al. 29249 BOOTES network optical upper limit BOOTES upper limits
Optical Fu et al. 29250 Nanshan/NEXT optical upper limit Xinjiang Astro. Obs. upper limits
Optical Kim et al. 29255 Assy optical upper limit 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 84 234 147 >20.5
uFC 296 546 246 >19.8
white 84 753 172 >20.7
v 625 645 19 >17.6
b 551 743 39 >19.1
u 296 719 265 >19.9

Table 3. UVOT observations reported by Breeveld and Laha (GCN Circ. 29259). 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.

January 6, 2021