Swift Observations of GRB 201104B

B. Sbarufatti (PSU), J.A. Kennea (PSU) and F.E. Marshall (NASA/GSFC) for the Swift team

1. Introduction

At 17:33:46 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 201104B (trigger=1004168) (Sbarufatti et al. GCN Circ. 28825). Swift slewed immediately to the burst. 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.

Marshall et al. (GCN Circ. 28827) reported the detection with UVOT of an optical afterglow. Vielfaure et al. (GCN Circ. 28840) determined a redshift of 1.954 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

As reported by Lien et al. (GCN Circ. 28839), the BAT ground-calculated position is RA, Dec = 5.215, 7.834 deg which is RA(J2000) = 00h20m51.5s Dec(J2000) = +07°50'03.6" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 53%.

The mask-weighted light curve (Figure 1) shows several overlapping pulses that start at ~T-9 s and end at ~T+1 s. The main peak occurs at ~T-8 s. T90 (15-350 keV) is 8.66 ± 0.16 s (estimated error including systematics).

The time-averaged spectrum from T-8.86 to T+0.88 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.47 ± 0.07. The fluence in the 15-150 keV band is 1.8 ± 0.1 x 10-6 erg cm-2. This fluence is larger than that of 56% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-8.52 s in the 15-150 keV band is 4.1 ± 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/1004168/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Kennea et al. (GCN Circ. 28850). We have analysed 4.9 ks of XRT data for GRB 201104B, from 105 s to 131.7 ks after the BAT trigger. The data comprise 347 s in Windowed Timing (WT) mode 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=0.11 (+0.11, -0.12), followed by a break at T+338 s to an α of 1.21 (+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 1.80 ± 0.07. The best-fitting absorption column is 1.12 (+0.30, -0.28) x 1022 cm-2, at a redshift of 1.954, in addition to the Galactic value of 5.7 x 1020 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.7 x 10-11 (4.5 x 10-11) erg cm-2 count-1.

4. UVOT Observations and Analysis

Analysis of the UVOT data was reported by Marshall et al. (GCN Circ. 28827). 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 0.045 mag. in the direction of the GRB (Schlegel et al. 1998).

The Swift/UVOT began settled observations of the field of GRB 201104B 56 s after the BAT trigger (Oates and Sbarufatti GCN Circ. 28851). A source consistent with the XRT position and UVOT position (Marshall et al. GCN Circ. 28827) 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 0.048 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
00h20m51.65s +07°50'29.4" 0.43" UVOT-refined Marshall et al. GCN Circ. 28827
00h20m51.46s +07°50'30.7" 2.3" XRT-final UKSSDC
00h20m51.46s +07°50'30.7" 2.3" XRT-refined Kennea et al. GCN Circ. 28850
00h20m51.5s +07°50'03.6" 1.0' BAT-refined Lien et al. GCN Circ. 28839

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 28826 MASTER optical observation MASTER
Optical Vielfaure et al. 28840 VLT/X-shooter redshift VLT redshift
Optical Belkin et al. 28852 TSHAO optical observation Zeiss-1000 detection
Optical Belkin et al. 28856 Kitab optical observations
Optical Gupta et al. 28860 1.3m DFOT optical observations Devasthal Opt.Tel. detection
Optical Hosokawa et al. 28870 MITSuME Akeno optical observation MITSuME Akeno marginal detection
Optical Belkin et al. 28871 continued TSHAO optical observation Zeiss-1000 detection
Optical Belkin et al. 29198 Maidanak and CrAO optical observations CrAO detection

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
white 56 206 147 18.63 ± 0.06
u 268 459 188 19.45 ± 0.23

Table 3. UVOT observations reported by Marshall et al. (GCN Circ. 28827). The start and stop times of the exposures are given in seconds since the BAT trigger. The preliminary detections and 3-σ upper limits are given. No correction has been made for extinction in the Milky Way.

December 30, 2020