Swift Observations of GRB 201020B

B. Sbarufatti (PSU) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

Malacaria and Meegan (GCN Circ. 28710) reported the GRB detection. At 17:33:54.33 UT on 20 October 2020, the Fermi Gamma-Ray Burst Monitor (GBM) triggered and located GRB 201020B (trigger 624908039 / 201020732). Table 1 contains the best reported positions from Swift.

Xu et al. (GCN Circ. 28719) reported the position from Xinjiang Astro. Obs. for the optical afterglow of this GRB. Breeveld and Lien (GCN Circ. 28743) reported the detection with UVOT of an optical afterglow. Kann et al. (GCN Circ. 28765) determined a redshift of 0.804 from GTC. Table 2 is a summary of GCN Circulars about this GRB from observatories other than Swift.

2. BAT Observations and Analysis

BAT did not observe this burst.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Sbarufatti et al. (GCN Circ. 28740).

The light curve (Figure 2) is consistent with a constant source of mean count rate 1.5 x 10^-1 ct s^-1. With the currently available dataset we cannot constrain the temporal evolution.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.2 (+0.4, -0.3). The best-fitting absorption column is 4.0 (+1.7, -1.4) x 10²¹ cm^-2, in excess of the Galactic value of 1.4 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 3.3 x 10^-11 (5.7 x 10^-11) erg cm^-2 count^-1.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations (Breeveld and Lien GCN Circ. 28743) of the field of GRB 201020B 72102 s after the Fermi GMB/LAT trigger (Fermi GBM team, GCN Circ. 28702; Malacaria and Meegan, GCN Circ. 28710; Arimoto et al., GCN Circ. 28716). A source consistent with the Master position (Lipunov et al., GCN Circ. 28718), also detected by Xu et al., (GCN Circ. 28719), Ridnaia et al. (GCN Circ. 28723), Belkin et al. (GCN Circ. 28723) and Sbarufatti et al. (GCN Circ. 28740), 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 E_B-V of 0.15 mag. in the direction of the GRB (Schlegel et al. 1998).

Figure 1. The BAT light curve is not available.

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

05^h01^m52.86^s +77°04'05.8" 0.54" UVOT ?

05^h01^m52.03^s +77°04'05.9" 2.0" XRT-final UKSSDC

05^h01^m52.03^s +77°04'05.9" 2.0" XRT Sbarufatti et al. GCN Circ. 28740

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Lipunov et al. 28704 Fermi GRB 201020B: Global MASTER-Net
observations report MASTER

Optical Lipunov et al. 28718 MASTER bright optical afterglow
detection MASTER detection

Optical Xu et al. 28719 Nanshan/NEXT optical afterglow
confirmation Xinjiang Astro. Obs. detection

Optical Belkin et al. 28725 Mondy optical observations Mondy detection

Optical Vinko et al. 28750 Konkoly Obs. optical detection Konkoly detection

Optical Kumar et al. 28753 GROWTH-India Telescope optical
observations GROWTH-India

Optical Kann et al. 28765 Redshift from GTC/OSIRIS GTC redshift

Optical Belkin et al. 28768 continued Mondy optical observations Mondy detection

Optical Vinko et al. 28769 follow-up observations at Konkoly Konkoly

Optical Dimple et al. 28772 1.3m DFOT optical observations Devasthal Opt.Tel.

Optical Kumar et al. 28776 HCT and GIT continued optical follow-up
observations Himalayan Chandra Telescope detection

Optical Kumar et al. 28781 1.3m DFOT continued optical follow-up
observations Devasthal Opt.Tel.

Optical Belkin et al. 29191 Assy, Koshka, Mondy and Maidanak
optical observations Maidanak detection

Gamma-ray Fermi 28702 Fermi GBM Final Real-time Localization Fermi GBM

Gamma-ray Malacaria and Meegan 28710 Fermi GBM detection Fermi GBM E_peak=136.9±3.4 keV
T₉₀=16 seconds
Fluence=3.929±0.040x10^-5erg cm^-2
(93^rd percentile for long GRBs)

Gamma-ray Ursi et al. 28714 AGILE detection AGILE

Gamma-ray Arimoto et al. 28716 Fermi-LAT detection Fermi LAT E_max=6 GeV

Gamma-ray Ridnaia et al. 28723 Konus-Wind detection Konus-Wind E_peak=193 (-23,+23) keV
Duration=~53.7 seconds
Fluence=4.72(-0.61,+0.70)x10^-5erg cm^-2

Other Nicastro et al. 28773 follow-up observations with Savelli
Telescope

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

Filter T_start(s) T_stop(s) Exp(s) Mag

white 73636 73973 332 19.54 ± 0.09

v 73979 74316 331 19.30 ± 0.28

u 72102 72821 708 19.12 ± 0.11

u 73293 73631 332 19.41 ± 0.19

RA (J2000)	Dec (J2000)	Error	Note	Reference
05^h01^m52.86^s	+77°04'05.8"	0.54"	UVOT	?
05^h01^m52.03^s	+77°04'05.9"	2.0"	XRT-final	UKSSDC
05^h01^m52.03^s	+77°04'05.9"	2.0"	XRT	Sbarufatti et al. GCN Circ. 28740

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Lipunov et al.	28704	Fermi GRB 201020B: Global MASTER-Net observations report	MASTER
Optical	Lipunov et al.	28718	MASTER bright optical afterglow detection	MASTER	detection
Optical	Xu et al.	28719	Nanshan/NEXT optical afterglow confirmation	Xinjiang Astro. Obs.	detection
Optical	Belkin et al.	28725	Mondy optical observations	Mondy	detection
Optical	Vinko et al.	28750	Konkoly Obs. optical detection	Konkoly	detection
Optical	Kumar et al.	28753	GROWTH-India Telescope optical observations	GROWTH-India
Optical	Kann et al.	28765	Redshift from GTC/OSIRIS	GTC	redshift
Optical	Belkin et al.	28768	continued Mondy optical observations	Mondy	detection
Optical	Vinko et al.	28769	follow-up observations at Konkoly	Konkoly
Optical	Dimple et al.	28772	1.3m DFOT optical observations	Devasthal Opt.Tel.
Optical	Kumar et al.	28776	HCT and GIT continued optical follow-up observations	Himalayan Chandra Telescope	detection
Optical	Kumar et al.	28781	1.3m DFOT continued optical follow-up observations	Devasthal Opt.Tel.
Optical	Belkin et al.	29191	Assy, Koshka, Mondy and Maidanak optical observations	Maidanak	detection
Gamma-ray	Fermi	28702	Fermi GBM Final Real-time Localization	Fermi GBM
Gamma-ray	Malacaria and Meegan	28710	Fermi GBM detection	Fermi GBM	E_peak=136.9±3.4 keV T₉₀=16 seconds Fluence=3.929±0.040x10^-5erg cm^-2 (93^rd percentile for long GRBs)
Gamma-ray	Ursi et al.	28714	AGILE detection	AGILE
Gamma-ray	Arimoto et al.	28716	Fermi-LAT detection	Fermi LAT	E_max=6 GeV
Gamma-ray	Ridnaia et al.	28723	Konus-Wind detection	Konus-Wind	E_peak=193 (-23,+23) keV Duration=~53.7 seconds Fluence=4.72(-0.61,+0.70)x10^-5erg cm^-2
Other	Nicastro et al.	28773	follow-up observations with Savelli Telescope

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white	73636	73973	332	19.54 ± 0.09
v	73979	74316	331	19.30 ± 0.28
u	72102	72821	708	19.12 ± 0.11
u	73293	73631	332	19.41 ± 0.19

Table 3. UVOT observations reported by Breeveld and Lien (GCN Circ. 28743). 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 29, 2020