Swift Observations of GRB 210619B

P. D'Avanzo (INAF-OAB), K.L. Page (U. Leicester) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 23:59:25 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210619B (trigger=1056757) (D'Avanzo et al. GCN Circ. 30261). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 105° from the Sun (8.6 hours West) and 116° from the 70%-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.

D'Avanzo et al. (GCN Circ. 30261) reported the discovery with UVOT of an optical afterglow. de Ugarte Postigo et al. (GCN Circ. 30272) determined a redshift of 1.937 from GTC. 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. 30282), the BAT ground-calculated position is RA, Dec = 319.718, 33.850 deg which is RA(J2000) = 21^h18^m52.3^s Dec(J2000) = +33°50'58.6" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 36%.

The mask-weighted light curve (Figure 1) shows a bright pulse that starts at ~T₀ and peaks at ~T+1 s, and is followed by several overlapping pulses that last till ~T+105 s. T₉₀ (15-350 keV) is 60.90 ± 0.28 s (estimated error including systematics).

The time-averaged spectrum from T-0.25 to T+105.38 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.41 ± 0.02. The fluence in the 15-150 keV band is 9.5 ± 0.1 x 10^-5 erg cm^-2. This fluence is larger than that of 99.8% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.95 s in the 15-150 keV band is 115.0 ± 2.2 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/1056757/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Page et al. (GCN Circ. 30269). We have analysed 63 ks of XRT data for GRB 210619B, from 208 s to 1945.6 ks after the BAT trigger. The data comprise 1.5 ks in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 30267).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=0.78 (+0.08, -0.13). At T+505 s the decay steepens to an α of 0.975 (+0.014, -0.016) before breaking again at T+12.2 ks to a final decay with index α=1.472 ± 0.029.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.858 ± 0.016. The best-fitting absorption column is 6.7 ± 0.8 x 10²¹ cm^-2, at a redshift of 1.937, in addition to the Galactic value of 2.0 x 10²¹ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.92 ± 0.06 and a best-fitting absorption column of 6.8 (+2.7, -2.6) x 10²¹ cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.8 x 10^-11 (5.3 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 2.0 x 10²¹ cm^-2
Intrinsic column: 6.8 (+2.7, -2.6) x 10²¹ cm^-2 at z=1.937
Photon index: 1.92 ± 0.06

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01056757.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210619B 136 s after the BAT trigger (Kuin and D'Avanzo GCN Circ. 30278). A source consistent with the XRT position (Beardmore et al., GCN Circ. 30267) was detected in the initial UVOT exposures consistent with the reported afterglow by Lipunov et al. (GCN Circ. 30259), Jelinek et al. (GCN Circ. 30263), Kong (GCN Circ. 30265), Kellegrin et al. (GCN Circ. 30268), and Zheng & Filippenko (GCN Circ. 30273). 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 0.173 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

21^h18^m52.38^s +33°51'01.6" 0.42" UVOT-refined Kuin and D'Avanzo GCN Circ. 30278

21^h18^m52.39^s +33°51'00.2" 1.9" XRT-final UKSSDC

21^h18^m52.44^s +33°51'00.1" 2.1" XRT-enhanced Beardmore et al. GCN Circ. 30267

21^h18^m52.3^s +33°50'58.6" 1.0' BAT-refined Lien et al. GCN Circ. 30282

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Jelinek et al. 30263 Ondrejov D50 detection D50 detection

Optical Kong 30265 iTelescope T18 detection iTelescope detection

Optical Pellegrin and Anandagoda 30268 SARA-KP 0.9m Optical Afterglow Detection SARA detection

Optical Perley 30271 Liverpool telescope imaging of a red
afterglow Liverpool Telescope

Optical de Ugarte Postigo et al. 30272 Redshift from OSIRIS/GTC GTC redshift

Optical Zheng and Filippenko 30273 KAIT Optical Detection KAIT detection

Optical Blazek et al. 30274 Liverpool Telescope r'-band observations Liverpool Telescope detection

Optical Kann et al. 30275 CAHA 2.2m photometric monitoring CAHA detection

Optical Xin et al. 30277 Xinglong TNT optical observations TNT

Optical Shrestha et al. 30280 Liverpool Telescope First Hour
Observations Liverpool Telescope

Optical Jelinek et al. 30281 steepening of the decay observed at the
D50 D50

Optical Kumar et al. 30286 GIT optical follow-up and confirmation
of jet-break GROWTH-India Tel. detection

Optical D'Avanzo et al. 30288 REM optical/NIR afterglow detection REM detection

Optical D'Avanzo et al. 30289 TNG NIR observations of the afterglow TNG

Optical Cunningham et al. 30290 SEDM Optical Observations Spectral Energy Distribution Machine at Palomar

Optical Moskvitin and Maslennikova 30291 SAO RAS optical observations SAO RAS detection

Optical 30292 iTelescope optical afterglow observation iTelescope

Optical Hu et al. 30293 1.5m OSN optical observation Obs.de Sierra Nevada detection

Optical Zhu et al. 30294 NOT optical observations NOT detection

Optical Belkin et al. 30299 AbAO optical observations Abastumani Astro. Obs. detection

Optical Moskvitin and Maslennikova 30303 Further SAO RAS optical observations SAO RAS detection

Optical Romanov and Lane 30305 Abbey Ridge Observatory optical
afterglow observation Abbey Ridge Obs.

Optical Karpov et al. 30308 Mini-MegaTORTORA early optical
observations TORTORA light curve

Optical Moskvitin and Maslennikova 30309 Even further SAO RAS optical
observations SAO RAS detection

Optical Vinko et al. 30320 optical afterglow detection from
Konkoly Observatory Konkoly detection

Optical Kann et al. 30338 Deep CAHA 2.2m detection and late-time
light curve behavior CAHA light curve

Optical Zaznobin et al. 30343 Sayan observatory 1.6-m telescope
observations Sayan Obs. detection

Optical Belkin et al. 30791 Maidanak and Assy optical observations,
broken power law parametrization Assy-Turgen Obs. detection

Radio Smith et al. 30361 JCMT SCUBA-2 sub-mm observations SCUBA

Radio Laskar et al. 30386 ALMA detection ALMA detection

Gamma-ray Zhao et al. 30264 GECAM detection GECAM

Gamma-ray Axelsson et al. 30270 Fermi-LAT detection Fermi LAT E_max=8.3 GeV

Gamma-ray Svinkin et al. 30276 Konus-Wind detection Konus-Wind E_peak=261 (-13,+14) keV
Fluence=4.60(-0.13,+0.13)x10^-4erg cm^-2

Gamma-ray Poolakkil and Meegan 30279 Fermi GBM detection Fermi GBM E_peak=210±3 keV
T₉₀=55 seconds
Fluence=307.93±0.9830x10^-6erg cm^-2
(brighter than 99.9% of long GRBs)

Gamma-ray Levin et al. 30283 SRG/ART-XC detection SRG/ART

Gamma-ray Kawakubo et al. 30284 CALET Gamma-Ray Burst Monitor detection CALET

Gamma-ray Minaev et al. 30304 SPI-ACS/INTEGRAL observations INTEGRAL

Gamma-ray Marisaldi et al. 30315 ASIM observation Atmosphere-Space International Monitor

Other 30301 possibility of lensed visible afterglow?

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 136 285 147 14.67 ± 0.02

white 794 814 19 16.66 ± 0.06

v 679 699 19 15.59 ± 0.10

b 604 624 20 16.39 ± 0.08

u 348 598 246 15.60 ± 0.03

w1 729 1107 39 17.22 ± 0.19

m2 704 1776 117 >19.3

w2 655 1726 136 19.20 ± 0.31

RA (J2000)	Dec (J2000)	Error	Note	Reference
21^h18^m52.38^s	+33°51'01.6"	0.42"	UVOT-refined	Kuin and D'Avanzo GCN Circ. 30278
21^h18^m52.39^s	+33°51'00.2"	1.9"	XRT-final	UKSSDC
21^h18^m52.44^s	+33°51'00.1"	2.1"	XRT-enhanced	Beardmore et al. GCN Circ. 30267
21^h18^m52.3^s	+33°50'58.6"	1.0'	BAT-refined	Lien et al. GCN Circ. 30282

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Jelinek et al.	30263	Ondrejov D50 detection	D50	detection
Optical	Kong	30265	iTelescope T18 detection	iTelescope	detection
Optical	Pellegrin and Anandagoda	30268	SARA-KP 0.9m Optical Afterglow Detection	SARA	detection
Optical	Perley	30271	Liverpool telescope imaging of a red afterglow	Liverpool Telescope
Optical	de Ugarte Postigo et al.	30272	Redshift from OSIRIS/GTC	GTC	redshift
Optical	Zheng and Filippenko	30273	KAIT Optical Detection	KAIT	detection
Optical	Blazek et al.	30274	Liverpool Telescope r'-band observations	Liverpool Telescope	detection
Optical	Kann et al.	30275	CAHA 2.2m photometric monitoring	CAHA	detection
Optical	Xin et al.	30277	Xinglong TNT optical observations	TNT
Optical	Shrestha et al.	30280	Liverpool Telescope First Hour Observations	Liverpool Telescope
Optical	Jelinek et al.	30281	steepening of the decay observed at the D50	D50
Optical	Kumar et al.	30286	GIT optical follow-up and confirmation of jet-break	GROWTH-India Tel.	detection
Optical	D'Avanzo et al.	30288	REM optical/NIR afterglow detection	REM	detection
Optical	D'Avanzo et al.	30289	TNG NIR observations of the afterglow	TNG
Optical	Cunningham et al.	30290	SEDM Optical Observations	Spectral Energy Distribution Machine at Palomar
Optical	Moskvitin and Maslennikova	30291	SAO RAS optical observations	SAO RAS	detection
Optical		30292	iTelescope optical afterglow observation	iTelescope
Optical	Hu et al.	30293	1.5m OSN optical observation	Obs.de Sierra Nevada	detection
Optical	Zhu et al.	30294	NOT optical observations	NOT	detection
Optical	Belkin et al.	30299	AbAO optical observations	Abastumani Astro. Obs.	detection
Optical	Moskvitin and Maslennikova	30303	Further SAO RAS optical observations	SAO RAS	detection
Optical	Romanov and Lane	30305	Abbey Ridge Observatory optical afterglow observation	Abbey Ridge Obs.
Optical	Karpov et al.	30308	Mini-MegaTORTORA early optical observations	TORTORA	light curve
Optical	Moskvitin and Maslennikova	30309	Even further SAO RAS optical observations	SAO RAS	detection
Optical	Vinko et al.	30320	optical afterglow detection from Konkoly Observatory	Konkoly	detection
Optical	Kann et al.	30338	Deep CAHA 2.2m detection and late-time light curve behavior	CAHA	light curve
Optical	Zaznobin et al.	30343	Sayan observatory 1.6-m telescope observations	Sayan Obs.	detection
Optical	Belkin et al.	30791	Maidanak and Assy optical observations, broken power law parametrization	Assy-Turgen Obs.	detection
Radio	Smith et al.	30361	JCMT SCUBA-2 sub-mm observations	SCUBA
Radio	Laskar et al.	30386	ALMA detection	ALMA	detection
Gamma-ray	Zhao et al.	30264	GECAM detection	GECAM
Gamma-ray	Axelsson et al.	30270	Fermi-LAT detection	Fermi LAT	E_max=8.3 GeV
Gamma-ray	Svinkin et al.	30276	Konus-Wind detection	Konus-Wind	E_peak=261 (-13,+14) keV Fluence=4.60(-0.13,+0.13)x10^-4erg cm^-2
Gamma-ray	Poolakkil and Meegan	30279	Fermi GBM detection	Fermi GBM	E_peak=210±3 keV T₉₀=55 seconds Fluence=307.93±0.9830x10^-6erg cm^-2 (brighter than 99.9% of long GRBs)
Gamma-ray	Levin et al.	30283	SRG/ART-XC detection	SRG/ART
Gamma-ray	Kawakubo et al.	30284	CALET Gamma-Ray Burst Monitor detection	CALET
Gamma-ray	Minaev et al.	30304	SPI-ACS/INTEGRAL observations	INTEGRAL
Gamma-ray	Marisaldi et al.	30315	ASIM observation	Atmosphere-Space International Monitor
Other		30301	possibility of lensed visible afterglow?

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white	136	285	147	14.67 ± 0.02
white	794	814	19	16.66 ± 0.06
v	679	699	19	15.59 ± 0.10
b	604	624	20	16.39 ± 0.08
u	348	598	246	15.60 ± 0.03
w1	729	1107	39	17.22 ± 0.19
m2	704	1776	117	>19.3
w2	655	1726	136	19.20 ± 0.31

Table 3. UVOT observations reported by Kuin and D'Avanzo (GCN Circ. 30278). 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.

September 10, 2021