Swift Observations of GRB 210610A

K.L. Page (U. Leicester), D.N. Burrows (PSU) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 15:03:43 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210610A (trigger=1054627) (Page et al. GCN Circ. 30160). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 115° from the Sun (8.4 hours East) and 113° from the 0%-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.

Page et al. (GCN Circ. 30160) reported the discovery with UVOT of an optical afterglow. Zhu et al. (GCN Circ. 30164) determined a redshift of 3.54 from Xinglong, and Dutta et al. (GCN Circ. 30200) determined a redshift of 3.5. 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 Barthelmy et al. (GCN Circ. 30186), the BAT ground-calculated position is RA, Dec = 204.284, 14.476 deg which is RA(J2000) = 13h37m08.2s Dec(J2000) = +14°28'32.5" with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 26%.

The mask-weighted light curve (Figure 1) shows a structure with two overlapping pulses that starts at ~T-6 s and ends at ~T+11 s. The two peaks occur at ~T-2 s and ~T0, respectively. T90 (15-350 keV) is 13.62 ± 3.15 s (estimated error including systematics).

The time-averaged spectrum from T-5.98 to T+11.35 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.41 ± 0.19. The fluence in the 15-150 keV band is 1.0 ± 0.1 x 10-6 erg cm-2. This fluence is larger than that of 38% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.29 s in the 15-150 keV band is 2.5 ± 0.4 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/1054627/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Burrows et al. (GCN Circ. 30192). We have analysed 22 ks of XRT data for GRB 210610A, from 97 s to 457.5 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 30165).

The late-time light curve (Figure 2) (from T0+4.1 ks) can be modelled with a power-law decay with a decay index of α=1.12 (+0.08, -0.07).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.84 (+0.12, -0.10). The best-fitting absorption column is 2.3 (+8.9, -2.3) x 1021 cm-2, at a redshift of 3.54, in addition to the Galactic value of 2.2 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 3.5 x 10-11 (3.7 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 2.2 x 1020 cm-2
Intrinsic column: 2.3 (+8.9, -2.3) x 1021 cm-2 at z=3.54
Photon index: 1.84 (+0.12, -0.10)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01054627.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210610A 94 s after the BAT trigger (Siegel et al. GCN Circ. 30246). A fading source consistent with the XRT position (Goad et al. GCN Circ. 30165) and the previously reported optical counterpart (Hosokawa et al., GCN. 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 0.032 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
13h37m07.59s +14°27'55.0" 0.43" UVOT-refined Siegel et al. GCN Circ. 30246
13h37m07.74s +14°27'55.4" 2.0" XRT-final UKSSDC
13h37m07.71s +14°27'55.4" 2.3" XRT-enhanced Goad et al. GCN Circ. 30165
13h37m08.2s +14°28'32.5" 1.7' BAT-refined Barthelmy et al. GCN Circ. 30186

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Hosokawa et al. 30161 MITSuME Akeno optical observation MITSuME Akeno detection
Optical Xu et al. 30162 Xinglong-2.16m optical afterglow
observations		 Xinglong detection
Optical Kumar et al. 30163 GIT optical detection GROWTH-India Tel. detection
Optical Zhu et al. 30164 Xinglong-2.16m redshift Xinglong redshift
Optical Lipunov et al. 30166 Swift GRB 210610A: MASTER optical
counterpart observation		 MASTER detection
Optical Lipunov et al. 30167 Swift GRB 210610A: Global MASTER-Net
observations report 		MASTER
Optical de Wet et al. 30168 MeerLICHT multi-colour photometry MeerLICHT detection
Optical Horiuchi et al. 30169 MITSuME Ishigaki optical observation MITSuME detection
Optical Lipunov et al. 30173 Swift GRB210610.83: Global MASTER-Net
OT detection		 MASTER detection
Optical Sun et al. 30185 1.5m OSN optical observation Obs.de Sierra Nevada
Optical Watson et al. 30191 RATIR Optical and NIR Detection RATIR detection
Optical Zheng and Filippenko 30203 KAIT Optical Detection KAIT detection
Optical Kann et al. 30211 CAHA 2.2m observations and light-curve
behavior		 CAHA detection
Optical Pankov et al. 30212 CrAO/ZTSH optical observations CrAO detection
Optical Belkin et al. 30214 CrAO/ZTSH optical observations CrAO detection
Optical Moskvitin and Maslennikova 30229 SAO RAS optical observations SAO RAS detection
Optical Kann et al. 30232 CAHA 2.2m Second Epoch CAHA detection
Optical Belkin et al. 30775 Maidanak and AbAO optical observations Abastumani Astro. Obs. detection
Radio Smith et al. 30359 JCMT SCUBA-2 sub-mm observations SCUBA
X-ray Dutta et al. 30200 Spectroscopy and redshift confirmation
with the Himalayan Chandra Telescope		 Chandra spectroscopy
Gamma-ray Frederiks et al. 30197 Konus-Wind detection Konus-Wind Epeak=148 (-63,+87) keV
Gamma-ray Veres et al. 30233 Fermi GBM observations Fermi GBM Epeak=247±74 keV
T90=8 seconds
Fluence=1.78±0.23x10-6erg cm-2
(27th percentile for long GRBs)

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 (fc) 94 244 147 17.09±0.03
white 587 892 68 17.92±0.07
v 636 829 38 16.74±0.14
v 4299 4498 196 >18.92
b 562 754 38 17.69±0.12
b 16163 17070 885 20.57±0.23
b 23125 45112 1552 >21.16
u 307 557 245 >20.02
uvw1 685 4908 216 >19.37
uvm2 4503 4703 196 >19.16
uvw2 4094 4294 196 >19.10

Table 3. UVOT observations reported by Siegel et al. (GCN Circ. 30246). 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 6, 2021