Swift Observations of GRB 210610B

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

1. Introduction

At 19:51:27 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210610B (trigger=1054681) (Page et al. GCN Circ. 30170). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 140° from the Sun (11.0 hours East) and 137° 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. 30170) reported the discovery with UVOT of an optical afterglow. Fynbo et al. (GCN Circ. 30182) determined a redshift of 1.13 from NOT, Fu et al. (GCN Circ. 30188) determined a redshift of 1.13, de Ugarte Postigo et al. (GCN Circ. 30194) determined a redshift of 1.1345 from GTC, and Dutta et al. (GCN Circ. 30201) determined a redshift of 1.13. 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 Krimm et al. (GCN Circ. 30207), the BAT ground-calculated position is RA, Dec = 243.929, 14.398 deg which is RA(J2000) = 16h15m42.8s Dec(J2000) = +14°23'54.5" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 24%.

The mask-weighted light curve (Figure 1) shows several overlapping pulses that start at ~T-12 and end at ~T+140 s. The main peak occurs at ~T+8 s. T90 (15-350 keV) is 69.38 ± 2.53 s (estimated error including systematics).

The time-averaged spectrum from T-12.04 to T+142.47 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.98 ± 0.11, and Epeak of 339.3 ± 218.6 keV (χ2 27.43 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 3.6 ± 0.1 x 10-5 erg cm-2 and the 1-s peak flux measured from T+7.95 s in the 15-150 keV band is 13.5 ± 0.7 ph cm-2 s-1. This fluence is larger than that of 99.3% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). A fit to a simple power law gives a photon index of 1.17 ± 0.03 (χ2 36.54 for 57 d.o.f.). 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/1054681/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Gropp et al. (GCN Circ. 30208). We have analysed 5.8 ks of XRT data for GRB 210610B, from 87 s to 81.7 ks after the BAT trigger. The data comprise 1.3 ks in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode.

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=1.13 (+0.23, -0.34). At T+133 s the decay steepens to an α of 1.86 (+0.06, -0.05). The light curve breaks again at T+397 s to a decay with α=0.77 (+0.06, -0.05), before a final break at T+1153 s s after which the decay index is 1.105 (+0.023, -0.022).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.847 ± 0.022. The best-fitting absorption column is 4.4 ± 2.5 x 1020 cm-2, at a redshift of 1.13, in addition to the Galactic value of 3.9 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.92 ± 0.12 and a best-fitting absorption column of 1.1 (+1.4, -1.1) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.4 x 10-11 (3.9 x 10-11) erg cm-2 count-1.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210610B 92 s after the BAT trigger (Siegel et al. GCN Circ. 30247). A fading source consistent with the XRT position (Osborne et al. GCN Circ. 30189) and the previously reported optical counterpart (Kumar et al., GCN Circ. 30174; Rumyantsev et al. GCN Circ. 30175; Hu et al., GCN Circ. 30177; de Wet et al., GCN Circ. 30180; Romanov, GCN Circ. 30181; Fynbo et al., GCN Circ. 30182; Moskvitin, GCN Circ. 30187; Fu et al., GCN Circ. 30188) 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 0.044 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
16h15m40.40s +14°23'56.9" 0.42" UVOT-refined Siegel et al. GCN Circ. 30247
16h15m40.47s +14°23'56.5" 1.5" XRT-final UKSSDC
16h15m40.52s +14°23'55.7" 2.2" XRT-enhanced Osborne et al. GCN Circ. 30189
16h15m42.8s +14°23'54.5" 1.0' BAT-refined Krimm et al. GCN Circ. 30207

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Kumar et al. 30174 GIT optical detection GROWTH-India Tel. detection
Optical Rumyantsev et al. 30175 CrAO/ZTSh optical observations CrAO detection
Optical Lipunov et al. 30176 Swift GRB 210610B: Global MASTER-Net
observations report 		MASTER
Optical Hu et al. 30177 BOOTES-2/TELMA optical observation BOOTES-2 detection
Optical Rumyantsev et al. 30178 CrAO/ZTSh optical afterglow photometry CrAO
Optical de Wet et al. 30180 MeerLICHT multi-colour photometry MeerLICHT
Optical 30181 iTelescope optical afterglow
observations		 iTelescope
Optical Fynbo et al. 30182 NOT optical observations and tentative
redshift		 NOT redshift
Optical Moskvitin 30187 SAO RAS optical observations SAO RAS detection
Optical Becerra et al. 30190 RATIR Optical and NIR Observations RATIR
Optical Mong et al. 30193 GOTO confirmation of afterglow detection Gravitational-wave Optical Transient Observer detection
Optical de Ugarte Postigo et al. 30194 Redshift confirmation from GTC GTC redshift
Optical Marchini et al. 30198 University of Siena Observatory optical
photometry of the afterglow		 detection
Optical Zheng and Filippenko 30204 KAIT Optical Detection KAIT detection
Optical Vreeswijk 30205 optical afterglow observations
Optical Pankov et al. 30213 CrAO/ZTSH optical observations
(correction to the GCN Circ. 30212)		 CrAO detection
Optical Pankov et al. 30215 CrAO/ZTSH optical observations and
light curve		 CrAO light curve
Optical Perley 30216 Liverpool Telescope optical photometry Liverpool Telescope
Optical Noto et al. 30220 MITSuME Akeno optical observation MITSuME Akeno detection
Optical Anandagoda and Pellegrin 30221 SARA-KP 0.9m Optical Afterglow Detection SARA detection
Optical Ibrahimov et al. 30226 optical photometry of afterglow at
INASAN/Simeiz
Optical Zheng and Filippenko 30227 Continued KAIT Optical Detection KAIT detection
Optical Romanov and Lane 30228 optical observations from Burke-Gaffney
Observatory and Abbey Ridge Observatory		 Abbey Ridge Obs.
Optical Moskvitin and Maslennikova 30230 Further SAO RAS optical observations SAO RAS detection
Optical Nissinen and Oksanen 30231 iTelescope optical afterglow
observations		 iTelescope detection
Optical D'Avanzo et al. 30238 REM optical/NIR observations of the
afterglow		 REM detection
Optical Pankov et al. 30243 AbAO optical observations Abastumani Astro. Obs. detection
Optical Pankov et al. 30245 CrAO/ZTSH continued optical
observations and light curve		 CrAO light curve
Optical G et al. 30316 T-CAT optical observations detection
Optical Kim et al. 30614 Assy optical observations Assy-Turgen Obs. detection
Optical Pankov et al. 30988 Sintesz-Newton/CrAO optical observations CrAO detection
Radio Laskar et al. 30217 ALMA detection ALMA detection
Radio Alexander et al. 30218 VLA detection VLA detection
Radio Smith et al. 30360 JCMT SCUBA-2 sub-mm observations SCUBA
X-ray Dutta et al. 30201 Spectroscopy and redshift confirmation
with the Himalayan Chandra Telescope		 Chandra spectroscopy
Gamma-ray Biltzinger et al. 30171 BALROG localization (Fermi Trigger
645047470 / GRB 210610827)		 BALROG
Gamma-ray Ursi et al. 30195 AGILE detection AGILE
Gamma-ray Frederiks et al. 30196 Konus-Wind detection Konus-Wind Epeak=257 (-14,+15) keV
Gamma-ray Malacaria and Hristov 30199 Fermi GBM detection Fermi GBM Epeak=414.3±11.7 keV
T90=55 seconds
Fluence=1.73±0.03x10-5erg cm-2
(brighter than 85% of long GRBs)
Other Fu et al. 30188 Photometry and phot-redshift from
Legacy Survey, PanSTAR, and SDSS		 redshift
Other Perley et al. 30206 Zwicky Transient Facility afterglow
detection		 detection
correction to GCN Circ. 30181 corrects Circ. 30181

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) 92 242 147 13.63±1.10
white 584 1355 225 15.24±0.02
white 172034 172423 377 20.22±0.14
white 286441 292869 328 20.71±0.23
white 452008 515231 4066 21.69±0.16
v 634 1405 97 15.63±0.05
v 6631 51903 659 18.26±0.09
b 560 1331 77 15.78±0.04
b 81150 133027 865 20.08±0.14
u (fc) 304 554 245 13.86±0.03
u 707 1306 58 14.93±0.04
u 4850 5040 186 16.68±0.05
u 45786 46567 761 18.06±0.05
u 131743 132650 885 20.00±0.18
u 365977 418267 3970 21.42±0.29
uvw1 683 1282 58 14.51±0.06
uvw1 4645 4845 196 16.26±0.07
uvw1 7041 45779 989 17.54±0.06
uvw1 125974 137480 881 19.53±0.20
uvw1 200561 247267 1111 19.95±0.24
uvm2 6836 7036 196 16.61±0.10
uvw2 783 1034 38 15.05±0.09
uvw2 50521 51421 885 18.44±0.10

Table 3. UVOT observations reported by Siegel et al. (GCN Circ. 30247). 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.

October 26, 2021