Swift Observations of GRB 210606A

B. Sbarufatti (PSU) for the Swift team

1. Introduction

Poolakkil and Meegan (GCN Circ. 30210) reported the GRB detection. At 03:56:02.12 UT on 6 June 2021, the Fermi Gamma-Ray Burst Monitor (GBM) triggered and located GRB 210606A (trigger 644644567 / 210606164) which was also detected by the Swift/BAT-GUANO (DeLaunay et al. 2021, GCN Circ. 30134) The Fermi GBM Final Real-time Localization (GCN Circ. 30131) is consistent with the Swift position. Table 1 contains the best reported positions from Swift.

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. 30172). Four uncatalogued X-ray sources are detected consistent with being within 394 arcsec of the Swift/BAT position, of which one ("Source 1") is fading with 4.6-sigma significance, and is therefore likely the GRB afterglow.

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.1 (+0.9, -0.6).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.8 (+0.5, -0.4). The best-fitting absorption column is 1.4 (+1.6, -1.0) x 10²¹ cm^-2, consistent with the Galactic value of 3.8 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 (4.3 x 10^-11) erg cm^-2 count^-1.

4. UVOT Observations and Analysis

UVOT results are not available.

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

11^h23^m45.86^s +00°48'46.5" 2.3" XRT-final UKSSDC

11^h23^m45.86^s +00°48'46.5" 2.3" XRT Sbarufatti et al. GCN Circ. 30172

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Lipunov et al. 30132 Fermi GRB 210606A: Global MASTER-Net
observations report MASTER

Optical Pereyra et al. 30135 Fermi GRB 210606A: DDOTI Upper Limits
on the Afterglow Deca-Degree Optical Imager upper limits

Optical Sun et al. 30141 1.5m OSN optical upper limit Obs.de Sierra Nevada upper limits

Gamma-ray Fermi 30131 Fermi GBM Final Real-time Localization Fermi GBM

Gamma-ray Poolakkil and Meegan 30210 Fermi GBM detection Fermi GBM E_peak=195±62 keV
T₉₀=11 seconds
Fluence=0.845±0.144x10^-6erg cm^-2
(11^st percentile for long GRBs)

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

RA (J2000)	Dec (J2000)	Error	Note	Reference
11^h23^m45.86^s	+00°48'46.5"	2.3"	XRT-final	UKSSDC
11^h23^m45.86^s	+00°48'46.5"	2.3"	XRT	Sbarufatti et al. GCN Circ. 30172

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Lipunov et al.	30132	Fermi GRB 210606A: Global MASTER-Net observations report	MASTER
Optical	Pereyra et al.	30135	Fermi GRB 210606A: DDOTI Upper Limits on the Afterglow	Deca-Degree Optical Imager	upper limits
Optical	Sun et al.	30141	1.5m OSN optical upper limit	Obs.de Sierra Nevada	upper limits
Gamma-ray	Fermi	30131	Fermi GBM Final Real-time Localization	Fermi GBM
Gamma-ray	Poolakkil and Meegan	30210	Fermi GBM detection	Fermi GBM	E_peak=195±62 keV T₉₀=11 seconds Fluence=0.845±0.144x10^-6erg cm^-2 (11^st percentile for long GRBs)

June 12, 2021