Swift Observations of GRB 210411C

T. Sbarrato (INAF-OAB), J.D. Gropp (PSU) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 15:05:51 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210411C (trigger=1042398) (Sbarrato et al. GCN Circ. 29794). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 90° from the Sun (5.6 hours West) and 84° 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.

Sbarrato et al. (GCN Circ. 29794) reported the discovery with UVOT of an optical afterglow. de Ugarte Postigo et al. (GCN Circ. 29806) determined a redshift of 2.826 from VLT. 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 Markwardt et al. (GCN Circ. 29810), the BAT ground-calculated position is RA, Dec = 296.601, -39.399 deg which is RA(J2000) = 19^h46^m24.2^s Dec(J2000) = -39°23'57.8" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 79%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T₀ and ends at ~T+17 s. The two major peaks occur at ~T+4 s and ~T+11 s, respectively. T₉₀ (15-350 keV) is 12.80 ± 0.60 s (estimated error including systematics).

The time-averaged spectrum from T-0.45 to T+14.44 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.60 ± 0.40, and E_peak of 14.8 ± 10.8 keV (χ² 51.24 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 1.2 ± 0.1 x 10^-6 erg cm^-2 and the 1-s peak flux measured from T+3.87 s in the 15-150 keV band is 4.8 ± 0.3 ph cm^-2 s^-1. This fluence is larger than that of 45% 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 2.62 ± 0.09 (χ² 68.87 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/1042398/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Gropp et al. (GCN Circ. 29807). We have analysed 11 ks of XRT data for GRB 210411C, from 66 s to 231.2 ks after the BAT trigger. The data comprise 136 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 29800).

The late-time light curve (Figure 2) (from T0+5.4 ks) can be modelled with a power-law decay with a decay index of α=1.43 (+0.16, -0.15).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.73 (+0.17, -0.16). The best-fitting absorption column is 5.8 (+4.5, -4.1) x 10²¹ cm^-2, at a redshift of 2.826, in addition to the Galactic value of 8.1 x 10²⁰ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.88 (+0.13, -0.10) and a best-fitting absorption column of 1.1 (+4.9, -1.1) x 10²¹ cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10^-11 (4.4 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 8.1 x 10²⁰ cm^-2
Intrinsic column: 1.1 (+4.9, -1.1) x 10²¹ cm^-2 at z=2.826
Photon index: 1.88 (+0.13, -0.10)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01042398.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210411C 66 s after the BAT trigger (Kuin and Sbarrato GCN Circ. 29811). A source consistent with the XRT position (Goad et al. GCN Circ. 29800) 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 0.081 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

19^h46^m26.81^s -39°23'52.2" 0.43" UVOT-refined Kuin and Sbarrato GCN Circ. 29811

19^h46^m26.74^s -39°23'53.4" 1.9" XRT-final UKSSDC

19^h46^m26.69^s -39°23'52.6" 2.0" XRT-enhanced Goad et al. GCN Circ. 29800

19^h46^m24.2^s -39°23'57.8" 1.0' BAT-refined Markwardt et al. GCN Circ. 29810

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Strausbaugh and Cucchiara 29802 LCO Optical Afterglow Candidate LCO detection

Optical Hu et al. 29803 BOOTES-4/MET optical upper limit BOOTES-4 upper limits

Optical Lipunov et al. 29804 Swift GRB 210411C: Global MASTER-Net
observations report MASTER

Optical de Ugarte Postigo et al. 29806 VLT/X-shooter redshift VLT redshift

Optical Pessi et al. 29808 ePESSTO+ NTT optical observations PESSTO detection

Optical Belkin et al. 29809 Chilescope optical observations detection

X-ray Aimuratov et al. 29823 A very significant early X-ray afterglow light curve

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 66 216 147 17.16 ± 0.03

v 609 801 39 17.43 ± 0.18

b 535 555 20 17.82 ± 0.17

u 279 528 246 17.82 ± 0.07

w1 658 1280 78 >19.7

m2 633 5836 83 >19.6

w2 584 5600 255 >20.1

RA (J2000)	Dec (J2000)	Error	Note	Reference
19^h46^m26.81^s	-39°23'52.2"	0.43"	UVOT-refined	Kuin and Sbarrato GCN Circ. 29811
19^h46^m26.74^s	-39°23'53.4"	1.9"	XRT-final	UKSSDC
19^h46^m26.69^s	-39°23'52.6"	2.0"	XRT-enhanced	Goad et al. GCN Circ. 29800
19^h46^m24.2^s	-39°23'57.8"	1.0'	BAT-refined	Markwardt et al. GCN Circ. 29810

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Strausbaugh and Cucchiara	29802	LCO Optical Afterglow Candidate	LCO	detection
Optical	Hu et al.	29803	BOOTES-4/MET optical upper limit	BOOTES-4	upper limits
Optical	Lipunov et al.	29804	Swift GRB 210411C: Global MASTER-Net observations report	MASTER
Optical	de Ugarte Postigo et al.	29806	VLT/X-shooter redshift	VLT	redshift
Optical	Pessi et al.	29808	ePESSTO+ NTT optical observations	PESSTO	detection
Optical	Belkin et al.	29809	Chilescope optical observations		detection
X-ray	Aimuratov et al.	29823	A very significant early X-ray afterglow		light curve

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white	66	216	147	17.16 ± 0.03
v	609	801	39	17.43 ± 0.18
b	535	555	20	17.82 ± 0.17
u	279	528	246	17.82 ± 0.07
w1	658	1280	78	>19.7
m2	633	5836	83	>19.6
w2	584	5600	255	>20.1

Table 3. UVOT observations reported by Kuin and Sbarrato (GCN Circ. 29811). 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.

April 17, 2021