Swift Observations of GRB 210402A

K.L. Page (U. Leicester), P.A. Evans (U. Leicester) and S.J. LaPorte (PSU) for the Swift team

1. Introduction

Page et al. (GCN Circ. 29743) reported the initial Swift results. BAT has triggered on a new GRB 210402A (Trigger #1040123) causing Swift to slew to the location and make follow-up observations. 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.

Xu et al. (GCN Circ. 29757) reported the position from NOT for the optical afterglow of this GRB. 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 Laha et al. (GCN Circ. 29761), the BAT ground-calculated position is RA, Dec = 198.265, 44.510 deg which is RA(J2000) = 13h13m03.7s Dec(J2000) = +44°30'36.1" with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment). The partial coding was 69%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T-50 s and ends at ~T+310 s. The main peak occurs at ~T+174 s. T90 (15-350 keV) is 238.7 ± 18.8 s (estimated error including systematics).

The time-averaged spectrum from T-51.60 to T+310.20 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.49 ± 0.27, and Epeak of 63.6 ± 22.9 keV (χ2 58.82 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 8.6 ± 0.4 x 10-6 erg cm-2 and the 1-s peak flux measured from T+173.53 s in the 15-150 keV band is 1.6 ± 0.2 ph cm-2 s-1. This fluence is larger than that of 91.0% 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 1.87 ± 0.06 (χ2 65.10 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/1040123/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 29750). We have analysed 24 ks of XRT data for GRB 210402A, from 160 s to 339.9 ks after the BAT trigger. The data comprise 1.1 ks in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode.

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=0.93 ± 0.13.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.811 (+0.027, -0.026). The best-fitting absorption column is 3.25 ± 0.13 x 1021 cm-2, in excess of the Galactic value of 1.5 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.19 (+0.20, -0.19) and a best-fitting absorption column of 3.5 ± 0.8 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 (5.8 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 3.5 ± 0.8 x 1021 cm-2
Galactic foreground: 1.5 x 1020 cm-2
Excess significance: 7.3 σ
Photon index: 2.19 (+0.20, -0.19)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01040123.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210402A 73 s after the BAT trigger (LaPorte and Page GCN Circ. 29764). No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 29750) 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 0.018 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
13h13m02.22s +44°30'52.3" 2.0" XRT-final UKSSDC
13h13m02.22s +44°30'52.3" 2.0" XRT-refined Evans et al. GCN Circ. 29750
13h13m03.7s +44°30'36.1" 1.2' BAT-refined Laha et al. GCN Circ. 29761

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 29741 Swift GRB210402.92: Global MASTER-Net
observations report 		MASTER
Optical Zhu et al. 29745 NOT optical upper limits NOT upper limits
Optical Leonini et al. 29747 Montarrenti Observatory upper limit Montarrenti upper limits
Optical Butler et al. 29752 DDOTI Upper Limits Deca-Degree Optical Imager upper limits
Optical Rossi 29753 LBT observations LBT
Optical Hu et al. 29755 BOOTES-network early optical upper limit BOOTES upper limits
Optical Xu et al. 29757 NOT optical afterglow candidate NOT detection
Optical Zhu et al. 29762 Further NOT optical observations NOT detection
Optical Zhirkov et al. 29765 MASTER prompt and follow up optical
observation 		MASTER
Gamma-ray Roberts and Meegan 29763 Fermi GBM detection Fermi GBM Fluence=1.699±0.008x10-6erg cm-2

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
whiteFC 163 313 147 >20.8
uFC 321 571 246 >20.3
white 163 1718 411 >21.4
v 653 5581 229 >19.6
b 578 1694 117 >20.2
u 321 1669 304 >20.2
w1 1108 1644 78 >19.2
w2 1207 1227 19 >18.4

Table 3. UVOT observations reported by LaPorte and Page (GCN Circ. 29764). The start and stop times of the exposures are given in seconds since the BAT trigger. The preliminary 3-σ upper limits are given. No correction has been made for extinction in the Milky Way.

April 7, 2021