Swift Observations of GRB 210509A

K.L. Page (U. Leicester), J.A. Kennea (PSU) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 14:32:20 UT, the Swift Burst Alert Telescope (BAT) triggered and located a GRB 210509A (trigger= 1047531) (Page et al. GCN Circ. 29970). Swift did not slew immediately due to an observing constraint. At the time of the trigger, the initial BAT position was 56° from the Sun (3.7 hours West) and 34° from the 4%-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.

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 Lien et al. (GCN Circ. 29975), the BAT ground-calculated position is RA, Dec = 350.363, 6.650 deg which is RA(J2000) = 23h21m27.2s Dec(J2000) = +06°39'01.1" with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment). The partial coding was 95%.

The mask-weighted light curve (Figure 1) shows several overlapping pulses that start at ~T-2 s and end at ~T+8 s. The burst went out of the BAT FOV at ~T+77 s. T90 (15-350 keV) is 8.22 ± 0.75 s (estimated error including systematics).

The time-averaged spectrum from T-1.65 to T+7.63 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.76 ± 1.00, and Epeak of 33.0 ± 8.3 keV (χ2 70.16 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 3.1 ± 0.4 x 10-7 erg cm-2 and the 1-s peak flux measured from T+3.24 s in the 15-150 keV band is 1.2 ± 0.2 ph cm-2 s-1. This fluence is larger than that of 13% 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 2.18 ± 0.17 (χ2 78.23 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/1047531/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Kennea et al. (GCN Circ. 29977). We have analysed 9.8 ks of XRT data for GRB 210509A, from 3.2 ks to 79.5 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. 29973).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.12 (+0.17, -0.15).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.63 (+0.36, -0.16). The best-fitting absorption column is consistent with the Galactic value of 7.1 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 4.0 x 10-11 (4.4 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 7.1 ± 8.9 x 1020 cm-2
Galactic foreground: 7.1 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 1.63 (+0.36, -0.16)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01047531.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210509A 3202 s after the BAT trigger (Kuin and Page GCN Circ. 29978). No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 29971) 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 0.074 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
23h21m30.15s +06°40'16.5" 2.5" XRT-final UKSSDC
23h21m30.15s +06°40'16.5" 2.5" XRT-enhanced Goad et al. GCN Circ. 29973
23h21m27.2s +06°39'01.1" 1.5' BAT-refined Lien et al. GCN Circ. 29975

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Sun et al. 29972 BOOTES-4/MET optical upper limit BOOTES-4 upper limits
Optical Lipunov et al. 29974 Swift GRB 210509A: Global MASTER-Net
observations report 		MASTER
Optical Watson et al. 29979 RATIR Optical and NIR Upper Limits RATIR upper limits

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 3202 3352 147 >20.8
white 3202 4585 344 >21.0
v 3358 4934 332 >19.0
b 4180 4380 197 >19.5
u 3974 4174 197 >19.6
w1 3769 3969 197 >19.5
m2 3563 3763 197 >20.3
w2 4592 4792 197 >19.6

Table 3. UVOT observations reported by Kuin and Page (GCN Circ. 29978). 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.

May 11, 2021