Swift Observations of GRB 200324A

S. Dichiara (NASA/GSFC/UMCP), V. D'Elia (ASDC) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 16:39:07 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200324A (trigger=963260) (Dichiara et al. GCN Circ. 27428). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 128° from the Sun (9.4 hours West) and 133° 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.

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 Ukwatta et al. (GCN Circ. 27443), the BAT ground-calculated position is RA, Dec = 222.682, 35.931 deg which is RA(J2000) = 14h50m43.7s Dec(J2000) = +35°55'51.0" with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows a weak pulse that starts at ~T-40 s, peaks at ~T0, and lasts until >~T+80 s. The burst was temporarily out of the BAT FOV from T+100.39 s to T+119.39 s. There might be additional burst emission during this period. Due to this data gap, the standard data analysis pipeline is unable to estimate an accurate T90.

The time-averaged spectrum from T-38.1 to T+100.4 s (before the burst went out of the BAT FOV) is best fit by a simple power-law model.

The power law index of the time-averaged spectrum is 1.72 ± 0.15. The fluence in the 15-150 keV band is 2.3 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 63% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.07 s in the 15-150 keV band is 0.7 ± 0.2 ph cm-2 s-1. 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/963260/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Elia et al. (GCN Circ. 27435). We have analysed 15 ks of XRT data for GRB 200324A, from 181 s to 120.9 ks after the BAT trigger. The data comprise 136 s in Windowed Timing (WT) mode (the first 1 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 27430).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=3.78 (+0.55, -0.23). At T+334 s the decay flattens to an α of 1.89 (+0.59, -0.23). The light curve breaks again at T+874 s to a decay with α=0.89 (+0.09, -0.10), before a final break at T+18.4 ks s after which the decay index is 1.9 (+0.6, -0.3).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.48 (+0.13, -0.11). The best-fitting absorption column is 1.49 (+0.28, -0.26) x 1021 cm-2, in excess of the Galactic value of 1.1 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.94 (+0.18, -0.17) and a best-fitting absorption column of 2.7 ± 0.7 x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.8 x 10-11 (5.3 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.7 ± 0.7 x 1021 cm-2
Galactic foreground: 1.1 x 1020 cm-2
Excess significance: 6.4 σ
Photon index: 1.94 (+0.18, -0.17)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00963260.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200324A 215 s after the BAT trigger (Breeveld and Dichiara GCN Circ. 27439). No optical afterglow consistent with the XRT position (Beardmore et al. GCN Circ. 27430) 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 0.01 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
14h50m41.73s +35°56'30.3" 1.4" XRT-final UKSSDC
14h50m41.70s +35°56'30.2" 1.7" XRT-enhanced Beardmore et al. GCN Circ. 27430
14h50m43.7s +35°55'51.0" 1.1' BAT-refined Ukwatta et al. GCN Circ. 27443

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 27427 Swift GRB200324.69: Global MASTER-Net
observations report 		MASTER
Optical Hosokawa et al. 27429 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Stecklum et al. 27431 Tautenburg observations Tautenburg
Optical Fynbo et al. 27432 NOT optical upper limit NOT upper limits
Optical Vinko et al. 27433 Konkoly Obs. optical upper limit Konkoly upper limits
Optical Strausbaugh and Cucchiara 27436 LCO Optical Upper Limit LCO upper limits
Optical Belkin et al. 27440 TSHAO optical upper limit Zeiss-1000 upper limits
Optical Nakamura and Sakamoto 27445 AROMA-N Optical Observation AROMA-N

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 215 365 147 >21.0
uFC 374 624 246 >20.6
white 215 1376 373 >21.7
v 704 1427 78 >18.9
b 630 1527 97 >19.8
u 374 1327 304 >20.2
w1 754 1477 78 >19.1
w2 1381 1401 19 >19.1

Table 3. UVOT observations reported by Breeveld and Dichiara (GCN Circ. 27439). 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.

March 27, 2020