Swift Observations of GRB 200528A

E. Ambrosi (INAF-IASFPA), M. Perri (ASDC) and F.E. Marshall (NASA/GSFC) for the Swift team

1. Introduction

At 10:27:23 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200528A (trigger=974827) (Ambrosi et al. GCN Circ. 27832). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 82° from the Sun (7.4 hours East) and 47° from the 31%-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 Barthelmy et al. (GCN Circ. 27839), the BAT ground-calculated position is RA, Dec = 176.637, 58.185 deg which is RA(J2000) = 11h46m32.8s Dec(J2000) = +58°11'07.3" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 62%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure rising just before T0, peaking around T+35 s and decaying to background by T+120 s. T90 (15-350 keV) is 58.7 ± 1.4 s (estimated error including systematics).

The time-averaged spectrum from T-1.02 to T+112.88 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.62 ± 0.14, and Epeak of 40.1 ± 5.5 keV (χ2 25.28 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 1.8 ± 0.0 x 10-5 erg cm-2 and the 1-s peak flux measured from T+33.82 s in the 15-150 keV band is 17.5 ± 0.5 ph cm-2 s-1. This fluence is larger than that of 96.4% 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 2.14 ± 0.03 (χ2 69.65 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/974827/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Perri et al. (GCN Circ. 27852). We have analysed 15 ks of XRT data for GRB 200528A, from 73 s to 344.2 ks after the BAT trigger. The data comprise 226 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 27836).

The late-time light curve (Figure 2) (from T0+5.1 ks) can be modelled with a power-law decay with a decay index of α=1.09 ± 0.05.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.30 ± 0.05. The best-fitting absorption column is 3.63 (+0.19, -0.18) x 1021 cm-2, in excess of the Galactic value of 1.2 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.24 (+0.16, -0.15) and a best-fitting absorption column of 3.8 (+0.7, -0.6) 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 (6.1 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 3.8 (+0.7, -0.6) x 1021 cm-2
Galactic foreground: 1.2 x 1020 cm-2
Excess significance: 9.8 σ
Photon index: 2.24 (+0.16, -0.15)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00974827.

4. UVOT Observations and Analysis

Analysis of the UVOT data was reported by Marshall and Ambrosi (GCN Circ. 27840). 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
11h46m32.99s +58°11'33.3" 1.5" XRT-final UKSSDC
11h46m32.99s +58°11'33.3" 1.8" XRT-enhanced Evans et al. GCN Circ. 27836
11h46m32.8s +58°11'07.3" 1.0' BAT-refined Barthelmy et al. GCN Circ. 27839

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Adachi et al. 27834 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Lipunov et al. 27835 Swift GRB 200528A: Global MASTER-Net
observations report 		MASTER
Optical Zhu et al. 27837 Nanshan/NEXT optical upper limit Xinjiang Astro. Obs. upper limits
Optical Kumar et al. 27838 1.3m DFOT Optical Upper Limits Devasthal Opt.Tel. upper limits
Optical Malesani et al. 27842 NOT optical upper limits NOT upper limits
Optical Belkin et al. 27849 Assy optical upper limit upper limits
Optical Kann et al. 27860 CAHA Upper Limit CAHA upper limits
Gamma-ray Fermi 27831 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Fletcher et al. 27841 Fermi GBM Observation Fermi GBM Epeak=55.5±0.8 keV
T90=56 seconds
Fluence=2.54±0.02x10-5erg cm-2
(89th percentile for long GRBs)
Gamma-ray Pal'shin et al. 27855 CALET Gamma-Ray Burst Monitor detection CALET
Gamma-ray Tsvetkova et al. 27883 Konus-Wind observation Konus-Wind Epeak=64 (-5,+5) keV
Fluence=2.12(-0.07,+0.07)x10-5erg cm-2
Other de Ugarte Postigo and Kann 27833 Potential host galaxy in PanSTARRS,
SDSS and WISE		 detection

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 81 231 147 >21.1
u 293 305 12 >18.7
v 5743 5942 197 >18.8
b 5127 5327 169 >20.7
uvw2 5538 5738 197 >21.0

Table 3. UVOT observations reported by Marshall and Ambrosi (GCN Circ. 27840). 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.

June 3, 2020