Swift Observations of GRB 200925B

S.B. Cenko (GSFC), B. Sbarufatti (PSU) and S.R. Oates (U.Birmingham) for the Swift team

1. Introduction

At 21:50:37 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200925B (trigger=997453) (Cenko et al. GCN Circ. 28499). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 86° from the Sun (4.2 hours East) and 106° from the 69%-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.

Oates and Cenko (GCN Circ. 28507) reported the detection with UVOT of an optical afterglow. 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 Stamatikos et al. (GCN Circ. 28506), the BAT ground-calculated position is RA, Dec = 246.892, 78.393 deg which is RA(J2000) = 16h27m34.1s Dec(J2000) = +78°23'33.5" with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment). The partial coding was 50%.

The mask-weighted light curve (Figure 1) shows two peaks, the first beginning ~T-2 s, peaking at T+5 s, and decaying by T+12 s. The second, weaker, peak runs from T+16 to T+22 s. T90 (15-350 keV) is 18.25 ± 0.97 s (estimated error including systematics).

The time-averaged spectrum from T+0.20 to T+19.59 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.01 ± 0.14. The fluence in the 15-150 keV band is 1.0 ± 0.1 x 10-6 erg cm-2. This fluence is larger than that of 38% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+4.38 s in the 15-150 keV band is 3.0 ± 0.3 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/997453/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Sbarufatti et al. (GCN Circ. 28508). We have analysed 10 ks of XRT data for GRB 200925B, from 87 s to 86.4 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 28500).

The late-time light curve (Figure 2) (from T0+29.2 ks) can be modelled with a power-law decay with a decay index of α=1.1 ± 0.4.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.12 (+0.19, -0.18). The best-fitting absorption column is 7.5 (+4.4, -2.9) x 1020 cm-2, consistent with the Galactic value of 4.6 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 3.1 x 10-11 (3.8 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 7.5 (+4.4, -2.9) x 1020 cm-2
Galactic foreground: 4.6 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 2.12 (+0.19, -0.18)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00997453.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200925B 82 s after the BAT trigger (Oates and Cenko GCN Circ. 28507). A source consistent with the XRT position (Beardmore et al. GCN Circ. 28503) and the optical position reported by the OSN (de Ugarte Postigo et al., GCN Circ. 28501) and MASTER (Lipunov et al., GCN Circ. 28502) 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 0.04 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
16h27m08.58s +78°23'26.4" 1.4" XRT-final UKSSDC
16h27m08.45s +78°23'25.9" 1.8" XRT-enhanced Beardmore et al. GCN Circ. 28503
16h27m34.1s +78°23'33.5" 1.2' BAT-refined Stamatikos et al. GCN Circ. 28506

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical de Ugarte Postigo et al. 28501 OSN Afterglow Candidate Discovery Obs.de Sierra Nevada detection
Optical Lipunov et al. 28502 Swift GRB 200925B: Global MASTER-Net
OT detection		 MASTER detection
Optical Grossan et al. 28511 NUTTelA-TAO / BSTI Early Measurements
(Preliminary)		 NUTTelA

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 82 232 147 19.80 ± 0.20
white 574 1714 264 20.77 ± 0.37
v 623 1763 136 >18.9
b 549 1689 117 >19.6
uFC 294 544 246 >19.7
u 294 1663 343 >19.9
w1 672 1813 136 >19.0
m2 1249 1441 39 >17.7
w2 599 1739 117 >18.9

Table 3. UVOT observations reported by Oates and Cenko (GCN Circ. 28507). 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.

September 28, 2020