Swift Observations of GRB 200127A

M.J. Moss (GWU), J.A. Kennea (PSU) and A. Belles (PSU) for the Swift team

1. Introduction

At 14:44:31 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200127A (trigger=952791) (Moss et al. GCN Circ. 26895). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 65° from the Sun (0.9 hours West) and 71° from the 7%-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 Palmer et al. (GCN Circ. 26939), the BAT ground-calculated position is RA, Dec = 295.206, 44.934 deg which is RA(J2000) = 19h40m49.4s Dec(J2000) = +44°56'03.5" with an uncertainty of 2.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 50%.

The mask-weighted light curve (Figure 1) shows a long weak emission that starts around ~T-150 s and ends at ~T+90 s. T90 (15-350 keV) is 216.9 ± 23.5 s (estimated error including systematics).

The time-averaged spectrum from T-144.22 to T+92.97 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.64 ± 0.19. The fluence in the 15-150 keV band is 2.4 ± 0.3 x 10-6 erg cm-2. This fluence is larger than that of 65% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+41.80 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/952791/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Kennea et al. (GCN Circ. 26901).

We have analysed 8.3 ks of XRT data for GRB 200127A, from 130 s to 18.2 ks after the BAT trigger. The data comprise 215 s in Windowed Timing (WT) mode (the first 9 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode.

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=1.1 (+0.6, -2.6). At T+172 s the decay steepens to an α of 2.58 (+0.18, -0.15) before breaking again at T+648 s to a final decay with index α=1.32 (+0.09, -0.12).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.98 (+0.05, -0.04). The best-fitting absorption column is consistent with the Galactic value of 2.2 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.75 (+0.12, -0.11) and a best-fitting absorption column consistent with the Galactic value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.1 x 10-11 (5.1 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.2 (+0.3, -0.0) x 1021 cm-2
Galactic foreground: 2.2 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 1.75 (+0.12, -0.11)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00952791.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200127A 150 s after the BAT trigger (Belles and Moss GCN Circ. 26919). No optical afterglow consistent with the XRT position (Evans et al., GCN Circ. 26902) 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 0.16 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
19h40m46.32s +44°54'01.0" 1.4" XRT-final UKSSDC
19h40m46.32s +44°54'01.0" 1.4" XRT-enhanced Evans et al. GCN Circ. 26902
19h40m49.4s +44°56'03.5" 2.4' BAT-refined Palmer et al. GCN Circ. 26939

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 26896 Swift GRB200127.61: Global MASTER-Net
observations report 		MASTER
Optical Belkin et al. 26899 AbAO optical upper limit Abastumani Astro. Obs. upper limits
Optical Liu et al. 26904 Nanshan-0.6m optical upper limit Xinjiang Astro. Obs. upper limits
Optical Fynbo et al. 26913 Nordic Optical telecope optical upper
limit		 upper limits
Optical Belkin et al. 26914 Mondy optical upper limit Mondy 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 150 300 147 >20.4
uFC 310 560 246 >19.4
white 150 11742 1368 >22.3
v 640 16568 1317 >20.2
b 566 10830 1121 >21.3
u 310 6701 638 >20.2
w1 690 18173 1095 >21.1
m2 838 17473 1298 >20.6
w2 615 12437 1110 >20.5

Table 3. UVOT observations reported by Belles and Moss (GCN Circ. 26919). 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.

January 30, 2020