Swift Observations of GRB 180925A

J.D. Gropp (PSU), B. Sbarufatti (INAF-OAB/PSU) and S.W.K. Emery (UCL-MSSL) for the Swift team

1. Introduction

At 14:37:31 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180925A (trigger=863421) (Gropp et al. GCN Circ. 23268). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 106° from the Sun (8.9 hours East) and 74° from the 100%-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.

Emery and Gropp (GCN Circ. 23274) 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 Ukwatta et al. (GCN Circ. 23273), the BAT ground-calculated position is RA, Dec = 315.212, -64.450 deg which is RA(J2000) = 21h00m50.9s Dec(J2000) = -64°26'59.2" with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment). The partial coding was 15%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T-5 s and ends at ~T+105 s. The main peak occurs at ~ T+1 s. T90 (15-350 keV) is 81.70 ± 23.25 s (estimated error including systematics).

The time-averaged spectrum from T-4.44 to T+103.96 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.53 ± 0.13. The fluence in the 15-150 keV band is 4.1 ± 0.3 x 10-6 erg cm-2. This fluence is larger than that of 78% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.87 s in the 15-150 keV band is 3.1 ± 0.6 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/863421/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Sbarufatti et al. (GCN Circ. 23272). We have analysed 13 ks of XRT data for GRB 180925A, from 115 s to 104.8 ks after the BAT trigger. The data comprise 191 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 23270).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=1.87 (+0.12, -0.11). At T+265 s the decay steepens to an α of 5.8 (+0.9, -0.5) before breaking again at T+385 s to a final decay with index α=1.22 ± 0.06.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.42 ± 0.08. The best-fitting absorption column is 1.3 ± 0.4 x 1021 cm-2, in excess of the Galactic value of 3.3 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.77 ± 0.17 and a best-fitting absorption column of 1.5 (+0.6, -0.5) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.9 x 10-11 (4.7 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.5 (+0.6, -0.5) x 1021 cm-2
Galactic foreground: 3.3 x 1020 cm-2
Excess significance: 3.5 σ
Photon index: 1.77 ± 0.17

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00863421.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180925A 119 s after the BAT trigger (Emery and Gropp GCN Circ. 23274). A fading source consistent with the XRT position (Goad et al. GCN Circ. 23271) 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 0.03 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
21h00m48.70s -64°27'24.82" 0.22" UVOT-refined Emery and Gropp GCN Circ. 23274
21h00m48.71s -64°27'23.8" 1.4" XRT-final UKSSDC
21h00m48.18s -64°27'24.8" 3.8" XRT-enhanced Goad et al. GCN Circ. 23271
21h00m50.9s -64°26'59.2" 1.5' BAT-refined Ukwatta et al. GCN Circ. 23273

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Malesani et al. 23276 VLT optical observations VLT
Gamma-ray Luo et al. 23275 Insight-HXMT/HE detection Insight-HXMT T90=23.23 seconds
Gamma-ray Kozlova et al. 23291 Konus-Wind observation Konus-Wind Epeak=528 (-135,+239) keV
Duration=~26 seconds
Fluence=1.44(-0.23,+0.32)x10-5erg cm-2
Gamma-ray Sharma et al. 23304 Astrosat CZTI detection CZTI

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

Filter Tstart(s) Tstop(s) Exp(s) Mag
white 119 269 147 19.73 ± 0.17
white 557 6943 674 20.32 ± 0.13
v 607 7354 549 >19.66
b 533 6739 347 20.89 ± 0.37
u 277 6533 579 19.78 ± 0.21
uvw1 657 7755 540 20.07 ± 0.29
uvm2 1062 7559 413 >20.14
uvw2 583 7149 510 >20.60

Table 3. UVOT observations reported by Emery and Gropp (GCN Circ. 23274). 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.

October 4, 2018