Swift Observations of GRB 180224A

A.Y. Lien (GSFC/UMBC), A. Melandri (INAF-OAB) and S.J. LaPorte (PSU) for the Swift team

1. Introduction

At 22:21:07 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180224A (trigger=811561) (Lien et al. GCN Circ. 22442). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 131° from the Sun (9.0 hours West) and 97° from the 68%-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.

Lien et al. (GCN Circ. 22442) reported the discovery with UVOT of an optical afterglow. Vladimirov et al. (GCN Circ. 22444) reported the position from MASTER for the optical afterglow of this GRB. 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 Sakamoto et al. (GCN Circ. 22447), the BAT ground-calculated position is RA, Dec = 202.675, 38.073 deg which is RA(J2000) = 13h30m41.9s Dec(J2000) = +38°04'23.0" with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 93%.

The mask-weighted light curve (Figure 1) shows a FRED-like pulse that starts at ~ T0, peaks at ~ T+1 s, and ends at ~T+15 s. T90 (15-350 keV) is 10.9 ± 3.6 s (estimated error including systematics).

The time-averaged spectrum from T-0.48 to T+14.57 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.11 ± 0.18. The fluence in the 15-150 keV band is 4.7 ± 0.5 x 10-7 erg cm-2. This fluence is larger than that of 18% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.40 s in the 15-150 keV band is 1.4 ± 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/811561/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Melandri et al. (GCN Circ. 22445). We have analysed 7.8 ks of XRT data for GRB 180224A, from 84 s to 56.7 ks after the BAT trigger. The data comprise 35 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. 22443).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=0.70 ± 0.08, followed by a break at T+3499 s to an α of 2.04 (+0.30, -0.25).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.65 (+0.17, -0.16). The best-fitting absorption column is 9.2 (+5.4, -4.7) x 1020 cm-2, in excess of the Galactic value of 8.2 x 1019 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.1 x 10-11 (4.6 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 9.2 (+5.4, -4.7) x 1020 cm-2
Galactic foreground: 8.2 x 1019 cm-2
Excess significance: 2.9 σ
Photon index: 1.65 (+0.17, -0.16)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00811561.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180224A 85 s after the BAT trigger (LaPorte and Lien GCN Circ. 22454). A fading source consistent with the XRT position (Evans et al. GCN Circ. 22443) also detected by (Lipunov et al. GCN Circ. 22444; Watson et al. GCN Circ. 22446; Pozanenko et al. GCN Circ. 22448) 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
13h30m44.10s +38°04'44.5" 0.46" UVOT-refined LaPorte and Lien GCN Circ. 22454
13h30m44.10s +38°04'43.5" 1.4" XRT-final UKSSDC
13h30m44.12s +38°04'43.5" 1.7" XRT-enhanced Evans et al. GCN Circ. 22443
13h30m41.9s +38°04'23.0" 1.3' BAT-refined Sakamoto et al. GCN Circ. 22447

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Vladimirov et al. 22444 Global MASTER-Net bright OT detection MASTER detection
Optical Watson et al. 22446 RATIR Optical Observations RATIR
Optical Mazaeva et al. 22448 AbAO optical observations Abastumani Astro. Obs. detection
Optical Izzo and Noschese 22452 OASDG optical observations S.Di Giacomo Obs.

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 61 211 294 18.9 ± 0.07
white 553 5337 287 20.7 ± 0.33
v 603 1397 97 >18.4
b 552 1348 78 >19.5
u 273 5063 500 >20.3
w1 652 1446 97 >18.7
m2 824 844 19 >18.2
w2 751 771 19 >17.6

Table 3. UVOT observations reported by LaPorte and Lien (GCN Circ. 22454). 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.

February 27, 2018