Swift Observations of GRB 180222A

J.K. Cannizzo (NASA/UMBC), A. D'Ai (INAF-IASFPA) and S.A. Sebzda (PSU) for the Swift team

1. Introduction

At 13:10:03 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180222A (trigger=811235) (Cannizzo et al. GCN Circ. 22433). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 75° from the Sun (5.3 hours West) and 148° from the 41%-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. 22436), the BAT ground-calculated position is RA, Dec = 256.537, -36.819 deg which is RA(J2000) = 17^h06^m08.8^s Dec(J2000) = -36°49'08.2" with an uncertainty of 1.6 arcmin, (radius, sys+stat, 90% containment). The partial coding was 98%.

The mask-weighted light curve (Figure 1) consists of a single FRED pulse beginning at ~T-8 s, peaking at T+0 s and decaying to background by T+60 s. T₉₀ (15-350 keV) is 64.0 ± 22.6 s (estimated error including systematics).

The time-averaged spectrum from -15.1 to 48.9 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.40 ± 0.21. The fluence in the 15-150 keV band is 7.9 ± 1.0 x 10^-7 erg cm^-2. This fluence is larger than that of 33% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+8.44 s in the 15-150 keV band is 0.4 ± 0.1 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/811235/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Ai et al. (GCN Circ. 22437). We have analysed 12 ks of XRT data for GRB 180222A, from 82 s to 115.8 ks after the BAT trigger. The data comprise 54 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 22435).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=2.25 (+0.19, -0.17).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.7 (+0.6, -0.5). The best-fitting absorption column is 1.7 (+0.9, -0.8) x 10²² cm^-2, in excess of the Galactic value of 6.6 x 10²¹ cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.4 x 10^-11 (9.2 x 10^-11) erg cm^-2 count^-1.

A summary of the WT-mode spectrum is thus:
Total column: 1.7 (+0.9, -0.8) x 10²² cm^-2
Galactic foreground: 6.6 x 10²¹ cm^-2
Excess significance: 2.3 σ
Photon index: 1.7 (+0.6, -0.5)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00811235.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180222A 83 s after the BAT trigger (Sebzda and Cannizzo GCN Circ. 22439). No optical afterglow consistent with the XRT position 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 E_B-V of 1.68 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

17^h06^m10.25^s -36°48'46.0" 6.5" XRT-final UKSSDC

17^h06^m10.03^s -36°48'43.4" 6.8" XRT-enhanced Beardmore et al. GCN Circ. 22435

17^h06^m08.8^s -36°49'08.2" 1.6' BAT-refined Palmer et al. GCN Circ. 22436

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Gamma-ray Mereghetti et al. 22434 A long GRB detected by INTEGRAL INTEGRAL Fluence=6x10^-7erg cm^-2

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

Filter T_start(s) T_stop(s) Exp(s) Mag

white_FC 83 233 147 >20.6

u_FC 295 545 246 >19.5

white 83 1196 353 >21.6

v 624 1073 58 >18.0

b 551 1171 58 >18.6

u 295 1146 285 >19.7

w1 673 1122 58 >18.0

m2 649 1097 58 >18.0

w2 600 1221 78 >19.3

RA (J2000)	Dec (J2000)	Error	Note	Reference
17^h06^m10.25^s	-36°48'46.0"	6.5"	XRT-final	UKSSDC
17^h06^m10.03^s	-36°48'43.4"	6.8"	XRT-enhanced	Beardmore et al. GCN Circ. 22435
17^h06^m08.8^s	-36°49'08.2"	1.6'	BAT-refined	Palmer et al. GCN Circ. 22436

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Gamma-ray	Mereghetti et al.	22434	A long GRB detected by INTEGRAL	INTEGRAL	Fluence=6x10^-7erg cm^-2

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white_FC	83	233	147	>20.6
u_FC	295	545	246	>19.5
white	83	1196	353	>21.6
v	624	1073	58	>18.0
b	551	1171	58	>18.6
u	295	1146	285	>19.7
w1	673	1122	58	>18.0
m2	649	1097	58	>18.0
w2	600	1221	78	>19.3

Table 3. UVOT observations reported by Sebzda and Cannizzo (GCN Circ. 22439). 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.

February 24, 2018