Swift Observations of GRB 220117A

A. Melandri (INAF-OAB), J.A. Kennea (PSU) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 16:18:51 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220117A (trigger=1093592) (Melandri et al. GCN Circ. 31466). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 124° from the Sun (10.1 hours East) and 59° 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.

Palmerio et al. (GCN Circ. 31480) reported the position from VLT for the optical afterglow of this GRB. Palmerio et al. (GCN Circ. 31480) determined a redshift of 4.961 from VLT. 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. 31485), the BAT ground-calculated position is RA, Dec = 91.590, -28.402 deg which is RA(J2000) = 06^h06^m21.5^s Dec(J2000) = -28°24'08.5" with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 26%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T+15 s and ends at ~T+65 s. The two larger peaks occur at ~T+15 s and ~T+44 s, respectively. T₉₀ (15-350 keV) is 49.81 ± 2.37 s (estimated error including systematics).

The time-averaged spectrum from T+14.79 to T+66.14 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.84 ± 0.18. The fluence in the 15-150 keV band is 1.6 ± 0.2 x 10^-6 erg cm^-2. This fluence is larger than that of 53% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+14.81 s in the 15-150 keV band is 1.9 ± 0.4 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/1093592/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Kennea et al. (GCN Circ. 31477). We have analysed 12 ks of XRT data for GRB 220117A, from 142 s to 103.1 ks after the BAT trigger. The data comprise 98 s in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 31472).

The late-time light curve (Figure 2) (from T0+4.3 ks) can be modelled with a power-law decay with a decay index of α=1.59 (+0.09, -0.08).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.54 ± 0.08. The best-fitting absorption column is 9.3 (+1.4, -1.3) x 10²² cm^-2, at a redshift of 4.961, in addition to the Galactic value of 2.6 x 10²⁰ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.79 ± 0.10 and a best-fitting absorption column of 9 (+15, -9) x 10²¹ cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 x 10^-11 (3.8 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 2.6 x 10²⁰ cm^-2
Intrinsic column: 9 (+15, -9) x 10²¹ cm^-2 at z=4.961
Photon index: 1.79 ± 0.10

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01093592.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 220117A 162 s after the BAT trigger (Breeveld and Melandri GCN Circ. 31482). No optical afterglow consistent with the XRT position (Beardmore et al. GCN Circ. 31472) 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 0.028 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

06^h06^m17.40^s -28°26'14.6" 2.0" XRT-final UKSSDC

06^h06^m17.32^s -28°26'15.5" 2.3" XRT-enhanced Beardmore et al. GCN Circ. 31472

06^h06^m21.5^s -28°24'08.5" 1.7' BAT-refined Palmer et al. GCN Circ. 31485

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Fu et al. 31467 Nanshan/NEXT optical observations and a
nearby PanSTARRS source Xinjiang Astro. Obs.

Optical Lipunov et al. 31470 Swift GRB 220117A: Global MASTER-Net
observations report MASTER

Optical Palmerio et al. 31480 VLT/X-shooter optical afterglow and
redshift VLT redshift

Optical Strausbaugh and Cucchiara 31481 LCO Optical Observations LCO

Optical Ito et al. 31488 MITSuME Akeno optical upper limits MITSuME Akeno upper limits

Optical Volnova et al. 31520 Terskol and AbAO optical upper limits Abastumani Astro. Obs. upper limits

Optical Guelbenzu et al. 31522 GROND observations GROND upper limits

Gamma-ray Veres 31487 Fermi GBM Sub-Threshold Detection Fermi GBM E_peak=70 keV

Gamma-ray Frederiks et al. 31511 Konus-Wind detection and joint
Konus-Wind + Swift-BAT spectral
analysis Konus-Wind E_peak=71 (-11,+17) keV

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 162 311 147 >20.5

u_FC 320 570 246 >20.0

white 162 998 306 >20.7

v 650 845 39 >18.4

b 576 770 39 >19.4

u 320 744 265 >20.0

w1 700 720 19 >19.3

m2 675 869 39 >18.7

w2 626 821 39 >18.9

RA (J2000)	Dec (J2000)	Error	Note	Reference
06^h06^m17.40^s	-28°26'14.6"	2.0"	XRT-final	UKSSDC
06^h06^m17.32^s	-28°26'15.5"	2.3"	XRT-enhanced	Beardmore et al. GCN Circ. 31472
06^h06^m21.5^s	-28°24'08.5"	1.7'	BAT-refined	Palmer et al. GCN Circ. 31485

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Fu et al.	31467	Nanshan/NEXT optical observations and a nearby PanSTARRS source	Xinjiang Astro. Obs.
Optical	Lipunov et al.	31470	Swift GRB 220117A: Global MASTER-Net observations report	MASTER
Optical	Palmerio et al.	31480	VLT/X-shooter optical afterglow and redshift	VLT	redshift
Optical	Strausbaugh and Cucchiara	31481	LCO Optical Observations	LCO
Optical	Ito et al.	31488	MITSuME Akeno optical upper limits	MITSuME Akeno	upper limits
Optical	Volnova et al.	31520	Terskol and AbAO optical upper limits	Abastumani Astro. Obs.	upper limits
Optical	Guelbenzu et al.	31522	GROND observations	GROND	upper limits
Gamma-ray	Veres	31487	Fermi GBM Sub-Threshold Detection	Fermi GBM	E_peak=70 keV
Gamma-ray	Frederiks et al.	31511	Konus-Wind detection and joint Konus-Wind + Swift-BAT spectral analysis	Konus-Wind	E_peak=71 (-11,+17) keV

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white_FC	162	311	147	>20.5
u_FC	320	570	246	>20.0
white	162	998	306	>20.7
v	650	845	39	>18.4
b	576	770	39	>19.4
u	320	744	265	>20.0
w1	700	720	19	>19.3
m2	675	869	39	>18.7
w2	626	821	39	>18.9

Table 3. UVOT observations reported by Breeveld and Melandri (GCN Circ. 31482). 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 25, 2022