Swift Observations of GRB 220408A

R. Caputo (GSFC), P.A. Evans (U. Leicester) and S.R. Oates (U.Birmingham) for the Swift team

1. Introduction

At 05:46:04 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220408A (trigger=1101675) (Caputo et al. GCN Circ. 31848). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 126° from the Sun (11.6 hours West) and 80° from the 40%-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.

Caputo et al. (GCN Circ. 31848) 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 Palmer et al. (GCN Circ. 31874), the BAT ground-calculated position is RA, Dec = 202.399, 47.065 deg which is RA(J2000) = 13h29m35.7s Dec(J2000) = +47°03'55.4" with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment). The partial coding was 34%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at T-2 s and peaks at T+3 s. There is a weak emission which peaks at T+17 s. T90 (15-350 keV) is 17.25 ± 1.10 s (estimated error including systematics).

The time-averaged spectrum from T-0.55 to T+18.26 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.19 ± 0.57, and Epeak of 35.3 ± 8.8 keV (χ2 62.40 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 1.2 ± 0.1 x 10-6 erg cm-2 and the 1-s peak flux measured from T+2.48 s in the 15-150 keV band is 4.9 ± 0.4 ph cm-2 s-1. This fluence is larger than that of 45% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). A fit to a simple power law gives a photon index of 2.14 ± 0.13 (χ2 72.77 for 57 d.o.f.). 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/1101675/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 31861). We have analysed 16 ks of XRT data for GRB 220408A, from 93 s to 234.2 ks after the BAT trigger. The data comprise 10 s in Windowed Timing (WT) mode (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. 31852).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=0.46 (+0.16, -0.15), followed by a break at T+2344 s to an α of 1.41 (+0.14, -0.13).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.91 (+0.23, -0.22). The best-fitting absorption column is 1.7 (+0.8, -0.7) x 1021 cm-2, in excess of the Galactic value of 2.0 x 1020 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10-11 (4.7 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.7 (+0.8, -0.7) x 1021 cm-2
Galactic foreground: 2.0 x 1020 cm-2
Excess significance: 3.6 σ
Photon index: 1.91 (+0.23, -0.22)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01101675.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 220408A 112 s after the BAT trigger (Oates and Caputo GCN Circ. 31855). A source consistent with the XRT position (Beardmore et al., GCN Circ. 31852) is detected in the initial white UVOT exposure only. 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 0.042 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
13h29m39.72s +47°04'09.5" 0.91" UVOT-initial Caputo et al. GCN Circ. 31848
13h29m39.98s +47°04'08.8" 2.0" XRT-final UKSSDC
13h29m39.98s +47°04'08.8" 2.0" XRT-enhanced Beardmore et al. GCN Circ. 31852
13h29m35.7s +47°03'55.4" 1.2' BAT-refined Palmer et al. GCN Circ. 31874

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 31849 Fermi GRB 220408A: Global MASTER-Net
observations report 		MASTER
Optical Zheng and Filippenko 31850 KAIT Optical Observations KAIT marginal detection
Optical Jiang et al. 31853 Xinglong-2.16m optical upper limit Xinglong upper limits
Optical Kumar et al. 31857 GIT optical upper limit GROWTH-India Tel. upper limits
Optical Hu et al. 31859 BOOTES-5/JGT optical afterglow detection BOOTES detection
Optical Strausbaugh and Cucchiara 31862 LCO Optical Afterglow Detection LCO detection
Optical Zhu et al. 31864 NOT optical observations and archival
detection		 NOT detection
Optical Paek et al. 31867 LOAO, DOAO, and, SAO optical upper limit DOAO upper limits
Optical Kumar et al. 31868 1.3m DFOT Optical upper limit DFOT upper limits
Optical Watson et al. 31870 RATIR Optical and NIR Observations RATIR upper limits
Optical O'Connor et al. 31875 Lowell Discovery Telescope Optical
Observations		 Lowell Discovery Telescope
Optical Beradze et al. 31884 GRANDMA observations GRANDMA
Optical Kumar et al. 31893 HCT optical upper limit Himalayan Chandra Telescope upper limits
Optical Noonan et al. 32327 VIRT Optical Upper Limit Virgin Island Robotic Telescope upper limits
X-ray Campana 31851 Chandra upper limit at the location Chandra upper limits
Gamma-ray Fermi 31847 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Bissaldi 31856 Fermi GBM observation Fermi GBM Epeak=48.8±2.3 keV
T90=20 seconds
Fluence=1.33±0.05x10-6erg cm-2
(brighter than 19% of long GRBs)
Other Minaev and Pozanenko 31865 redshift lower limit estimating redshift

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 112 262 147 20.30 ± 0.22
white 605 799 40 >21.4
v 655 675 19 >19.3
b 581 774 39 >20.7
u 325 575 246 >20.2
w1 704 724 19 >17.9
m2 679 699 19 >17.4
w2 631 650 19 >17.7

Table 3. UVOT observations reported by Oates and Caputo (GCN Circ. 31855). 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.

July 6, 2022