Swift Observations of GRB 220325A

M. Ferro (INAF-OAB), K.L. Page (U. Leicester) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 17:16:23 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220325A (trigger=1099310) (Ferro et al. GCN Circ. 31787). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 95° from the Sun (6.3 hours West) and 23° from the 45%-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 Laha et al. (GCN Circ. 31799), the BAT ground-calculated position is RA, Dec = 269.469, -7.055 deg which is RA(J2000) = 17h57m52.7s Dec(J2000) = -07°03'18.4" with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment). The partial coding was 52%.

The BAT light curve (Figure 1) showed a complex structure with a duration of about 6 s. T90 (15-350 keV) is 3.50 ± 0.56 s (estimated error including systematics).

The time-averaged spectrum from T-0.97 to T+3.02 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.69 ± 1.13, and Epeak of 39.3 ± 8.9 keV (χ2 63.15 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 2.9 ± 0.4 x 10-7 erg cm-2 and the 1-s peak flux measured from T+0.53 s in the 15-150 keV band is 2.0 ± 0.3 ph cm-2 s-1. This fluence is larger than that of 12% 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 2.06 ± 0.17 (χ2 69.80 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/1099310/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Page et al. (GCN Circ. 31794). We have analysed 24 ks of XRT data for GRB 220325A, from 95 s to 275.4 ks after the BAT trigger. The data comprise 8 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 Osborne et al. (GCN Circ. 31790).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.72 ± 0.04.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.59 (+0.27, -0.21). The best-fitting absorption column is 4.4 (+2.0, -0.5) x 1021 cm-2, consistent with the Galactic value of 3.9 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 5.1 x 10-11 (6.6 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 4.4 (+2.0, -0.5) x 1021 cm-2
Galactic foreground: 3.9 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 1.59 (+0.27, -0.21)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01099310.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 220325A 114 s after the BAT trigger (Kuin and Ferro GCN Circ. 31801). No optical afterglow consistent with the XRT position (Osborne et al. GCN Circ. 31790) 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 0.915 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
17h57m57.00s -07°02'09.3" 2.2" XRT-final UKSSDC
17h57m57.01s -07°02'09.3" 2.4" XRT-enhanced Osborne et al. GCN Circ. 31790
17h57m52.7s -07°03'18.4" 1.5' BAT-refined Laha et al. GCN Circ. 31799

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 31789 MASTER Global Robotic Net optical
observations 		MASTER
Optical Gupta et al. 31793 1.3m DFOT Optical upper limit DFOT upper limits
Optical Strausbaugh and Cucchiara 31795 LCO Optical Upper Limits LCO upper limits
Optical Guelbenzu et al. 31797 Tautenburg observations Tautenburg
Gamma-ray Wood 31791 Fermi GBM Sub-Threshold Detection Fermi GBM Epeak=70 keV
Other Andrade et al. 31804 GRANDMA observations

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 114 264 147 >20.8
uFC 326 576 246 >20.1
white 114 5035 488 >21.0
v 657 1234 78 >18.7
b 582 4893 255 >20.1
u 326 4688 481 >20.3
w1 707 726 19 >17.9

Table 3. UVOT observations reported by Kuin and Ferro (GCN Circ. 31801). 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.

March 30, 2022