Swift Observations of GRB 220501A

A. D'Ai (INAF-IASFPA) for the Swift team

1. Introduction

At 19:51:51 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220501A (trigger=1104842) (D'Ai et al. GCN Circ. 31982). Swift did not slew to the burst location due to an observing constraint. At the time of the trigger, the initial BAT position was 45° from the Sun (3.1 hours East) and 35° from the 1%-illuminated Moon. Table 1 contains the best reported positions from Swift.

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 Krimm et al. (GCN Circ. 31993), the BAT ground-calculated position is RA, Dec = 85.583, 14.010 deg which is RA(J2000) = 05^h42^m20.0^s Dec(J2000) = +14°00'36.5" with an uncertainty of 1.6 arcmin, (radius, sys+stat, 90% containment). The partial coding was 92%.

The mask-weighted light curve (Figure 1) shows a FRED-like pulse that starts at ~T-1 s and peaks at ~T+5 s. It is followed by a second weak pulse at ~T+200 s. T₉₀ (15-350 keV) is 202.24 ± 20.60 s (estimated error including systematics).

The time-averaged spectrum from T-1.21 to T+224.20 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.64 ± 0.62, and E_peak of 72.3 ± 28.6 keV (χ² 44.65 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 2.2 ± 0.2 x 10^-6 erg cm^-2 and the 1-s peak flux measured from T+4.28 s in the 15-150 keV band is 1.2 ± 0.2 ph cm^-2 s^-1. This fluence is larger than that of 62% 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 1.54 ± 0.13 (χ² 52.35 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/1104842/BA/.

3. XRT Observations and Analysis

XRT refined results are not available.

4. UVOT Observations and Analysis

UVOT results are not available.

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.

RA (J2000) Dec (J2000) Error Note Reference

05^h42^m20.0^s +14°00'36.5" 1.6' BAT-refined Krimm et al. GCN Circ. 31993

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Klotz et al. 31984 GRANDMA/TJML Optical Observations GRANDMA

Optical Hu et al. 31990 BOOTES-1 optical upper limit BOOTES-1 upper limits

Gamma-ray Fermi 31983 Fermi GBM Final Real-time Localization Fermi GBM

Gamma-ray Roberts and Meegan 31988 Fermi GBM observation Fermi GBM E_peak=114±11 keV
T₉₀=51 seconds
Fluence=3.53±0.24x10^-6erg cm^-2
(brighter than 47% of long GRBs)

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

RA (J2000)	Dec (J2000)	Error	Note	Reference
05^h42^m20.0^s	+14°00'36.5"	1.6'	BAT-refined	Krimm et al. GCN Circ. 31993

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Klotz et al.	31984	GRANDMA/TJML Optical Observations	GRANDMA
Optical	Hu et al.	31990	BOOTES-1 optical upper limit	BOOTES-1	upper limits
Gamma-ray	Fermi	31983	Fermi GBM Final Real-time Localization	Fermi GBM
Gamma-ray	Roberts and Meegan	31988	Fermi GBM observation	Fermi GBM	E_peak=114±11 keV T₉₀=51 seconds Fluence=3.53±0.24x10^-6erg cm^-2 (brighter than 47% of long GRBs)

May 3, 2022