Swift Observations of GRB 190828B

S. Dichiara (NASA/GSFC/UMCP), P.A. Evans (U. Leicester) and S.R. Oates (U.Warwick) for the Swift team

1. Introduction

At 12:59:59.42 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190828B (trigger=922808) (Dichiara et al. GCN Circ. 25517). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 90° from the Sun (6.3 hours East) and 106° from the 5%-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 Krimm et al. (GCN Circ. 25534), the BAT ground-calculated position is RA, Dec = 251.838, 27.287 deg which is RA(J2000) = 16^h47^m21.2^s Dec(J2000) = +27°17'12.9" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows a multi-peak structure that starts at ~T-40 s and ends at ~T+50 s. There are roughly three main peaks that occurs at ~T₀, ~T+11 s, and ~T+31 s, respectively. T₉₀ (15-350 keV) is 66.6 ± 6.2 s (estimated error including systematics).

The time-averaged spectrum from T-42.4 to T+53.3 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.29 ± 0.07. The fluence in the 15-150 keV band is 4.2 ± 0.2 x 10^-6 erg cm^-2. This fluence is larger than that of 78% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+10.74 s in the 15-150 keV band is 2.3 ± 0.2 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/922808/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 25546). We have analysed 20 ks of XRT data for GRB 190828B, from 45 s to 384.8 ks after the BAT trigger. The data comprise 7 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. 25521).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=2.25 (+0.16, -0.14). At T+235 s the decay flattens to an α of -0.20 (+0.19, -0.20). The light curve breaks again at T+1028 s to a decay with α=0.95 (+0.06, -0.05), before a final break at T+42.2 ks s after which the decay index is 1.8 (+0.5, -0.4).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.56 (+0.11, -0.09). The best-fitting absorption column is 7.8 (+3.8, -1.0) x 10²⁰ cm^-2, consistent with the Galactic value of 6.8 x 10²⁰ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.91 ± 0.10 and a best-fitting absorption column of 1.25 (+0.29, -0.27) x 10²¹ cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10^-11 (4.4 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.25 (+0.29, -0.27) x 10²¹ cm^-2
Galactic foreground: 6.8 x 10²⁰ cm^-2
Excess significance: 3.4 σ
Photon index: 1.91 ± 0.10

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00922808.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190828B 63 s after the BAT trigger (Oates and Dichiara GCN Circ. 25544). No optical afterglow consistent with the XRT position (Beardmore et al. GCN Circ. 25521) 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 0.10 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

16^h47^m19.51^s +27°16'50.8" 1.4" XRT-final UKSSDC

16^h47^m19.51^s +27°16'51.3" 1.7" XRT-enhanced Beardmore et al. GCN Circ. 25521

16^h47^m21.2^s +27°17'12.9" 1.0' BAT-refined Krimm et al. GCN Circ. 25534

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Yu et al. 25524 Dabancheng-0.5m optical upper limit upper limits

Optical Brosio et al. 25548 optical upper limit from 0.5m Savelli
Robotic Telescope upper limits

Optical Fynbo et al. 25550 VLT/X-shooter optical afterglow
candidate VLT detection

Gamma-ray Fermi 25516 Fermi GBM Final Real-time Localization Fermi GBM

Gamma-ray Roberts et al. 25535 Fermi GBM observation Fermi GBM E_peak=237±22 keV
T₉₀=55 seconds
Fluence=1.24±0.06x10^-5erg cm^-2
(80^th percentile for long GRBs)

Gamma-ray Gaikwad et al. 25771 AstroSat CZTI detection CZTI T₉₀=31.9 seconds

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 63 213 147 >20.8

u_FC 275 525 246 >20.1

white 63 1717 411 >21.4

v 605 1767 136 >19.1

b 530 1693 117 >20.0

u 275 1668 363 >20.4

w1 654 1644 117 >19.2

m2 630 5626 259 >19.4

w2 580 1743 136 >19.3

RA (J2000)	Dec (J2000)	Error	Note	Reference
16^h47^m19.51^s	+27°16'50.8"	1.4"	XRT-final	UKSSDC
16^h47^m19.51^s	+27°16'51.3"	1.7"	XRT-enhanced	Beardmore et al. GCN Circ. 25521
16^h47^m21.2^s	+27°17'12.9"	1.0'	BAT-refined	Krimm et al. GCN Circ. 25534

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Yu et al.	25524	Dabancheng-0.5m optical upper limit		upper limits
Optical	Brosio et al.	25548	optical upper limit from 0.5m Savelli Robotic Telescope		upper limits
Optical	Fynbo et al.	25550	VLT/X-shooter optical afterglow candidate	VLT	detection
Gamma-ray	Fermi	25516	Fermi GBM Final Real-time Localization	Fermi GBM
Gamma-ray	Roberts et al.	25535	Fermi GBM observation	Fermi GBM	E_peak=237±22 keV T₉₀=55 seconds Fluence=1.24±0.06x10^-5erg cm^-2 (80^th percentile for long GRBs)
Gamma-ray	Gaikwad et al.	25771	AstroSat CZTI detection	CZTI	T₉₀=31.9 seconds

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white_FC	63	213	147	>20.8
u_FC	275	525	246	>20.1
white	63	1717	411	>21.4
v	605	1767	136	>19.1
b	530	1693	117	>20.0
u	275	1668	363	>20.4
w1	654	1644	117	>19.2
m2	630	5626	259	>19.4
w2	580	1743	136	>19.3

Table 3. UVOT observations reported by Oates and Dichiara (GCN Circ. 25544). 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.

September 18, 2019