Swift Observations of GRB 190701A

T.N. Ukwatta (LANL), E. Ambrosi (INAF-IASFPA) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 09:45:20 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190701A (trigger=912310) (Ukwatta et al. GCN Circ. 24934). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 61° from the Sun (4.8 hours West) and 53° from the 3%-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 Lien et al. (GCN Circ. 24973), the BAT ground-calculated position is RA, Dec = 28.131, 58.899 deg which is RA(J2000) = 01h52m31.5s Dec(J2000) = +58°53'56.9" with an uncertainty of 2.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 52%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure. The main period of emission includes three broad peaks between roughly T+0 s and T+100 s, peaking about T+38 s. T90 (15-350 keV) is 52.4 ± 4.0 s (estimated error including systematics).

The time-averaged spectrum from T+35.0 to T+90.0 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.91 ± 0.27. The fluence in the 15-150 keV band is 8.7 ± 1.5 x 10-7 erg cm-2. This fluence is larger than that of 35% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+37.92 s in the 15-150 keV band is 1.2 ± 0.3 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/912310/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Ambrosi et al. (GCN Circ. 24961). We have analysed 16 ks of XRT data for GRB 190701A, from 144 s to 166.3 ks after the BAT trigger. The data comprise 10 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 24935).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=8.0 (+0.0, -0.9). At T+241 s the decay flattens to an α of 0.36 (+0.10, -0.18) before breaking again at T+7227 s to a final decay with index α=1.20 (+0.28, -0.20).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.82 (+0.21, -0.13). The best-fitting absorption column is consistent with the Galactic value of 6.4 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 4.7 x 10-11 (7.2 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 6.4 ± 1.1 x 1021 cm-2
Galactic foreground: 6.4 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 1.82 (+0.21, -0.13)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00912310.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190701A 145 s after the BAT trigger (Siegel and Ukwatta GCN Circ. 24955). No optical afterglow consistent with the enhanced XRT position (Osborne et al. GCN Circ. 24940) 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 0.59 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
01h52m19.52s +58°54'33.7" 1.5" XRT-final UKSSDC
01h52m19.54s +58°54'33.9" 1.9" XRT-enhanced Osborne et al. GCN Circ. 24940
01h52m31.5s +58°53'56.9" 2.4' BAT-refined Lien et al. GCN Circ. 24973

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Becerra et al. 24946 COATLI Optical Observations COATLI
Optical Kumar et al. 24954 No candidates/counterpart in optical
follow-up from GROWTH-India Telescope		 GROWTH-India detection
X-ray Setoguchi et al. 24937 MAXI/GSC detection MAXI detection

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 145 295 147 >20.3
uFC 303 553 246 >19.8
white 145 6776 727 >21.2
v 629 11104 409 >19.3
b 559 6570 432 >20.6
u 303 6365 659 >20.6
w1 677 6160 432 >19.9
m2 652 5955 432 >19.3
w2 610 10926 1158 >20.2

Table 3. UVOT observations reported by Siegel and Ukwatta (GCN Circ. 24955). 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.

July 4, 2019