Swift Observations of GRB 190630C

T.N. Ukwatta (LANL), M.G. Bernardini (INAF-OAB) and K.K. Simpson (PSU) for the Swift team

1. Introduction

At 23:52:59 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190630C (trigger=912239) (Ukwatta et al. GCN Circ. 24927). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 164° from the Sun (11.0 hours West) and 142° from the 4%-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. 24941), the BAT ground-calculated position is RA, Dec = 293.890, -32.759 deg which is RA(J2000) = 19h35m33.6s Dec(J2000) = -32°45'32.5" with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 57%.

The mask-weighted light curve (Figure 1) shows a single FRED-shaped burst, beginning at ~T-5 s, peaking at T+0 s, and declining to background by ~T+40 s. A spacecraft slew took the burst position out of the BAT field of view just before T+500 s. T90 (15-350 keV) is 38.4 ± 9.3 s (estimated error including systematics).

The time-averaged spectrum from T-1.92 to T+50.23 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.96 ± 0.11. The fluence in the 15-150 keV band is 2.3 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 63% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.12 s in the 15-150 keV band is 2.0 ± 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/912239/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Bernardini et al. (GCN Circ. 24958).

We have analysed 5.7 ks of XRT data for GRB 190630C, from 155 s to 57.3 ks after the BAT trigger. The data comprise 56 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 24930).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=4.3 (+0.7, -0.5), followed by a break at T+310 s to an α of 1.06 (+0.12, -0.23).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 3.09 (+0.33, -0.29). The best-fitting absorption column is 2.7 ± 0.7 x 1021 cm-2, in excess of the Galactic value of 1.1 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.2 ± 0.3 and a best-fitting absorption column of 2.0 (+1.0, -0.9) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.2 x 10-11 (4.8 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.0 (+1.0, -0.9) x 1021 cm-2
Galactic foreground: 1.1 x 1021 cm-2
Excess significance: 1.7 σ
Photon index: 2.2 ± 0.3

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00912239.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190630C 136 s after the BAT trigger (Simpson and Ukwatta GCN Circ. 24945). No optical afterglow consistent with the XRT position (Goad et al. GCN Circ. 24930) 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 0.11 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
19h35m31.32s -32°44'38.9" 1.8" XRT-final UKSSDC
19h35m31.28s -32°44'38.6" 1.9" XRT-enhanced Goad et al. GCN Circ. 24930
19h35m33.6s -32°45'32.5" 1.1' BAT-refined Krimm et al. GCN Circ. 24941

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Zheng and Filippenko 24944 KAIT Optical Upper Limit KAIT upper limits
Optical Belkin et al. 24947 ISON-Castelgrande optical upper limit upper limits
Gamma-ray Sharma et al. 24939 AstroSat CZTI detection CZTI T90=21.3 seconds

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 157 307 147 >20.5
white 157 307 147 >20.5
v 136 412 32 >18.1
w2 367 387 19 >20.1

Table 3. UVOT observations reported by Simpson and Ukwatta (GCN Circ. 24945). 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 2, 2019