Swift Observations of GRB 190604B

K.L. Page (U. Leicester), J.A. Kennea (PSU) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 14:57:15 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190604B (trigger=906654) (Page et al. GCN Circ. 24742). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 139° from the Sun (9.0 hours West) and 152° from the 2%-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.

Breeveld and Page (GCN Circ. 24747) reported the detection with UVOT of an optical afterglow. 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 Markwardt et al. (GCN Circ. 24752), the BAT ground-calculated position is RA, Dec = 297.546, -32.975 deg which is RA(J2000) = 19h50m10.9s Dec(J2000) = -32°58'31.1" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 64%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts from ~T-40 s and ends at ~T+240 s. There are two major peaks that occur at ~T0 and ~T+158 s, respectively. T90 (15-350 keV) is 220.2 ± 4.7 s (estimated error including systematics).

The time-averaged spectrum from T-39.776 s to T+239.504 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.81 ± 0.04. The fluence in the 15-150 keV band is 1.77 ± 0.04 x 10-5 erg cm-2. This fluence is larger than that of 96.4% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+157.54 s in the 15-150 keV band is 7.3 ± 0.4 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/906654/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Kennea et al. (GCN Circ. 24749). We have analysed 14 ks of XRT data for GRB 190604B, from 120 s to 85.8 ks after the BAT trigger. The data comprise 285 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 24743).

The late-time light curve (Figure 2) (from T0+4.9 ks) can be modelled with a power-law decay with a decay index of α=1.15 ± 0.06.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.40 ± 0.03. The best-fitting absorption column is 1.49 (+0.16, -0.13) x 1021 cm-2, consistent with the Galactic value of 1.4 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.02 ± 0.11 and a best-fitting absorption column of 2.9 ± 0.4 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 (5.3 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.9 ± 0.4 x 1021 cm-2
Galactic foreground: 1.4 x 1021 cm-2
Excess significance: 6.1 σ
Photon index: 2.02 ± 0.11

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00906654.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190604B 128 s after the BAT trigger (Breeveld and Page GCN Circ. 24747). A faint source is detected in the initial white UVOT exposure at a position consistent with the enhanced XRT source position (Evans et al., GCN Circ. 24746). 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 0.15 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
19h50m09.82s -32°58'38.9" 0.61" UVOT ?
19h50m09.74s -32°58'40.6" 1.4" XRT-final UKSSDC
19h50m09.74s -32°58'40.9" 1.9" XRT-enhanced Evans et al. GCN Circ. 24746
19h50m10.9s -32°58'31.1" 1.0' BAT-refined Markwardt et al. GCN Circ. 24752

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Khandagale et al. 24748 GROWTH-India telescope followup - no
optical counterpart found		 GROWTH-India detection
Optical Hu et al. 24750 BOOTES-3 early optical limits BOOTES-3 upper limits
Optical Belkin et al. 24754 Kitab observatory optical upper limit BOOTES-3 upper limits
Optical Izzo et al. 24762 VLT Afterglow Detection VLT 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
white 128 276 146 19.90 ± 0.24
white 618 812 39 >19.63
v 669 861 39 >17.95
b 594 787 39 >18.98
u 338 588 246 >19.71
uvw1 718 738 20 >17.44
uvw2 645 837 39 >18.09

Table 3. UVOT observations reported by Breeveld and Page (GCN Circ. 24747). The start and stop times of the exposures are given in seconds since the BAT trigger. The preliminary detections and 3-σ upper limits are given. No correction has been made for extinction in the Milky Way.

June 7, 2019