Swift Observations of GRB 190610A

P.A. Evans (U. Leicester), A.P. Beardmore (U. Leicester) and N.P.M. Kuin (UCL/MSSL) for the Swift team

1. Introduction

At 11:27:45 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190610A (trigger=907754) (Evans et al. GCN Circ. 24775). Swift did not slew to the burst location due to an observing constraint. At the time of the trigger, the initial BAT position was 44° from the Sun (2.1 hours West) and 129° from the 53%-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 Palmer et al. (GCN Circ. 24783), the BAT ground-calculated position is RA, Dec = 46.244, -7.661 deg which is RA(J2000) = 03h04m58.6s Dec(J2000) = -07°39'38.7" with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows a short spike that starts ~T+0.2, peaks at ~T+0.3, and ends ~T+0.9. Due to the SAA, data beyond ~T+35 s are unavailable. T90 (15-350 keV) is 0.62 ± 0.11 s (estimated error including systematics).

The time-averaged spectrum from T+0.21 to T+0.89 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 0.32 ± 0.27. The fluence in the 15-150 keV band is 4.1 ± 0.5 x 10-7 erg cm-2. This fluence is larger than that of 92% of the short GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.05 s in the 15-150 keV band is 3.9 ± 0.5 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/907754/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans (GCN Circ. 24898). Swift-XRT has carried out further observations of GRB 190610A (Palmer et al. GCN Circ. 24783, Beardmore et al., GCN Circ. 24883), in order to ascertain whether the faint source in the BAT error circle ("Source 7") is fading. Marginal evidence of fading has been detected. The original detection had a count rate of 1.76 (+0.83, -0.64) x 10-3 ct s-1. Thus the source appears to be fading, but only at the 2.1-sigma level. Given the low significance of the fading, the faintness of the source, and the fact that low-level variability is not uncommon among X-ray sources, we cannot say with confidence that this is definitely the afterglow of GRB 190610A, however it remains a tantalising possibility.

The details of the source can be viewed, and the light curve (Figure 2) rebinned, at: http://www.swift.ac.uk/ToO_GRBs/00020896/Source7.php

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190610A 292282 s after the BAT trigger (Kuin and Evans GCN Circ. 24834). No optical afterglow consistent with the BAT position or the XRT candidate position (Beardmore et al., GCN Circ. 24833) has been 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 0.09 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.

RA (J2000) Dec (J2000) Error Note Reference
03h04m58.6s -07°39'38.7" 1.9' BAT-refined Palmer et al. GCN Circ. 24783

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Gamma-ray Cai et al. 24782 Insight-HXMT/HE detection Insight-HXMT T90=0.56 seconds
Gamma-ray Tamura et al. 24805 CALET Gamma-Ray Burst Monitor detection CALET
Gamma-ray Frederiks et al. 24828 Konus-Wind observation of short/hard
GRB 190610A		 Konus-Wind Epeak=903 (-181,+251) keV

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
v 292282 300459 4890 >20.3

Table 3. UVOT observation reported by Kuin and Evans (GCN Circ. 24834). The start and stop time of the exposure are given in seconds since the BAT trigger. The preliminary 3-σ upper limit is given. No correction has been made for extinction in the Milky Way.

June 29, 2019