Swift Observations of GRB 190515B

M.J. Moss (George Washington University), A.P. Beardmore (U. Leicester) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 16:04:21 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190515B (trigger=903845) (Moss et al. GCN Circ. 24553). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 49° from the Sun (3.1 hours East) and 93° from the 87%-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. 24564), the BAT ground-calculated position is RA, Dec = 98.826, 52.322 deg which is RA(J2000) = 06h35m18.3s Dec(J2000) = +52°19'17.6" with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment). The partial coding was 67%.

The mask-weighted light curve (Figure 1) shows a single-pulse structure that starts at ~T-30 s, peaks at ~T0, and ends at ~T+30 s. T90 (15-350 keV) is 46.4 ± 12.3 s (estimated error including systematics).

The time-averaged spectrum from T-30.84 to T+31.02 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.74 ± 0.15. The fluence in the 15-150 keV band is 1.6 ± 0.1 x 10-6 erg cm-2. This fluence is larger than that of 53% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.39 s in the 15-150 keV band is 1.1 ± 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/903845/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Beardmore et al. (GCN Circ. 24561). We have analysed 11 ks of XRT data for GRB 190515B, from 73 s to 90.4 ks after the BAT trigger. The data comprise 66 s in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 24554).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=3.95 (+0.20, -0.19). At T+294 s the decay flattens to an α of 0.39 (+0.09, -0.10) before breaking again at T+9885 s to a final decay with index α=1.20 (+0.25, -0.20).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 3.3 ± 0.3. The best-fitting absorption column is 2.1 (+0.6, -0.5) x 1021 cm-2, in excess of the Galactic value of 1.3 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.97 (+0.24, -0.15) and a best-fitting absorption column of 1.34 (+0.69, -0.07) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 x 10-11 (4.4 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.34 (+0.69, -0.07) x 1021 cm-2
Galactic foreground: 1.3 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 1.97 (+0.24, -0.15)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00903845.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 190515B 93 s after the BAT trigger (Siegel and Moss GCN Circ. 24568). No optical afterglow consistent with the XRT position (Osborne et al. GCN Circ. 24557) 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 0.09 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.

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
06h35m23.62s +52°19'02.3" 1.5" XRT-final UKSSDC
06h35m23.45s +52°18'59.9" 2.3" XRT-enhanced Osborne et al. GCN Circ. 24557
06h35m18.3s +52°19'17.6" 1.5' BAT-refined Krimm et al. GCN Circ. 24564

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Belkin et al. 24556 Kitab Observatory optical upper limit upper limits
Optical de Ugarte Postigo et al. 24558 Optical observations from OSN Obs.de Sierra Nevada
Optical Hu et al. 24567 BOOTES-4/MET and 2.2m CAHA optical
limits		 BOOTES-4 upper limits

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 93 242 147 >19.9
uFC 305 555 246 >19.4
white 93 605 167 >20.0
v 636 4442 216 >18.9
b 561 754 39 >18.7
u 305 730 265 >19.5
w1 685 4722 88 >18.9
m2 661 4647 216 >19.6
w2 612 4237 216 >19.0

Table 3. UVOT observations reported by Siegel and Moss (GCN Circ. 24568). 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.

May 17, 2019