Swift Observations of GRB 180715A

T.N. Ukwatta (LANL), S. Campana (INAF-OAB) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 18:07:05 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180715A (trigger=848048) (Ukwatta et al. GCN Circ. 22947). Swift could not immediately slew to the location due to the Earth limb constraint. At the time of the trigger, the initial BAT position was 118° from the Sun (8.0 hours East) and 83° from the 10%-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 Ukwatta et al. (GCN Circ. 22951), the BAT ground-calculated position is RA, Dec = 235.085, -0.899 deg which is RA(J2000) = 15h40m20.5s Dec(J2000) = -00°53'57.6" with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 85%.

The mask-weighted light curve (Figure 1) shows a double-peaked structure that starts at ~T-0.1 and ends at ~T+0.6, The two peaks occur at ~T+0.1 and ~T+0.5, respectively. T90 (15-350 keV) is 0.68 ± 0.09 s (estimated error including systematics).

The time-averaged spectrum from T-0.14 to T+0.63 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 0.74 ± 0.27. The fluence in the 15-150 keV band is 1.3 ± 0.2 x 10-7 erg cm-2. This fluence is larger than that of 79% of the short GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.26 s in the 15-150 keV band is 1.2 ± 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/848048/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Kennea et al. (GCN Circ. 22953). We have analysed 14 ks of Photon Counting (PC) mode XRT data for the Swift/BAT-detected burst GRB 180715A (Ukwatta et al. GCN Circ. 22951), collected between T0+11.0 ks and T0+67.7 ks. Five uncatalogued X-ray sources have been detected consistent with being within 296 arcsec of the Swift/BAT position, however none of them is above the RASS limit or shows definitive signs of fading.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180715A 4312 s after the BAT trigger (Siegel and Ukwatta GCN Circ. 22967). No optical afterglow consistent with the BAT position (Ukwatta et al., GCN Circ. 22951) 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 0.13 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
15h40m20.5s -00°53'57.6" 2.0' BAT-refined Ukwatta et al. GCN Circ. 22951

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Guidorzi et al. 22949 LCO Sutherland possible optical
candidate		 LCO possible detection
Optical Lyman et al. 22950 GOTO optical limits Gravitational-wave Optical Transient Observer upper limits
Optical Watson et al. 22954 COATLI Optical Observations COATLI
Optical Guidorzi et al. 22955 LCO Sutherland observations LCO
Optical Fong et al. 22968 Gemini imaging upper limits Gemini upper limits
Optical Vlasenko et al. 22978 MASTER optical observation MASTER
Radio Bright et al. 23217 15 GHz upper limits from the AMI Short
GRB Program		 AMI upper limits
Gamma-ray Hamburg and Meegan 22952 Fermi GBM detection Fermi GBM Epeak=852±182 keV
T90=0.7 seconds
Fluence=1.7±0.1x10-6erg cm-2
(89th percentile for short GRBs)
Gamma-ray Torii et al. 22956 CALET Gamma-Ray Burst Monitor detection CALET Epeak=707 (-195, +337) keV
Gamma-ray Sharma et al. 23012 AstroSat CZTI detection CZTI T90=0.9 seconds
Gamma-ray Cannady et al. 23059 CALET CAL gamma-ray analysis CALET

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 4312 4462 147 >20.6
white 4312 5695 344 >21.0
v 4469 4669 197 >19.1
b 5290 5490 197 >20.1
u 5085 5284 197 >19.8
w1 4879 5079 197 >19.5
m2 4674 4874 197 >19.7
w2 5701 5726 25 >18.7

Table 3. UVOT observations reported by Siegel and Ukwatta (GCN Circ. 22967). 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.

September 11, 2018