Swift Observations of GRB 180314A

P. D'Avanzo (INAF-OAB), S.J. LaPorte (PSU) and S.R. Oates (U.Warwick) for the Swift team

1. Introduction

At 00:43:31 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180314A (trigger=814129) (D'Avanzo et al. GCN Circ. 22478). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 103° from the Sun (7.0 hours East) and 126° from the 12%-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.

D'Avanzo et al. (GCN Circ. 22478) reported the discovery with UVOT of an optical afterglow. Sbarufatti et al. (GCN Circ. 22484) determined a redshift of 1.445 from VLT. 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. 22490), the BAT ground-calculated position is RA, Dec = 99.208, -24.530 deg which is RA(J2000) = 06h36m50.0s Dec(J2000) = -24°31'47.7" with an uncertainty of 3.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 24%.

The mask-weighted light curve (Figure 1) shows a few overlapping pulses that starts at ~ T-12 s and ends at ~T+110 s. The main peak occurs at ~T+4 s. Note that the burst location came in to the BAT FOV at ~T-32 s, and thus the burst emission might start before it is in the BAT FOV. T90 (15-350 keV) is 51.2 ± 22.3 s (estimated error including systematics).

The time-averaged spectrum from T-11.80 to T+109.75 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.83 ± 0.25, and Epeak of 94.9 ± 17.8 keV (χ2 47.72 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 1.1 ± 0.04 x 10-5 erg cm-2 and the 1-s peak flux measured from T+3.05 s in the 15-150 keV band is 7.9 ± 0.6 ph cm-2 s-1. This fluence is larger than that of 93.2% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). A fit to a simple power law gives a photon index of 1.50 ± 0.05 (χ2 72.57 for 57 d.o.f.). 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/814129/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by LaPorte et al. (GCN Circ. 22486). We have analysed 24 ks of XRT data for GRB 180314A, from 165 s to 340.9 ks after the BAT trigger. The data comprise 181 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 22480).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=2.60 ± 0.17. At T+328 s the decay steepens to an α of 5.8 (+2.2, -1.1) before breaking again at T+399 s to a final decay with index α=0.970 (+0.026, -0.025).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.34 (+0.08, -0.04). The best-fitting absorption column is consistent with the Galactic value of 1.5 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.77 ± 0.07 and a best-fitting absorption column consistent with the Galactic value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.9 x 10-11 (4.7 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.5 x 1021 cm-2
Intrinsic column: 1.5 (+0.9, -0.0) x 1021 cm-2 at z=1.445
Photon index: 1.77 ± 0.07

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00814129.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180314A 167 s after the BAT trigger (Oates and D'Avanzo GCN Circ. 22483). A source consistent with the XRT position (Osborne et al. GCN Circ. 22480) 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 0.08 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
06h37m03.68s -24°29'46.0" 0.42" UVOT-refined Oates and D'Avanzo GCN Circ. 22483
06h37m03.70s -24°29'46.5" 1.4" XRT-final UKSSDC
06h37m03.65s -24°29'46.6" 1.8" XRT-enhanced Osborne et al. GCN Circ. 22480
06h36m50.0s -24°31'47.7" 3.1' BAT-refined Ukwatta et al. GCN Circ. 22490

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Bolmer 22479 GROND detection of the afterglow GROND detection
Optical Lipunov et al. 22481 MASTER-Net OT dtetction MASTER
Optical Melandri et al. 22482 REM optical/NIR observations REM detection
Optical Sbarufatti et al. 22484 VLT/X-shooter redshift VLT redshift
Optical Guidorzi et al. 22487 LCO Siding Springs observations LCO detection
Optical Klotz et al. 22488 TAROT La Silla observatory optical
observations		 TAROT detection
Optical Shiraishi et al. 22505 MITSuME Okayama optical upper limits MITSuME Okayama upper limits
Gamma-ray Bissaldi and Roberts 22485 Fermi GBM observation Fermi GBM Epeak=104±3 keV
T90=28 seconds
Fluence=1.52±0.03x10-5erg cm-2
(83rd percentile for long GRBs)
Gamma-ray Tsvetkova et al. 22513 Konus-Wind observation Konus-Wind Epeak=111 (-6,+6) keV
Duration=~22 seconds
Fluence=1.47(-0.06,+0.06)x10-5erg cm-2
Gamma-ray Sharma et al. 22515 AstroSat CZTI detection CZTI T90=37.3 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 167 317 147 16.28 ± 0.03
white 606 626 19 17.86 ± 0.15
v 829 849 19 17.07 ± 0.25
b 755 774 19 18.09 ± 0.28
u 326 576 246 16.95 ± 0.06
w1 705 1131 39 17.98 ± 0.33
m2 680 1106 58 18.12 ± 0.35
w2 631 1578 117 18.75 ± 0.30

Table 3. UVOT observations reported by Oates and D'Avanzo (GCN Circ. 22483). 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.

March 18, 2018