Swift Observations of GRB 180325A

E. Troja (NASA/GSFC/UMCP), K.L. Page (U. Leicester) and F.E. Marshall (NASA/GSFC) for the Swift team

1. Introduction

At 01:53:02 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180325A (trigger=817564) (Troja et al. GCN Circ. 22532). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 143° from the Sun (10.2 hours East) and 53° from the 55%-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.

Troja et al. (GCN Circ. 22532) reported the discovery with UVOT of an optical afterglow. Heintz et al. (GCN Circ. 22535) determined a redshift of 2.25 from NOT, and D’Avanzo et al. (GCN Circ. 22555) determined a redshift of 2.248 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 Lien et al. (GCN Circ. 22545), the BAT ground-calculated position is RA, Dec = 157.416, 24.461 deg which is RA(J2000) = 10h29m39.9s Dec(J2000) = +24°27'39.7" with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 89%.

The mask-weighted light curve (Figure 1) shows two separate pulses. The first pulse starts and peaks at ~T0, and ends at ~T+10 s. The second brighter pulse starts at ~T+70 s, peaks at ~T+80 s, and ends at ~T+110 s. T90 (15-350 keV) is 94.1 ± 1.5 s (estimated error including systematics).

The time-averaged spectrum from T-0.0 to T+107.9 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.18 ± 0.05. The fluence in the 15-150 keV band is 6.5 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 87% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+80.75 s in the 15-150 keV band is 9.4 ± 0.3 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/817564/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Page et al. (GCN Circ. 22540). We have analysed 14 ks of XRT data for GRB 180325A, from 61 s to 190.5 ks after the BAT trigger. The data comprise 329 s in Windowed Timing (WT) mode (the first 10 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. 22539).

The late-time light curve (Figure 2) (from T0+4.8 ks) can be modelled with a power-law decay with a decay index of α=2.11 (+0.07, -0.06).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.60 ± 0.09. The best-fitting absorption column is 1.7 (+0.6, -0.5) x 1022 cm-2, at a redshift of 2.25, in addition to the Galactic value of 1.3 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.86 ± 0.09 and a best-fitting absorption column of 1.4 ± 0.4 x 1022 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10-11 (4.2 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.3 x 1020 cm-2
Intrinsic column: 1.4 ± 0.4 x 1022 cm-2 at z=2.25
Photon index: 1.86 ± 0.09

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00817564.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180325A 82 s after the BAT trigger (Marshall and Troja GCN Circ. 22549). A fading source consistent with the enhanced XRT position (Osborne et al. GCN Circ. 22539) 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 0.02 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
10h29m42.59s +24°27'48.6" 0.44" UVOT-refined Marshall and Troja GCN Circ. 22549
10h29m42.57s +24°27'48.9" 1.4" XRT-final UKSSDC
10h29m42.56s +24°27'49.0" 1.5" XRT-enhanced Osborne et al. GCN Circ. 22539
10h29m39.9s +24°27'39.7" 1.1' BAT-refined Lien et al. GCN Circ. 22545

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Guidorzi et al. 22534 prompt LT observations Liverpool Telescope detection
Optical Heintz et al. 22535 NOT redshift NOT redshift
Optical D'Avanzo et al. 22536 prompt REM optical/NIR detection REM detection
Optical Watson et al. 22537 RATIR Optical Observations RATIR
Optical Littlefield and Garnavich 22538 Optical Photometry with SLKT Pan-STARRS light curve
Optical Strobl et al. 22541 D50 early optical follow-up D50
Optical Klotz et al. 22542 TAROT Calern observatory optical
observations		 TAROT detection
Optical Lipunov et al. 22543 Global MASTER-Net OT prompt
observations 		MASTER
Optical Schweyer and Kann 22544 GROND observations GROND detection
Optical Malesani and Fynbo 22551 further NOT observations NOT light curve
Optical D’Avanzo et al. 22555 VLT/X-shooter spectroscopic observations VLT
Gamma-ray Frederiks et al. 22546 Konus-Wind observation Konus-Wind Epeak=306 (-39,+50) keV
Gamma-ray Sharma et al. 22557 AstroSat CZTI detection CZTI T90=15.7 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
white 82 232 147 18.48 ± 0.05
white 5039 5239 197 20.75 ± 0.23
v 5450 5650 197 >18.8
b 4834 6385 309 >20.1
u 295 6265 353 >20.0
w1 5860 6060 197 >20.1
m2 5654 5854 197 >19.4
w2 5245 12343 980 >20.9

Table 3. UVOT observations reported by Marshall and Troja (GCN Circ. 22549). 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 30, 2018