Swift Observations of GRB 180410A

J.D. Gropp (PSU), P.A. Evans (U. Leicester) and F.E. Marshall (NASA/GSFC) for the Swift team

1. Introduction

At 08:02:59 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180410A or a Galactic transient (trigger=824063) (Gropp et al. GCN Circ. 22620). Swift slewed immediately to the source. At the time of the trigger, the initial BAT position was 76° from the Sun (5.2 hours East) and 142° from the 31%-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 Lien et al. (GCN Circ. 22628), the BAT ground-calculated position is RA, Dec = 95.974, 12.804 deg which is RA(J2000) = 06h23m53.7s Dec(J2000) = +12°48'15.2" with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment). The partial coding was 53%.

The mask-weighted light curve (Figure 1) shows a weak-long pulse that starts and peak at ~ T+25 s, and ends at ~T+150 s. T90 (15-350 keV) is 100.7 ± 13.8 s (estimated error including systematics).

The time-averaged spectrum from T+25.16 to T+153.67 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.46 ± 0.13. The fluence in the 15-150 keV band is 2.2 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 62% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+25.70 s in the 15-150 keV band is 0.7 ± 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/824063/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 22629).

We have analysed 6.8 ks of XRT data for GRB 180410A, from 176 s to 45.6 ks after the BAT trigger. The data comprise 201 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 22622).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=1.39 (+0.22, -0.28). At T+321 s the decay steepens to an α of 2.9 (+0.4, -0.3) before breaking again at T+866 s to a final decay with index α=1.48 (+0.15, -0.13).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.87 (+0.07, -0.05). The best-fitting absorption column is consistent with the Galactic value of 6.6 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.45 (+0.21, -0.19) and a best-fitting absorption column of 7.7 (+2.3, -1.2) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 6.2 x 10-11 (8.1 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 7.7 (+2.3, -1.2) x 1021 cm-2
Galactic foreground: 6.6 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 1.45 (+0.21, -0.19)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00824063.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180410A 179 s after the BAT trigger (Marshall and Gropp GCN Circ. 22627). No optical afterglow consistent with the enhanced XRT position (Evans et al. GCN Circ. 22622) 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 0.92 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
06h23m49.71s +12°48'41.1" 1.5" XRT-final UKSSDC
06h23m49.72s +12°48'41.1" 1.8" XRT-enhanced Evans et al. GCN Circ. 22622
06h23m53.7s +12°48'15.2" 1.9' BAT-refined Lien et al. GCN Circ. 22628

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Itoh et al. 22623 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Watson et al. 22631 RATIR Optical Observations RATIR upper limits
Gamma-ray Bissaldi 22636 Fermi GBM observation Fermi GBM T90=100 seconds
Fluence=3.1±0.4x10-6erg cm-2
(43rd percentile for long GRBs)

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 179 329 147 >20.6
uFC 338 587 246 >20.0
white 179 2068 450 >20.9
v 669 1943 156 >19.2
b 594 2043 156 >20.1
u 338 2017 382 >20.4
w1 718 1993 136 >20.1
m2 693 1968 136 >20.3
w2 644 2093 175 >19.9

Table 3. UVOT observations reported by Marshall and Gropp (GCN Circ. 22627). 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.

April 13, 2018