Swift Observations of GRB 181030A

E. Sonbas (Adiyaman Univ.), P. D'Avanzo (INAF-OAB) and S.W.K. Emery (UCL-MSSL) for the Swift team

1. Introduction

At 01:27:22 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 181030A (trigger=869494) (Sonbas et al. GCN Circ. 23389). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 72° from the Sun (4.5 hours West) and 39° from the 68%-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.

Sonbas et al. (GCN Circ. 23389) reported the discovery with UVOT of an optical afterglow. Moskvitin and Spiridonova (GCN Circ. 23390) reported the position from SAO RAS for the optical afterglow of this GRB. 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 Cummings et al. (GCN Circ. 23397), the BAT ground-calculated position is RA, Dec = 146.916, 14.657 deg which is RA(J2000) = 09h47m39.8s Dec(J2000) = +14°39'26.5" with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 71%.

The mask-weighted light curve (Figure 1) shows a long pulse that starts at ~T0 and peaks at ~T+70 s. The main pulse emission ends at ~T+200 s, with some weak emission lasting till ~T+400 s. T90 (15-350 keV) is 304.7 ± 76.9 s (estimated error including systematics).

The time-averaged spectrum from T+3.2 to T+443.0 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.77 ± 0.09. The fluence in the 15-150 keV band is 6.2 ± 0.3 x 10-6 erg cm-2. This fluence is larger than that of 86% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+71.33 s in the 15-150 keV band is 0.8 ± 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/869494/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Avanzo et al. (GCN Circ. 23394). We have analysed 15 ks of XRT data for GRB 181030A, from 148 s to 91.0 ks after the BAT trigger. The data comprise 121 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 23391).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=0.3 (+0.5, -0.7), followed by a break at T+179 s to an α of 1.819 (+0.023, -0.022).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.06 (+0.05, -0.04). The best-fitting absorption column is 5.3 (+1.9, -1.8) x 1020 cm-2, in excess of the Galactic value of 3.5 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.56 (+0.17, -0.14) and a best-fitting absorption column of 5.3 (+4.8, -1.8) x 1020 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.2 x 10-11 (4.5 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 5.3 (+4.8, -1.8) x 1020 cm-2
Galactic foreground: 3.5 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 1.56 (+0.17, -0.14)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00869494.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 181030A 152 s after the BAT trigger (Emery and Sonbas GCN Circ. 23400). A fading source is detected in the initial UVOT exposures at a position consistent with the XRT position (Beardmore et al., GCN Circ. 23391) and also with SAO RAS (Moskvitin et al., GCN Circ. 23390) and MASTER Net (Lipunov et al., GCN Circ. 23392). 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 0.03 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
09h47m42.27s +14°39'26.3" 1.5" XRT-final UKSSDC
09h47m42.25s +14°39'26.3" 1.7" XRT-enhanced Beardmore et al. GCN Circ. 23391
09h47m39.8s +14°39'26.5" 1.1' BAT-refined Cummings et al. GCN Circ. 23397

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Moskvitin and Spiridonova 23390 SAO RAS optical observations SAO RAS detection
Optical Lipunov et al. 23392 MASTER Net OT detection MASTER detection
Optical Watson et al. 23396 RATIR Optical and NIR Observations RATIR 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 152 270 116 19.34 ± 0.30
white 3978 5614 393 >20.7
v 4389 6009 378 >18.9
b 3774 17487 635 >20.1
u 3569 17236 1278 >20.3
uvw1 4799 11747 852 >20.2
uvm2 4594 22498 1201 >20.7
uvw2 4184 5819 393 >20.1

Table 3. UVOT observations reported by Emery and Sonbas (GCN Circ. 23400). 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.

November 1, 2018