Swift Observations of GRB 181201A

S. Mereghetti (IASF-Milano) and M.H. Siegel (PSU) for the Swift team

1. Introduction

Mereghetti et al. (GCN Circ. 23469) reported the GRB detection. A gamma ray burst lasting about 180 s has been detected by IBAS in the IBIS/ISGRI data at 02:38 UT of 1 December 2018 Table 1 contains the best reported positions from Swift.

Podesta et al. (GCN Circ. 23470) reported the position from MASTER for the optical afterglow of this GRB. Siegel and Cannizzo (GCN Circ. 23499) reported the detection with UVOT of an optical afterglow. Bolmer and Schady (GCN Circ. 23486) determined a redshift from GROND, and Izzo et al. (GCN Circ. 23488) determined a redshift of 0.450 from VLT/FORS2. Table 2 is a summary of GCN Circulars about this GRB from observatories other than Swift.

2. BAT Observations and Analysis

BAT did not observe this burst.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Page et al. (GCN Circ. 23474).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.44 ± 0.18.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.74 (+0.08, -0.07). The best-fitting absorption column is 6.1 (+2.0, -0.9) x 1020 cm-2, consistent with the Galactic value of 5.2 x 1020 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.8 x 10-11 (4.2 x 10-11) erg cm-2 count-1.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations (Siegel and Cannizzo GCN Circ. 23499) of the field of GRB 18201A 14.3 ks after the INTEGRAL trigger (Mereghetti et al., GCN Circ. 23469). We confirm the optical afterglow initially reported by MASTER (Podeta et al., GCN Circ. 23470, Kong, GCN Circ. 23475, Volnova et al., GCN Circ. 23477, Mazaeva et al., GCN Circ. 23479, Watson et al., GCN Circ. 23481, Belkin et al., GCN Circ. 23485, Bolmer and Schady, GCN Circ. 23486, Cai et al., GCN Circ. 23491). 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).

Figure 1. The BAT light curve is not available.

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
21h17m11.17s -12°37'51.0" 0.2" UVOT-refined Siegel and Cannizzo GCN Circ. 23499
21h17m11.20s -12°37'50.9" 1.4" XRT-final UKSSDC
21h17m11.20s -12°37'51.0" 2.0" XRT Page et al. GCN Circ. 23474

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Podesta et al. 23470 MASTER-Net OT detection MASTER detection
Optical Kong 23475 iTelescope optical afterglow detection iTelescope detection
Optical Lipunov et al. 23476 MASTER-Net OT continued observation MASTER
Optical Volnova et al. 23477 optical observations in SAAO detection
Optical Heintz et al. 23478 NOT optical observations NOT
Optical Mazaeva et al. 23479 Zeiss-1000/CrAO optical observations CrAO detection
Optical Watson et al. 23481 COATLI Optical Detection COATLI detection
Optical Lipunov et al. 23484 Global MASTER-Net prompt optical
descovery and afterglow observations		 MASTER
Optical Belkin et al. 23485 optical observations in Chilescope
observatory		 detection
Optical Bolmer and Schady 23486 GROND detection of the afterglow and
redshift upper limit of z < 3		 GROND redshift
Optical Izzo et al. 23488 VLT/FORS2 tentative spectroscopic
redshift		 VLT/FORS2 redshift
Optical Watson et al. 23492 Further COATLI Optical Monitoring COATLI
Optical Volnova et al. 23497 continued optical observations in SAAO
and Chilesope
Optical Ramsay et al. 23503 GOTO optical observations Gravitational-wave Optical Transient Observer
Optical Bolmer and Schady 23504 Continued GROND observations show that
afterglow is still bright		 GROND detection
Optical Reva et al. 23507 continued optical observations in TSHAO Zeiss-1000
Optical Srivastava et al. 23510 Optical afterglow measurements by
GROWTH-India		 GROWTH-India detection
Optical Belkin et al. 23514 continued optical observations in TSHAO
and Chilesope		 Zeiss-1000
Optical Mazaeva et al. 23522 continued optical observations in Mondy Mondy
Radio Laskar et al. 23518 ALMA detection of a fading mm afterglow ALMA detection
Radio Laskar et al. 23519 VLA detection VLA detection
Gamma-ray Mereghetti et al. 23469 a long GRB detected by INTEGRAL INTEGRAL
Gamma-ray Arimoto et al. 23480 Fermi-LAT detection Fermi LAT Emax=25.5 GeV
Gamma-ray Cai et al. 23491 Insight-HXMT/HE detection Insight-HXMT T90=15.18 seconds
Gamma-ray Svinkin et al. 23495 Konus-Wind observation Konus-Wind Epeak=152 (-6,+6) keV
Fluence=1.99(-0.06,+0.06)x10-4erg cm-2
Gamma-ray Khanam et al. 23501 AstroSat CZTI detection CZTI T90=19.2 seconds
Other Belkin et al. 23601 photometric signature of the SN light curve

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

Filter Tstart(s) Tstop(s) Exp(s) Mag
uvw2 14310 26435 4901 15.39 ± 0.02
white 56416 74104 924 17.53 ± 0.03
white 148341 165117 1162 18.71 ± 0.04
v 56713 74204 535 17.70 ± 0.10
v 160066 165389 389 18.70 ± 0.26
u 56120 77881 2757 17.25 ± 0.03
u 148114 164276 2896 18.30 ± 0.05

Table 3. UVOT observations reported by Siegel and Cannizzo (GCN Circ. 23499). 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.

January 4, 2019