Swift Observations of GRB 150101B

J.R. Cummings (NASA/UMBC) for the Swift team

1. Introduction

Cummings (GCN Circ. 17267) reported the initial Swift results. It triggered Fermi GBM, trigger number 441818617. Table 1 contains the best reported positions from Swift.

Castro-Tirado et al. (GCN Circ. 17278) determined a redshift of 0.093 from GTC, and Levan et al. (GCN Circ. 17281) determined a redshift of 0.134 from VLT. Table 2 is a summary of GCN Circulars about this GRB from observatories other than Swift.

2. BAT Observations and Analysis

The lightcurve (Figure 1) shows a single peak with a T90 of 0.018 ± 0.006 seconds. There appears to be significant spectral lag.

3. XRT Observations and Analysis

We have analysed 23 ks of XRT data for the Swift-detected burst GRB 150101B, from 139.2 ks to 2612.4 ks after the Swift trigger. The data are entirely in Photon Counting (PC) mode.

The late-time light curve (Figure 2) (from T0+139 ks) is consistent with a constant source of mean count rate 1.6 x 10-2 ct s-1. A power-law fit gives an index of -0.07 (+0.11, -0.10).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.30 (+0.48, -0.29). The best-fitting absorption column is 4.9 (+9.4, -1.4) x 1020 cm-2, consistent with the Galactic value of 3.5 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 2.8 x 10-11 (3.4 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 4.9 (+9.4, -1.4) x 1020 cm-2
Galactic foreground: 3.5 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 2.30 (+0.48, -0.29)

4. UVOT Observations and Analysis

UVOT results are not available.

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
12h32m04.93s -10°56'00.8" 1.8" XRT-final UKSSDC
12h32m05.09s -10°56'02.0" 2.4" XRT Cummings et al. GCN Circ. 17268
12h32m10.5s -10°57'21" 2.5' BAT-initial Cummings GCN Circ. 17267

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Fong et al. 17271 Swift J123205.1-105602: Magellan
candidate afterglow		 Magellan detection
Optical Castro-Tirado et al. 17278 potential host galaxy redshift by GTC GTC redshift
Optical Levan et al. 17281 Swift J123205.1-105602: VLT
observations and redshift		 VLT redshift
Optical van der Horst et al. 17309 Swift J123205.1-105602: continued WSRT
radio and VLT optical observations		 VLT
Optical Levan et al. 17321 Swift J123205.1-105602, deep VLT
observations		 VLT
Optical D'Avanzo et al. 17326 Swift J123205.1-105602: TNG NIR
follow-up		 TNG
Optical Fong et al. 17333 Confirmation of a fading optical
counterpart		 Gemini detection
Radio van der Horst et al. 17286 Swift J123205.1-105602: WSRT radio
observation		 WSRT
Radio Fong 17288 Swift J123205.1-105602: 9.8 GHz VLA
observations		 VLA
X-ray Fong et al. 17285 Swift J123205.1-105602: X-ray
counterpart is likely an AGN		 detection
X-ray Troja et al. 17289 Swift J123205.1-105602: Chandra
observations		 Chandra
X-ray Campana 17318 Swift J123205.1-105602: XMM-Newton
observation		 XMM
X-ray Levan et al. 17431 Swift J123205.1-105602: Second epoch
Chandra observations		 Chandra
Gamma-ray Stanbro 17276 Fermi GBM detection Fermi GBM T90=0.08 seconds
Fluence=1.09±0.14x10-7erg cm-2
(10th percentile for short GRBs)

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

Table 3. Found no UVOT observations.

May 22, 2015