GCN Circular 17281
Subject
GRB 150101B/Swift J123205.1-105602: VLT observations and redshift
Date
2015-01-04T21:00:02Z (10 years ago)
From
Andrew Levan at U.of Leicester <A.J.Levan@warwick.ac.uk>
A.J. Levan (U. Warwick), J. Hjorth (DARK/NBI), K. Wiersema, N.R. Tanvir (U. Leicester) report on behalf of a larger collaboration:
"We obtained imaging and spectroscopy of GRB 150101B/Swift J123205.1-105602 (Cummings et al. ATel 6871) with the ESO VLT on 4 Jan 2015, beginning at approximately 06:40 UT. At this epoch 1280s of I-band and1080s of z-band observations were obtained with FORS2 in ~1" seeing. The known galaxy, 2MASX J12320498-1056010, is clearly detected in these images, and its light extends under the position of the candidate optical counterpart reported by Fong et al. (GCN 17271). We do not detect any point source at this location, although the combination of differing seeing, band and the underlying host galaxy mean that we cannot make any strong statements about the variability of the source. We additionally note that the 90% confidence of the refined XRT position (http://www.swift.ac.uk/xrt_live_cat/00020464/) currently includes the nucleus of 2MASX J12320498-1056010, but not the source identified by Fong et al.
In addition we obtained spectroscopic observations of 2MASX J12320498-1056010, covering the wavelength range 5300-8500A. The continuum is well detected across this wavelength range, with several prominent absorption features, most notably Mg b, and Na D at a preliminary redshift of z=0.134. We do not see any emission lines, although H-beta, [OIII] (4959,5007) and H-alpha all fall within our wavelength range at this redshift. We are not sensitive to the line identified by Castro-Tirado et al. (GCN 17278) as [OII] at z=0.093 since it lies blueward of our spectrum, but note that z=0.093 is not consistent with the observed absorption features. The absence of emission lines implies that the galaxy is a passive system. With an absolute magnitude of M_K ~ -25.5 (Vega), it lies the bright end of the luminosity function.
The properties of the galaxy are broadly similar to those of some short-GRBs (most notably GRB 050509B, Gehrels et al. 2005 Nature 437 851; Fong et al. 2013 ApJ 769 56). At z=0.134 the isotropic energy release, E_iso, would be approximately 1e48 erg, again in the energy range seen for other short-GRBs. However, the soft gamma-ray spectrum of the prompt emission is unusual, as is the long lived X-ray emission of the potential counterpart. The X-rays persist for several days post burst, at a luminosity of approximately 2e43 ergs/s. This is typical of an AGN, but such bright sources are unusual in the small BAT error box. The log(N)-log(S) of Manners et al. (2003 MNRAS 343 293), predicts <10 sources of this flux or brighter per square degree (<~0.05 in a region the size of the BAT error box). Furthermore, the optical spectrum does not show obvious signs of AGN activity. Hence it is likely, but not yet conclusive, that the X-ray source, and galaxy are associated with GRB 150101B/Swift J123205.1-105602.
If associated with a short-GRB we might expect the development of kilonova emission over the next few days which may peak at a H-band magnitude of around H(AB)=22-23, if similar to that seen in GRB 130603B (Tanvir et al. 2013 Nature 500 547, Berger et al. 2013 ApJ 774 23). Alternatively, the soft gamma-ray emission, prolonged X-rays, and location close to the nucleus of its host galaxy could be indicative of an unusual AGN outburst or tidal disruption flare.
We thank Dimitri Mawet and the staff of Paranal for their excellent assistance with these observations."