Abstract
ABSTRACT We present X-ray and optical observations of the short duration gamma-ray burst GRB 071227 and its host at z = 0.381, obtained using Swift, Gemini South, and the Very Large Telescope. We identify a short-lived and moderately bright optical transient, with flux significantly in excess of that expected from a simple extrapolation of the X-ray spectrum at 0.2–0.3 d after burst. We fit the SED with afterglow models allowing for high extinction and thermal emission models that approximate a kilonova to assess the excess’ origins. While some kilonova contribution is plausible, it is not favoured due to the low temperature and high luminosity required, implying superluminal expansion and a large ejecta mass of ∼0.1 M$\odot$. We find, instead, that the transient is broadly consistent with power-law spectra with additional dust extinction of E(B − V) ∼ 0.4 mag, although a possibly thermal excess remains in the z band. We investigate the host, a spiral galaxy with an edge-on orientation, resolving its spectrum along its major axis to construct the galaxy rotation curve and analyse the star formation and chemical properties. The integrated host emission shows evidence for high extinction, consistent with the afterglow findings. The metallicity and extinction are consistent with previous studies of this host and indicate the galaxy is a typical, but dusty, late-type SGRB host.
Highlights
The detection of gravitational wave signal GW 170817 (Abbott et al 2017), an inspiral and merger of a binary neutron star system, coupled with the coincident detection of the short gamma-ray burst (SGRB) GRB 170817A by both the Fermi Gamma-ray Burst Monitor (Fermi-GBM ) and INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) spacecraft (Goldstein et al 2017; Savchenko et al 2017), has greatly strengthened the link between SGRBs and a compact binary progenitor
We retrieved the X-Ray Telescope (XRT) data from the UK Swift Science Data Centre (UKSSDC)1 and corrected it for line-of-sight absorption using the ratio of counts-to-flux unabsorbed to counts-to-flux observed from the fit to the late-time photon counting mode spectrum on the UKSSDC
No further flares were identified by Margutti et al and we find no evidence for one in the XRT light curve, a conclusion reached by Bernardini et al (2011)
Summary
Both short and long GRBs produce broadband afterglows, which provide a great deal of information about the properties of the jet opening angles, energetics and circumburst medium (Sari et al 1999; Piran 2004). A kilonova is a radioactively powered and rapidly evolving transient, peaking in the optical or NIR depending on time, the progenitor properties and viewing angle (Metzger 2017) This is due to the behaviour of the different types of ejecta produced during and following the merger process: dynamical and disk wind ejecta (Rosswog 2015). D’Avanzo et al (2009) is a notable counterpart to this paper, presenting optical photometry of the GRB and photometric and spectroscopic analysis of the host as we do here. Their observations were performed using the FOcal Reducer and low dispersion Spectrograph (FORS2) instrument on the Very Large Telescope (VLT).
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