Abstract
Over the last decade there has been immense progress in the follow-up of short and long GRBs, resulting in a significant rise in the detection rate of X-ray and optical afterglows, in the determination of GRB redshifts, and of the identification of the underlying host galaxies. Nevertheless, our theoretical understanding on the progenitors and central engines powering these vast explosions is lagging behind, and a newly identified class of `ultra-long' GRBs has fuelled speculation on the existence of a new channel of GRB formation. In this paper we present high signal-to-noise X-shooter observations of the host galaxy of GRB130925A, which is the fourth unambiguously identified ultra-long GRB, with prompt gamma-ray emission detected for ~20ks. The GRB line of sight was close to the host galaxy nucleus, and our spectroscopic observations cover both this region along the bulge/disk of the galaxy, in addition to a bright star-forming region within the outskirts of the galaxy. From our broad wavelength coverage we obtain accurate metallicity and dust-extinction measurements at both the galaxy nucleus, and an outer star-forming region, and measure a super-solar metallicity at both locations, placing this galaxy within the 10-20% most metal-rich GRB host galaxies. Such a high metal enrichment has implications on the progenitor models of both long and ultra-long GRBs, although the edge-on orientation of the host galaxy does not allow us to rule out a large metallicity variation along our line of sight. The spatially resolved spectroscopic data presented in this paper offer important insight into variations in the metal and dust abundance within GRB host galaxies. They also illustrate the need for IFU observations on a larger sample of GRB host galaxies at varies metallicities to provide a more quantitative view on the relation between the GRB circumburst and the galaxy-whole properties.
Highlights
In this paper we have presented a detailed analysis on the spectroscopic X-Shooter observations taken of the host galaxy of the ultra-long duration gamma-ray bursts (GRBs) 130925A
The position of the slit fell along the disk of the galaxy, and fortuitously crossed the GRB explosion sight, close to the nucleus of the host galaxy, and a star forming region located in the outskirts of the galaxy
Our observations suggest that the GRB occurred in a metal-rich region of the host galaxy, with a higher dust content than that of a neighbouring HII region, which is in apparent conflict with the favoured progenitor models of both standard long duration GRBs, and the more recently discovered but more poorly understood class of ultra-long GRBs
Summary
The collapsar model (Woosley 1993; Woosley & MacFadyen 1999; Hirschi et al 2005; Yoon & Langer 2005; Woosley & Heger 2006) has been largely successful at describing the overall observed properties of long-duration gamma-ray bursts (GRBs), and has been validated by the detection of supernovae coincident with the GRB explosions (GRB 980425/SN1998bw; Galama et al 1998, GRB030329/SN2003dh; Hjorth et al 2003, Stanek et al 2003). The host galaxies of heavily dust extinguished GRBs (i.e. AV > 1 mag) typically have larger stellar masses, luminosities and dust masses than the hosts of optically bright GRBs (Perley et al 2009; Krühler et al 2011; Rossi et al 2012; Perley et al 2013; Hunt et al 2014; Schady et al 2014), and there are an appreciable number of GRB host galaxies with near-solar or super-solar metallicities (Prochaska et al 2007; Levesque et al 2010b; Savaglio et al 2012; Elliott et al 2013; Graham & Fruchter 2013) These more massive and metal-rich GRB host galaxies do not represent the majority of the GRB host galaxy sample, they do call for certain details of the classically accepted collapsar model to be reviewed, such as the effect of differential rotation (Georgy et al 2012), and binary system models (Podsiadlowski et al 2004; Izzard et al 2004; Petrovic et al 2005; van den Heuvel & Portegies Zwart 2013), or alternatively, for a re-assessment of how representative the galaxywhole, or even the GRB line of sight properties are of the conditions within the GRB local environment.
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