Swift J2058+0516 Redshift Determination: A Swift J1644/GRB110328A-like Event?

ATel #3426; S. B. Cenko (UC Berkeley), R. M. Quimby (Caltech), A. Rau (MPE Garching), S. R. Kulkarni, A. Horesh (Caltech)
on 11 Jun 2011; 19:25 UT
Credential Certification: S. Bradley Cenko (cenko@astro.berkeley.edu)

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We obtained a series of moderate-resolution optical spectra of the optical counterpart (ATEL #3390, ATEL #3425) of the hard X-ray transient Swift J2058+0516 (ATEL #3384) with the DEIMOS spectrograph mounted on Keck II telescope beginning at 13:42 UT on 2011 June 1. Superimposed on a relatively flat continuum, we identify a series of narrow absorption lines corresponding to Mg II and Fe II at a common redshift of z = 1.185. No significant emission features are visible over the observed wavelength range (4500-9500 A).

If we assume this galaxy is the host of the Swift J2058+0516, the observed X-ray counterpart (ATEL #3384) has a luminosity of L_X ~ 4e47 erg / s approximately 9 days after discovery by the Swift-BAT hard X-ray transient monitor. This is significantly larger than the distribution of gamma-ray burst X-ray afterglow luminosities at this stage, but is instead reminiscent of the recently discovered transient Swift J1644/GRB110328A (Levan et al, astro-ph/1104.3356; Bloom et al, astro-ph/1104.3257; Burrows et al., astro-ph/1104.4787; Zauderberg et al., Nature, submitted). Other similarities include:

- Faint optical counterpart (relative to the X-ray emission)
- Requirement for beaming for the outflow to be sub-Eddington (for M < 10^9 Msun, expected given the faintness of the optical counterpart)
- Lack of evidence for AGN activity (e.g., broad Mg II emission) in the host spectrum

If we simply the scale the observed radio counterpart of Swift J1644/GRB110328A to the inferred distance of Swift J2058+0516, we expect a flux density of ~ 300 uJy at 5 GHz, and ~ 700 uJy at 22 GHz.