Swift J2058+0516: GROND detection of candidate optical counterpart

ATel #3390; Arne Rau, Jochen Greiner, Felipe Olivares E. (all MPE Garching)
on 30 May 2011; 13:31 UT
Credential Certification: Arne Rau (arau@mpe.mpg.de)

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Starting May 29, 2011 at 8:56 UT we continued observations of the new transient Swift J2058.4+0516 (Krimm et al. 2011, ATEL #3384; Greiner et al. 2011, ATEL #3385) with GROND (Greiner et al. 2008, PASP 120, 405), the 7-channel imager mounted at the 2.2m MPI/ESO telescope at La Silla Observatory (Chile).

In a 60min exposure we detected a single, unresolved source within the 1.7 arcsec Swift/XRT error circle (Krimm et al. 2011, ATEL #3384) at

RA = 20:58:19.90 (314.58292 deg)
Dec = +05:13:32.2 (5.22562 deg)

The uncertainty in both coordinates is ~0.3 arcsec rms.

We measured the following magnitudes (all in AB system):

g = 23.5 +/- 0.1
r = 23.6 +/- 0.1
i = 23.6 +/- 0.2
z > 22.5
J > 21.2
H > 20.6
K > 18.2

These magnitudes were derived by calibrating the images against SDSS and 2MASS field stars and are not corrected for Galactic foreground reddening of E(B-V)~0.1mag (Schlegel et al. 1998). Upper limits are 3 sigma.

A GROND r'-band image is available at http://www.mpe.mpg.de/~jcg/xrt/Swift2058p0516.html

If the soft X-ray flux remained at the same level as reported by Krimm et al. (2011, ATEL #3384) the X-ray to optical flux ratio would be 3x10^4. This is challenging for both, an AGN, and a LMXB/CV interpretation. The blue color hints towards an accretion disk origin of the optical emission. In case of a Galactic source (D<10kpc) the donor would have to be of M-type or later and the X-ray luminosity would be ~5x10^35 (D/10kpc)^2 erg/s. Optical spectroscopic observations will be important to reveal the true nature of this source.

No cataloged radio source has been found within <3arcmin from the GROND position.