Spectroscopic classification of PSN J13215756+3843229 in NGC 5112 at the Nordic Optical Telescope

ATel #7223; N. Elias-Rosa (INAF- Osservatorio Astronomico di Padova), A. Morales-Garoffolo (ICE/CSIC-IEEC), A. Pastorello, L. Tartaglia, S. Benetti, E. Cappellaro, G. Terreran, L. Tomasella, M. Turatto (INAF- Osservatorio Astronomico di Padova), U. Noebauer, E. Ishida, S. Taubenberger (MPA Garching), Jussi Harmanen (NOT/Univ. of Turku)
on 14 Mar 2015; 17:38 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: N. Elias-Rosa (nancy.elias@oapd.inaf.it)

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We report the spectroscopic classification of PSN J13215756+3843229 obtained through observations with the 2.5-m Nordic Optical Telescope (equipped with ALFOSC; range 330-910 nm, resolution 1.8 nm).

A low signal-to-noise spectrogram of PSN J13215756+3843229 has been obtained on 2015 March 11.23 UT, showing a very weak continuum, and is dominated by narrow (though resolved) Balmer lines in emission. The full-width-half maximum inferred from the Halpha and Hbeta is about 1300 km/s.

Accounting for the unfiltered discovery magnitude (m = 18) reported by Gagliano et al. (TOCP), and adopting for the host galaxy NGC 5112 a Tully-Fisher distance of 19 Mpc (Tully et al. 2009, AJ 138, 323), which is consistent with the distance estimate obtained via Virgo-corrected recessional velocity (e.g. Mould et al. 2000, ApJ, 529, 786), we infer an absolute magnitude of -13.4 for the transient. Subsequent follow up observations obtained with the Asiago 1.82 m Copernico Telescope + AFOSC on March 13.03 UT, showed that the object has the following magnitudes: r = 19.44 (0.08) and i = 19.84 (0.09).

We also inspected some archival Hubble Space Telescope (HST) WFPC2 images in F450W (~B), F606W (~V), and F814W (~I), obtained on 2001-11-12 (Prog.Id 9042 PI: S. Smartt). We used the r-, and i-band images of the transient obtained with AFOSC on 2015 March 13.03 UT to geometrically match the HST mosaics. The transient is projected over an extended structure, although a source can be discriminated within the 0.35" total positional uncertainty. This source was also detected by the Dolphot package (Dolphin 2000 PASP, 112, 1383), with estimated magnitudes of 22.2, 22.0, and 21.9 in filters F450W, F606W and F814W, respectively. Adopting the above mentioned distance distance, and a Galactic dust reddening E(B-V) = 0.01 (Schlafly & Finkbeiner, 2011, ApJ, 737, 103) to NGC 5112, we obtain the following absolute magnitudes for the candidate precursor star: -9.2, -9.5 and -9.6 in the approximately B, V and I bands, respectively, with an intrinsic colour B-V = 0.4 mag. We note that, given the complex environment that surrounds the transient position we cannot rule out that the progenitor is not the source indicated above.

Accounting for the spectroscopic classification, the absolute magnitude of the transient and the photometric analysis on the site of the precursor, we suggest that PSN J13215756+3843229 is a SN impostor, possibly an outburst of an LBV.

Information on this transient, discovered by R. Gagliano, D. Post, J. Newton, and T. Puckett, is available in the "Bright Supernova" website (http://www.rochesterastronomy.org/snimages/) and in the CBAT "Transient Object Followup Reports" website (http://www.cbat.eps.harvard.edu/index.html).