An Unusual Optical Transient from Palomar-Quest: Possible Peculiar Supernova?

ATel #827; A. Mahabal, S. G. Djorgovski (Caltech), C. Baltay, and D. Rabinowitz (Yale), on behalf of the Palomar-Quest survey team; the Nearby Supernova Factory; A. Gal-Yam and A. Soderberg (Caltech); D. Stern (JPL), M. Dickinson, E. MacDonald (NOAO) and S. Juneau (University of Arizona)
on 6 Jun 2006; 16:40 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Ashish Mahabal (aam@astro.caltech.edu)

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An optical transient (OT) was detected in the Palomar-Quest scans taken of 20 May 2006 UT, in comparison with a combination of reference images taken in May and June 2002, by the SNfactory processing pipeline. The transient had a magnitude of R ~ 18.9 mag, and is located at RA = 14:39:33.21 and Dec = +05:46:36.6 (J2000), approximately 2 arcsec W and 0.5 arcsec S off a nucleus of an anonymous galaxy. We give it a provisional spatiotemporal designation OT060520:143933+054636.

The initial spectrum was taken using the SNfactory's SNIFS integral-field spectrograph at the UH 2.2-m telescope on Mauna Kea on 23 May 2006 UT. The spectrum revealed a number of possible P-Cygni type features, but much narrower than what is commonly seen in SN spectra, as well as a prominent rollover in the red part of the spectrum, also atypical of known SNe. It could not be readily classified in the terms of known SN types, AGN, or other commonly observed variable sources.

A spectrum of the OT and the apparent host galaxy was obtained at the Keck-I 10-m telescope using LRIS long-slit spectrograph on 25 May 2006 UT by D. Stern, M. Dickinson, E. MacDonald and S. Juneau. This gives a redshift z = 0.1608 for the host, on the basis of strong and extended [O II] 3727 and H-alpha emission line s (at this z, H-alpha is unfortunately in the middle of the atmospheric A-band), as well as the Ca II H+K absorption lines. Both the images and the spectra of the host are indicative of an actively star forming disk galaxy.

The spectrum of the OT has evolved relative to the initial SNIFS observation, with relatively weak, broader features, but still unlike those normally seen in the SN spectra; and a less prominent rollover in the red. However, the most strik ing feature is a strong Mg II 2799 absorption doublet seen at the host galaxy's redshift in the spectrum of the OT. It is comparable to the strongest Mg II absorbers seen in the spectra of GRB afterglows, and is indicative of a relatively dense ISM along the line of sight, consistent with an explosion in a young, star-forming region.

An additional spectrum was obtained at the Palomar 200-inch telescope using DBSP long-slit spectrograph on 31 May 2006 UT, suggestive of a further evolution.

A preliminary examination of the photometric monitoring of the source at the Palomar 60-inch telescope by A. Soderberg et al. does not show significant variations in flux over the past few days. These observations continue, and a more detailed analysis will be presented later.

At this redshift, in the standard cosmology with h=0.7, Omega=0.27, Lambda=0.73, the luminosity distance is 772.4 Mpc. The observed flux in the restframe B band corresponds to the absolute magnitude M_B ~ -19.5 to -20, i.e., a very luminous object. We also note the very blue color, even without any correction for the extinction in the host.

While the physical nature of the OT is still uncertain, it is likely related to a peculiar Type-Ib SN phenomenon; the closest analog we could find in the literature may be SN1984L. If the object is a SN, it is certainly a very unusual one. Finding charts, spectra, and further updates about this source can be found at: http://www.astro.caltech.edu/~aam/science/OT060520_143933+054636/