Color Me Intrigued: the Discovery of iPTF 16fnm, a Supernova 2002cx-like Object

Abstract

Modern wide-field, optical time-domain surveys must solve a basic optimization problem: maximize the number of transient discoveries or minimize the follow-up needed for the new discoveries. Here, we describe the Color Me Intrigued experiment, the first from the intermediate Palomar Transient Factory (iPTF) to search for transients simultaneously in the $g_{\mathrm{PTF}}$- and $R_{\mathrm{PTF}}$-bands. During the course of this experiment we discovered iPTF16fnm, a new member of the 02cx-like subclass of type Ia supernovae (SNe). iPTF16fnm peaked at $M_{g_{\mathrm{PTF}}}=-15.09\pm 0.17\mathrm{mag}$, making it the second least-luminous known type Ia SN. iPTF 16fnm exhibits all the hallmarks of the 02cx-like class: (i) low luminosity at peak, (ii) low ejecta velocities, and (iii) a non-nebular spectra several months after peak. Spectroscopically, iPTF16fnm exhibits a striking resemblence to 2 other low-luminosity 02cx-like SNe: SNe 2007qd and 2010ae. iPTF16fnm and SN 2005hk decline at nearly the same rate, despite a 3 mag difference in brightness at peak. When considering the full subclass of 02cx-like SNe, we do not find evidence for a tight correlation between peak luminosity and decline rate in either the $g^{\prime }$ or $r^{\prime }$ band. We further examine the $g^{\prime }-r^{\prime }$ evolution of 02cx-like SNe and find that their unique color evolution can be used to separate them from 91bg-like and normal type Ia SNe. This selection function will be especially important in the spectroscopically incomplete Zwicky Transient Facility/Large Synoptic Survey Telescope era. We measure the relative rate of 02cx-like SNe to normal SNe Ia and find $r_{N_{02cx}/N_{Ia}}=25_{-18.5}^{+75}\%$. Finally, we close by recommending that LSST periodically evaluate, and possibly update, its observing cadence to maximize transient science.

13 pages, 8 figures, 3 table - submitted to ApJ

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