Spectroscopic Classification of Extragalactic Transients from CRTS

TL;DR

This study presents the CRTS spectroscopic follow-up of extragalactic transients, revealing a SN-dominated population with a substantial fraction of Type-Ia, Type-II, SLSN-I, and interacting SNe (including Ia-CSM and Ibn). It documents notable events such as SN 2008iy with a pre-supernova outburst, CSS111225 with extreme rebrightening, and SN 2009ny, while highlighting a prevalence of transients in very low-luminosity host galaxies and sub-solar metallicities. The authors discuss challenges in distinguishing TDEs and AGN-driven variability from SNe in galactic nuclei, emphasizing multi-epoch spectra and precise astrometry. They also address observational biases toward luminous, long-duration events and the need for deeper, higher-cadence spectroscopy for upcoming large surveys. Overall, the work provides a concrete, spectroscopy-driven census of extragalactic transients in CRTS and outlines implications for future transient architecture and follow-up strategies.

Abstract

The Catalina Real-time Transient Survey (CRTS) carried out a public survey for optical transients between 2007 and 2019, discovering more than 16,000 transient candidates. Here we present the spectra and highlight the results of the spectroscopic follow-up of CRTS extragalactic transients. As expected, we find that the bulk of these transients are normal supernovae. However, as we prioritised transients exhibiting unusual features or environments during our spectroscopic follow-up, we focus on the rarer types of transients. These objects include more than a dozen type-I superluminous supernovae and dozens of type-I and II supernovae that underwent circumstellar medium interactions. We highlight several specific supernovae, including a new analysis of SN 2008iy, a type-IIn which exhibited a bright pre-supernova outburst event similar to SN 2009ip and lasted more than 1800 days; CSS111225:140122+161705, a type-I supernova that showed an extreme 2.5 magnitude rebrightening event more than 200 days after its initial outburst; and SN 2009ny, a type-Ibn supernova that exhibited strong helium emission lines similar to SN 2002ao. We confirm our previous finding that numerous CRTS transients are associated with galaxies of extremely low luminosity. We discuss the difficulty in determining the origin of transients associated with outbursts in active galactic nuclei (AGN), tidal disruption events, and type-IIn supernovae. As an example, we present CSS150120:110008+385352, a CRTS transient similar to CSS100217:102913+404220 that occurred within a quiescent AGN and peaked at Mv = -23.6.

Figures (15)

• Figure 1: Example spectra for nearby type-Ia supernovae discovered by CRTS. The age of events in days (in parentheses) relative to maximum light are marked and sorted from youngest (top) to oldest (bottom).
• Figure 2: As in Figure 1, but for a samples of type-IIP supernovae spanning a range of ages, sorted vertically from youngest to oldest.
• Figure 3: Spectra of SLSN-I discovered by CRTS compared to known PTF SLSN-I PTF09cnd and PTF12dam. In the left panel, from top to bottom we present CRTS CSS121210 (= CSS121210:112615+144039), SN 2011ke (= CSS110406:135058+261642), MLS160616 (= MLS160616:160420+392813), and CSS140609 (= CSS140609:224011+095209). In the right panel, we present CRTS SLSN-I CSS141223 (= CSS141223:113342+004332, 2015bn), CSS150522 (= CSS150522:180139+381750) CSS150123 (= CSS150123:102121+524752), SN 2011kf (= CSS111230:143658+163057) and SN 2017dwh (= CSS170425:143443+312917).
• Figure 4: Spectra of type-IIn supernovae discovered by CRTS. In the left panel, we show the spectra of type-IIn supernovae discovered by CRTS with best matching SNID Blo07. The spectra span post-maximum ages (in parentheses) rangeing from 3 to 660 days. In the right panel, we show the diversity of $\rm H_{\alpha}$ line profiles for these same events.
• Figure 5: The nightly-combined rest-frame lightcurves of SN 2008iy (red, solid line) compared with other supernova light curves matched to peak magnitude and timescale (using stretch factor $S$). Left panel: the observed absolute magnitude light curve for SN 2008iy (red points; triangles upper limits, squares detections); type-IIn supernova SN 2006gy Smi07 scaled with $S=4.8$ and matched to 2008iy in peak brightness (blue squares and dashed-line); empirical type-Ia supernova model Parab-18 (black line, Goldhaber et al. 2001) scaled with $S=18.2$ and matched in peak brightness. Right panel: normalised fluxes for the same set of supernova light curves and stretches factors as left panel.