Search

We investigate the spectroscopic characteristics of Intermediate Luminosity Red Transients (ILRTs), a class of elusive objects with peak luminosity between that of classical novae and standard supernovae. We present the extensive optical and near-infrared (NIR) spectroscopic monitoring of four ILRTs, namely NGC 300 2008OT-1, AT 2019abn, AT 2019ahd and AT 2019udc. First we focus on the evolution of the most prominent spectral features observed in the low resolution spectra, then we discuss more in detail the high resolution spectrum collected for NGC 300 2008OT-1 with the Very Large Telescope equipped with UVES. Finally we analyse late time spectra of NGC 300 2008OT-1 and AT 2019ahd through comparisons with both synthetic and observed spectra. Balmer and Ca lines dominate the optical spectra, revealing the presence of slowly moving circumstellar medium (CSM) around the objects. The line luminosity of H$\alpha$, H$\beta$ and Ca II NIR triplet presents a double peaked evolution with time, possibly indicative of interaction between fast ejecta and the slow CSM. The high resolution spectrum of NGC 300 2008OT-1 reveals a complex circumstellar environment, with the transient being surrounded by a slow ($\sim$30 km s$^{-1}$) progenitor wind. At late epochs, optical spectra of NGC 300 2008OT-1 and AT 2019ahd show broad ($\sim$2500 km s$^{-1}$) emission features at $\sim$6170 A and $\sim$7000 A which are unprecedented for ILRTs. We find that these lines originate most likely from the blending of several narrow lines, possibly of iron-peak elements., Comment: 28 pages, 19 figures. Submitted to A&A

We investigate the photometric characteristics of a sample of Intermediate Luminosity Red Transients (ILRTs), a class of elusive objects with peak luminosity between that of classical novae and standard supernovae. We present the multi-wavelength photometric follow-up of four ILRTs, namely NGC 300 2008OT-1, AT 2019abn, AT 2019ahd and AT 2019udc. Through the analysis and modelling of their spectral energy distribution and bolometric light curves we infer the physical parameters associated with these transients. All four objects display a single peaked light curve which ends in a linear decline in magnitudes at late phases. A flux excess with respect to a single black body emission is detected in the infrared domain for three objects in our sample, a few months after maximum. This feature, commonly found in ILRTs, is interpreted as a sign of dust formation. Mid infrared monitoring of NGC 300 2008OT-1 761 days after maximum allows us to infer the presence of $\sim$10$^{-3}$-10$^{-5}$ M$_{\odot}$ of dust, depending on the chemical composition and the grain size adopted. The late time decline of the bolometric light curves of the considered ILRTs is shallower than expected for $^{56}$Ni decay, hence requiring an additional powering mechanism. James Webb Space Telescope observations of AT 2019abn prove that the object has faded below its progenitor luminosity in the mid-infrared domain, five years after its peak. Together with the disappearance of NGC 300 2008OT-1 in Spitzer images seven years after its discovery, this supports the terminal explosion scenario for ILRTs. With a simple semi-analytical model we try to reproduce the observed bolometric light curves in the context of few M$_{\odot}$ of material ejected at few 10$^{3}$ km s$^{-1}$ and enshrouded in an optically thick circumstellar medium., Comment: 21 pages, 15 figures plus 20 additional pages of data in appendix. Submitted to A&A

We present JWST spectral and photometric observations of the Type IIP supernova (SN) 2022acko at ~50 days past explosion. These data are the first JWST spectral observations of a core-collapse SN. We identify ~30 different H I features, other features associated with products produced from the CNO cycle, and s-process elements such as Sc II and Ba II. By combining the JWST spectra with ground-based optical and NIR spectra, we construct a full Spectral Energy Distribution from 0.4 to 25 microns and find that the JWST spectra are fully consistent with the simultaneous JWST photometry. The data lack signatures of CO formation and we estimate a limit on the CO mass of < 10^{-8} solar mass. We demonstrate how the CO fundamental band limits can be used to probe underlying physics during stellar evolution, explosion, and the environment. The observations indicate little mixing between the H envelope and C/O core in the ejecta and show no evidence of dust. The data presented here set a critical baseline for future JWST observations, where possible molecular and dust formation may be seen.

An analysis leveraging 170 optical spectra of 35 stripped-envelope (SE) core-collapse supernovae observed by the Carnegie Supernova Project-I and published in a companion paper is presented. Mean template spectra are constructed for the SNe IIb, Ib and Ic sub-types and parent ions associated with designated spectral features are identified with the aid of the spectral synthesis code SYNAPPS. Our modeled mean spectra suggest the ~6150~\AA\ feature in SNe~IIb may have an underlying contribution due to silicon, while the same feature in some SNe Ib may have an underlying contribution due to hydrogen. Standard spectral line diagnostics consisting of pseudo-equivalent widths (pEW) and blue-shifted Doppler velocity are measured for each of the spectral features. Correlation matrices and rolling mean values of both spectral diagnostics are constructed. A Principle Component Analysis (PCA) is applied to various wavelength ranges of the entire data set and suggests clear separation among the different SE SN sub-types, which follows from trends previously identified in the literature. In addition, our finds reveal the presence of two SNe IIb sub-types, a handful of SNe Ib displaying signatures of weak, high-velocity hydrogen, and a single SN~Ic with evidence of weak helium features. Our PCA results can be leveraged to obtain robust sub-typing of SE SN based on a single spectrum taken during the so-called photospheric phase, separating SNe IIb from SNe Ib with ~80 percent completion., Comment: Version submitted to the publishers

We present 170 optical spectra of 35 low-redshift stripped-envelope core-collapse supernovae observed by the Carnegie Supernova Project-I between 2004 and 2009. The data extend from as early as -19 days (d) prior to the epoch of B-band maximum to +322 d, with the vast majority obtained during the so-called photospheric phase covering the weeks around peak luminosity. In addition to histogram plots characterizing the red-shift distribution, number of spectra per object, and the phase distribution of the sample, spectroscopic classification is also provided following standard criteria. The CSP-I spectra are electronically available and a detailed analysis of the data set is presented in a companion paper being the fifth and final paper of the series, Comment: Updated version that went to the publishers

We present a JWST/MIRI low-resolution mid-infrared (MIR) spectroscopic observation of the normal Type Ia supernova (SN Ia) SN 2021aefx at +323 days past rest-frame B-band maximum light. The spectrum ranges from 4-14 um, and shows many unique qualities including a flat-topped [Ar III] 8.991 um profile, a strongly tilted [Co III] 11.888 um feature, and multiple stable Ni lines. These features provide critical information about the physics of the explosion. The observations are compared to synthetic spectra from detailed NLTE multi-dimensional models. The results of the best-fitting model are used to identify the components of the spectral blends and provide a quantitative comparison to the explosion physics. Emission line profiles and the presence of electron capture (EC) elements are used to constrain the mass of the exploding white dwarf (WD) and the chemical asymmetries in the ejecta. We show that the observations of SN 2021aefx are consistent with an off-center delayed-detonation explosion of a near-Chandrasekhar mass (Mch) WD at a viewing angle of -30 degrees relative to the point of the deflagration-to-detonation transition. From the strength of the stable Ni lines we determine that there is little to no mixing in the central regions of the ejecta. Based on both the presence of stable Ni and the Ar velocity distributions, we obtain a strict lower limit of 1.2 Msun of the initial WD, implying that most sub-Mch explosions models are not viable models for SN 2021aefx. The analysis here shows the crucial importance of MIR spectra for distinguishing between explosion scenarios for SNe Ia., Comment: 21 pages, 9 figures, 4 tables, accepted to ApJL; updated to accepted version

We present ultraviolet (UV) to near-infrared (NIR) observations and analysis of the nearby Type Ia supernova SN 2021fxy. Our observations include UV photometry from Swift/UVOT, UV spectroscopy from HST/STIS, and high-cadence optical photometry with the Swope 1-m telescope capturing intra-night rises during the early light curve. Early $B-V$ colours show SN 2021fxy is the first "shallow-silicon" (SS) SN Ia to follow a red-to-blue evolution, compared to other SS objects which show blue colours from the earliest observations. Comparisons to other spectroscopically normal SNe Ia with HST UV spectra reveal SN 2021fxy is one of several SNe Ia with flux suppression in the mid-UV. These SNe also show blue-shifted mid-UV spectral features and strong high-velocity Ca II features. One possible origin of this mid-UV suppression is the increased effective opacity in the UV due to increased line blanketing from high velocity material, but differences in the explosion mechanism cannot be ruled out. Among SNe Ia with mid-UV suppression, SNe 2021fxy and 2017erp show substantial similarities in their optical properties despite belonging to different Branch subgroups, and UV flux differences of the same order as those found between SNe 2011fe and 2011by. Differential comparisons to multiple sets of synthetic SN Ia UV spectra reveal this UV flux difference likely originates from a luminosity difference between SNe 2021fxy and 2017erp, and not differing progenitor metallicities as suggested for SNe 2011by and 2011fe. These comparisons illustrate the complicated nature of UV spectral formation, and the need for more UV spectra to determine the physical source of SNe Ia UV diversity., Comment: 26 pages, 19 figures, 9 tables; submitted to MNRAS, posted after receiving referee comments

We present high-cadence photometric and spectroscopic observations of SN~2020jfo in ultraviolet and optical/near-infrared bands starting from $\sim 3$ to $\sim 434$ days after the explosion, including the earliest data with the 10.4\,m GTC. SN~2020jfo is a hydrogen-rich Type II SN with a relatively short plateau duration ($67.0 \pm 0.6$ days). When compared to other Type II supernovae (SNe) of similar or shorter plateau lengths, SN~2020jfo exhibits a fainter peak absolute $V$-band magnitude ($M_V = -16.90 \pm 0.34$ mag). SN~2020jfo shows significant H$\alpha$ absorption in the plateau phase similar to that of typical SNe~II. The emission line of stable [Ni~II] $\lambda$7378, mostly seen in low-luminosity SNe~II, is very prominent in the nebular-phase spectra of SN~2020jfo. Using the relative strengths of [Ni~II] $\lambda$7378 and [Fe~II] $\lambda$7155, we derive the Ni/Fe production (abundance) ratio of 0.08--0.10, which is $\sim 1.5$ times the solar value. The progenitor mass of SN~2020jfo from nebular-phase spectral modelling and semi-analytical modelling falls in the range of 12--15\,$M_\odot$. Furthermore, semi-analytical modelling suggests a massive H envelope in the progenitor of SN~2020jfo, which is unlikely for SNe~II having short plateaus., Comment: 20 pages (plus 5 pages appendix), 19 figures, Accepted for publication in MNRAS

If high-mass stars (>20-25 Msun) are the progenitors of stripped-envelope (SE) supernovae (SNe), their massive ejecta should lead to broad, long-duration lightcurves (LCs). Instead, literature samples of SE~SNe have reported relatively narrow LCs with ejecta masses between 1-4 Msun that favor progenitors <20-25 Msun. Working with the untargeted sample of (i)PTF SNe to better constrain their rates, we search for SE~SNe with broad LCs. Using a simple LC stretch compared to a template to measure broadness, we identified eight significantly broader Type~Ibc SNe after applying quantitative sample selection criteria. The LCs, colors, and spectra of these SNe are found to evolve more slowly relative to typical Type~Ibc SNe, proportional with the stretch. Bolometric LC modeling and their nebular spectra indicate high ejecta and nickel masses, assuming radioactive decay powering. Additionally, these objects are preferentially located in low-metallicity host galaxies with high star-formation rates, which may account for their massive progenitors, as well as their relative absence from the literature. Our study thus supports the link between broad LCs (as measured by stretch) and high-mass progenitor stars in SE~SNe with independent evidence from bolometric LC modeling, nebular spectra, host environment properties, and photometric evolution. In the first systematic search of its kind using an untargeted sample, we use the stretch distribution to identify a higher than previously appreciated fraction of SE~SNe with broad LCs (~13%). Correcting for Malmquist and LC duration observational biases, we conservatively estimate that a minimum of ~6% of SE~SNe are consistent with high-mass progenitors. This result has implications for the progenitor channels of SE~SNe, late stages of massive stellar evolution, oxygen fraction in the universe, and formation channels for stellar-mass black holes.(Abridged), Comment: 38 pages, 24 figures, 12 tables. Submitted to A&A

We present photometric and spectroscopic data on three extragalactic luminous red novae (LRNe): AT2018bwo, AT2021afy, and AT2021blu. AT2018bwo was discovered in NGC45 (at 6.8 Mpc) a few weeks after the outburst onset. During the monitoring period, the transient reached a peak luminosity of 10^40 erg/s. AT2021afy, hosted by UGC10043 (49.2 Mpc), showed a double-peaked light curve, with the two peaks reaching a similar luminosity of 2.1(+-0.6)x10^41 erg/s. For AT2021blu in UGC5829, (8.6 Mpc), the pre-outburst phase was well-monitored by several photometric surveys, and the object showed a slow luminosity rise before the outburst. The light curve of AT2021blu was sampled with an unprecedented cadence until the object disappeared behind the Sun, and it was then recovered at late phases. The light curve of AT2021blu shows a double peak, with a prominent early maximum reaching a luminosity of 6.5x10^40 erg/s, which is half of that of AT2021afy. The spectra of AT2021afy and AT2021blu display the expected evolution for LRNe: a blue continuum dominated by prominent Balmer lines in emission during the first peak, and a redder continuum consistent with that of a K-type star with narrow absorption metal lines during the second, broad maximum. The spectra of AT2018bwo are markedly different, with a very red continuum dominated by broad molecular features in absorption. As these spectra closely resemble those of LRNe after the second peak, AT2018bwo was probably discovered at the very late evolutionary stages. This would explain its fast evolution and the spectral properties compatible with that of an M-type star. From the analysis of deep frames of the LRN sites years before the outburst, and considerations of the light curves, the quiescent progenitor systems of the three LRNe were likely massive, with primaries ranging from 13Mo for AT2018bwo, to 13-18Mo for AT2021blu, and over 40Mo for AT2021afy., Comment: 33 pages, 19 figures, 7 tables (plus 3 available at the CDS). Accepted for publication in A&A

Type Ia Supernovae (SNe Ia) have been extensively used as standardisable candles in the optical for several decades. However, SNe Ia have shown to be more homogeneous in the near-infrared (NIR), where the effect of dust extinction is also attenuated. In this work, we explore the possibility of using a low number of NIR observations for accurate distance estimations, given the homogeneity at these wavelengths. We found that one epoch in $J$ and/or $H$ band, plus good $gr$-band coverage, gives an accurate estimation of peak magnitudes in $J$ ($J_{max}$) and $H$ ($H_{max}$) bands. The use of a single NIR epoch only introduces an additional scatter of $\sim0.05$ mag for epochs around the time of $B$-band peak magnitude ($T_{max}$). We also tested the effect of optical cadence and signal-to-noise ratio (S/N) in the estimation of $T_{max}$ and its uncertainty propagation to the NIR peak magnitudes. Both cadence and S/N have a similar contribution, where we constrained the introduced scatter of each to $<0.02$ mag in $J_{max}$ and $<0.01$ in $H_{max}$. However, these effects are expected to be negligible, provided the data quality is comparable to that obtained for observations of nearby SNe ($z\lesssim0.1$). The effect of S/N in the NIR was tested as well. For SNe Ia at $0.08

We present the analysis of SN 2020wnt, an unusual hydrogen-poor super-luminous supernova (SLSN-I), at a redshift of 0.032. The light curves of SN 2020wnt are characterised by an early bump lasting $\sim5$ days, followed by a bright main peak. The SN reaches a peak absolute magnitude of M$_{r}^{max}=-20.52\pm0.03$ mag at $\sim77.5$ days from explosion. This magnitude is at the lower end of the luminosity distribution of SLSNe-I, but the rise-time is one of the longest reported to date. Unlike other SLSNe-I, the spectra of SN 2020wnt do not show O II, but strong lines of C II and Si II are detected. Spectroscopically, SN 2020wnt resembles the Type Ic SN 2007gr, but its evolution is significantly slower. Comparing the bolometric light curve to hydrodynamical models, we find that SN 2020wnt luminosity can be explained by radioactive powering. The progenitor of SN 2020wnt is likely a massive and extended star with a pre-SN mass of 80 M$_\odot$ and a pre-SN radius of 15 R$_\odot$ that experiences a very energetic explosion of $45\times10^{51}$ erg, producing 4 M$_\odot$ of $^{56}$Ni. In this framework, the first peak results from a post-shock cooling phase for an extended progenitor, and the luminous main peak is due to a large nickel production. These characteristics are compatible with the pair-instability SN scenario. We note, however, that a significant contribution of interaction with circumstellar material cannot be ruled out., Comment: Accepted in MNRAS (Sept 2022). 20 pages, 17 figures and 2 tables in the main text; 6 tables in the appendix

We present 75 near-infrared (NIR; 0.8$-$2.5 $\mu$m) spectra of 34 stripped-envelope core-collapse supernovae (SESNe) obtained by the Carnegie Supernova Project-II (CSP-II), encompassing optical spectroscopic Types IIb, Ib, Ic, and Ic-BL. The spectra range in phase from pre-maximum to 80 days past maximum. This unique data set constitutes the largest NIR spectroscopic sample of SESNe to date. NIR spectroscopy provides observables with additional information that is not available in the optical. Specifically, the NIR contains the resonance lines of He I and allows a more detailed look at whether Type Ic supernovae are completely stripped of their outer He layer. The NIR spectra of SESNe have broad similarities, but closer examination through statistical means reveals a strong dichotomy between NIR "He-rich" and "He-poor" SNe. These NIR subgroups correspond almost perfectly to the optical IIb/Ib and Ic/Ic-BL types, respectively. The largest difference between the two groups is observed in the 2 $\mu$m region, near the He I $\lambda$2.0581 $\mu$m line. The division between the two groups is not an arbitrary one along a continuous sequence. Early spectra of He-rich SESNe show much stronger He I $\lambda$2.0581 $\mu$m absorption compared to the He-poor group, but with a wide range of profile shapes. The same line also provides evidence for trace amounts of He in half of our SNe in the He-poor group.

We present the results of our monitoring campaigns of the luminous red novae (LRNe) AT 2020hat in NGC 5068 and AT 2020kog in NGC 6106. The two objects were imaged (and detected) before their discovery by routine survey operations. They show a general trend of slow luminosity rise, lasting at least a few months. The subsequent major LRN outbursts were extensively followed in photometry and spectroscopy. The light curves present an initial short-duration peak, followed by a redder plateau phase. AT 2020kog is a moderately luminous event peaking at ~7 x 10^40 erg/s, while AT 2020hat is almost one order of magnitude fainter than AT 2020kog, although it is still more luminous than V838 Mon. In analogy with other LRNe, the spectra of AT 2020kog change significantly with time. They resemble those of type IIn supernovae at early phases, then they become similar to those of K-type stars during the plateau, and to M-type stars at very late phases. In contrast, AT 2020hat already shows a redder continuum at early epochs, and its spectrum shows the late appearance of molecular bands. A moderate-resolution spectrum of AT 2020hat taken at +37 d after maximum shows a forest of narrow P Cygni lines of metals with velocities of 180 km/s, along with an Halpha emission with a full-width at half-maximum velocity of 250 km/s. For AT 2020hat, a robust constraint on its quiescent progenitor is provided by archival images of the Hubble Space Telescope. The progenitor is clearly detected as a mid-K type star, with an absolute magnitude of MF606W = -3.33+-0.09 mag and a colour of F606W-F814W = 1.14+-0.05 mag, which are inconsistent with the expectations from a massive star that could later produce a core-collapse supernova. Although quite peculiar, the two objects nicely match the progenitor versus light curve absolute magnitude correlations discussed in the literature., Comment: 14 pages, 9 figures, 2 tables. Accepted for publication in A&A

We present the follow-up campaign of the luminous red nova (LRN) AT~2019zhd, the third event of this class observed in M 31. The object was followed by several sky surveys for about five months before the outburst, during which it showed a slow luminosity rise. In this phase, the absolute magnitude ranged from M_r=-2.8+-0.2 mag to M_r=-5.6+-0.1 mag. Then, over a four-five day period, AT 2019zhd experienced a major brightening, reaching at peak M_r=-9.61+-0.08 mag, and an optical luminosity of 1.4x10^39 erg/s. After a fast decline, the light curve settled onto a short-duration plateau in the red bands. Although less pronounced, this feature is reminiscent of the second red maximum observed in other LRNe. This phase was followed by a rapid linear decline in all bands. At maximum, the spectra show a blue continuum with prominent Balmer emission lines. The post-maximum spectra show a much redder continuum, resembling that of an intermediate-type star. In this phase, Halpha becomes very weak, Hbeta is no longer detectable and a forest of narrow absorption metal lines now dominate the spectrum. The latest spectra, obtained during the post-plateau decline, show a very red continuum (T_eff ~ 3000 K) with broad molecular bands of TiO, similar to those of M-type stars. The long-lasting, slow photometric rise observed before the peak resembles that of LRN V1309 Sco, which was interpreted as the signature of the common-envelope ejection. The subsequent outburst is likely due to the gas outflow following a stellar merging event. The inspection of archival HST images taken 22 years before the LRN discovery reveals a faint red source (M_F555W=0.21+-0.14 mag, with F555W-F814W = 2.96+-0.12 mag) at the position of AT 2019zhd, which is the most likely quiescent precursor. The source is consistent with expectations for a binary system including a predominant M5-type star., Comment: 17 pages, 10 figures, 3 tables. Accepted for publication in A&A

Type Ic supernovae represent the explosions of the most stripped massive stars, but their progenitors and explosion mechanisms remain unclear. Larger samples of observed supernovae can help characterize the population of these transients. We present an analysis of 44 spectroscopically normal Type Ic supernovae, with focus on the light curves. The photometric data were obtained over 7 years with the Palomar Transient Factory (PTF) and its continuation, the intermediate Palomar Transient Factory (iPTF). This is the first homogeneous and large sample of SNe Ic from an untargeted survey, and we aim to estimate explosion parameters for the sample. We present K-corrected Bgriz light curves of these SNe, obtained through photometry on template-subtracted images. We performed an analysis on the shape of the $r$-band light curves and confirmed the correlation between the rise parameter Delta m_{-10} and the decline parameter Delta m_{15}. Peak r-band absolute magnitudes have an average of -17.71 +- 0.85 mag. To derive the explosion epochs, we fit the r-band lightcurves to a template derived from a well-sampled light curve. We computed the bolometric light curves using r and g band data, g-r colors and bolometric corrections. Bolometric light curves and Fe II lambda 5169 velocities at peak were used to fit to the Arnett semianalytic model in order to estimate the ejecta mass M_{ej}, the explosion energy E_{K} and the mass of radioactive nickel (M(56) Ni) for each SN. Including 41 SNe, we find average values of =4.50 +-0.79 msun, =1.79 +- 0.29 x10^{51} erg, and = 0.19 +- 0.03 msun. The explosion-parameter distributions are comparable to those available in the literature, but our large sample also includes some transients with narrow and very broad light curves leading to more extreme ejecta masses values., Comment: Submitted to A&A

The Type Ia supernova (SN Ia) LSQ14fmg exhibits exaggerated properties which may help to reveal the origin of the "super-Chandrasekhar" (or 03fg-like) group. The optical spectrum is typical of a 03fg-like SN Ia, but the light curves are unlike those of any SNe Ia observed. The light curves of LSQ14fmg rise extremely slowly. At -23 rest-frame days relative to B-band maximum, LSQ14fmg is already brighter than $M_V$=-19 mag before host extinction correction. The observed color curves show a flat evolution from the earliest observation to approximately one week after maximum. The near-infrared light curves peak brighter than -20.5 mag in the J and H bands, far more luminous than any 03fg-like SNe Ia with near-infrared observations. At one month past maximum, the optical light curves decline rapidly. The early, slow rise and flat color evolution are interpreted to result from an additional excess flux from a power source other than the radioactive decay of the synthesized $^{56}Ni$. The excess flux matches the interaction with a typical superwind of an asymptotic giant branch (AGB) star in density structure, mass-loss rate, and duration. The rapid decline starting at around one month past B-band maximum may be an indication of rapid cooling by active carbon monoxide (CO) formation, which requires a low temperature and high density environment. These peculiarities point to an AGB progenitor near the end of its evolution and the core degenerate scenario as the likely explosion mechanism for LSQ14fmg., Comment: 22 pages, 15 figures, accepted for publication in ApJ

Context: Type Ibn supernovae are a rare class of stripped envelope supernovae interacting with a helium-rich CSM. The majority of the SNe Ibn reported display a surprising homogeneity in their fast lightcurves and starforming hosts. Aims: We present the discovery and study of SN 2020bqj (ZTF20aalrqbu), a SN Ibn with a long-duration peak plateau lasting 40 days and hosted by a faint low-mass galaxy. We aim to explain its peculiar properties using an extensive data set. Methods: We compare the evolution of SN 2020bqj with SNe Ibn from the literature. We fit the bolometric and multi-band lightcurves with different powering mechanism models. Results: The risetime, peak magnitude and spectral features of SN 2020bqj are consistent with those of most SNe Ibn, but the SN is a clear outlier based on its bright, long-lasting peak plateau and low host mass. We show through modeling that the lightcurve can be powered predominantly by shock heating from the interaction of the SN ejecta and a dense CSM. The peculiar Type Ibn SN 2011hw is a close analog to SN 2020bqj, suggesting a similar progenitor and CSM scenario. In this scenario a very massive progenitor star in the transitional phase between a luminous blue variable and a compact Wolf-Rayet star undergoes core-collapse, embedded in a dense helium-rich CSM with an elevated opacity compared to normal SNe Ibn, due to the presence of residual hydrogen. This scenario is consistent with the observed properties of SN 2020bqj and the modeling results. Conclusions: SN 2020bqj is a compelling example of a transitional SN Ibn/IIn based on not only its spectral features, but also its lightcurve, host galaxy properties and the inferred progenitor properties. The strong similarity with SN 2011hw suggests this subclass may be the result of a progenitor in a stellar evolution phase that is distinct from those of progenitors of regular SNe Ibn., Comment: 22 pages, 14 figures, 5 tables. Accepted for publication in Astronomy & Astrophysics. Abstract abridged for arXiv submission

We present multi-wavelength observations of two gap transients followed by the Carnegie Supernova Project-II and supplemented with data obtained by a number of different programs. Here in the first of two papers, we focus on the intermediate luminosity red transient (ILRT) designated SNhunt120, while in a companion paper we examine the luminous red novae AT 2014ej. Our data set for SNhunt120 consists of an early optical discovery, estimated to be within 3 days after outburst, the subsequent optical and near-infrared broadband followup extending over a $\sim$2 month period, two visual- and two near-infrared wavelength spectra, and Spitzer Space Telescope observations extending from early ($+$28 d) to late ($+$1155 d) phases. SNhunt120 resembles other ILRTs such as NGC 300-2008-OT and SN 2008S, and like these other ILRTs, SNhunt120 exhibits prevalent mid-infrared emission at both early and late phases. From the comparison of SNhunt120 and other ILRTs to electron-capture supernova simulations, we find that the current models underestimate the explosion kinetic energy and thereby produce synthetic light curves that over-estimate the luminosity. Finally, examination of pre-outburst Hubble Space Telescope images yields no progenitor detection., Comment: Paper 1 of a series of 2. Eight pages of text, 14 figures, and 10 tables. Replaced to match the version published on A&A

We present optical and near-infrared broadband photometry and optical spectra of AT 2014ej from the the Carnegie Supernova Project-II. These observations are complemented with data from the CHilean Automatic Supernova sEarch, the Public ESO Spectroscopic Survey of Transient Objects, and from the Backyard Observatory Supernova Search. Observational signatures of AT 2014ej reveal that it is similar to other members of the gap-transient subclass known as luminous red novae (LRNe), including the ubiquitous double hump light curve and spectral properties akin to the LRN SN 2017jfs. A medium-dispersion, visual-wavelength spectrum of AT 2014ej taken the Magellan Clay telescope, exhibits a P Cygni H$\alpha$ feature characterized by a blue velocity at zero intensity of $\approx 110$ km s$^{-1}$ and a P Cygni minimum velocity of $\approx70$ km s$^{-1}$, and which we attribute to emission from a circumstellar wind. Inspection of pre-outbust Hubble Space Telescope images yields no conclusive progenitor detection. In comparison with a sample of LRNe from the literature, AT 2014ej lies among the brighter end of the luminosity distribution. Comparison of the ultra-violet, optical, infrared (UVOIR) light curves of well-observed LRNe to common-envelope evolution models from the literature, indicates the models under predict the luminosity of the comparison sample at all phases and also produce inconsistent time-scales of the secondary peak. Future efforts to model LRNe should expand upon the current parameter space explored and therefore may consider more massive systems and a wider range of dynamical timescales., Comment: Paper 2 of a series of 2 papers. Replaced to match the version published on A&A. 8 pages of text, 15 figures, 10 tables

We present a new method to photometrically delineate between various sub-types of type Ia supernovae (SNe Ia). Using the color-stretch parameters, $s_{BV}$ or $s_{gr}$, and the time of i-band primary maximum relative to the B-band or g-band maximum it is demonstrated that 2003fg-like, 1991bg-like, and 2002cx-like SNe Ia can readily be identified. In the cases of these extreme SNe Ia, their primary i-band maximum occurs after the time of the B or g band maxima. We suggest that the timing of the i-band maximum can reveal the physical state of the SN Ia explosion as it traces: i) the speed of the recombination front of iron group elements in the ejecta, ii) the temperature evolution and rate of adiabatic cooling in the ejecta and, iii) the presence of interaction with a stellar envelope. This photometric sub-typing can be used in conjunction with other SNe analysis, such as the Branch diagram, to examine the physics and diversity of SNe Ia. The results here can also be used to screen out non-Ia SNe from cosmological samples that do not have complete spectroscopic typing. Finally, as future surveys like LSST create large databases of light curves of many objects this photometric identification can be used to readily identify and study the rates and bulk properties of peculiar SNe Ia., Comment: accepted for publication in ApjL, one additional 03fg-like SNe add compared to the previous version

Supernova LSQ13abf was discovered soon after explosion by the La Silla-QUEST Survey and followed by the CSP II at optical and near-IR wavelengths. Our analysis indicates LSQ13abf was discovered within two days of explosion and its first 10 days of evolution reveal a B-band light curve with an abrupt drop in luminosity. Contemporaneously, the V-band light curve exhibits a rise towards a first peak and the r- and i-band light curves show no early peak. The early light-curve evolution of LSQ13abf is reminiscent of the post explosion cooling phase observed in the Type Ib SN 2008D, and the similarity between the two objects extends over weeks. Spectroscopically, LSQ13abf resembles SN 2008D with P Cygni He I features that strengthen over time. Spectral energy distributions are constructed from broad-band photometry, and by fitting black-body (BB) functions a UVOIR light curve is constructed, and the underlying BB-temperature and BB-radius profiles are estimated. Explosion parameters are estimated by simultaneously fitting an Arnett model to the UVOIR light curve and the velocity evolution derived from spectral features, and a post-shock breakout cooling model to the first two epochs of the bolometric evolution. This combined model suggests an explosion energy of 1.3x10$^{51}$ ergs, a relatively high ejecta mass of 5.94 M$_{\odot}$, a Ni mass of 0.16 M$_{\odot}$, and a progenitor-star radius of 28.0 R$_{\odot}$. The ejecta mass suggests the origins of LSQ13abf lie with a >25 M$_{\odot}$ ZAMS progenitor and its radius is three and nine times larger than values estimated from the same analysis applied to observations of SNe 2008D and 1999ex, respectively. Alternatively, comparison of hydrodynamical simulations of >20-25 M$_{\odot}$ ZAMS progenitors that evolve to pre-SN envelope masses around 10 M$_{\odot}$ and extended (~100 R$_{\odot}$) envelopes also match the observations of LSQ13abf., Comment: 29 pages, including 9 of text, 17 figures, and 11 tables. Includes response to referee's comments. Constructive comments are welcome. Abstract is abridged

Supernovae (SNe) Type Ibn are rapidly evolving and bright (M$_\text{R,peak}$ $\sim-19$) transients interacting with He-rich circumstellar material (CSM). SN 2018bcc, detected by the ZTF shortly after explosion, provides the best constraints on the shape of the rising light curve (LC) of a fast Type Ibn. Aims: We used the high-quality data set of SN 2018bcc to study observational signatures of the class. Additionally, the powering mechanism of SN 2018bcc offers insights into the debated progenitor connection of Type Ibn SNe. Methods: We compared well-constrained LC properties obtained from empirical models are compared with the literature. We fit the pseudo-bolometric LC with semi-analytical models powered by radioactive decay and CSM interaction. Finally, we modeled the line profiles and emissivity of the prominent He I lines, in order to study the formation of P-Cygni profiles and estimate CSM properties. Results: SN 2018bcc had a rise time to peak of $5.6^{+0.2}_{-0.1}$ days in the restframe with a rising shape power-law index close to 2, and seems to be a typical rapidly evolving Type Ibn SN. The spectrum lacked signatures of SN-like ejecta and was dominated by over 15 He emission features at 20 days past peak, alongside Ca and Mg, all with V$_{\text{FWHM}} \sim 2000~\text{km}~\text{s}^{-1}$. The luminous and rapidly evolving LC could be powered by CSM interaction but not by the decay of radioactive $^{56}$Ni. Modeling of the He I lines indicated a dense and optically thick CSM that can explain the P-Cygni profiles. Conclusions: Like other rapidly-evolving Type Ibn SNe, SN 2018bcc is a luminous transient with a rapid rise to peak powered by shock interaction inside a dense and He-rich CSM. Its spectra do not support the existence of two Type Ibn spectral classes. We also note the remarkable observational match to pulsational pair instability (PPI) SN models., Comment: Accepted for publication in the extragalactic astronomy section of A&A. 24 pages, 17 Figures. Data are made public via WiseRep

We present a sample of supernovae Type IIn (SNe IIn) from the untargeted, magnitude-limited surveys of the Palomar Transient Factory (PTF) and its successor, the intermediate PTF (iPTF). The SNe IIn found and followed by the PTF/iPTF were used to select a sample of 42 events with useful constraints on the rise times as well as with available post-peak photometry. The sample SNe were discovered in 2009-2016 and have at least one low-resolution classification spectrum, as well as photometry from the P48 and P60 telescopes at Palomar Observatory. We study the light-curve properties of these SNe IIn using spline fits (for the peak and the declining portion) and template matching (for the rising portion). We find that the typical rise times are divided into fast and slow risers at $20\pm6$ d and $50\pm11$ d, respectively. The decline rates are possibly divided into two clusters, but this division has weak statistical significance. We find no significant correlation between the peak luminosity of SNe IIn and their rise times, but the more luminous SNe IIn are generally found to be more long-lasting. Slowly rising SNe IIn are generally found to decline slowly. The SNe in our sample were hosted by galaxies of absolute magnitude $-22 \lesssim M_g \lesssim -13$ mag. The K-corrections at light-curve peak of the SNe IIn in our sample are found to be within 0.2 mag for the observer's frame $r$-band, for SNe at redshifts $z < 0.25$. By applying K-corrections and also including ostensibly "superluminous" SNe IIn, we find that the peak magnitudes are $M_{\rm peak}^{r} = -19.18\pm1.32$ mag. We conclude that the occurrence of conspicuous light-curve bumps in SNe IIn, such as in iPTF13z, are limited to $1.4^{+14.6}_{-1.0} \%$ of the SNe IIn. We also investigate a possible sub-type of SNe IIn with a fast rise to a $\gtrsim 50$ d plateau followed by a slow, linear decline., Comment: Matches journal version. Table 2 on CDS. Durations in Fig. 13 corrected, conclusions unchanged. Abstract abridged. 33 pages, 22 figures, 7 tables

We present observations and analysis of 18 stripped-envelope supernovae observed during 2013 -- 2018. This sample consists of 5 H/He-rich SNe, 6 H-poor/He-rich SNe, 3 narrow lined SNe Ic and 4 broad lined SNe Ic. The peak luminosity and characteristic time-scales of the bolometric light curves are calculated, and the light curves modelled to derive 56Ni and ejecta masses (MNi and Mej). Additionally, the temperature evolution and spectral line velocity-curves of each SN are examined. Analysis of the [O I] line in the nebular phase of eight SNe suggests their progenitors had initial masses $<20$ Msun. The bolometric light curve properties are examined in combination with those of other SE events from the literature. The resulting dataset gives the Mej distribution for 80 SE-SNe, the largest such sample in the literature to date, and shows that SNe Ib have the lowest median Mej, followed by narrow lined SNe Ic, H/He-rich SNe, broad lined SNe Ic, and finally gamma-ray burst SNe. SNe Ic-6/7 show the largest spread of Mej, ranging from $\sim 1.2 - 11$ Msun, considerably greater than any other subtype. For all SE-SNe $<$Mej$>=2.8\pm{1.5}$ Msun which further strengthens the evidence that SE-SNe arise from low mass progenitors which are typically $<5$ Msun at the time of explosion, again suggesting Mzams $<25$ Msun. The low $<$Mej$>$ and lack of clear bimodality in the distribution implies $<30$ Msun progenitors and that envelope stripping via binary interaction is the dominant evolutionary pathway of these SNe., Comment: Accepted for publication in MNRAS, no changes to the previous submission

We study 34 Type Ic supernovae that have broad spectral features (SNe Ic-BL). We obtained our photometric data with the Palomar Transient Factory (PTF) and its continuation, the intermediate Palomar Transient Factory (iPTF). This is the first large, homogeneous sample of SNe Ic-BL from an untargeted survey. Furthermore, given the high cadence of (i)PTF, most of these SNe were discovered soon after explosion. We present K-corrected $Bgriz$ light curves of these SNe, obtained through photometry on template-subtracted images. We analyzed the shape of the $r$-band light curves, finding a correlation between the decline parameter $\Delta m_{15}$ and the rise parameter $\Delta m_{-10}$. We studied the SN colors and, based on $g-r$, we estimated the host-galaxy extinction. Peak $r$-band absolute magnitudes have an average of $-18.6\pm0.5$ mag. We fit each $r$-band light curve with that of SN 1998bw (scaled and stretched) to derive the explosion epochs. We computed the bolometric light curves using bolometric corrections, $r$-band data, and $g-r$ colors. Expansion velocities from Fe II were obtained by fitting spectral templates of SNe Ic. Bolometric light curves and velocities at peak were fitted using the semianalytic Arnett model to estimate ejecta mass $M_{\rm ej}$, explosion energy $E_{K}$ and $^{56}$Ni mass $M(^{56}$Ni). We find average values of $M_{\rm ej} = 4\pm3~{\rm M}_{\odot}$, $E_{K} = (7\pm6) \times 10^{51}~$erg, and $M(^{56}$Ni) $= 0.31\pm0.16~{\rm M}_{\odot}$. We also estimated the degree of $^{56}$Ni mixing using scaling relations derived from hydrodynamical models and we find that all the SNe are strongly mixed. The derived explosion parameters imply that at least 21% of the progenitors of SNe Ic-BL are compatible with massive ($>28~{\rm M}_{\odot}$), possibly single stars, whereas at least 64% might come from less massive stars in close binary systems., Comment: 53 pages, 40 figures, 6 tables, accepted for publication in Astronomy and Astrophysics. Abstract abridged to fit the arXiv limit

We present multi-wavelength observations of the tidal disruption event (TDE) iPTF15af, discovered by the intermediate Palomar Transient Factory (iPTF) survey at redshift $z=0.07897$. The optical and ultraviolet (UV) light curves of the transient show a slow decay over five months, in agreement with previous optically discovered TDEs. It also has a comparable black-body peak luminosity of $L_{\rm{peak}} \approx 1.5 \times 10^{44}$ erg/s. The inferred temperature from the optical and UV data shows a value of (3$-$5) $\times 10^4$ K. The transient is not detected in X-rays up to $L_X < 3 \times 10^{42}$erg/s within the first five months after discovery. The optical spectra exhibit two distinct broad emission lines in the He II region, and at later times also H$\alpha$ emission. Additionally, emission from [N III] and [O III] is detected, likely produced by the Bowen fluorescence effect. UV spectra reveal broad emission and absorption lines associated with high-ionization states of N V, C IV, Si IV, and possibly P V. These features, analogous to those of broad absorption line quasars (BAL QSOs), require an absorber with column densities $N_{\rm{H}} > 10^{23}$ cm$^{-2}$. This optically thick gas would also explain the non-detection in soft X-rays. The profile of the absorption lines with the highest column density material at the largest velocity is opposite that of BAL QSOs. We suggest that radiation pressure generated by the TDE flare at early times could have provided the initial acceleration mechanism for this gas. Spectral UV line monitoring of future TDEs could test this proposal., Comment: 20 pages, 12 figures, published in ApJ

We present an analysis of 507 spectra of 173 stripped-envelope (SE) supernovae (SNe) discovered by the untargeted Palomar Transient Factory (PTF) and intermediate PTF (iPTF) surveys. Our sample contains 55 Type IIb SNe (SNe IIb), 45 Type Ib SNe (SNe Ib), 56 Type Ic SNe (SNe Ic), and 17 Type Ib/c SNe (SNe Ib/c). We compare the SE SN subtypes via measurements of the pseudo-equivalent widths (pEWs) and velocities of the He I $\lambda\lambda5876, 7065$ and O I $\lambda7774$ absorption lines. Consistent with previous work, we find that SNe Ic show higher pEWs and velocities in O I $\lambda7774$ compared to SNe IIb and Ib. The pEWs of the He I $\lambda\lambda5876, 7065$ lines are similar in SNe Ib and IIb after maximum light. The He I $\lambda\lambda5876, 7065$ velocities at maximum light are higher in SNe Ib compared to SNe IIb. We have identified an anticorrelation between the He I $\lambda7065$ pEW and O I $\lambda7774$ velocity among SNe IIb and Ib. This can be interpreted as a continuum in the amount of He present at the time of explosion. It has been suggested that SNe Ib and Ic have similar amounts of He, and that lower mixing could be responsible for hiding He in SNe Ic. However, our data contradict this mixing hypothesis. The observed difference in the expansion rate of the ejecta around maximum light of SNe Ic ($V_{\mathrm{m}}=\sqrt{2E_{\mathrm{k}}/M_{\mathrm{ej}}}\approx15,000$ km s$^{-1}$) and SNe Ib ($V_{\mathrm{m}}\approx9000$ km s$^{-1}$) would imply an average He mass difference of $\sim1.4$ $M_{\odot}$, if the other explosion parameters are assumed to be unchanged between the SE SN subtypes. We conclude that SNe Ic do not hide He but lose He due to envelope stripping., Comment: 18 pages, 6 figures, accepted for publication in Astronomy and Astrophysics

iPTF15dtg is a Type Ic supernova (SN) showing a broad light curve around maximum light, consistent with massive ejecta if we assume a radioactive-powering scenario. We study the late-time light curve of iPTF15dtg, which turned out to be extraordinarily luminous for a stripped-envelope (SE) SN. We compare the observed light curves to those of other SE SNe and also with models for the $^{56}$Co decay. We analyze and compare the spectra to nebular spectra of other SE SNe. We build a bolometric light curve and fit it with different models, including powering by radioactivity, magnetar powering, as well as a combination of the two. Between 150 d and 750 d past explosion, iPTF15dtg's luminosity declined by merely two magnitudes instead of the six magnitudes expected from $^{56}$Co decay. This is the first spectroscopically-regular SE SN showing this behavior. The model with both radioactivity and magnetar powering provides the best fit to the light curve and appears to be the more realistic powering mechanism. An alternative mechanism might be CSM interaction. However, the spectra of iPTF15dtg are very similar to those of other SE SNe, and do not show signs of strong CSM interaction. iPTF15dtg is the first spectroscopically-regular SE SN whose light curve displays such clear signs of a magnetar contributing to the powering of the late time light curve. Given this result, the mass of the ejecta needs to be revised to a lower value, and therefore the progenitor mass could be significantly lower than the previously estimated $>$35 $M_{\odot}$., Comment: 9 pages, 8 figures, accepted for publication in Astronomy and Astrophysics

We performed a wide-area (2000 deg$^{2}$) g and I band experiment as part of a two month extension to the Intermediate Palomar Transient Factory. We discovered 36 extragalactic transients including iPTF17lf, a highly reddened local SN Ia, iPTF17bkj, a new member of the rare class of transitional Ibn/IIn supernovae, and iPTF17be, a candidate luminous blue variable outburst. We do not detect any luminous red novae and place an upper limit on their rate. We show that adding a slow-cadence I band component to upcoming surveys such as the Zwicky Transient Facility will improve the photometric selection of cool and dusty transients., Comment: 14 pages, 17 figures. Accepted to PASP

We study PTF11mnb, a He-poor supernova (SN) whose pre-peak light curves (LCs) resemble those of SN 2005bf, a peculiar double-peaked stripped-envelope (SE) SN. LCs, colors and spectral properties are compared to those of SN 2005bf and normal SE SNe. A bolometric LC is built and modeled with the SNEC hydrodynamical code explosion of a MESA progenitor star, as well as with semi-analytic models. The LC of PTF11mnb turns out to be similar to that of SN 2005bf until $\sim$50 d, when the main (secondary) peaks occur at $-18.5$ mag. The early peak occurs at $\sim$20 d, and is about 1.0 mag fainter. After the main peak, the decline rate of PTF11mnb is remarkably slower than that of SN 2005bf, and it traces the $^{56}$Co decay rate. The spectra of PTF11mnb reveal no traces of He unlike in the case of SN Ib 2005bf. The bolometric LC is well reproduced by the explosion of a massive ($M_{ej} =$ 7.8 $M_{\odot}$), He-poor star with a double-peaked $^{56}$Ni distribution, a total $^{56}$Ni mass of 0.59 $M_{\odot}$ and an explosion energy of 2.2$\times$10$^{51}$ erg. Alternatively, a normal SN Ib/c explosion [M($^{56}$Ni)$=$0.11 $M_{\odot}$, $E_{K}$ = 0.2$\times$10$^{51}$ erg, $M_{ej} =$ 1 $M_{\odot}$] can power the first peak while a magnetar ($B$=5.0$\times$10$^{14}$ G, $P=18.1$ ms) provides energy for the main peak. The early $g$-band LC implies a radius of at least 30 $R_{\odot}$. If PTF11mnb arose from a massive He-poor star characterized by a double-peaked $^{56}$Ni distribution, the ejecta mass and the absence of He imply a large ZAMS mass ($\sim85 M_{\odot}$) for the progenitor, which most likely was a Wolf-Rayet star, surrounded by an extended envelope formed either by a pre-SN eruption or due to a binary configuration. Alternatively, PTF11mnb could be powered by a normal SE SN during the first peak and by a magnetar afterwards., Comment: Abstract abridged to fit allowed limit; revised version after addressing referee's comments; comments are welcome, 14 pages, 12 figures

Wide-field surveys are discovering a growing number of rare transients whose physical origin is not yet well understood. Here, we present optical and UV data and analysis of iPTF16asu, a luminous, rapidly-evolving, high velocity, stripped-envelope supernova. With a rest-frame rise-time of just 4 days and a peak absolute magnitude of $M_{\rm g}=-20.4$ mag, the light curve of iPTF16asu is faster and more luminous than previous rapid transients. The spectra of iPTF16asu show a featureless, blue continuum near peak that develops into a Type Ic-BL spectrum on the decline. We show that while the late-time light curve could plausibly be powered by $^{56}$Ni decay, the early emission requires a different energy source. Non-detections in the X-ray and radio strongly constrain any associated gamma-ray burst to be low-luminosity. We suggest that the early emission may have been powered by either a rapidly spinning-down magnetar, or by shock breakout in an extended envelope of a very energetic explosion. In either scenario a central engine is required, making iPTF16asu an intriguing transition object between superluminous supernovae, Type Ic-BL supernovae, and low-energy gamma-ray bursts., Comment: ApJ in press; matches published version. Minor changes following referee's comments; conclusions unchanged

Type Ibn supernovae (SNe Ibn) are thought to be the core-collapse explosions of massive stars whose ejecta interact with He-rich circumstellar material (CSM). We report the discovery of a SN Ibn, with the longest rise-time ever observed, OGLE-2014-SN-131. We discuss the potential powering mechanisms and the progenitor nature of this peculiar stripped-envelope (SE), circumstellar-interacting SN. Optical photometry and spectroscopy were obtained with multiple telescopes including VLT, NTT, and GROND. We compare light curves and spectra with those of other known SNe Ibn and Ibc. CSM velocities are derived from the spectral analysis. The SN light curve is modeled under different assumptions about its powering mechanism (${^{56}}$Ni decay, CSM-interaction, magnetar) in order to estimate the SN progenitor parameters. OGLE-2014-SN-131 spectroscopically resembles SNe Ibn such as SN 2010al. Its peak luminosity and post-peak colors are also similar to those of other SNe Ibn. However, it shows an unprecedentedly long rise-time and a much broader light curve compared to other SNe Ibn. Its bolometric light curve can be reproduced by magnetar and CSM-interaction models, but not by a ${^{56}}$Ni-decay powering model. To explain the unusually long rise-time, the broad light curve, the light curve decline, and the spectra characterized by narrow emission lines, we favor a powering mechanism where the SN ejecta are interacting with a dense CSM. The progenitor of OGLE-2014-SN-131 was likely a Wolf-Rayet star with a mass greater than that of a typical SN Ibn progenitor, which expelled the CSM that the SN is interacting with., Comment: 14 pages, 11 Figures. Accepted for publication in A&A

Context: Research on supernovae (SNe) over the past decade has confirmed that there is a distinct class of events which are much more luminous (by $\sim2$ mag) than canonical core-collapse SNe (CCSNe). These events with visual peak magnitudes $\lesssim-21$ are called superluminous SNe (SLSNe). Aims: There are a few intermediate events which have luminosities between these two classes. Here we study one such object, SN 2012aa. Methods: The optical photometric and spectroscopic follow-up observations of the event were conducted over a time span of about 120 days. Results: With V_abs at peak ~-20 mag, the SN is an intermediate-luminosity transient between regular SNe Ibc and SLSNe. It also exhibits an unusual secondary bump after the maximum in its light curve. We interpret this as a manifestation of SN-shock interaction with the CSM. If we would assume a $^{56}$Ni-powered ejecta, the bolometric light curve requires roughly 1.3 M_sun of $^{56}$Ni and an ejected mass of ~14 M_sun. This would also imply a high kinetic energy of the explosion, ~5.4$\times10^{51}$ ergs. On the other hand, the unusually broad light curve along with the secondary peak indicate the possibility of interaction with CSM. The third alternative is the presence of a central engine releasing spin energy that eventually powers the light curve over a long time. The host of the SN is a star-forming Sa/Sb/Sbc galaxy. Conclusions: Although the spectral properties and velocity evolution of SN 2012aa are comparable to those of normal SNe Ibc, its broad light curve along with a large peak luminosity distinguish it from canonical CCSNe, suggesting the event to be an intermediate-luminosity transient between CCSNe and SLSNe at least in terms of peak luminosity. We argue that SN 2012aa belongs to a subclass where CSM interaction plays a significant role in powering the SN, at least during the initial stages of evolution., Comment: 21 pages, 15 figures, accepted for publication in Astronomy & Astrophysics

We investigate two stripped-envelope supernovae (SNe) discovered in the nearby galaxy NGC 5806 by the (i)PTF. These SNe, designated PTF12os/SN 2012P and iPTF13bvn, exploded at a similar distance from the host-galaxy center. We classify PTF12os as a Type IIb SN based on our spectral sequence; iPTF13bvn has previously been classified as Type Ib having a likely progenitor with zero age main sequence (ZAMS) mass below ~17 solar masses. Our main objective is to constrain the explosion parameters of iPTF12os and iPTF13bvn, and to put constraints on the SN progenitors. We present comprehensive datasets on the SNe, and introduce a new reference-subtraction pipeline (FPipe) currently in use by the iPTF. We perform a detailed study of the light curves (LCs) and spectral evolution of the SNe. The bolometric LCs are modeled using the hydrodynamical code HYDE. We use nebular models and late-time spectra to constrain the ZAMS mass of the progenitors. We perform image registration of ground-based images of PTF12os to archival HST images of NGC 5806 to identify a potential progenitor candidate. Our nebular spectra of iPTF13bvn indicate a low ZAMS mass of ~12 solar masses for the progenitor. The late-time spectra of PTF12os are consistent with a ZAMS mass of ~15 solar masses. We successfully identify a progenitor candidate to PTF12os using archival HST images. This source is consistent with being a cluster of massive stars. Our hydrodynamical modeling suggests that the progenitor of PTF12os had a compact He core with a mass of 3.25 solar masses, and that 0.063 solar masses of strongly mixed 56Ni was synthesized. Spectral comparisons to the Type IIb SN 2011dh indicate that the progenitor of PTF12os was surrounded by a hydrogen envelope with a mass lower than 0.02 solar masses. We also find tentative evidence that the progenitor of iPTF13bvn could have been surrounded by a small amount of hydrogen., Comment: 34 pages, 18 figures, accepted by A&A

Type Ic supernovae (SNe Ic) arise from the core-collapse of H (and He) poor stars, which could be either single WR stars or lower-mass stars stripped of their envelope by a companion. Their light curves are radioactively powered and usually show a fast rise to peak ($\sim$10-15 d), without any early (first few days) emission bumps (with the exception of broad-lined SNe Ic) as sometimes seen for other types of stripped-envelope SNe (e.g., Type IIb SN 1993J and Type Ib SN 2008D). We have studied iPTF15dtg, a spectroscopically normal SN Ic with an early excess in the optical light curves followed by a long ($\sim$30 d) rise to the main peak. It is the first spectroscopically-normal double-peaked SN Ic observed. We aim to determine the properties of this explosion and of its progenitor star. Optical photometry and spectroscopy of iPTF15dtg was obtained with multiple telescopes. The resulting light curves and spectral sequence are analyzed and modelled with hydrodynamical and analytical models, with particular focus on the early emission. Results. iPTF15dtg is a slow rising SN Ic, similar to SN 2011bm. Hydrodynamical modelling of the bolometric properties reveals a large ejecta mass ($\sim$10 $M_{\odot}$) and strong $^{56}$Ni mixing. The luminous early emission can be reproduced if we account for the presence of an extended ($\sim$500 R$_{\odot}$), low-mass ($\sim$0.045 M$_{\odot}$) envelope around the progenitor star. Alternative scenarios for the early peak, such as the interaction with a companion, a shock-breakout (SBO) cooling tail from the progenitor surface, or a magnetar-driven SBO are not favored. The large ejecta mass and the presence of H and He free extended material around the star suggest that the progenitor of iPTF15dtg was a massive ($\gtrsim$ 35 M$_{\odot}$) WR star suffering strong mass loss., Comment: Resubmitted to Astronomy and Astrophysics, 13 pages, 14 figures