The SED Machine: a robotic spectrograph for fast transient classification

Current time domain facilities are finding several hundreds of transient astronomical events a year. The discovery rate is expected to increase in the future as soon as new surveys such as the Zwicky Transient Facility (ZTF) and the Large Synoptic Sky Survey (LSST) come on line. At the present time, the rate at which transients are classified is approximately one order or magnitude lower than the discovery rate, leading to an increasing "follow-up drought". Existing telescopes with moderate aperture can help address this deficit when equipped with spectrographs optimized for spectral classification. Here, we provide an overview of the design, operations and first results of the Spectral Energy Distribution Machine (SEDM), operating on the Palomar 60-inch telescope (P60). The instrument is optimized for classification and high observing efficiency. It combines a low-resolution (R$\sim$100) integral field unit (IFU) spectrograph with "Rainbow Camera" (RC), a multi-band field acquisition camera which also serves as multi-band (ugri) photometer. The SEDM was commissioned during the operation of the intermediate Palomar Transient Factory (iPTF) and has already proved lived up to its promise. The success of the SEDM demonstrates the value of spectrographs optimized to spectral classification. Introduction of similar spectrographs on existing telescopes will help alleviate the follow-up drought and thereby accelerate the rate of discoveries.Comment: 21 pages, 20 figure

Search for precursor eruptions among Type IIb supernovae

The progenitor stars of several Type IIb supernovae (SNe) show indications for extended hydrogen envelopes. These envelopes might be the outcome of luminous energetic pre-explosion events, so-called precursor eruptions. We use the Palomar Transient Factory (PTF) pre-explosion observations of a sample of 27 nearby Type IIb SNe to look for such precursors during the final years prior to the SN explosion. No precursors are found when combining the observations in 15-day bins, and we calculate the absolute-magnitude-dependent upper limit on the precursor rate. At the 90% confidence level, Type IIb SNe have on average $<0.86$ precursors as bright as absolute $R$-band magnitude $-14$ in the final 3.5 years before the explosion and $<0.56$ events over the final year. In contrast, precursors among SNe IIn have a $\gtrsim 5$ times higher rate. The kinetic energy required to unbind a low-mass stellar envelope is comparable to the radiated energy of a few-weeks-long precursor which would be detectable for the closest SNe in our sample. Therefore, mass ejections, if they are common in such SNe, are radiatively inefficient or have durations longer than months. Indeed, when using 60-day bins a faint precursor candidate is detected prior to SN 2012cs ($\sim2$% false-alarm probability). We also report the detection of the progenitor of SN 2011dh which does not show detectable variability over the final two years before the explosion. The suggested progenitor of SN 2012P is still present, and hence is likely a compact star cluster, or an unrelated object.Comment: 16 pages, 10 figure

Lightcurve and spectral modelling of the Type IIb SN 2020acat. Evidence for a strong Ni bubble effect on the diffusion time

We use the light curve and spectral synthesis code JEKYLL to calculate a set of macroscopically mixed Type IIb supernova (SN) models, which are compared to both previously published and new late-phase observations of SN 2020acat. The models differ in the initial mass, the radial mixing and expansion of the radioactive material, and the properties of the hydrogen envelope. The best match to the photospheric and nebular spectra and lightcurves of SN 2020acat is found for a model with an initial mass of 17 solar masses, strong radial mixing and expansion of the radioactive material, and a 0.1 solar mass hydrogen envelope with a low hydrogen mass-fraction of 0.27. The most interesting result is that strong expansion of the clumps containing radioactive material seems to be required to fit the observations of SN 2020acat both in the diffusion phase and the nebular phase. These "Ni bubbles" are expected to expand due to heating from radioactive decays, but the degree of expansion is poorly constrained. Without strong expansion there is a tension between the diffusion phase and the subsequent evolution, and models that fit the nebular phase produce a diffusion peak that is too broad. The diffusion phase lightcurve is sensitive to the expansion of the "Ni bubbles", as the resulting Swiss-cheese-like geometry decreases the effective opacity and therefore the diffusion time. This effect has not been taken into account in previous lightcurve modelling of stripped-envelope SNe, which may lead to a systematic underestimate of their ejecta masses. It should be emphasized, though, that JEKYLL is limited to a geometry that is spherically symmetric on average, and large-scale asymmetries may also play a role. The relatively high initial mass found for the progenitor of SN 2020acat places it at the upper end of the mass distribution of Type IIb SN progenitors, and a single star origin can not be excluded.Comment: Accepted for publication by Astronomy and Astrophysic

Supernova PTF12glz: a possible shock breakout driven through an aspherical wind

We present visible-light and ultraviolet (UV) observations of the supernova PTF12glz. The SN was discovered and monitored in near-UV and R bands as part of a joint GALEX and Palomar Transient Factory campaign. It is among the most energetic Type IIn supernovae observed to date (~10^51erg). If the radiated energy mainly came from the thermalization of the shock kinetic energy, we show that PTF12glz was surrounded by ~1 solar mass of circumstellar material (CSM) prior to its explosive death. PTF12glz shows a puzzling peculiarity: at early times, while the freely expanding ejecta are presumably masked by the optically thick CSM, the radius of the blackbody that best fits the observations grows at ~8000km/s. Such a velocity is characteristic of fast moving ejecta rather than optically thick CSM. This phase of radial expansion takes place before any spectroscopic signature of expanding ejecta appears in the spectrum and while both the spectroscopic data and the bolometric luminosity seem to indicate that the CSM is optically thick. We propose a geometrical solution to this puzzle, involving an aspherical structure of the CSM around PTF12glz. By modeling radiative diffusion through a slab of CSM, we show that an aspherical geometry of the CSM can result in a growing effective radius. This simple model also allows us to recover the decreasing blackbody temperature of PTF12glz. SLAB-Diffusion, the code we wrote to model the radiative diffusion of photons through a slab of CSM and evaluate the observed radius and temperature, is made available on-line.Comment: Sumbitted to ApJ. Comments are welcom

AT2019wxt: An ultra-stripped supernova candidate discovered in electromagnetic follow-up of a gravitational wave trigger

We present optical, radio and X-ray observations of a rapidly-evolving transient AT2019wxt (PS19hgw), discovered during the search for an electromagnetic (EM) counterpart to the gravitational-wave (GW) trigger S191213g (LIGO Scientific Collaboration & Virgo Collaboration 2019a). Although S191213g was not confirmed as a significant GW event in the off-line analysis of LIGO-Virgo data, AT2019wxt remained an interesting transient due its peculiar nature. The optical/NIR light curve of AT2019wxt displayed a double-peaked structure evolving rapidly in a manner analogous to currently know ultra-stripped supernovae (USSNe) candidates. This double-peaked structure suggests presence of an extended envelope around the progenitor, best modelled with two-components: i) early-time shock-cooling emission and ii) late-time radioactive $^{56}$Ni decay. We constrain the ejecta mass of AT2019wxt at $M_{ej} \approx{0.20 M_{\odot}}$ which indicates a significantly stripped progenitor that was possibly in a binary system. We also followed-up AT2019wxt with long-term Chandra and Jansky Very Large Array observations spanning $\sim$260 days. We detected no definitive counterparts at the location of AT2019wxt in these long-term X-ray and radio observational campaigns. We establish the X-ray upper limit at $9.93\times10^{-17}$ erg cm$^{-2}$ s$^{-1}$ and detect an excess radio emission from the region of AT2019wxt. However, there is little evidence for SN1993J- or GW170817-like variability of the radio flux over the course of our observations. A substantial host galaxy contribution to the measured radio flux is likely. The discovery and early-time peak capture of AT2019wxt in optical/NIR observation during EMGW follow-up observations highlights the need of dedicated early, multi-band photometric observations to identify USSNe.Comment: 20 pages, 14 figures, Submitted to Ap