The luminous late-time emission of the type Ic supernova iPTF15dtg - evidence for powering from a magnetar?

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 Astrophysic

The Broad Absorption Line Tidal Disruption Event iPTF15af: Optical and Ultraviolet Evolution

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 Ap

iPTF15dtg: a double-peaked Type Ic supernova from a massive progenitor

Context. Type Ic supernovae (SNe Ic) arise from the core-collapse of H- (and He-) poor stars, which could either be single Wolf-Rayet (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 (~10−15 d), without any early (in the 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). Aims. We have studied iPTF15dtg, a spectroscopically normal SN Ic with an early excess in the optical light curves followed by a long (~30 d) rise to the main peak. It is the first spectroscopically-normal double-peaked SN Ic to be observed. Our aim is to determine the properties of this explosion and of its progenitor star. Methods. Optical photometry and spectroscopy of iPTF15dtg was obtained with multiple telescopes. The resulting light curves and spectral sequence are analyzed and modeled 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 modeling of the bolometric properties reveals a large ejecta mass (~10 M_⊙) and strong ^(56)Ni mixing. The luminous early emission can be reproduced if we account for the presence of an extended (≳500 R_⊙), low-mass (≳0.045 M_⊙) 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. Conclusions. 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 (≳35 M_⊙) WR star that experienced strong mass loss

An updated measurement of the Hubble constant from near-infrared observations of Type Ia supernovae

We present a measurement of the Hubble constant ($H_0$) using type Ia supernova (SNe Ia) in the near-infrared (NIR) from the recently updated sample of SNe Ia in nearby galaxies with distances measured via Cepheid period-luminosity relations by the SHOES project. We collect public near-infrared photometry of up to 19 calibrator SNe Ia and further 57 SNe Ia in the Hubble flow ($z>0.01$), and directly measure their peak magnitudes in the $J$ and $H$ band by Gaussian processes and spline interpolation. Calibrator peak magnitudes together with Cepheid-based distances are used to estimate the average absolute magnitude in each band, while Hubble-flow SNe are used to constrain the zero-point intercept of the magnitude-redshift relation. Our baseline result of $H_0$ is $72.3\pm1.4$ (stat) $\pm1.4$ (syst) km s$^{-1}$ Mpc$^{-1}$ in the $J$ band and $72.3\pm1.3$ (stat) $\pm1.4$ (syst) km s$^{-1}$ Mpc$^{-1}$ in the $H$ band, where the systematic uncertainties include the standard deviation of up to 21 variations of the analysis, the 0.7\% distance scale systematic from SHOES Cepheid anchors, a photometric zeropoint systematic, and a cosmic variance systematic. Our final measurement represents a measurement with a precision of 2.8\% in both bands. The variant with the largest change in $H_0$ is when limiting the sample to SNe from CSP and CfA programmes, noteworthy because these are the best calibrated, yielding $H_0\sim75$ km s$^{-1}$ Mpc$^{-1}$ in both bands. We demonstrate stretch and reddening corrections are still useful in the NIR to standardize SN Ia NIR peak magnitudes. Based on our results, in order to improve the precision of the $H_0$ measurement with SNe Ia in the NIR in the future, we would need to increase the number of calibrator SNe Ia, be able to extend the Hubble-Lema\^itre diagram to higher-z, and include standardization procedures to help reducing the NIR intrinsic scatter.Comment: 15 pages, 8 figures. Accepted in A&

The Carnegie Supernova Project-I. Spectroscopic analysis of stripped-envelope supernovae

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: Re-submitted to A&A after addressing constructive comments from the referee. Comments are welcomed, particularly notice to any work that should be reference

The Carnegie Supernova Project-I. Optical spectroscopy of stripped-envelope supernovae

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 seriesComment: Resubmitted to A&A after address referee's comments. Comments welcomed, and let us know if we missed to reference your paper

Testing the Homogeneity of Type Ia Supernovae in the Near-Infrared for Accurate Distance Estimations

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 $0.08<z\lesssim0.1$, NIR observations with better S/N than that found in the CSP sample is necessary to constrain the introduced scatter to a minimum ($\lesssim0.05$ mag). These results provide confidence for our FLOWS project that aims in using SNe Ia with public ZTF optical light curves and few NIR epochs to map out the peculiar velocity field of the local Universe. This will allow us to determine the distribution of dark matter in our own supercluster, Laniakea, and test the standard cosmological model by measuring the growth rate of structures, parameterised by $fD$, and the Hubble-Lema\^itre constant, $H_0$.Comment: Accepted in A&