The first direct double neutron star merger detection: implications for cosmic nucleosynthesis

The astrophysical r-process site where about half of the elements heavier than iron are produced has been a puzzle for several decades. Here we discuss the role of neutron star mergers (NSMs) in the light of the first direct detection of such an event in both gravitational (GW) and electromagnetic (EM) waves. We analyse bolometric and NIR lightcurves of the first detected double neutron star merger and compare them to nuclear reaction network-based macronova models. The slope of the bolometric lightcurve is consistent with the radioactive decay of neutron star ejecta with $Y_e \lesssim 0.3$ (but not larger), which provides strong evidence for an r-process origin of the electromagnetic emission. This rules out in particular "nickel winds" as major source of the emission. We find that the NIR lightcurves can be well fitted either with or without lanthanide-rich ejecta. Our limits on the ejecta mass together with estimated rates directly confirm earlier purely theoretical or indirect observational conclusions that double neutron star mergers are indeed a major site of cosmic nucleosynthesis. If the ejecta mass was {\em typical}, NSMs can easily produce {\em all} of the estimated Galactic r-process matter, and --depending on the real rate-- potentially even more. This could be a hint that the event ejected a particularly large amount of mass, maybe due to a substantial difference between the component masses. This would be compatible with the mass limits obtained from the GW-observation. The recent observations suggests that NSMs are responsible for a broad range of r-process nuclei and that they are at least a major, but likely the dominant r-process site in the Universe.Comment: 11 pages, 8 figures; accepted for A \&

Lyapunov exponents in a Sachdev-Ye-Kitaev-type model with population imbalance in the conformal limit and beyond

The Sachdev-Ye-Kitaev (SYK) model shows chaotic behavior with a maximal Lyapunov exponent. In this paper, we investigate the four-point function of a SYK-type model numerically, which gives us access to its Lyapunov exponent. The model consists of two sets of Majorana fermions, called A and B, and the interactions are restricted to being exclusively pairwise between the two sets, not within the sets. We find that the Lyapunov exponent is still maximal at strong coupling. Furthermore, we show that even though the conformal dimensions of the A and B fermions change with the population ratio, the Lyapunov exponent remains constant, not just in the conformal limit where it is maximal, but also in the intermediate and weak coupling regimes.Comment: 12 pages, 8 figures. Comments welcom

Metallicity at the explosion sites of interacting transients

Context. Some circumstellar-interacting (CSI) supernovae (SNe) are produced by the explosions of massive stars that have lost mass shortly before the SN explosion. There is evidence that the precursors of some SNe IIn were luminous blue variable (LBV) stars. For a small number of CSI SNe, outbursts have been observed before the SN explosion. Eruptive events of massive stars are named as SN impostors (SN IMs) and whether they herald a forthcoming SN or not is still unclear. The large variety of observational properties of CSI SNe suggests the existence of other progenitors, such as red supergiant (RSG) stars with superwinds. Furthermore, the role of metallicity in the mass loss of CSI SN progenitors is still largely unexplored. Aims. Our goal is to gain insight on the nature of the progenitor stars of CSI SNe by studying their environments, in particular the metallicity at their locations. Methods. We obtain metallicity measurements at the location of 60 transients (including SNe IIn, SNe Ibn, and SN IMs), via emission-line diagnostic on optical spectra obtained at the Nordic Optical Telescope and through public archives. Metallicity values from the literature complement our sample. We compare the metallicity distributions among the different CSI SN subtypes and to those of other core-collapse SN types. We also search for possible correlations between metallicity and CSI SN observational properties. Results. We find that SN IMs tend to occur in environments with lower metallicity than those of SNe IIn. Among SNe IIn, SN IIn-L(1998S-like) SNe show higher metallicities, similar to those of SNe IIL/P, whereas long-lasting SNe IIn (1988Z-like) show lower metallicities, similar to those of SN IMs. The metallicity distribution of SNe IIn can be reproduced by combining the metallicity distributions of SN IMs (that may be produced by major outbursts of massive stars like LBVs) and SNe IIP (produced by RSGs). The same applies to the distributions of the Normalized Cumulative Rank (NCR) values, which quantifies the SN association to H II regions. For SNe IIn, we find larger mass-loss rates and higher CSM velocities at higher metallicities. The luminosity increment in the optical bands during SN IM outbursts tend to be larger at higher metallicity, whereas the SN IM quiescent optical luminosities tend to be lower. Conclusions. The difference in metallicity between SNe IIn and SN IMs suggests that LBVs are only one of the progenitor channels for SNe IIn, with 1988Z-like and 1998S-like SNe possibly arising from LBVs and RSGs, respectively. Finally, even though linedriven winds likely do not primarily drive the late mass-loss of CSI SN progenitors, metallicity has some impact on the observational properties of these transients. Key words. supernovae: general - stars: evolution - galaxies: abundancesComment: Submitted to Astronomy and Astrophysics on 28/02/2015; submitted to arXiv after the 1st referee repor

Probing gas and dust in the tidal tail of NGC 5221 with the type Ia supernova iPTF16abc

Context. Type Ia supernovae (SNe Ia) can be used to address numerous questions in astrophysics and cosmology. Due to their well known spectral and photometric properties, SNe Ia are well suited to study gas and dust along the lines-of-sight to the explosions. For example, narrow Na I D and Ca II H&K absorption lines can be studied easily, because of the well-defined spectral continuum of SNe Ia around these features. Aims. We study the gas and dust along the line-of-sight to iPTF16abc, which occurred in an unusual location, in a tidal arm, 80 kpc from centre of the galaxy NGC 5221. Methods. Using a time-series of high-resolution spectra, we examine narrow Na I D and Ca II H&K absorption features for variations in time, which would be indicative for circumstellar (CS) matter. Furthermore, we take advantage of the well known photometric properties of SNe Ia to determine reddening due to dust along the line-of-sight. Results. From the lack of variations in Na I D and Ca II H&K, we determine that none of the detected absorption features originate from the CS medium of iPTF16abc. While the Na I D and Ca II H&K absorption is found to be optically thick, a negligible amount of reddening points to a small column of interstellar dust. Conclusions. We find that the gas along the line-of-sight to iPTF16abc is typical of what might be found in the interstellar medium (ISM) within a galaxy. It suggests that we are observing gas that has been tidally stripped during an interaction of NGC 5221 with one of its neighbouring galaxies in the past $\sim10^9$ years. In the future, the gas clouds could become the locations of star formation. On a longer time scale, the clouds might diffuse, enriching the circum-galactic medium (CGM) with metals. The gas profile along the line-of-sight should be useful for future studies of the dynamics of the galaxy group containing NGC 5221.Comment: 8 pages, 6 figure

Rapid "Turn-on" of type 1 AGN in a quiescent early type galaxy SDSS1115+0544

We present a detailed study of a transient in the center of SDSS1115+0544 based on the extensive UV, optical, mid-IR light curves (LC) and spectra over 1200 days. The host galaxy is a quiescent early type galaxy at $z$ = 0.0899 with a blackhole mass of $2\times10^7M_\odot$. The transient underwent a 2.5 magnitude brightening over $\sim120$ days, reaching a peak $V$-band luminosity (extinction corrected) of $-20.9$ magnitude, then fading 0.5 magnitude over 200 days, settling into a plateau of $>600$ days. Following the optical brightening are the significant mid-IR flares at $3.4$ and $4.5\mu$m, with a peak time delay of $\sim180$ days. The mid-IR LCs are explained as the echo of UV photons by a dust medium with a radius of $5\times10^{17}$ cm, consistent with $\rm E(B-V)$ of 0.58 inferred from the spectra. This event is very energetic with an extinction corrected $L_{bol} \sim 4\times10^{44}$ erg s$^{-1}$. Optical spectra over 400 days in the plateau phase revealed newly formed broad H$\alpha, \beta$ emission with a FWHM of $\sim3750$ km s$^{-1}$ and narrow coronal lines such as [Fe VII], [Ne V]. This flare also has a steeply rising UV continuum, detected by multi-epoch $Swift$ data at $+700$ to $+900$ days post optical peak. The broad Balmer lines and the UV continuum do not show significant temporal variations. The slow evolving LCs over 1200 days, the constant Balmer lines and UV continuum at late-times rule out TDE and SN IIn as the physical model for this event. We propose that this event is a `turn-on' AGN, transitioning from a quiescent state to a type 1 AGN with a sub-Eddington accretion rate of $0.017M_\odot$/yr. This change occurred on a very short time scale of $\sim 120- 200$ days. The discovery of such a rapid `turn-on' AGN poses challenges to accretion disk theories and may indicate such event is not extremely rare.Comment: Comments are welcome. Emails to the first author. Accepted for publication in Ap