We present observations of PS16dtm (also known as SN 2016ezh), a luminous transient that occurred at the nucleus of a narrow-line Seyfert 1 galaxy hosting a 106 M black hole. The light curve shows ...that PS16dtm exhibited a plateau phase for ∼100 days, during which it showed no color evolution, maintained a blackbody temperature of K, and radiated at approximately the Eddington luminosity of the supermassive black hole (SMBH). The spectra exhibit multicomponent hydrogen emission lines and strong Fe ii emission, show little time evolution, and closely resemble the spectra of NLS1s while being distinct from those of Type IIn supernovae (SNe IIn). Moreover, PS16dtm is undetected in the X-rays to a limit an order of magnitude below an archival X-ray detection of its host galaxy. These observations strongly link PS16dtm to activity associated with the SMBH and are difficult to reconcile with an SN origin or known forms of active galactic nucleus (AGN) variability. Therefore, we argue that PS16dtm is a tidal disruption event (TDE) in which the accretion of the stellar debris powers the rise in the continuum and excitation of the preexisting broad-line region, while obscuring the X-ray-emitting region of the preexisting AGN disk. We predict that PS16dtm will remain bright for years and that the X-ray emission will reappear on a similar timescale as the accretion rate declines. Placing PS16dtm in the context of other TDEs, we find that TDEs in AGN galaxies are more efficient and reach Eddington luminosities, likely due to interaction of the stellar debris with the preexisting accretion disk.
ABSTRACT We report the discovery of transient radio emission from the nearby optically discovered tidal disruption event (TDE) ASASSN-14li (distance of 90 Mpc), making it the first typical TDE ...detected in the radio, and unambiguously pointing to the formation of a non-relativistic outflow with a kinetic energy of (4-10) × 1047 erg, a velocity of 12,000-36,000 km s−1, and a mass of 3 × 10−5-7 × 10−4 M . We show that the outflow was ejected on 2014 August 11-25, in agreement with an independent estimate of the timing of super-Eddington accretion based on the optical, ultraviolet, and X-ray observations, and that the ejected mass corresponds to about 1%-10% of the mass accreted in the super-Eddington phase. The temporal evolution of the radio emission also uncovers the circumnuclear density profile, on a scale of about 0.01 pc, a scale that cannot be probed via direct measurements even in the nearest supermassive black holes. Our discovery of radio emission from the nearest well-studied TDE to date, with a radio luminosity lower than all previous limits, indicates that non-relativistic outflows are ubiquitous in TDEs, and that future, more sensitive, radio surveys will uncover similar events.
We present the first effort to aggregate, homogenize, and uniformly model the combined ultraviolet, optical, and near-infrared data set for the electromagnetic counterpart of the binary neutron star ...merger GW170817. By assembling all of the available data from 18 different papers and 46 different instruments, we are able to identify and mitigate systematic offsets between individual data sets and to identify clear outlying measurements, with the resulting pruned and adjusted data set offering an opportunity to expand the study of the kilonova. The unified data set includes 647 individual flux measurements, spanning 0.45-29.4 days post-merger, and thus has greater constraining power for physical models than any single data set. We test a number of semi-analytical models and find that the data are well modeled with a three-component kilonova model: a "blue" lanthanide-poor component ( cm2 g−1) with and an intermediate opacity "purple" component ( cm2 g−1) with and and a "red" lanthanide-rich component ( cm2 g−1) with and . We further explore the possibility of ejecta asymmetry and its impact on the estimated parameters. From the inferred parameters we draw conclusions about the physical mechanisms responsible for the various ejecta components, the properties of the neutron stars, and, combined with an up-to-date merger rate, the implications for r-process enrichment via this channel. To facilitate future studies of this keystone event we make the unified data set and our modeling code public.
ABSTRACT
We present and analyse a new tidal disruption event (TDE), AT2017eqx at redshift z = 0.1089, discovered by Pan-STARRS and ATLAS. The position of the transient is consistent with the nucleus ...of its host galaxy; the spectrum shows a persistent blackbody temperature T ≳ 20 000 K with broad H i and He ii emission; and it peaks at a blackbody luminosity of L ≈ 1044 erg s−1. The lines are initially centred at zero velocity, but by 100 d, the H i lines disappear while the He ii develops a blueshift of ≳ 5000 km s−1. Both the early- and late-time morphologies have been seen in other TDEs, but the complete transition between them is unprecedented. The evolution can be explained by combining an extended atmosphere, undergoing slow contraction, with a wind in the polar direction becoming visible at late times. Our observations confirm that a lack of hydrogen a TDE spectrum does not indicate a stripped star, while the proposed model implies that much of the diversity in TDEs may be due to the observer viewing angle. Modelling the light curve suggests AT2017eqx resulted from the complete disruption of a solar-mass star by a black hole of ∼106.3 M⊙. The host is another Balmer-strong absorption galaxy, though fainter and less centrally concentrated than most TDE hosts. Radio limits rule out a relativistic jet, while X-ray limits at 500 d are among the deepest for a TDE at this phase.
We present UV, optical, and near-infrared (NIR) photometry of the first electromagnetic counterpart to a gravitational wave source from Advanced Laser Interferometer Gravitational-wave Observatory ...(LIGO)/Virgo, the binary neutron star merger GW170817. Our data set extends from the discovery of the optical counterpart at 0.47-18.5 days post-merger, and includes observations with the Dark Energy Camera (DECam), Gemini-South/FLAMINGOS-2 (GS/F2), and the Hubble Space Telescope (HST). The spectral energy distribution (SED) inferred from this photometry at 0.6 days is well described by a blackbody model with T 8300 K, a radius of R 4.5 × 10 14 cm (corresponding to an expansion velocity of v 0.3 c ), and a bolometric luminosity of L bol 5 × 10 41 erg s−1. At 1.5 days we find a multi-component SED across the optical and NIR, and subsequently we observe rapid fading in the UV and blue optical bands and significant reddening of the optical/NIR colors. Modeling the entire data set, we find that models with heating from radioactive decay of 56Ni, or those with only a single component of opacity from r-process elements, fail to capture the rapid optical decline and red optical/NIR colors. Instead, models with two components consistent with lanthanide-poor and lanthanide-rich ejecta provide a good fit to the data; the resulting "blue" component has M ej blue 0.01 M and v ej blue 0.3 c , and the "red" component has M ej red 0.04 M and v ej red 0.1 c . These ejecta masses are broadly consistent with the estimated r-process production rate required to explain the Milky Way r-process abundances, providing the first evidence that binary neutron star (BNS) mergers can be a dominant site of r-process enrichment.
We present Spitzer Space Telescope 3.6 and 4.5 m observations of the binary neutron star merger GW170817 at 43, 74, and 264 days post-merger. Using the final observation as a template, we uncover a ...source at the position of GW170817 at 4.5 m with a brightness of 22.9 0.3 AB mag at 43 days and 23.8 0.3 AB mag at 74 days (the uncertainty is dominated by systematics from the image subtraction); no obvious source is detected at 3.6 m to a 3 limit of >23.3 AB mag in both epochs. The measured brightness is dimmer by a factor of about 2-3 times compared to our previously published kilonova model, which is based on UV, optical, and near-infrared data at 30 days. However, the observed fading rate and color (m3.6-m4.5 0 AB mag) are consistent with our model. We suggest that the discrepancy is likely due to a transition to the nebular phase, or a reduced thermalization efficiency at such late time. Using the Spitzer data as a guide, we briefly discuss the prospects of observing future binary neutron star mergers with Spitzer (in the Laser Interferometer Gravitational-Wave Observatory (LIGO)/Virgo Observing Run 3) and the James Webb Space Telescope (in LIGO/Virgo Observing Run 4 and beyond).
Solitary stars that wander too close to their galactic centres can become tidally disrupted, if the tidal forces due to the supermassive black hole residing there overcome the self-gravity of the ...star. If the star is only partially disrupted, so that a fraction survives as a self-bound object, this remaining core will experience a net gain in specific orbital energy, which translates into a velocity ‘kick’ of up to ∼103 km s−1. In this paper, we present the result of smoothed particle hydrodynamics simulations of such partial disruptions, and analyse the velocity kick imparted on the surviving core. We compare γ = 5/3 and γ = 4/3 polytropes disrupted in both a Newtonian potential, and a generalized potential that reproduces most relativistic effects around a Schwarzschild black hole either exactly or to excellent precision. For the Newtonian case, we confirm the results of previous studies that the kick velocity of the surviving core is virtually independent of the ratio of the black hole to stellar mass, and is a function of the impact parameter β alone, reaching at most the escape velocity of the original star. For a given β, relativistic effects become increasingly important for larger black hole masses. In particular, we find that the kick velocity increases with the black hole mass, making larger kicks more common than in the Newtonian case, as low-β encounters are statistically more likely than high-β encounters. The analysis of the tidal tensor for the generalized potential shows that our results are robust lower limits on the true relativistic kick velocities, and are generally in very good agreement with the exact results.
We describe the simulated data sample for the Photometric Large Synoptic Survey Telescope (LSST) Astronomical Time Series Classification Challenge (PLAsTiCC), a publicly available challenge to ...classify transient and variable events that will be observed by the LSST, a new facility expected to start in the early 2020s. The challenge was hosted by Kaggle, ran from 2018 September 28 to December 17, and included 1094 teams competing for prizes. Here we provide details of the 18 transient and variable source models, which were not revealed until after the challenge, and release the model libraries at https://doi.org/10.5281/zenodo.2612896. We describe the LSST Operations Simulator used to predict realistic observing conditions, and we describe the publicly available SNANA simulation code used to transform the models into observed fluxes and uncertainties in the LSST passbands (ugrizy). Although PLAsTiCC has finished, the publicly available models and simulation tools are being used within the astronomy community to further improve classification, and to study contamination in photometrically identified samples of SN Ia used to measure properties of dark energy. Our simulation framework will continue serving as a platform to improve the PLAsTiCC models, and to develop new models.
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