ABSTRACT
We present optical spectroscopic and Swift UVOT/XRT observations of the X-ray and UV/optical bright tidal disruption event (TDE) candidate AT 2018fyk/ASASSN–18ul discovered by ASAS–SN. The ...Swift light curve is atypical for a TDE, entering a plateau after ∼40 d of decline from peak. After 80 d the UV/optical light curve breaks again to decline further, while the X-ray emission becomes brighter and harder. In addition to broad H, He, and potentially O/Fe lines, narrow emission lines emerge in the optical spectra during the plateau phase. We identify both high-ionization (O iii) and low-ionization (Fe ii) lines, which are visible for ∼45 d. We similarly identify Fe ii lines in optical spectra of ASASSN–15oi 330 d after discovery, indicating that a class of Fe-rich TDEs exists. The spectral similarity between AT 2018fyk, narrow-line Seyfert 1 galaxies, and some extreme coronal line emitters suggests that TDEs are capable of creating similar physical conditions in the nuclei of galaxies. The Fe ii lines can be associated with the formation of a compact accretion disc, as the emergence of low-ionization emission lines requires optically thick, high-density gas. Taken together with the plateau in X-ray and UV/optical luminosity this indicates that emission from the central source is efficiently reprocessed into UV/optical wavelengths. Such a two-component light curve is very similar to that seen in the TDE candidate ASASSN–15lh, and is a natural consequence of a relativistic orbital pericentre.
Most stellar remnants so far have been found in binary systems, where they interact with matter from their companions. Isolated neutron stars and black holes are difficult to find as they are dark, ...yet they are predicted to exist in our Galaxy in vast numbers. We explored the OGLE-III data base of 150 million objects observed in years 2001–2009 and found 59 microlensing events exhibiting a parallax effect due to the Earth's motion around the Sun. Combining parallax and brightness measurements from microlensing light curves with expected proper motions in the Milky Way, we identified 13 microlensing events which are consistent with having a white dwarf, neutron star or a black hole lens and we estimated their masses and distances. The most massive of our black hole candidates has 9.3 M⊙ and is at a distance of 2.4 kpc. The distribution of masses of our candidates indicates a continuum in mass distribution with no mass gap between neutron stars and black holes. We also present predictions on how such events will be observed by the astrometric Gaia mission.
ABSTRACT
We present the results from Nordic Optical Telescope and X-shooter follow-up campaigns of the tidal disruption event (TDE) iPTF16fnl, covering the first ∼100 d after the transient discovery. ...We followed the source photometrically until the TDE emission was no longer detected above the host galaxy light. The bolometric luminosity evolution of the TDE is consistent with an exponential decay with e-folding constant t0 = 17.6 ± 0.2 d. The early-time spectra of the transient are dominated by broad He ii λ4686, H $\beta$, H $\alpha$, and N iii λ4100 emission lines. The latter is known to be produced together with the N iii λ4640 in the Bowen fluorescence mechanism. Due to the medium-resolution X-shooter spectra we have been able to separate the Bowen blend contribution from the broad He ii emission line. The detection of the Bowen fluorescence lines in iPTF16fnl place this transient among the N-rich TDE subset. In the late-time X-shooter spectra, narrow emission lines of O iii and N ii originating from the host galaxy are detected, suggesting that the host galaxy harbours a weak active galactic nucleus in its core. The properties of all broad emission lines evolve with time. The equivalent widths follow an exponential decay compatible with the bolometric luminosity evolution. The full width at half-maximum of the broad lines decline with time and the line profiles develop a narrow core at later epochs. Overall, the optical emission of iPTF16fnl can be explained by being produced in an optically thick region in which high densities favour the Bowen fluorescence mechanism and where multiple electron scatterings are responsible for the line broadening.
We investigate the influence of the matter along the line of sight and in the lens environment on the image configurations, relative time delays, and the resulting models of strong gravitational ...lensing. The distribution of matter in space and properties of gravitationally bound haloes are based on the Millennium Simulation. In our numerical experiments we consider isolated lens in a uniform universe model and the same lens surrounded by close neighbours and/or objects close to the line of sight which gives four different descriptions of the light propagation. We compare the results of the lens modelling which neglects effects of the environment and line of sight, when applied to image configurations resulting from approaches partially or fully taking into account these effects. We show that for a source at the redshift z 2 the effects are indeed important and may prevent successful fitting of lens models in a substantial part of simulated image configurations, especially when the relative time delays are taken into account. To have good constraints on the models we limit ourselves to configurations of four images. We consider 80 lenses and large number of source positions in each case. The influence of the lens neighbourhood and the line of sight introduces the spread into the fitted values of the deflection angles which translates into the spread in the lens velocity dispersion of ∼4 per cent. Similarly for the lens axis ratio we get the spread of ∼10 per cent and for the Hubble's constant of ∼6 per cent. When averaged over all lenses and all image configurations considered, the median fitted values of the parameters (including the Hubble's constant) do not differ more than 1 per cent from their values used in simulations.
We investigate strong lensing by non-singular finite isothermal ellipsoids taking into account the influence of the matter along the line of sight and in the close lens vicinity. We compare three ...descriptions of light propagation: the full approach taking into account all matter inhomogeneities along the rays; the single plane approach, where we take into account the influence of the strong lens neighbours but neglect the foreground and background objects; and the single lens approach. In each case, we simulate many strong lensing configurations placing a point source at the same redshift but in different locations inside the region surrounded by caustics. We further analyse configurations of four or five images. For every simulated strong lensing configuration, we attempt to fit a simplified lens model using a single isothermal ellipsoid or a single isothermal ellipsoid with external shear. The single lens fits to configurations obtained in the full approach are rejected in majority of cases with 95 per cent significance. For configurations obtained in the single plane approach, the rejection rate is substantially lower. Also the inclusion of external shear in simplified modelling improves the chances of obtaining acceptable fits, but the problem is not solved completely. The quantitative estimates of the rates of rejection of simplified models depend on the required accuracy of the models, and we present few illustrative examples, which show that both matter close to the lens and matter along the rays do have important influence on lens modelling. We also estimate the typical value of the external shear and compare the fitted parameters of the simplified models with the parameters of the lenses used in the simulations.
ABSTRACT
Numerical relativity predicts that the coalescence of a black hole (BH) binary causes the newly formed BH to recoil, and evidence for such recoils has been found in the gravitational waves ...observed during the merger of stellar-mass BHs. Recoiling (super)massive BHs are expected to reside in hypercompact stellar clusters (HCSCs). Simulations of galaxy assembly predict that hundreds of HCSCs should be present in the halo of a Milky Way (MW)-type galaxy, and a fraction of those around the MW should have magnitudes within the sensitivity limit of existing surveys. However, recoiling BHs and their HCSCs are still waiting to be securely identified. With the goal of enabling searches through recent and forthcoming data bases, we improve over existing literature to produce realistic renditions of HCSCs bound to BHs with a mass of 105 M⊙. Including the effects of a population of blue stragglers, we simulate their appearance in Pan-STARRS and in forthcoming Euclid images. We also derive broad-band spectra and the corresponding multiwavelength colours, finding that the great majority of the simulated HCSCs fall on the colour–colour loci defined by stars and galaxies, with their spectra resembling those of giant K-type stars. We discuss the clusters properties, search strategies, and possible interlopers.
ABSTRACT
We present the results of a multiwavelength follow-up campaign for the luminous nuclear transient Gaia16aax, which was first identified in 2016 January. The transient is spatially consistent ...with the nucleus of an active galaxy at z = 0.25, hosting a black hole of mass ${\sim }6\times 10^8\, \mathrm{M}_\odot$. The nucleus brightened by more than 1 mag in the Gaia G band over a time-scale of less than 1 yr, before fading back to its pre-outburst state over the following 3 yr. The optical spectra of the source show broad Balmer lines similar to the ones present in a pre-outburst spectrum. During the outburst, the H α and H β emission lines develop a secondary peak. We also report on the discovery of two transients with similar light-curve evolution and spectra: Gaia16aka and Gaia16ajq. We consider possible scenarios to explain the observed outbursts. We exclude that the transient event could be caused by a microlensing event, variable dust absorption or a tidal encounter between a neutron star and a stellar mass black hole in the accretion disc. We consider variability in the accretion flow in the inner part of the disc, or a tidal disruption event of a star ${\ge } 1 \, \mathrm{M}_{\odot }$ by a rapidly spinning supermassive black hole as the most plausible scenarios. We note that the similarity between the light curves of the three Gaia transients may be a function of the Gaia alerts selection criteria.
Gaia transients in galactic nuclei Kostrzewa-Rutkowska, Z; Jonker, P G; Hodgkin, S T ...
Monthly notices of the Royal Astronomical Society,
11/2018, Letnik:
481, Številka:
1
Journal Article
Abstract
We present the first estimate of the Galactic nova rate based on optical transient surveys covering the entire sky. Using data from the All-Sky Automated Survey for Supernovae (ASAS-SN) and ...Gaia—the only two all-sky surveys to report classical nova candidates—we find 39 confirmed Galactic novae and 7 additional unconfirmed candidates discovered from 2019 to 2021, yielding a nova discovery rate of ≈14 yr
−1
. Using accurate Galactic stellar mass models and three-dimensional dust maps and incorporating realistic nova light curves, we have built a sophisticated Galactic nova model to estimate the fraction of Galactic novae discovered by these surveys over this time period. The observing capabilities of each survey are distinct: the high cadence of ASAS-SN makes it sensitive to fast novae, while the broad observing filter and high spatial resolution of Gaia make it more sensitive to highly reddened novae across the entire Galactic plane and bulge. Despite these differences, we find that ASAS-SN and Gaia give consistent Galactic nova rates, with a final joint nova rate of 26 ± 5 yr
−1
. This inferred nova rate is substantially lower than found by many other recent studies. Critically assessing the systematic uncertainties in the Galactic nova rate, we argue that the role of faint, fast-fading novae has likely been overestimated, but that subtle details in the operation of transient alert pipelines can have large, sometimes unappreciated effects on transient recovery efficiency. Our predicted nova rate can be directly tested with forthcoming red/near-infrared transient surveys in the southern hemisphere.
ABSTRACT
Over the past few years, ∼30 extragalactic fast X-ray transients (FXRTs) have been discovered, mainly in Chandra and XMM-Newton data. Their nature remains unclear, with proposed origins, ...including a double neutron star merger, a tidal disruption event involving an intermediate-mass black hole and a white dwarf, or a supernova shock breakout. A decisive differentiation between these three promising mechanisms for their origin requires an understanding of the FXRT energetics, environments, and/or host properties. We present optical observations obtained with the Very Large Telescope for the FXRTs XRT 000519 and XRT 110103 and Gran Telescopio Canarias observations for XRT 000519 designed to search for host galaxies of these FXRTs. In the gs, rs, and R-band images, we detect an extended source on the north-west side of the $\sim \, 1^{\prime \prime }$ (68 per cent confidence) error circle of the X-ray position of XRT 000519 with a Kron magnitude of gs = 26.29 ± 0.09 (AB magnitude). We discuss the XRT 000519 association with the probable host candidate for various possible distances, and we conclude that if XRT 000519 is associated with the host candidate a supernova shock breakout scenario is likely excluded. No host galaxy is found near XRT 110103 down to a limiting magnitude of R > 25.8.
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