273 research outputs found
A unified model for tidal disruption events
In the past few years wide-field optical and UV transient surveys as well as
X-ray telescopes have allowed us to identify a few dozen candidate tidal
disruption events (TDEs). While in theory the physical processes in TDEs are
expected to be ubiquitous, a few distinct classes of TDEs have been observed.
Some TDEs radiate mainly in NUV/optical while others produce prominent X-rays.
Moreover, relativistic jets have been observed in only a handful of TDEs. This
diversity might be related to the details of the super-Eddington accretion and
emission physics relevant to TDE disks. In this Letter, we utilize novel
three-dimensional general relativistic radiation magnetohydrodynamics
simulations to study the super-Eddington compact disk phase expected in TDEs.
Consistent with previous studies, geometrically thick disks, wide-angle
optically-thick fast outflows and relativistic jets are produced. The outflow
density and velocity depend sensitively on the inclination angle, and hence so
does the reprocessing of emission produced from the inner disk. We then use
Monte-Carlo radiative transfer to calculate the reprocessed spectra and find
that that the observed ratio of optical to X-ray fluxes increases with
increasing inclination angle. This naturally leads to a unified model for
different classes of TDEs in which the spectral properties of the TDE depend
mainly on the viewing-angle of the observer with respect to the orientation of
the disk.Comment: Accepted to ApJ Letter
Distinguishing Tidal Disruption Events from Impostors
Recent claimed detections of tidal disruption events (TDEs) in
multi-wavelength data have opened potential new windows into the evolution and
properties of otherwise dormant supermassive black holes (SMBHs) in the centres
of galaxies. At present, there are several dozen TDE candidates, which share
some properties and differ in others. The range in properties is broad enough
to overlap other transient types, such as active galactic nuclei (AGN) and
supernovae (SNe), which can make TDE classification ambiguous. A further
complication is that "TDE signatures" have not been uniformly observed to
similar sensitivities or even targeted across all candidates. This chapter
reviews those events that are unusual relative to other TDEs, including the
possibility of TDEs in pre-existing AGN, and summarises those characteristics
thought to best distinguish TDEs from continuously accreting AGN, strongly
flaring AGN, SNe, and Gamma-Ray Bursts (GRBs), as well as other potential
impostors like stellar collisions, "micro-TDEs," and circumbinary accretion
flows. We conclude that multiple observables should be used to classify any one
event as a TDE. We also consider the TDE candidate population as a whole,
which, for certain host galaxy or SMBH characteristics, is distinguishable
statistically from non-TDEs, suggesting that at least some TDE candidates do in
fact arise from SMBH-disrupted stars.Comment: 57 pages, 17 figures. Accepted for publication in Springer Space
Science Reviews. Chapter in ISSI review "The Tidal Disruption of Stars by
Massive Black Holes" vol. 79. Corrected several typos from previous
submissio
Mechanical Properties of a Library of Low-Band-Gap Polymers
The
mechanical properties of low-band-gap polymers are important
for the long-term survivability of roll-to-roll processed organic
electronic devices. Such devices, e.g., solar cells, displays, and
thin-film transistors, must survive the rigors of roll-to-roll coating
and also thermal and mechanical forces in the outdoor environment
and in stretchable and ultraflexible form factors. This paper measures
the stiffness (tensile modulus), ductility (crack-onset strain), or
both of a combinatorial library of 51 low-band-gap polymers. The purpose
of this study is to systematically screen a library of low-band-gap
polymers to better understand the connection between molecular structures
and mechanical properties in order to design conjugated polymers that
permit mechanical robustness and even extreme deformability. While
one of the principal conclusions of these experiments is that the
structure of an isolated molecule only partially determines the mechanical
propertiesanother important codeterminant is the packing structuresome
general trends can be identified. (1) Fused rings tend to increase
the modulus and decrease the ductility. (2) Branched side chains have
the opposite effect. Despite the rigidity of the molecular structure,
the most deformable films can be surprisingly compliant (modulus ≥
150 MPa) and ductile (crack-onset strain ≤ 68%). This paper
concludes by proposing a new composite merit factor that combines
the power conversion efficiency in a fully solution processed device
obtained via roll and roll-to-roll coating and printing (as measured
in an earlier paper) and the mechanical deformability toward the goal
of producing modules that are both efficient and mechanically stable
Multi-Messenger Astronomy with Extremely Large Telescopes
The field of time-domain astrophysics has entered the era of Multi-messenger
Astronomy (MMA). One key science goal for the next decade (and beyond) will be
to characterize gravitational wave (GW) and neutrino sources using the next
generation of Extremely Large Telescopes (ELTs). These studies will have a
broad impact across astrophysics, informing our knowledge of the production and
enrichment history of the heaviest chemical elements, constrain the dense
matter equation of state, provide independent constraints on cosmology,
increase our understanding of particle acceleration in shocks and jets, and
study the lives of black holes in the universe. Future GW detectors will
greatly improve their sensitivity during the coming decade, as will
near-infrared telescopes capable of independently finding kilonovae from
neutron star mergers. However, the electromagnetic counterparts to
high-frequency (LIGO/Virgo band) GW sources will be distant and faint and thus
demand ELT capabilities for characterization. ELTs will be important and
necessary contributors to an advanced and complete multi-messenger network.Comment: White paper submitted to the Astro2020 Decadal Surve
Seventeen Tidal Disruption Events from the First Half of ZTF Survey Observations: Entering a New Era of Population Studies
While tidal disruption events (TDEs) have long been heralded as laboratories for the study of quiescent black holes, the small number of known TDEs and uncertainties in their emission mechanism have hindered progress towards this promise. Here present 17 new TDEs that have been detected recently by the Zwicky Transient Facility along with Swift UV and X-ray follow-up observations. Our homogeneous analysis of the optical/UV light curves, including 22 previously known TDEs from the literature, reveals a clean separation of light curve properties with spectroscopic class. The TDEs with Bowen fluorescence features in their optical spectra have smaller blackbody radii, as well as longer rise times and higher disruption rates compared to the rest of the sample. The Bowen fluorescence mechanism requires a high density which can be reached at smaller radii, which in turn yields longer diffusion timescales. Thus, the difference in rise times suggests the pre-peak TDE light curves are governed not by the fallback timescale, but instead by the diffusion of photons through the tidal debris. The small subset of TDEs that show only helium emission lines in their spectra have the longest rise times, the highest luminosities and the lowest rates. We also report, for the first time, the detection of soft X-ray flares from a TDE on day timescales. Based on the fact the flares peak at a luminosity similar to the optical/UV blackbody luminosity, we attribute them to brief glimpses through a reprocessing layer that otherwise obscures the inner accretion flow
Discovery of Highly Blueshifted Broad Balmer and Metastable Helium Absorption Lines in a Tidal Disruption Event
We report the discovery of nonstellar hydrogen Balmer and metastable helium absorption lines accompanying a transient, high-velocity (0.05c) broad absorption line (BAL) system in the optical spectra of the tidal disruption event (TDE) AT2018zr (z = 0.071). In the Hubble Space Telescope UV spectra, absorption of high- and low-ionization lines is also present at this velocity, making AT2018zr resemble a low-ionization BALQSO. We conclude that these transient absorption features are more likely to arise in fast outflows produced by the TDE than absorbed by the unbound debris. In accordance with the outflow picture, we are able to reproduce the flat-topped Hα emission in a spherically expanding medium without invoking the typical prescription of an elliptical disk. We also report the appearance of narrow (~1000 kms^(−1)) N III λ4640, He II λ4686, Hα, and Hβ emission in the late-time optical spectra of AT2018zr, which may be a result of UV continuum hardening at late times, as observed by Swift. Including AT2018zr, we find a high association rate (three out of four) of BALs in the UV spectra of TDEs. This suggests that outflows may be ubiquitous among TDEs and less sensitive to viewing angle effects compared to QSO outflows
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