| 1. |
- Dey, Lankeswar, et al.
(författare)
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Authenticating the Presence of a Relativistic Massive Black Hole Binary in OJ 287 Using Its General Relativity Centenary Flare : Improved Orbital Parameters
- 2018
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 866:1
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Tidskriftsartikel (refereegranskat)abstract
- Results from regular monitoring of relativistic compact binaries like PSR 1913+16 are consistent with the dominant (quadrupole) order emission of gravitational waves (GWs). We show that observations associated with the binary black hole (BBH) central engine of blazar OJ 287 demand the inclusion of gravitational radiation reaction effects beyond the quadrupolar order. It turns out that even the effects of certain hereditary contributions to GW emission are required to predict impact flare timings of OJ 287. We develop an approach that incorporates this effect into the BBH model for OJ 287. This allows us to demonstrate an excellent agreement between the observed impact flare timings and those predicted from ten orbital cycles of the BBH central engine model. The deduced rate of orbital period decay is nine orders of magnitude higher than the observed rate in PSR 1913+16, demonstrating again the relativistic nature of OJ 287's central engine. Finally, we argue that precise timing of the predicted 2019 impact flare should allow a test of the celebrated black hole no-hair theorem at the 10% level.
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| 2. |
- Gafton, Emanuel, 1989-
(författare)
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Attraction and Rejection : On the love–hate relationship between stars and black holes
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Solitary stars wandering too close to the supermassive black hole at the centre of their galaxy may become tidally disrupted, if the tidal forces due to the black hole overcome the self-gravity holding the star together. Depending on the strength of the encounter, the star may be partially disrupted, resulting in a surviving stellar core and two tidal arms, or may be completely disrupted, resulting in a long and thin tidal stream expected to fall back and circularize into an accretion disc (the two cases are illustrated on the cover of this thesis).While some aspects of a tidal disruption can be described analytically with reasonable accuracy, such an event is the highly non-linear outcome of the interplay between the stellar hydrodynamics and self-gravity, tidal accelerations from the black hole, radiation, potentially magnetic fields and, in extreme cases, nuclear reactions. In the vicinity of the black hole, general relativistic effects become important in determining both the fate of the star and the subsequent evolution of the debris stream.In this thesis we present a new approach for studying the relativistic regime of tidal disruptions. It combines an exact relativistic description of the hydrodynamical evolution of a test fluid in a fixed curved spacetime with a Newtonian treatment of the fluid's self-gravity. The method, though trivial to incorporate into existing Newtonian codes, yields very accurate results at minimal additional computational expense.Equipped with this new tool, we set out to systematically explore the parameter space of tidal disruptions, focusing on the effects of the impact parameter (describing the strength of the disruption) and of the black hole spin on the morphology and energetics of the resulting debris stream. We also study the effects of general relativity on partial disruptions, in order to determine the range of impact parameters at which partial disruptions occur for various black hole masses, and the effects of general relativity on the velocity kick imparted to the surviving core. Finally, we simulate the first part of a tidal disruption with our code and then use the resulting debris distribution as input for a grid-based, general relativistic magnetohydrodynamics code, with which we follow the formation and evolution of the resulting accretion disc.
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| 3. |
- Gafton, Emanuel, et al.
(författare)
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Relativistic effects on tidal disruption kicks of solitary stars
- 2015
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 449:1, s. 771-780
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Tidskriftsartikel (refereegranskat)abstract
- 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 similar to 10(3) 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 gamma = 5/3 and gamma = 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 beta alone, reaching at most the escape velocity of the original star. For a given beta, relativistic effects become increasingly important for larger black holemasses. 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-beta encounters are statistically more likely than high-beta 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.
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| 4. |
- Gafton, Emanuel, 1989-, et al.
(författare)
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Tidal disruptions by rotating black holes : effects of spin and impact parameter
- 2019
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 487:4, s. 4790-4808
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Tidskriftsartikel (refereegranskat)abstract
- We present the results of relativistic smoothed particle hydrodynamics simulations of tidal disruptions of stars by rotating supermassive black holes, for a wide range of impact parameters and black hole spins. For deep encounters, we find that: relativistic precession creates debris geometries impossible to obtain with the Newtonian equations; part of the fluid can be launched on plunging orbits, reducing the fallback rate and the mass of the resulting accretion disc; multiple squeezings and bounces at periapsis may generate distinctive X-ray signatures resulting from the associated shock breakout; disruptions can occur inside the marginally bound radius, if the angular momentum spread launches part of the debris on non-plunging orbits. Perhaps surprisingly, we also find relativistic effects important in partial disruptions, where the balance between self-gravity and tidal forces is so precarious that otherwise minor relativistic effects can have decisive consequences on the stellar fate. In between, where the star is fully disrupted but relativistic effects are mild, the difference resides in a gentler rise of the fallback rate, a later and smaller peak, and longer return times. However, relativistic precession always causes thicker debris streams, both in the bound part (speeding up circularization) and in the unbound part (accelerating and enhancing the production of separate transients). We discuss various properties of the disruption (compression at periapsis, shape and spread of the energy distribution) and potential observables (peak fallback rate, times of rise and decay, duration of super-Eddington fallback) as a function of the impact parameter and the black hole spin.
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| 5. |
- Melandri, A., et al.
(författare)
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The high-redshift gamma-ray burst GRB 140515A A comprehensive X-ray and optical study
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 581
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Tidskriftsartikel (refereegranskat)abstract
- High-redshift eamma-ray bursts (GRBs) offer several advantages When studying the distant Universe, providing unique information about the structure and properties of the galaxies in which they exploded. Spectroscopic identification with large ground-based telescopes has improved our knowledge of this kind of distant events. We present the multi-wavelength analysis of the high-z S'wift GRB GRB 1.40515A (z = 6.327). The best estimate of the neutral hydrogen fraction of the intergalactic medium towards the burst is X-HI <= 0.002. The spectral absorption lines detected for this event are the weakest lines ever observed in GRB afterglows, suggesting that GRB 140515A exploded in a very low-density environment. Its circtun-burst medium is characterised by an average extinction (A(V) similar to 0.1) that seems to be typical of z >= 6 events. The observed multi-band light curves are explained either with a very hard injected spectrum (p = 1.7) or with a multi-component emission (p = 2.1). In the second case a long-lasting central engine activity is needed in order to explain the late time X-ray emission. The possible origin of GRB 140515A in a Pop III (or in a Pop II star with a local environment enriched by Pop III) massive star is unlikely.
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| 6. |
- Perego, Albino, et al.
(författare)
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MODA : a new algorithm to compute optical depths in multidimensional hydrodynamic simulations
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 568, s. A11-
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Tidskriftsartikel (refereegranskat)abstract
- Aims, We introduce the multidimensional optical depth algorithm (MODA) for the calculation of optical depths in approximate multidimensional radiative transport schemes, equally applicable to neutrinos and photons. Motivated by (but not limited to) neutrino transport in three-dimensional simulations of core-collapse supernovae and neutron star mergers. our method makes no assumptions about the geometry of the matter distribution, apart from expecting optically transparent boundaries. Methods. Based on local information about opacities, the algorithm figures out an escape route that tends to minimize the optical depth without assuming any predefined paths for radiation. Its adaptivity makes it suitable for a variety of astrophysical settings with complicated geometry (e.g., core-collapse supernovae, compact baldly mergers, tidal disruptions, star formation, etc.). We implement the MODA algorithm into both a Eulerian hydrodynamics code with a fixed, uniform grid and into an SPH code where we use a tree structure that is otherwise used for searching neighbors and calculating gravity. Results. In a series of numerical experiments, we compare the MODA results with analytically known solutions. We also use snapshots from actual 3D simulations and compare the results of MODA with those obtained with other methods, such as the global and local ray-by-ray method. It turns out that MODA achieves excellent accuracy at a moderate computational cost In appendix we also discuss implementation details and parallelization strategies.
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| 7. |
- Sadowski, Aleksander, et al.
(författare)
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Magnetohydrodynamical simulations of a deep tidal disruption in general relativity
- 2016
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 458:4, s. 4250-4268
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Tidskriftsartikel (refereegranskat)abstract
- We perform hydro- and magnetohydrodynamical general-relativistic simulations of a tidal disruption of a 0.1 M-circle dot red dwarf approaching a 10(5) M-circle dot non-rotating massive black hole on a close (impact parameter beta = 10) elliptical (eccentricity e = 0.97) orbit. We track the debris self-interaction, circularization and the accompanying accretion through the black hole horizon. We find that the relativistic precession leads to the formation of a self-crossing shock. The dissipated kinetic energy heats up the incoming debris and efficiently generates a quasi-spherical outflow. The self-interaction is modulated because of the feedback exerted by the flow on itself. The debris quickly forms a thick, almost marginally bound disc that remains turbulent for many orbital periods. Initially, the accretion through the black hole horizon results from the self-interaction, while in the later stages it is dominated by the debris originally ejected in the shocked region, as it gradually falls back towards the hole. The effective viscosity in the debris disc stems from the original hydrodynamical turbulence, which dominates over the magnetic component. The radiative efficiency is very low because of low energetics of the gas crossing the horizon and large optical depth that results in photon trapping. Although the parameters of the simulated tidal disruption are probably not representative of most observed events, it is possible to extrapolate some of its properties towards more common configurations.
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| 8. |
- Taddia, Francesco, et al.
(författare)
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A metallicity study of 1987A-like supernova host galaxies
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 558
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Tidskriftsartikel (refereegranskat)abstract
- Context. The origin of the blue supergiant (BSG) progenitor of Supernova (SN) 1987A has long been debated, along with the role that its sub-solar metallicity played. We now have a sample of SN 1987A-like events that arise from the rare core collapse (CC) of massive (similar to 20 M-circle dot) and compact (less than or similar to 100 R-circle dot) BSGs. Aims. The metallicity of the explosion sites of the known BSG SNe is investigated, as well as the association of BSG SNe to star-forming regions. Methods. Both indirect and direct metallicity measurements of 13 BSG SN host galaxies are presented, and compared to those of other CC SN types. Indirect measurements are based on the known luminosity-metallicity relation and on published metallicity gradients of spiral galaxies. In order to provide direct metallicity measurements based on strong line diagnostics, we obtained spectra of each BSG SN host galaxy both at the exact SN explosion sites and at the positions of other H II regions. We also observed these hosts with narrow H alpha and broad R-band filters in order to produce continuum-subtracted H alpha images. This allows us to measure the degree of association between BSG SNe and star-forming regions, and to compare it to that of other SN types. Results. BSG SNe are found to explode either in low-luminosity galaxies or at large distances from the nuclei of luminous hosts. Therefore, their indirectly measured metallicities are typically lower than those of SNe IIP and Ibc. This result is confirmed by the direct metallicity estimates, which show slightly sub-solar oxygen abundances (12 + log (O/H) similar to 8.3-8.4 dex) for the local environments of BSG SNe, similar to that of the Large Magellanic Cloud (LMC), where SN 1987A exploded. However, we also note that two objects of our sample (SNe 1998A and 2004em) were found at near solar metallicity. SNe IIb have a metallicity distribution similar to that of our BSG SNe. Finally, we find that the degree of association to star-forming regions is similar among BSG SNe, SNe IIP and IIn. Conclusions. Our results suggest that LMC metal abundances play a role in the formation of some 1987A-like SNe. This would naturally fit in a single star scenario for the progenitors. However, the existence of two events at nearly solar metallicity suggests that also other channels, e.g. binarity, contribute to produce BSG SNe.
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| 9. |
- Tejeda, Emilio, et al.
(författare)
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Tidal disruptions by rotating black holes : relativistic hydrodynamics with Newtonian codes
- 2017
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 469:4, s. 4483-4503
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Tidskriftsartikel (refereegranskat)abstract
- We propose an approximate approach for studying the relativistic regime of stellar tidal disruptions by rotating massive black holes. It combines an exact relativistic description of the hydrodynamical evolution of a test fluid in a fixed curved space-time with a Newtonian treatment of the fluid's self-gravity. Explicit expressions for the equations of motion are derived for Kerr space-time using two different coordinate systems. We implement the new methodology within an existing Newtonian smoothed particle hydrodynamics code and show that including the additional physics involves very little extra computational cost. We carefully explore the validity of the novel approach by first testing its ability to recover geodesic motion, and then by comparing the outcome of tidal disruption simulations against previous relativistic studies. We further compare simulations in Boyer-Lindquist and Kerr-Schild coordinates and conclude that our approach allows accurate simulation even of tidal disruption events where the star penetrates deeply inside the tidal radius of a rotating black hole. Finally, we use the new method to study the effect of the black hole spin on the morphology and fallback rate of the debris streams resulting from tidal disruptions, finding that while the spin has little effect on the fallback rate, it does imprint heavily on the stream morphology, and can even be a determining factor in the survival or disruption of the star itself. Our methodology is discussed in detail as a reference for future astrophysical applications.
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