arXiv subjects

Curtis J. Saxton

Publications and source records attributed to Curtis J. Saxton.

At least 19 recordsLinked to original sources

arXiv2023

Ordering of timescales predicts applicability of quasi-linear theory in unstable flows

We discuss the applicability of quasilinear-type approximations for a turbulent system with a large range of spatial and temporal scales. We consider a paradigm fluid system of rotating convection with a vertical and horizontal temperature gradients. In particular, the interaction of rotating with the horizontal temperature gradient drives a ``thermal wind'' shear flow whose strength is controlled by a horizontal temperature gradient. Varying the parameters systematically alters the ordering of the shearing timescale, the convective timescale, and the correlation timescale. We demonstrate that quasilinear-type approximations work well when the shearing timescale or the correlation timescale is sufficiently short. In all cases, the Generalised Quasilinear approximation (GQL) systematically outperforms the Quasilinear approximation (QL). We discuss the consequences for statistical theories of turbulence interacting with mean gradients. We conclude with comments about the general applicability of these ideas across a wide variety of non-linear physical systems.

physics.flu-dyn↗

arXiv2019

Polarised radiative transfer, rotation measure fluctuations and large-scale magnetic fields

Faraday rotation measure at radio wavelengths is commonly used to diagnose large-scale magnetic fields. It is argued that the length-scales on which magnetic fields vary in large-scale diffuse astrophysical media can be inferred from correlations in the observed RM. RM is a variable which can be derived from the polarised radiative transfer equations in restrictive conditions. This paper assesses the usage of RMF (rotation measure fluctuation) analyses for magnetic field diagnostics in the framework of polarised radiative transfer. We use models of various magnetic field configurations and electron density distributions to show how density fluctuations could affect the correlation length of the magnetic fields inferred from the conventional RMF analyses. We caution against interpretations of RMF analyses when a characteristic density is ill-defined, e.g. in cases of log-normal distributed and fractal-like density structures. As the spatial correlations are generally not the same in the line-of-sight longitudinal direction and the sky plane direction, one also needs to clarify the context of RMF when inferring from observational data. In complex situations, a covariant polarised radiative transfer calculation is essential to capture all aspects of radiative and transport processes, which would otherwise ambiguate the interpretations of magnetism in galaxy clusters and larger-scale cosmological structures.

astro-ph.HE↗

arXiv2017

Spectral features of tidal-disruption candidates and alternative origins for such transient flares

UV and optically selected candidates for stellar tidal disruption events (TDE) often exhibit broad spectral features (HeII emission, H$α$ emission, or absorption lines) on a blackbody-like continuum (1e4K<T<1e5K). The lines presumably emit from TDE debris or circumnuclear clouds photoionized by the flare. Line velocities however are much lower than expected from a stellar disruption by supermassive black hole (SMBH), and are somewhat faster than expected for the broad line region (BLR) clouds of a persistently active galactic nucleus (AGN). The distinctive spectral states are not strongly related to observed luminosity and velocity, nor to SMBH mass estimates. We use exhaustive photoionization modelling to map the domain of fluxes and cloud properties that yield (e.g.) a He-overbright state where a large HeII(4686A)/H$α$ line-ratio creates an illusion of helium enrichment. Although observed line ratios occur in a plausible minority of cases, AGN-like illumination can not reproduce the observed equivalent widths. We therefore propose to explain these properties by a light-echo photoionization model: the initial flash of a hot blackbody (detonation) excites BLR clouds, which are then seen superimposed on continuum from a later, expanded, cooled stage of the central luminous source. The implied cloud mass is substellar, which may be inconsistent with a TDE. Given these and other inconsistencies with TDE models (e.g. host-galaxies distribution) we suggest to also consider alternative origins for these nuclear flares, which we briefly discuss (e.g. nuclear supernovae and starved/subluminous AGNs).

astro-ph.HE↗

arXiv2016

Dark matter concentrations in galactic nuclei according to polytropic models

We calculate the radial profiles of galaxies where the nuclear region is self-gravitating, consisting of self-interacting dark matter (SIDM) with $F$ degrees of freedom. For sufficiently high density this dark matter becomes collisional, regardless of its behaviour on galaxy scales. Our calculations show a spike in the central density profile, with properties determined by the dark matter microphysics, and the densities can reach the `mean density' of a black hole (from dividing the black-hole mass by the volume enclosed by the Schwarzschild radius). For a galaxy halo of given compactness ($χ=2GM/Rc^2$), certain values for the dark matter entropy yield a dense central object lacking an event horizon. For some soft equations of state of the SIDM (e.g. $F\ge6$), there are multiple horizonless solutions at given compactness. Although light propagates around and through a sphere composed of dark matter, it is gravitationally lensed and redshifted. While some calculations give non-singular solutions, others yield solutions with a central singularity. In all cases the density transitions smoothly from the central body to the dark-matter envelope around it, and to the galaxy's dark matter halo. We propose that pulsar timing observations will be able to distinguish between systems with a centrally dense dark matter sphere (for different equations of state) and conventional galactic nuclei that harbour a supermassive black hole.

astro-ph.GA↗

arXiv2015

High-energy neutrino fluxes from AGN populations inferred from X-ray surveys

High-energy neutrinos and photons are complementary messengers, probing violent astrophysical processes and structural evolution of the Universe. X-ray and neutrino observations jointly constrain conditions in active galactic nuclei (AGN) jets: their baryonic and leptonic contents, and particle production efficiency. Testing two standard neutrino production models for local source Cen A \citep{KT2008,BB2009}, we calculate the high-energy neutrino spectra of single AGN sources and derive the flux of high-energy neutrinos expected for the current epoch. Assuming that accretion determines both X-rays and particle creation, our parametric scaling relations predict neutrino yield in various AGN classes. We derive redshift-dependent number densities of each class, from {\it Chandra} and {\it Swift}/BAT X-ray luminosity functions \citep{SGB2008,ACS2009}. We integrate the neutrino spectrum expected from the cumulative history of AGN (correcting for cosmological and source effects, e.g. jet orientation and beaming). Both emission scenarios yield neutrino fluxes well above limits set by {\it IceCube} (by $\sim 4$--$10^6 \times$ at 1 PeV, depending on the assumed jet models for neutrino production). This implies that: (i) Cen A might not be a typical neutrino source as commonly assumed; (ii) both neutrino production models overestimate the efficiency; (iii) neutrino luminosity scales with accretion power differently among AGN classes and hence does not follow X-ray luminosity universally; (iv) some AGN are neutrino-quiet (e.g. below a power threshold for neutrino production); (v) neutrino and X-ray emission have different duty cycles (e.g. jets alternate between baryonic and leptonic flows); or (vi) some combination of the above.

astro-ph.HE↗

arXiv2014

Dark halo microphysics and massive black hole scaling relations in galaxies

We investigate the black hole (BH) scaling relation in galaxies using a model in which the galaxy halo and central BH are a self-gravitating sphere of dark matter (DM) with an isotropic, adiabatic equation of state. The equipotential where the escape velocity approaches the speed of light defines the horizon of the BH. We find that the BH mass ($m_\bullet$) depends on the DM entropy, when the effective thermal degrees of freedom ($F$) are specified. Relations between BH and galaxy properties arise naturally, with the BH mass and DM velocity dispersion following $m_\bullet\proptoσ^{F/2}$ (for global mean density set by external cosmogony). Imposing observationally derived constraints on $F$ provides insight into the microphysics of DM. Given that DM velocities and stellar velocities are comparable, the empirical correlation between $m_\bullet$ and stellar velocity dispersions $σ_\star$ implies that $7 6$ the dense dark envelope surrounding the BH approaches the mean density of the BH itself, while the outer halo can show a nearly uniform kpc-scale core resembling those observed in galaxies.

astro-ph.GA↗

arXiv2013

Gravitational and distributed heating effects of a cD galaxy on the hydrodynamical structure of its host cluster

We investigate the effects of a cD galaxy's gravity and AGN heating of the host galaxy cluster. We consider a standard prescription for the hydrodynamics, with the structures determined by mass continuity, momentum and energy conservation equations in spherical symmetry. The cluster comprises a dark matter halo (DM) and ionised X-ray emitting intracluster gas (ICM), which jointly determine the gravitational potential. The cD galaxy is an additive gravitational potential component. The DM assumes a polytropic equation of state (determined by its microphysics), which could be non-radiative self-interacting particles or more exotically interacting particles. The AGN provides distributed heating, counteracting radiative cooling. Stationary density and velocity dispersion profiles are obtained by numerically integrating the hydrodynamic equations with appropriate boundary conditions. The minimum gas temperature in the cluster core is higher when a cD galaxy is present than when it is absent. The solutions also yield a point-like mass concentration exceeding a minimum mass: presumably the AGN's supermassive black hole (SMBH). Consistency with observed SMBH masses constrains the possible DM equations of state. The constraints are looser when a cD galaxy is present. Distributed (AGN) heating alters cluster global properties, and also reduces the lower limits for the central point-mass, for the preferred DM models in which the dark particles have greater heat capacity than point particles. Eluding these constraints would require dominant non-spherical or anisotropic effects (e.g. bulk rotation, non-radial streaming, asymmetric lumps, or a strong magnetic field).

astro-ph.CO↗

arXiv2012

Galaxy stability within a self-interacting dark matter halo

This paper investigates spheroidal galaxies comprising a self-interacting dark matter halo (SIDM) plus de Vaucouleurs stellar distribution. These are coupled only via their shared gravitational field, which is computed consistently from the density profiles. Assuming conservation of mass, momentum and angular momentum, perturbation analyses reveal the galaxy's response to radial disturbance. The modes depend on fundamental dark matter properties, the stellar mass, and the halo's mass and radius. The coupling of stars and dark matter stabilises some haloes that would be unstable as one-fluid models. However the centrally densest haloes are unstable, causing radial flows of SIDM and stars (sometimes in opposite directions). Depending on the dark microphysics, some highly diffuse haloes are also unstable. Unstable galaxies might shed their outskirts or collapse. Observed elliptical galaxies appear to exist in the safe domain. Halo pulsations are possible. The innermost node of SIDM waves may occur within ten half-light radii. Induced stellar ripples may also occur at detectable radii if higher overtones are excited. If any SIDM exists, observational skotoseismology of galaxies could probe DM physics, measure the sizes of specific systems, and perhaps help explain peculiar objects (e.g. some shell galaxies, and the growth of red nuggets).

astro-ph.CO↗

arXiv2012

Long-term X-ray variability of Swift J1644+57

We studied the X-ray timing and spectral variability of the X-ray source Sw J1644+57, a candidate for a tidal disruption event. We have separated the long-term trend (an initial decline followed by a plateau) from the short-term dips in the Swift light-curve. Power spectra and Lomb-Scargle periodograms hint at possible periodic modulation. By using structure function analysis, we have shown that the dips were not random but occurred preferentially at time intervals ~ [2.3, 4.5, 9] x 10^5 s and their higher-order multiples. After the plateau epoch, dipping resumed at ~ [0.7, 1.4] x 10^6 s and their multiples. We have also found that the X-ray spectrum became much softer during each of the early dip, while the spectrum outside the dips became mildly harder in its long-term evolution. We propose that the jet in the system undergoes precession and nutation, which causes the collimated core of the jet briefly to go out of our line of sight. The combined effects of precession and nutation provide a natural explanation for the peculiar patterns of the dips. We interpret the slow hardening of the baseline flux as a transition from an extended, optically thin emission region to a compact, more opaque emission core at the base of the jet.

astro-ph.HE↗

arXiv2010

Time-dependent radio emission from evolving jets

We investigated the time-dependent radiative and dynamical properties of light supersonic jets launched into an external medium, using hydrodynamic simulations and numerical radiative transfer calculations. These involved various structural models for the ambient media, with density profiles appropriate for galactic and extragalactic systems. The radiative transfer formulation took full account of emission, absorption, re-emission, Faraday rotation and Faraday conversion explicitly. High time-resolution intensity maps were generated, frame-by-frame, to track the spatial hydrodynamical and radiative properties of the evolving jets. Intensity light curves were computed via integrating spatially over the emission maps. We apply the models to jets in active galactic nuclei (AGN). From the jet simulations and the time-dependent emission calculations we derived empirical relations for the emission intensity and size for jets at various evolutionary stages. The temporal properties of jet emission are not solely consequences of intrinsic variations in the hydrodynamics and thermal properties of the jet. They also depend on the interaction between the jet and the ambient medium. The interpretation of radio jet morphology therefore needs to take account of environmental factors. Our calculations have also shown that the environmental interactions can affect specific emitting features, such as internal shocks and hotspots. Quantification of the temporal evolution and spatial distribution of these bright features, together with the derived relations between jet size and emission, would enable us to set constraints on the hydrodynamics of AGN and the structure of the ambient medium.

astro-ph.GA↗

arXiv2010

Polytropic dark halos of elliptical galaxies

The kinematics of stars and planetary nebulae in early type galaxies provide vital clues to the enigmatic physics of their dark matter halos. We fit published data for fourteen such galaxies using a spherical, self-gravitating model with two components: (1) a Sersic stellar profile fixed according to photometric parameters, and (2) a polytropic dark matter halo that conforms consistently to the shared gravitational potential. The polytropic equation of state can describe extended theories of dark matter involving self-interaction, non-extensive thermostatistics, or boson condensation (in a classical limit). In such models, the flat-cored mass profiles widely observed in disc galaxies are due to innate dark physics, regardless of any baryonic agitation. One of the natural parameters of this scenario is the number of effective thermal degrees of freedom of dark matter (F_d) which is proportional to the dark heat capacity. By default we assume a cosmic ratio of baryonic and dark mass. Non-Sersic kinematic ideosyncrasies and possible non-sphericity thwart fitting in some cases. In all fourteen galaxies the fit with a polytropic dark halo improves or at least gives similar fits to the velocity dispersion profile, compared to a stars-only model. The good halo fits usually prefer F_d values from six to eight. This range complements the recently inferred limit of 7<F_d<10 (Saxton & Wu), derived from constraints on galaxy cluster core radii and black hole masses. However a degeneracy remains: radial orbital anisotropy or a depleted dark mass fraction could shift our models' preference towards lower F_d; whereas a loss of baryons would favour higher F_d.

astro-ph.CO↗

arXiv2008

Structures, oscillations and solitons in large-scale multi-component self-gravitating systems

We investigate the structure of dynamics of large self-gravitating astrophysical systems using a self-interacting two-component model. We consider two cases, galaxy clusters and cosmic walls, for illustrations. In both cases stability analyses are conducted using perturbative expansion. We have found that waves and solitons are easily generated in these systems. Our analysis shows that dark matter can be Jeans unstable in the very inner regions of galaxy clusters if it has a large internal degree of freedom. The dark matter core may collapse under external perturbations. We also discuss dark-matter oscillations in galaxy clusters and how mode growth and decay lead to heating of intracluster medium. Our analysis shows that dark-matter solitons with both positive and negative amplitudes can be excited in cosmic walls. Resonances in soliton interaction could enhance gas condensation. The co-existence of the two types of dark-matter solitons implies that bright filaments can arise in dark voids.

astro-ph↗

arXiv2008

Radial structure, inflow & central mass of stationary radiative galaxy clusters

We analyse the radial structure of self-gravitating spheres consisting of multiple interpenetrating fluids, such as the X-ray emitting gas and the dark halo of a galaxy cluster. In these dipolytropic models, adiabatic dark matter sits in equilibrium, while the gas develops a gradual, smooth, quasi-stationary cooling flow. Both affect and respond to the collective gravitational field. We find that all subsonic, radially continuous, steady solutions require a non-zero minimum central point mass. For Mpc-sized halos with 7 to 10 effective degrees of freedom (F2), the minimum central mass is compatible with observations of supermassive black holes. Smaller gas mass influxes enable smaller central masses for wider ranges of F2. The halo comprises a sharp spike around the central mass, embedded within a core of nearly constant density (at 10-10^2.5kpc scales), with outskirts that attenuate and naturally truncate at finite radius (several Mpc). The gas density resembles a broken power law in radius, but the temperature dips and peaks within the dark core. A finite minimum temperature occurs due to gravitational self-warming, without cold mass dropout nor needing regulatory heating. X-ray emission from the intracluster medium mimics a beta-model plus bright compact nucleus. Near-sonic points in the gas flow are bottlenecks to allowed steady solutions; the outermost are at kpc scales. These sites may preferentially develop cold mass dropout during strong perturbations off equilibrium. Within the sonic point, the profile of gas specific entropy is flatter than s r^{1/2}, but this is a shallow ramp and not an isentropic core. When F2 is large, the inner halo spike is only marginally Jeans stable in the central pc, suggesting that a large non-linear disturbance could trigger local dark collapse onto the central object.

astro-ph↗

arXiv2008

The effect of two-temperature post-shock accretion flow on the linear polarization pulse in magnetic cataclysmic variables

The temperatures of electrons and ions in the post-shock accretion region of a magnetic cataclysmic variable (mCV) will be equal at sufficiently high mass flow rates or for sufficiently weak magnetic fields. At lower mass flow rates or in stronger magnetic fields, efficient cyclotron cooling will cool the electrons faster than the electrons can cool the ions and a two-temperature flow will result. Here we investigate the differences in polarized radiation expected from mCV post-shock accretion columns modeled with one- and two-temperature hydrodynamics. In an mCV model with one accretion region, a magnetic field >~30 MG and a specific mass flow rate of ~0.5 g/cm/cm/s, along with a relatively generic geometric orientation of the system, we find that in the ultraviolet either a single linear polarization pulse per binary orbit or two pulses per binary orbit can be expected, depending on the accretion column hydrodynamic structure (one- or two-temperature) modeled. Under conditions where the physical flow is two-temperature, one pulse per orbit is predicted from a single accretion region where a one-temperature model predicts two pulses. The intensity light curves show similar pulse behavior but there is very little difference between the circular polarization predictions of one- and two-temperature models. Such discrepancies indicate that it is important to model some aspect of two-temperature flow in indirect imaging procedures, like Stokes imaging, especially at the edges of extended accretion regions, were the specific mass flow is low, and especially for ultraviolet data.

astro-ph↗

arXiv2007

X-ray emissions from two-temperature accretion flows within a dipole magnetic funnel

We investigate the hydrodynamics of accretion channelled by a dipolar magnetic field (funnel flows). We consider situations in which the electrons and ions in the flow cannot maintain thermal equilibrium (two-temperature effects) due to strong radiative loss, and determine the effects on the keV X-ray properties of the systems. We apply this model to investigate the accretion shocks of white dwarfs in magnetic cataclysmic variables. We have found that the incorporation of two-temperature effects could harden the keV X-rays. Also, the dipolar model yields harder X-ray spectra than the standard planar model if white dwarf is sufficiently massive (>~1M_sun). When fitting observed keV X-ray spectra of magnetic cataclysmic variables, the inclusion of two-temperature hydrodynamics and a dipolar accretion geometry lowers estimates for white-dwarf masses when compared with masses inferred from models excluding these effects. We find mass reductions <~9% in the most massive cases.

astro-ph↗

arXiv2005

Accretion in dipole magnetic fields: flow structure and X-ray emission of accreting white dwarfs

Field-channelled accretion flows occur in a variety of astrophysical objects, including T Tauri stars,magnetic cataclysmic variables and X-ray pulsars. We consider a curvilinear coordinate system and derive a general hydrodynamic formulation for accretion onto stellar objects confined by a stellar dipole magnetic field. The hydrodynamic equations are solved to determine the velocity, density and temperature profiles of the flow. We use accreting magnetic white-dwarf stars as an illustrative example of astrophysical applications. Our calculations show that the compressional heating due to the field geometry is as important as radiative cooling and gravity in determining the structure of the post-shock flow in accreting white-dwarf stars. The generalisation of the formulation to accretion flows channelled by higher-order fields and the applications to other astrophysical systems are discussed.

astro-ph↗

arXiv2005

Interactions of Jets with Inhomogeneous Cloudy Media

We present two-dimensional slab-jet simulations of jets in inhomogeneous media consisting of a tenuous hot medium populated with a small filling factor by warm, dense clouds. The simulations are relevant to the structure and dynamics of sources such as Gigahertz Peak Spectrum and Compact Steep Spectrum radio galaxies, High Redshift Radio Galaxies and radio galaxies in cooling flows. The jets are disrupted to a degree depending upon the filling factor of the clouds. With a small filling factor, the jet retains some forward momentum but also forms a halo or bubble around the source. At larger filling factors channels are formed in the cloud distribution through which the jet plasma flows and a hierarchical structure consisting of nested lobes and an outer enclosing bubble results. We suggest that the CSS quasar 3C48 is an example of a low filling factor jet - interstellar medium interaction whilst M87 may be an example of the higher filling factor type of interaction. Jet disruption occurs primarily as a result of Kelvin-Helmholtz instabilities driven by turbulence in the radio cocoon not through direct jet-cloud interactions, although there are some examples of these. In all radio galaxies whose morphology may be the result of jet interactions with an inhomogeneous interstellar medium we expect that the dense clouds will be optically observable as a result of radiative shocks driven by the pressure of the radio cocoon. We also expect that the radio galaxies will possess faint haloes of radio emitting material well beyond the observable jet structure.

astro-ph↗

arXiv2002

Production of ring-like structure in the cocoon of Hercules A

The radio lobes of the radio galaxy Hercules A contain intriguing ring-like structures concentric with the jet axis. To investigate the occurrence of such features, we have used hydrodynamic simulations of jets with a range of Mach numbers (from M=2 to 50) and densities (down to a ratio of 1E-4 relative to the background) to generate ray-traced images simulating synchrotron emission from the time-dependent shock structures. We compare these images with observations of Hercules A, and consider the physical nature and temporal evolution of the most plausible configurations. We find that the observed ring-like structures are well explained as nearly annular shocks propagating in the backflow surrounding the jet. We infer that the jet is oriented at between 30 and 70 degrees to the line of sight, consistent with radio depolarisation observations of Gizani & Leahy. The observational lack of hot-spots at the extremities of the radio lobes, and the possible presence of a buried hot-spot near the base of the western lobe, are explained in terms of the intrinsic brightness fluctuations and dynamics of the terminal shock of an ultra-light, low Mach number jet that surges along its axis due to intermittent pinching and obstruction by turbulent backflow in the cocoon. We conclude from the appearance of both sides of the Hercules A, that both jets are on the borderline of becoming fully turbulent.

astro-ph↗