arXiv subjects

A. R. King

Publications and source records attributed to A. R. King.

At least 37 records · Page 2Linked to original sources

arXiv2010

Retrograde Accretion and Merging Supermassive Black Holes

We investigate whether a circumbinary gas disc can coalesce a supermassive black hole binary system in the centre of a galaxy. This is known to be problematic for a prograde disc. We show that in contrast, interaction with a retrograde circumbinary disc is considerably more effective in shrinking the binary because there are no orbital resonances. The binary directly absorbs negative angular momentum from the circumbinary disc by capturing gas into a disc around the secondary black hole, or discs around both holes if the binary mass ratio is close to unity. In many cases the binary orbit becomes eccentric, shortening the pericentre distance as the eccentricity grows. In all cases the binary coalesces once it has absorbed the angular momentum of a gas mass comparable to that of the secondary black hole. Importantly, this conclusion is unaffected even if the gas inflow rate through the disc is formally super--Eddington for either hole. The coalescence timescale is therefore always $\sim M_2/\dot M$, where $M_2$ is the secondary black hole mass and $\dot M$ the inflow rate through the circumbinary disc.

astro-ph.HE↗

arXiv2010

Field and Globular Cluster LMXBs in NGC 4278

We report a detailed spectral analysis of the population of low-mass X-ray binaries (LMXBs) detected in the elliptical galaxy NGC~4278 with Chandra. Seven luminous sources were studied individually, four in globular clusters (GCs), and three in the stellar field. The range of (0.3-8 keV) $L_X$ for these sources suggests that they may be black hole binaries (BHBs). Comparison of our results with simulations allows us to discriminate between disk and power-law dominated emission, pointing to spectral/luminosity variability, reminiscent of Galactic BHBs. The BH masses derived from a comparison of our spectral results with the $L_X \sim T^4_{in}$ relation of Galactic BHBs are in the 5-15 $M_{\odot}$ range, as observed in the Milky Way. The analysis of joint spectra of sources selected in three luminosity ranges suggests that while the high luminosity sources have prominent thermal disk emission components, power-law components are likely to be important in the mid and low-luminosity spectra. Comparing low-luminosity average spectra, we find a relatively larger $N_H$ in the GC spectrum; we speculate that this may point to either a metallicity effect, or to intrinsic physical differences between field and GC accreting binaries. Analysis of average sample properties uncover a previously unreported $L_X - R_G$ correlation (where $R_G$ is the galactocentric radius) in the GC-LMXB sample, implying richer LMXB populations in more central GCs. No such trend is seen in the field LMXB sample. We can exclude that the GC $L_X - R_G$ correlation is the by-product of a luminosity effect, and suggest that it may be related to the presence of more compact GCs at smaller galactocentric radii, fostering more efficient binary formation.

astro-ph.HE↗

arXiv2010

The X-ray Spectra of the Luminous LMXBs in NGC 3379: Field and Globular Cluster Sources

From a deep multi-epoch Chandra observation of the elliptical galaxy NGC 3379 we report the spectral properties of eight luminous LMXBs (LX>1.2E38 erg/s). We also present a set of spectral simulations, produced to aid the interpretation of low-count single-component spectral modeling. These simulations demonstrate that it is possible to infer the spectral states of X-ray binaries from these simple models and thereby constrain the properties of the source. Of the eight LMXBs studied, three reside within globular clusters, and one is a confirmed field source. Due to the nature of the luminosity cut all sources are either neutron star binaries emitting at or above the Eddington luminosity or black hole binaries. The spectra from these sources are well described by single-component models, with parameters consistent with Galactic LMXB observations, where hard-state sources have a range in photon index of 1.5-1.9 and thermally dominated sources have inner disc temperatures between ~0.7-1.55 keV. The large variability observed in the brightest globular cluster source (LX>4E38 erg/s) suggests the presence of a black hole binary. At its most luminous this source is observed in a thermally dominated state with kT=1.5 keV, consistent with a black hole mass of ~4 Msol. This observation provides further evidence that globular clusters are able to retain such massive binaries. We also observed a source transitioning from a bright state (LX~1E39 erg/s), with prominent thermal and non-thermal components, to a less luminous hard state (LX=3.8E38 erg/s, Gamma=1.85). In its high flux emission this source exhibits a cool-disc component of ~0.14 keV, similar to spectra observed in some ultraluminous X-ray sources. Such a similarity indicates a possible link between `normal' stellar mass black holes in a high accretion state and ULXs.

astro-ph.CO↗

arXiv2010

AGN have Underweight Black Holes and Reach Eddington

Eddington outflows probably regulate the growth of supermassive black holes (SMBH) in AGN. I show that effect of the Rayleigh--Taylor instability on these outflows means that SMBH masses are likely to be a factor of a few below the $M - σ$ relation in AGN. This agrees with the suggestion by Batcheldor (2010) that the $M - σ$ relation defines an upper limit to the black hole mass. I further argue that observed AGN black holes must spend much of their lives accreting at the Eddington rate. This is already suggested by the low observed AGN fraction amongst all galaxies despite the need to grow to the masses required by the Soltan relation, and is reinforced by the suggested low SMBH masses. Most importantly, this is the simplest explanation of the recent discovery by Tombesi et al (2010a, b) of the widespread incidence of massive ultrafast X--ray outflows in a large sample of AGN.

astro-ph.CO↗

arXiv2010

The Accretion Disc Dynamo in the Solar Nebula

The nearest accretion disc to us in space if not time was the protosolar nebula. Remnants of this nebula thus potentially offer unique insight into how discs work. In particular the existence of chondrules, which must have formed in the disc as small molten droplets, requires strong and intermittent heating of disc material. We argue that this places important constraints on the way gravitational energy is released in accretion discs, which are not met by current shearing--box simulations of MRI--driven dynamos. A deeper understanding of accretion energy release in discs may require a better model for these dynamos.

astro-ph.SR↗

arXiv2009

Black Hole Outflows

I show that Eddington accretion episodes in AGN are likely to produce winds with velocities $v \sim 0.1c$ and ionization parameters up to $ξ\sim 10^4$ (cgs), implying the presence of resonance lines of helium-- and hydrogenlike iron. These properties are direct consequences of momentum and mass conservation respectively, and agree with recent X-ray observations of fast outflows from AGN. Because the wind is significantly subluminal, it can persist long after the AGN is observed to have become sub--Eddington. The wind creates a strong cooling shock as it interacts with the interstellar medium of the host galaxy, and this cooling region may be observable in an inverse Compton continuum and lower--excitation emission lines associated with lower velocities. The shell of matter swept up by the (`momentum--driven') shocked wind must propagate beyond the black hole's sphere of influence on a timescale $\la 3\times 10^5$ yr. Outside this radius the shell stalls unless the black hole mass has reached the value $M_σ$ implied by the $M - σ$ relation. If the wind shock did not cool, as suggested here, the resulting (`energy--driven') outflow would imply a far smaller SMBH mass than actually observed. In galaxies with large bulges the black hole may grow somewhat beyond this value, suggesting that the observed $M -σ$ relation may curve upwards at large $M$. Minor accretion events with small gas fractions can produce galaxy--wide outflows with velocities significantly exceeding $σ$, including fossil outflows in galaxies where there is little current AGN activity. Some rare cases may reveal the energy--driven outflows which sweep gas out of the galaxy and establish the black hole--bulge mass relation. However these require the quasar to be at the Eddington luminosity.

astro-ph.CO↗

arXiv2009

Black hole mergers: the first light

The coalescence of supermassive black hole binaries occurs via the emission of gravitational waves, that can impart a substantial recoil to the merged black hole. We consider the energy dissipation, that results if the recoiling black hole is surrounded by a thin circumbinary disc. Our results differ significantly from those of previous investigations. We show analytically that the dominant source of energy is often potential energy, released as gas in the outer disc attempts to circularize at smaller radii. Thus, dimensional estimates, that include only the kinetic energy gained by the disc gas, underestimate the real energy loss. This underestimate can exceed an order of magnitude, if the recoil is directed close to the disc plane. We use three dimensional Smooth Particle Hydrodynamics (SPH) simulations and two dimensional finite difference simulations to verify our analytic estimates. We also compute the bolometric light curve, which is found to vary strongly depending upon the kick angle. A prompt emission signature due to this mechanism may be observable for low mass (10^6 Solar mass) black holes whose recoil velocities exceed about 1000 km/s. Emission at earlier times can mainly result from the response of the disc to the loss of mass, as the black holes merge. We derive analytically the condition for this to happen.

astro-ph.HE↗

arXiv2009

Comparing GC and Field LMXBs in Elliptical Galaxies with deep Chandra and Hubble data

(abridged) We present a statistical study of the low-mass X-ray binary (LMXB) populations of three nearby, old elliptical galaxies: NGC 3379, NGC 4278, and NGC 4697. With a cumulative ~1 Ms Chandra ACIS observing time, we detect 90-170 LMXBs within the D25 ellipse of each galaxy. Cross-correlating Chandra X-ray sources and HST optical sources, we identify 75 globular cluster (GC) LMXBs and 112 field LMXBs. In the low luminosity range allowed by our deeper data (LX < 5 x 1037 erg s-1), we find a significant relative lack of GC-LMXBs, when compared with field sources. Using the co-added sample from the three galaxies, we find that the incompleteness-corrected X-ray luminosity functions (XLFs) of GC and field LMXBs differ at ~4# significance at LX < 5 x 1037 erg s-1. As previously reported, these XLFs are consistent at higher luminosities. Our observations may indicate a potential predominance of GC-LMXBs with donors evolved beyond the main sequence, when compared to current models, but their efficient formation requires relatively high initial binary fractions in clusters. The field LMXB XLF can be fitted with either a single power-law model plus a localized excess at a luminosity of 5-6 x 1037 erg s-1, or a broken power-law with a similar low-luminosity break. This XLF may be explained with NS-red-giant LMXBs, contributing to ~15% of total LMXBs population at ~5x1037 erg s-1. The difference in the GC and field XLFs is consistent with different origins and/or evolutionary paths between the two LMXB populations, although a fraction of the field sources are likely to have originated in GCs.

astro-ph.CO↗

arXiv2009

Black hole mergers: can gas discs solve the `final parsec' problem?

We compute the effect of an orbiting gas disc in promoting the coalescence of a central supermassive black hole binary. Unlike earlier studies, we consider a finite mass of gas with explicit time dependence: we do not assume that the gas necessarily adopts a steady state or a spatially constant accretion rate, i.e. that the merging black hole was somehow inserted into a pre--existing accretion disc. We consider the tidal torque of the binary on the disc, and the binary's gravitational radiation. We study the effects of star formation in the gas disc in a simple energy feedback framework. The disc spectrum differs in detail from that found before. In particular, tidal torques from the secondary black hole heat the edges of the gap, creating bright rims around the secondary. These rims do not in practice have uniform brightness either in azimuth or time, but can on average account for as much as 50 per cent of the integrated light from the disc. This may lead to detectable high--photon--energy variability on the relatively long orbital timescale of the secondary black hole, and thus offer a prospective signature of a coalescing black hole binary. We also find that the disc can drive the binary to merger on a reasonable timescale only if its mass is at least comparable with that of the secondary black hole, and if the initial binary separation is relatively small, i.e. $a_0 \lesssim 0.05$ pc. Star formation complicates the merger further by removing mass from the disc. In the feedback model we consider, this sets an effective limit to the disc mass. As a result, binary merging is unlikely unless the black hole mass ratio is $\la 0.001$. Gas discs thus appear not to be an effective solution to the `last parsec' problem for a significant class of mergers.

astro-ph.CO↗

arXiv2009

RS Ophiuchi: Thermonuclear Explosion or Disc Instability?

Sokoloski et al (2008) have recently reported evidence that the recurrent nova RS Ophiuchi produced a pair of highly collimated radio jets within days of its 2006 outburst. This suggests that an accretion disc must be present during the outburst. However in the standard picture of recurrent novae as thermonuclear events, any such disc must be expelled from the white dwarf vicinity, as the nuclear energy yield greatly exceeds its binding energy. We suggest instead that the outbursts of RS Oph are thermal--viscous instabilities in a disc irradiated by the central accreting white dwarf. The distinctive feature of RS Oph is the very large size of its accretion disc. Given this, it fits naturally into a consistent picture of systems with unstable accretion discs. This picture explains the presence and speed of the jets, the brightness and duration of the outburst, and its rise time and linear decay, as well as the faintness of the quiescence. By contrast, the hitherto standard picture of recurrent thermonuclear explosions has a number of severe difficulties. These include the presence of jets, the faintness of quiescence, and the fact the the accretion disc must be unstable unless it is far smaller than any reasonable estimate.

astro-ph.EP↗

arXiv2009

Episodic Random Accretion and the Cosmological Evolution of Supermassive Black Hole Spins

The growth of supermassive black holes (BHs) located at the centers of their host galaxies comes mainly from accretion of gas, but how to fuel them remains an outstanding unsolved problem in quasar evolution. This issue can be elucidated by quantifying the radiative efficiency parameter ($η$) as a function of redshift, which also provides constraints on the average spin of the BHs and its possible evolution with time. We derive a formalism to link $η$ with the luminosity density, BH mass density, and duty cycle of quasars, quantities we can estimate from existing quasar and galaxy survey data. We find that $η$ has a strong cosmological evolution: at z~2, $η\approx 0.3$, and by $z\approx 0$ it has decreased by an order of magnitude, to $η\approx 0.03$. We interpret this trend as evolution in BH spin, and we appeal to episodic, random accretion as the mechanism for reducing the spin. The observation that the fraction of radio-loud quasars decreases with increasing redshift is inconsistent with the popular notion that BH spin is a critical factor for generating strong radio jets. In agreement with previous studies, we show that the derived history of BH accretion closely follows the cosmic history of star formation, consistent with other evidence that BHs and their host galaxies coevolve.

astro-ph.GA↗

arXiv2009

The Growth of Supermassive Black Holes Across Cosmic Time

One of the main themes in extragalactic astronomy for the next decade will be the evolution of galaxies over cosmic time. Many future observatories, including JWST, ALMA, GMT, TMT and E-ELT will intensively observe starlight over a broad redshift range, out to the dawn of the modern Universe when the first galaxies formed. It has, however, become clear that the properties and evolution of galaxies are intimately linked to the growth of their central black holes. Understanding the formation of galaxies, and their subsequent evolution, will therefore be incomplete without similarly intensive observations of the accretion light from supermassive black holes (SMBH) in galactic nuclei. To make further progress, we need to chart the formation of typical SMBH at z>6, and their subsequent growth over cosmic time, which is most effectively achieved with X-ray observations. Recent technological developments in X-ray optics and instrumentation now bring this within our grasp, enabling capabilities fully matched to those expected from flagship observatories at longer wavelengths.

astro-ph.CO↗

arXiv2009

Deep Chandra Monitoring Observations of NGC 4278: Catalog of Source Properties

We present the properties of the discrete X-ray sources detected in our monitoring program of the globular cluster (GC) rich elliptical galaxy, NGC 4278, observed with Chandra ACIS-S in six separate pointings, resulting in a co-added exposure of 458-ks. From this deep observation, 236 sources have been detected within the region overlapped by all observations, 180 of which lie within the D25 ellipse of the galaxy. These 236 sources range in Lx from 3.5E36 erg/s (with 3sigma upper limit <1E37 erg/s) to ~2E40 erg/s, including the central nuclear source which has been classified as a LINER. From optical data, 39 X-ray sources have been determined to be coincident with a globular cluster, these sources tend to have high X-ray luminosity, with ten of these sources exhibiting Lx>1E38 erg/s. From X-ray source photometry, it has been determined that the majority of the 236 point sources that have well constrained colors, have values that are consistent with typical LMXB spectra, with 29 of these sources expected to be background objects from the logN-logS relation. There are 103 sources in this population that exhibit long-term variability, indicating that they are accreting compact objects. 3 of these sources have been identified as transient candidates, with a further 3 possible transients. Spectral variations have also been identified in the majority of the source population, where a diverse range of variability has been identified, indicating that there are many different source classes located within this galaxy.

astro-ph.CO↗

arXiv2009

Transient Low-Mass X-Ray Binary Populations in Elliptical Galaxies NGC 3379 and NGC 4278

We propose a physically motivated and self-consistent prescription for the modeling of transient neutron star (NS) low-mass X-ray binary (LMXB) properties, such as duty cycle (DC), outburst duration and recurrence time. We apply this prescription to the population synthesis (PS) models of field LMXBs presented by Fragos et al. (2008), and compare the transient LMXB population to the Chandra X-ray survey of the two elliptical galaxies NGC 3379 and NGC 4278, which revealed several transient sources (Brassington et al., 2008, 2009). We are able to exclude models with a constant DC for all transient systems, while models with a variable DC based on the properties of each system are consistent with the observed transient populations. We predict that the majority of the observed transient sources in these two galaxies are LMXBs with red giant donors. Our comparison suggests that LMXBs formed through evolution of primordial field binaries are dominant in globular cluster (GC) poor elliptical galaxies, while they still have a significant contribution in GC rich ones.

astro-ph.CO↗

arXiv2008

Masses, Beaming and Eddington Ratios in Ultraluminous X-ray Sources

I suggest that the beaming factor in bright ULXs varies as $b \propto \dot m^{-2}$, where $\dot m$ is the Eddington ratio for accretion. This is required by the observed universal $L_{\rm soft} \propto T^{-4}$ relation between soft--excess luminosity and temperature, and is reasonable on general physical grounds. The beam scaling means that all observable properties of bright ULXs depend essentially only on the Eddington ratio $\dot m$, and that these systems vary mainly because the beaming is sensitive to the Eddington ratio. This suggests that bright ULXs are stellar--mass systems accreting at Eddington ratios of order 10 -- 30, with beaming factors $b \ga 0.1$. Lower--luminosity ULXs follow bolometric (not soft--excess) $L \sim T^4$ correlations and probably represent {\it sub}--Eddington accretion on to black holes with masses $\sim 10\msun$. High--mass X-ray binaries containing black holes or neutron stars and undergoing rapid thermal-- or nuclear--timescale mass transfer are excellent candidates for explaining both types. If the $b \propto \dot m^{-2}$ scaling for bright ULXs can be extrapolated to the Eddington ratios found in SS433, some objects currently identified as AGN at modest redshifts might actually be ULXs (`pseudoblazars'). This may explain cases where the active source does not coincide with the centre of the host galaxy.

astro-ph↗

arXiv2008

Stellar disruption by a supermassive black hole: is the light curve really proportional to $t^{-5/3}$?

In this paper we revisit the arguments for the basis of the time evolution of the flares expected to arise when a star is disrupted by a supermassive black hole. We present a simple analytic model relating the lightcurve to the internal density structure of the star. We thus show that the standard lightcurve proportional to $t^{-5/3}$ only holds at late times. Close to the peak luminosity the lightcurve is shallower, deviating more strongly from $t^{-5/3}$ for more centrally concentrated (e.g. solar--type) stars. We test our model numerically by simulating the tidal disruption of several stellar models, described by simple polytropic spheres with index $γ$. The simulations agree with the analytical model given two considerations. First, the stars are somewhat inflated on reaching pericentre because of the effective reduction of gravity in the tidal field of the black hole. This is well described by a homologous expansion by a factor which becomes smaller as the polytropic index becomes larger. Second, for large polytropic indices wings appear in the tails of the energy distribution, indicating that some material is pushed further away from parabolic orbits by shocks in the tidal tails. In all our simulations, the $t^{-5/3}$ lightcurve is achieved only at late stages. In particular we predict that for solar type stars, this happens only after the luminosity has dropped by at least two magnitudes from the peak. We discuss our results in the light of recent observations of flares in otherwise quiescent galaxies and note the dependence of these results on further parameters, such as the star/hole mass ratio and the stellar orbit.

astro-ph↗

arXiv2008

Discovery of Hot Gas in Outflow in NGC3379

We report the discovery of a faint (L_x ~ 4 10^37 erg/s, 0.5-2 keV), out-flowing gaseous hot interstellar medium (ISM) in NGC 3379. This represents the lowest X-ray luminosity ever measured from a hot phase of the ISM in a nearby early type galaxy. The discovery of the hot ISM in a very deep Chandra observation was possible thanks to its unique spectral and spatial signatures, which distinguish it from the integrated stellar X-ray emission, responsible for most of the unresolved emission in the Chandra data. This hot component is found in a region of about 800 pc in radius at the center of the galaxy and has a total mass M~ 3 10^5 solar masses. Independent theoretical prediction of the characteristics of an ISM in this galaxy, based on the intrinsic properti es of NGC 3379, reproduce well the observed luminosity, temperature, and radial distribution and mass of the hot gas, and indicate that the gas is in an outflowing phase, predicted by models but not observed in any system so far.

astro-ph↗

arXiv2008

Deep Chandra Monitoring Observations of NGC 3379: Catalog of Source Properties

We present the properties of the discrete X-ray sources detected in our monitoring program of the `typical' elliptical galaxy, NGC 3379, observed with Chandra ACIS-S in five separate pointings, resulting in a co-added exposure of 324-ks. From this deep observation, 132 sources have been detected within the region overlapped by all observations, 98 of which lie within the D25 ellipse of the galaxy. These 132 sources range in Lx from 6E35 erg/s (with 3sigma upper limit <4E36 erg/s) to ~2E39 erg/s, including one source with Lx>1E39 erg/s, which has been classified as a ULX. From optical data, 10 X-ray sources have been determined to be coincident with a globular cluster, these sources tend to have high X-ray luminosity, with three of these sources exhibiting Lx> 1E38 erg/s. From X-ray source photometry, it has been determined that the majority of the 132 sources that have well constrained colors, have values that are consistent with typical LMXB spectra. Additionally to this, a sub-population of 10 sources has been found to exhibit very hard spectra and it is expected that most of these sources are absorbed background AGN. There are 64 sources in this population that exhibit long-term variability, indicating that they are accreting compact objects. Five of these sources have been identified as transient candidates, with a further 3 possible transients. Spectral variations have also been identified in the majority of the source population, where a diverse range of variability has been identified, indicating that there are many different source classes located within this galaxy.

astro-ph↗