Low Radiative Efficiency Accretion Flows Research Articles

Thin discs, thick discs and transition zones

Accretion onto a compact object must occur through a disc when the material has some initial angular momentum. Thin discs and the thicker low radiative efficiency accretion flows are solutions to this problem that have been widely studied and applied. This is an introduction to these accretion flows within the context of X-ray binaries and cataclysmic variables.

Accretion onto Nearby Supermassive Black Holes: [ITAL]Chandra[/ITAL] Constraints on the Dominant Cluster Galaxy NGC 6166

Chandra observations of low-luminosity supermassive black holes in nearby elliptical galaxies provide tight limits on both their nuclear luminosities and their Bondi accretion rates. We examine Chandra constraints on NGC 6166, the dominant galaxy in the cluster Abell 2199, which hosts an ~109 M☉ black hole. We measure a nuclear X-ray luminosity of LX, 1 keV ~ 1040 ergs s-1 and show that the density and temperature profiles of the hot interstellar medium imply a Bondi accretion rate of Bondi 3 × 10-2 M☉ yr-1. This accretion rate predicts a nuclear luminosity of ~1044 ergs s-1 for a canonical radiative efficiency of 10%. Unless the Bondi estimate is inappropriate and/or the accretion rate onto the black hole is significantly reduced, the observed nuclear flux constrains the radiative efficiency of the accretion flow to be η ~ 10-5. We show that low radiative efficiency accretion flows can explain the observed nuclear X-ray luminosity but that the power output from the jets in NGC 6166 is also important to the energetics of the system.

Are There Advection-dominated Accretion Flows in Nearby Giant Elliptical Galaxies?

Recently, it has been suggested that the hard X-ray power-law tails exhibited in the ASCA spectra of several nearby giant elliptical galaxies are caused by low radiative efficiency accretion flows (advection-dominated accretion flows [ADAFs]) onto a massive black hole in the centers of the galaxies. The estimated fluxes from these central ADAFs as derived from the ASCA X-ray spectral analyses indicate that they may be visible as X-ray point sources. We analyze archival ROSAT HRI images of three such galaxies, NGC 1399, 4636, and 4696, to determine whether X-ray point sources consistent with ASCA-derived fluxes are present in the centers of the galaxies. We find that the upper limit for the flux of a central X-ray point source in each of these galaxies, as determined by the ROSAT HRI data, is only marginally consistent with the predicted flux inferred from the ASCA spectra. We suggest that although a central point source, such as an ADAF associated with a massive black hole, cannot be completely ruled out, they can only account for a fraction of the flux of the observed hard X-ray power-law components.

X‐Ray Images of Hot Accretion Flows

We consider the X-ray emission due to bremsstrahlung processes from hot, low radiative-efficiency accretion flows around supermassive and galactic black holes. We calculate surface brightness profiles and Michelson visibility functions for a range of density profiles, rho ~ r^(-3/2+p), with 0 < p < 1, to allow for the presence of outflows. We find that although the 1 keV emitting region in these flows can always extend up to 10^6 Schwarzschild radii (R_S), their surface brightness profiles and visibility functions are strongly affected by the specific density profile. The advection-dominated solutions with no outflows (p=0) lead to centrally peaked profiles with characteristic sizes of only a few tens of R_S. Solutions with strong outflows (p~1) lead to flat intensity profiles with significantly larger characteristic sizes of up to 10^6 R_S. This implies that low luminosity galactic nuclei, such as M87, may appear as extended X-ray sources when observed with current X-ray imaging instruments. We show that X-ray brightness profiles and their associated visibility functions may be powerful probes for determining the relevant mode of accretion and, in turn, the properties of hot accretion flows. We discuss the implications of our results for observations with the Chandra X-ray Observatory and the planned X-ray interferometer MAXIM.

Hard X-ray emission from elliptical galaxies

ABSTRA C T We report the detection of hard X-ray emission components in the spectra of six nearby, giant elliptical galaxies observed with the ASCA satellite. The systems studied, which exhibit strong dynamical evidence for supermassive black holes in their nuclei, are M87, NGC 1399 and NGC 4696 (the dominant galaxies of the Virgo, Fornax and Centaurus clusters, respectively) and NGC 4472, 4636 and 4649 (three further giant ellipticals in the Virgo cluster). The ASCA data for all six sources provide clear evidence for hard emission components, which can be parametrized by power-law models with photon indices in the range Ga 0:6‐1:5 (mean value 1.2) and intrinsic 1‐10 keV luminosities of 2 10 40 ‐2 10 42 erg s 21 . Our results imply the identification of a new class of accreting X-ray source, with X-ray spectra significantly harder than those of binary X-ray sources, Seyfert nuclei or low-luminosity active galactic nuclei, and bolometric luminosities relatively dominated by their X-ray emission. We discuss various possible origins for the hard X-ray emission and argue that it is most likely to be due to accretion on to the central supermassive black holes, via low radiative efficiency accretion flows coupled with strong outflows. In the case of M87, our detected power-law flux is in good agreement with a previously reported measurement from ROSAT High Resolution Imager observations, which were able to resolve the jet from the nuclear X-ray emission components. We confirm previous results showing that the use of multiphase models in the analysis of the ASCA data leads to determinations of approximately solar emission-weighted metallicities for the X-ray gas in the galaxies. We also present results on the individual element abundances in NGC 4636.

Hard X-Ray Emission from Elliptical Galaxies and Its Contribution to the X-Ray Background

We explore the implications of the discovery of hard, power-law X-ray sources in the spectra of nearby elliptical galaxies for the origin of the X-ray background (XRB). The spectra of these sources are consistent with models of thermal bremsstrahlung emission from low radiative efficiency accretion flows around central supermassive black holes and are unique in that they approximately match the spectrum of the hard XRB. If such sources, with luminosities consistent with those observed in nearby elliptical galaxies, are present in most early-type galaxies, then their integrated emission may contribute significantly to the XRB. These sources may also contribute to the hard source counts detected in deep X-ray surveys.