On the Lack of Type I X-ray Bursts in Black Hole X-ray Binaries: Evidence for the Event Horizon?

Type I X-ray bursts are very common in neutron star X-ray binaries, but no Type I burst has been seen in the dozen or so binaries in which the accreting compact star is too massive to be a neutron star and therefore is identified as a black hole candidate. We have carried out a global linear stability analysis of the accumulating fuel on the surface of a compact star to identify the conditions under which thermonuclear bursts are triggered. Our analysis, which improves on previous calculations, reproduces the gross observational trends of bursts in neutron star systems. It further shows that, if black hole candidates have surfaces, they would very likely exhibit instabilities similar to those that lead to Type I bursts on neutron stars. The lack of bursts in black hole candidates is thus significant, and indicates that these objects have event horizons. We discuss possible caveats to this conclusion.Comment: 11 pages, 1 figure, to appear in 1 August 2002 edition of Astrophysical Journal Letters, significant changes to the methods, results unchange

Low-Luminosity Accretion in Black Hole X-ray Binaries and Active Galactic Nuclei

At luminosities below a few percent of Eddington, accreting black holes switch to a hard spectral state which is very different from the soft blackbody-like spectral state that is found at higher luminosities. The hard state is well-described by a two-temperature, optically thin, geometrically thick, advection-dominated accretion flow (ADAF) in which the ions are extremely hot (up to $10^{12}$ K near the black hole), the electrons are also hot ($\sim10^{9-10.5}$ K), and thermal Comptonization dominates the X-ray emission. The radiative efficiency of an ADAF decreases rapidly with decreasing mass accretion rate, becoming extremely low when a source reaches quiescence. ADAFs are expected to have strong outflows, which may explain why relativistic jets are often inferred from the radio emission of these sources. It has been suggested that most of the X-ray emission also comes from a jet, but this is less well established.Comment: To appear in "From X-ray Binaries to Quasars: Black Hole Accretion on All Mass Scales" edited by T. Maccarone, R. Fender, L. Ho, to be published as a special edition of "Astrophysics and Space Science" by Kluwe

Convective magneto-rotational instabilities in accretion disks

We present a study of instabilities occuring in thick magnetized accretion disks. We calculate the growth rates of these instabilities and characterise precisely the contribution of the magneto-rotational and the convective mechanism. All our calculations are performed in radially stratified disks in the cylindrical limit. The numerical calculations are performed using the appropriate local dispersion equation solver discussed in Blokland et al. (2005). A comparison with recent results by Narayan et al. (2002) shows excellent agreement with their approximate growth rates only if the disks are weakly magnetized. However, for disks close to equipartition, the dispersion equation from Narayan et al. (2002) loses its validity. Our calculations allow for a quantitative determination of the increase of the growth rate due to the magneto-rotational mechanism. We find that the increase of the growth rate for long wavelength convective modes caused by this mechanism is almost neglible. On the other hand, the growth rate of short wavelength instabilities can be significantly increased by this mechanism, reaching values up to 60%.Comment: 10 pages, 9 figures, Accepted for publication in Astronomy & Astrophysic

A late time acceleration of the universe with two scalar fields : many possibilities

In the present work, an attempt has been made to explain the recent cosmic acceleration with two mutually interacting scalar fields, one being the Brans-Dicke scalar field and the other a quintessence scalar field. Conditions have been derived for which the quintessence scalar field has an early oscillation and it grows during a later time to govern the dynamics of the universe.Comment: 8 pages, no figures. Accepted for publication in Mod. Phys. Lett.

The Event Horizon of M87

The 6 billion solar mass supermassive black hole at the center of the giant elliptical galaxy M87 powers a relativistic jet. Observations at millimeter wavelengths with the Event Horizon Telescope have localized the emission from the base of this jet to angular scales comparable to the putative black hole horizon. The jet might be powered directly by an accretion disk or by electromagnetic extraction of the rotational energy of the black hole. However, even the latter mechanism requires a confining thick accretion disk to maintain the required magnetic flux near the black hole. Therefore, regardless of the jet mechanism, the observed jet power in M87 implies a certain minimum mass accretion rate. If the central compact object in M87 were not a black hole but had a surface, this accretion would result in considerable thermal near-infrared and optical emission from the surface. Current flux limits on the nucleus of M87 strongly constrain any such surface emission. This rules out the presence of a surface and thereby provides indirect evidence for an event horizon.Comment: 9 pages, 2 figures, submitted to Ap

Where are all the gravastars? Limits upon the gravastar model from accreting black holes

The gravastar model, which postulates a strongly correlated thin shell of anisotropic matter surrounding a region of anti-de Sitter space, has been proposed as an alternative to black holes. We discuss constraints that present-day observations of well-known black hole candidates place on this model. We focus upon two black hole candidates known to have extraordinarily low luminosities: the supermassive black hole in the Galactic Center, Sagittarius A*, and the stellar-mass black hole, XTE J1118+480. We find that the length scale for modifications of the type discussed in Chapline et al. (2003) must be sub-Planckian.Comment: 11 pages, 4 figure

Event-Horizon-Telescope Evidence for Alignment of the Black Hole in the Center of the Milky Way with the Inner Stellar Disk

Observations of the black hole in the center of the Milky Way with the Event Horizon Telescope at 1.3 mm have revealed a size of the emitting region that is smaller than the size of the black-hole shadow. This can be reconciled with the spectral properties of the source, if the accretion flow is seen at a relatively high inclination (50-60 degrees). Such an inclination makes the angular momentum of the flow, and perhaps of the black hole, nearly aligned with the angular momenta of the orbits of stars that lie within 3 arcsec from the black hole. We discuss the implications of such an alignment for the properties of the black hole and of its accretion flow. We argue that future Event-Horizon-Telescope observations will not only refine the inclination of Sgr A* but also measure precisely its orientation on the plane of the sky.Comment: To appear in the Astrophysical Journa

A review of factors influencing the stress response in Australian marsupials

Many Australian marsupials are threatened species. In order to manage in situ and ex situ populations effectively, it is important to understand how marsupials respond to threats. Stress physiology (the study of the response of animals to challenging stimuli), a key approach in conservation physiology, can be used to characterize the physiological response of wildlife to threats. We reviewed the literature on the measurement of glucocorticoids (GCs), endocrine indicators of stress, in order to understand the stress response to conservation-relevant stressors in Australian marsupials and identified 29 studies. These studies employed a range of methods to measure GCs, with faecal glucocorticoid metabolite enzyme immunoassay being the most common method. The main stressors considered in studies of marsupials were capture and handling. To date, the benefits of stress physiology have yet to be harnessed fully in marsupial conservation. Despite a theoretical base dating back to the 1960s, GCs have only been used to understand how 21 of the 142 extant species of Australian marsupial respond to stressors. These studies include merely six of the 60 marsupial species of conservation concern (IUCN Near Threatened to Critically Endangered). Furthermore, the fitness consequences of stress for Australian marsupials are rarely examined. Individual and species differences in the physiological stress response also require further investigation, because significant species-specific variations in GC levels in response to stressors can shed light on why some individuals or species are more vulnerable to stress factors while others appear more resilient. This review summarizes trends, knowledge gaps and future research directions for stress physiology research in Australian marsupial conservation

Solar Neutrinos and the Eclipse Effect

The solar neutrino counting rate in a real time detector like Super--Kamiokanda, SNO, or Borexino is enhanced due to neutrino oscillations in the Moon during a partial or total solar eclipse. The enhancement is calculated as a function of the neutrino parameters in the case of three flavor mixing. This enhancement, if seen, can further help to determine the neutrino parameters.Comment: 24 Pages Revtex, 8 figures as one ps file. To appear in Phys. Rev. D; Some typos corrected and a reference adde