Low-luminosity Type Research Articles

On the Contribution of Active Galactic Nuclei to the Cosmic Background Radiation

We present the results of calculations of the cosmic AGN background spectrum from 3 keV (4\times 10^{-4} microns) to 1000 microns. These computations make use of the measured X-ray luminosity function and its evolution, as well as fits from synthesis models of the cosmic X-ray background (CXB) to predict the AGN contribution to the cosmic infrared background (CIRB) for different models of the location and distribution of the absorbing material. By comparing our results to observational constraints we conclude that the current deep Spitzer surveys can account for the entire AGN contribution to the CIRB at 24 microns, but these AGN are almost all Compton-thin. In fact, the calculations show that Compton-thick AGN are a small fraction of the CIRB for \lambda < 100 microns. For this reason, the most efficient method of identifying the Compton-thick AGN population is through hard X-ray imaging at energies >~ 40 keV. Selection of AGN based on their rest-frame near-IR colors will miss low luminosity type 2 AGN due to contamination from the host galaxy. Finally, the AGN that dominate the CXB must have star formation rates < 100 M_{sun} yr^{-1}, consistent with them having similar properties as the sources which dominate the CIRB at z ~1. Including the estimated re-radiated emission from star formation, AGN and their host galaxies may contribute ~30% of the CIRB at 70 microns, dropping to ~10% at 24 microns and ~1% at 1-10 microns.

Peculiar, low-luminosity Type II supernovae: low-energy explosions in massive progenitors?

A number of supernovae, classified as Type II, show remarkably peculiar properties such as an extremely low expansion velocity and an extraordinarily small amount of 56Ni in the ejecta. We present a joint analysis of the available observations for two of these peculiar Type II supernovae, SN 1997D and SN 1999br, using a comprehensive semi-analytic method that can reproduce the light curve and the evolution of the line velocity and continuum temperature. We find that these events are underenergetic with respect to a typical Type II supernova and that the inferred mass of the ejecta is relatively large. We discuss the possibility that these supernovae originate from the explosion of a massive progenitor in which the rate of early infall of stellar material on the collapsed core is large. Events of this type could form a black hole remnant, giving rise to significant fallback and late-time accretion.

Spectral properties of a sample of low luminosity type I X-ray bursters

I report on a sample of new type I X-ray bursters, firstly detected with the Wide Field Cameras on board BeppoSAX and then studied with the Narrow Field Instruments on a broad spectral range (0.1–200 keV). Properties of the transient/persistent emission are summarized and the broad band X-ray spectra discussed in detail for a few sources.

What Can the Accretion‐induced Collapse of White Dwarfs Really Explain?

The accretion-induced collapse (AIC) of a white dwarf into a neutron star has been invoked to explain gamma-ray bursts, Type Ia supernovae, and a number of problematic neutron star populations and specific binary systems. The ejecta from this collapse has also been claimed as a source of r-process nucleosynthesis. So far, most AIC studies have focused on determining the event rates from binary evolution models and less attention has been directed toward understanding the collapse itself. However, the collapse of a white dwarf into a neutron star is followed by the ejection of rare neutron-rich isotopes. The observed abundance of these chemical elements may set a more reliable limit on the rate at which AICs have taken place over the history of the Galaxy. In this paper, we present a thorough study of the collapse of a massive white dwarf in one- and two-dimensions and determine the amount and composition of the ejected material. We discuss the importance of the input physics (equation of state, neutrino transport, rotation) in determining these quantities. These simulations affirm that AICs are too baryon rich to produce gamma-ray bursts and do not eject enough nickel to explain Type Ia supernovae (with the possible exception of a small subclass of extremely low-luminosity Type Ias). Although nucleosynthesis constraints limit the number of neutron stars formed via AICs to 0.1% of the total Galactic neutron star population, AICs remain a viable scenario for forming systems of neutron stars that are difficult to explain with Type II core-collapse supernovae.

Soft X-ray Variability and the Covering Fraction of Active Galactic Nuclei

The variability of soft X-rays (0.2 – 2 keV) in some low-luminosity type 1 Seyferts may partly be due to an extrinsic mechanism: dense clouds of gas in the broad-line region, opaque to soft X-rays, move across our line of sight to the X-ray emitting portions of the accretion disk (Reichert, Mushotzky, and Holt 1986; Lawrence and Elvis 1982; Halpern 1984). As the clouds move, the covering fraction changes stochastically. Evidence for partial covering of the X-ray source in low-luminosity AGNs has been seen in soft X-ray spectra by Holt et al. (1980) and Reichert et al. (1985).

Low-frequency divergent X-ray variability in the Seyfert galaxy NGC4051

The X-ray emission from NGC4051, a nearby low luminosity Type 1 Seyfert galaxy, exhibits variations that are both rapid and apparently quasi-periodic1,2. Many Seyfert galaxies are variable3 but our knowledge of the form of variability has not advanced substantially since early well-resolved observations4 and attempts at statistical description5. Here we report an uninterrupted 62-h observation of NGC4051 with the EXOSAT observatory, giving an order of magnitude improvement in the quality of the time series, allowing comparison with X-ray binaries. The observed power spectral density is roughly proportional to 1/f, similar to Cyg X-l, and characteristic of turbulent processes. No preferred timescale or luminosity gradient has been seen so far. If this behaviour holds generally for active galactic nuclei, quantities derived from short samples will be subject to selection effects. Some naive interpretaions of variability are clearly invalid. Similar conclusions have recently been arrived at by McHardy et al. (see accompanying paper6).

The X-ray spectrum and time variability of narrow emission line galaxies

X-ray spectral and temporal observations are reported for six narrow emission line galaxies (NELGs), all of which are fitted by power-law X-ray spectra of energy slope 0.8 and have column densities in the line of sight greater than 1 x 10 to the 22nd atoms/sq cm. Three of the objects, NGC 526a, NGC 2110 and MCG-5-23-16 are variable in their X-ray flux, and the latter two, along with NGC 5506 and NGC 7582, showed detectable variability in at least one observation. The measured X-ray properties of these NELGs, which also included NGC 2992, strongly resemble those of previously-measured type 1 Seyferts of the same X-ray luminosity and lead to the conclusion of great similarity between the NELGs and low-luminosity type 1 Seyferts. The implications of these observations for the optical line-emitting region structure of these galaxies are discussed.