3,309 research outputs found
BAL QSOs and Extreme UFOs: the Eddington connection
We suggest a common physical origin connecting the fast, highly ionized winds
(UFOs) seen in nearby AGN, and the slower and less ionized winds of BAL QSOs.
The primary difference is the mass loss rate in the wind, which is ultimately
determined by the rate at which mass is fed towards the central supermassive
black hole (SMBH) on large scales. This is below the Eddington accretion rate
in most UFOs, and slightly super-Eddington in extreme UFOs such as PG1211+143,
but ranges up to times this in BAL QSOs. For UFOs this implies
black hole accretion rates and wind mass loss rates which are at most
comparable to Eddington, giving fast, highly-ionized winds. In contrast BAL QSO
black holes have mildly super-Eddington accretion rates, and drive winds whose
mass loss rates are significantly super-Eddington, and so are slower and less
ionized. This picture correctly predicts the velocities and ionization states
of the observed winds, including the recently-discovered one in SDSS
J1106+1939. We suggest that luminous AGN may evolve through a sequence from BAL
QSO through LoBAL to UFO-producing Seyfert or quasar as their Eddington factors
drop during the decay of a bright accretion event. LoBALs correspond to a
short-lived stage in which the AGN radiation pressure largely evacuates the
ionization cone, but before the large-scale accretion rate has dropped to the
Eddington value. We show that sub-Eddington wind rates would produce an relation lying above that observed. We conclude that significant SMBH
mass growth must occur in super-Eddington phases, either as BAL QSOs, extreme
UFOs, or obscured from direct observation.Comment: 8 pages, 5 figures; a higher quality version of Figure 5 available on
reques
The ultimate outcome of black hole - neutron star mergers
We present a simple, semi--analytical description for the final stages of
mergers of black hole (BH) -- neutron star (NS) systems. Such systems are of
much interest as gravitational wave sources and gamma--ray burst progenitors.
Numerical studies show that in general the neutron star is not disrupted at the
first phase of mass transfer. Instead, what remains of the neutron star is left
on a wider, eccentric, orbit. We consider the evolution of such systems as they
lose angular momentum via gravitational radiation and come into contact for
further phases of mass transfer. During each mass transfer event the neutron
star mass is reduced until a critical value where mass loss leads to a rapid
increase in the stellar radius. At this point Roche lobe overflow shreds what
remains of the neutron star, most of the mass forming a disc around the black
hole. Such a disc may be massive enough to power a gamma--ray burst. The mass
of the neutron star at the time of disruption (and therefore the disc mass) is
largely independent of the initial masses of the black hole and neutron star,
indicating that BH--NS star mergers may be standard candles.Comment: MNRAS, in pres
Progenitors of Long Gamma-ray Bursts
Pinpointing the progenitors of long duration gamma-ray bursts (LGRBs) remains
an extremely important question, although it is now clear that at least a
fraction of LGRBs originate in the core collapse of massive stars in type Ic
supernovae, the pathways to the production of these stars, and their initial
masses, remain uncertain. Rotation is thought to be vital in the creation of
LGRBs, and it is likely that black hole creation is also necessary. We suggest
that these two constraints can be met if the GRB progenitors are very massive
stars (>20 solar masses) and are formed in tight binary systems. Using simple
models we compare the predictions of this scenario with observations and find
that the location of GRBs on their host galaxies are suggestive of
main-sequence masses in excess of 20 solar masses, while 50% of the known
compact binary systems may have been sufficiently close to have had the
necessary rotation rates for GRB creation. Thus, massive stars in compact
binaries are a likely channel for at least some fraction of LGRBs.Comment: To appear in "Gamma-ray bursts: Prospects for GLAST", AIP Conference
proceedings 906, Editors M. Axelsson and F Ryd
Superhumps in Low-Mass X-Ray Binaries
We propose a mechanism for the superhump modulations observed in optical
photometry of at least two black hole X-ray transients (SXTs). As in extreme
mass-ratio cataclysmic variables (CVs), superhumps are assumed to result from
the presence of the 3:1 orbital resonance in the accretion disc. This causes
the disc to become non-axisymmetric and precess. However the mechanism for
superhump luminosity variations in low mass X-ray binaries (LMXBs) must differ
from that in CVs, where it is attributed to a tidally-driven modulation of the
disc's viscous dissipation, varying on the beat between the orbital and disc
precession period. By contrast in LMXBs, tidal dissipation in the outer
accretion disc is negligible: the optical emission is overwhelming dominated by
reprocessing of intercepted central X-rays. Thus a different origin for the
superhump modulation is required. Recent observations and numerical simulations
indicate that in an extreme mass-ratio system the disc area changes on the
superhump period. We deduce that the superhumps observed in SXTs arise from a
modulation of the reprocessed flux by the changing area. Therefore, unlike the
situation in CVs, where the superhump amplitude is inclination-independent,
superhumps should be best seen in low-inclination LMXBs, whereas an orbital
modulation from the heated face of the secondary star should be more prominent
at high inclinations. Modulation at the disc precession period (10s of days)
may indicate disc asymmetries such as warping. We comment on the orbital period
determinations of LMXBs, and the possibility and significance of possible
permanent superhump LMXBs.Comment: 6 pages, 1 encapsulated figure. MNRAS in press; replaced to correct
typographical error
The analysis of 108mAg, 166mHo and 94Nb in decommissioning waste
Contaminated waste consisting of various materials results from the decommissioning
of nuclear power plants. Disposal of Decommissioning Waste requires as much
measurement data for the radionuclides contained within the waste as possible. Data
that are obtained is then used to create an inventory of the radionuclides in the various
types of wastes. This research investigates a route to obtaining additional
measurement data for such inventories. The importance of these inventories is to
provide as much information as possible so that a reliable risk assessment can be
performed on the waste samples and the proposed method of disposal. [Continues.
Post-Fledging Dispersal of Burrowing Owls in Southwestern Idaho: Characterization of Movements and Use of Satellite Burrows
Using radiotelemetry, we monitored dispersing juvenile Western Burrowing Owls (Athene cunicularia hypugaea) within a migratory population in southwestern Idaho during 1994 and 1995. Owls remained within natal areas for an average (± SE) of 58 ± 3.4 days post-hatching before moving permanently beyond 300 m, which was our operational cutoff for dispersal from the natal area. On average, owls dispersed on 27 July (range: 15 July to 22 August), which was approximately 4 weeks after fledging. After initiating dispersal, juveniles continued moving farther away from their natal burrows and, by 61-65 days post-hatching, they had moved 0.6 ± 0.2 km. Each juvenile used 5.1 ± 1.2 satellite burrows, and individual satellite burrows were used for up to 14 days. The average date on which we last sighted radio-tagged juveniles was 13 August, and all but one juvenile departed the study area by early September. Our study illustrates the importance of satellite burrows to dispersing Burrowing Owls
A New Evolutionary Picture for CVs and LMXBs
We consider an alternative to the standard picture of CV and LMXB evolution,
namely the idea that most CVs (and by extension LMXBs) may not yet have had
time to evolve to their theoretical minimum orbital periods. We call this the
Binary Age Postulate (BAP). The observed short-period cutoff in the CV
histogram emerges naturally as the shortest period yet reached in the age of
the Galaxy, while the post-minimum-period space density problem is removed. The
idea has similar desirable consequences for LMXBs. In both cases systems with
nuclear-evolved secondary stars form a prominent part of the short-period
distributions. Properties such as the existence and nature of ultrashort-period
systems, and the spread in mass transfer rates at a given orbital period, are
naturally reproduced.Comment: 10 pages, 6 figures; to appear in 'The Physics of Cataclysmic
Variables and Related Objects', Goettingen, August 5-10, 200
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