2,297 research outputs found
Monolithically integrated active optical devices
Considerations relevant to the monolithic integration of optical detectors, lasers, and modulators with high speed amplifiers are discussed. Some design considerations for representative subsystems in the GaAs-AlGaAs and GaInAs-InP materials systems are described. Results of a detailed numerical design of an electro-optical birefringent filter for monolithic integration with a laser diode is described, and early experimental results on monolithic integration of broadband MESFET amplifiers with photoconductive detectors are reported
On the Dynamics of Suddenly Heated Accretion Disks around Neutron Stars
Type I X-ray bursts and superbursts on neutron stars release sudden and
intense radiation fields into their surroundings. Here, we consider the
possible effects of these powerful explosions on the structure of the accretion
disk. The goal is to account for the apparent evolution of the innermost
regions of the accretion disk around 4U 1820-30 during a superburst. Three
different processes are considered in detail: radiatively or thermally driven
outflows, inflow due to Poynting-Robertson drag, and a structural change to the
disk by X-ray heating. Radiatively driven winds with large column densities can
be launched from the inner disk, but only for L/L_{Edd} >~ 1, which is expected
to be obtained only at the onset of the burst. Furthermore, the predicted mass
outflow rate is less than the accretion rate in 4U 1820-30. Estimates of the
Poynting-Robertson or radiative drag timescale shows that it is a very
efficient means of removing angular momentum from the gas. However, the
analytical results are likely only applicable at the innermost edge of the
disk. X-ray heating gives a change in the disk scale height that is correlated
with the blackbody temperature, as seen in the evolution during the 4U 1820-30
superburst. If this change in the scale height can alter the surface density,
then the viscous time (with \alpha ~ 0.03-0.2) is the closest match to the 4U
1820-30 results. We expect, however, that all three processes are likely
ongoing when an accretion disk is subject to a sudden heating event.
Ultimately, a numerical simulation of a disk around a bursting neutron star is
required to determine the exact response of the disk. Magnetic truncation of
the accretion flow is also considered and applied to the 4U 1820-30 X-ray
reflection results.Comment: 14 pages, 6 figures, accepted by Ap
Lifting the Veil on Obscured Accretion: Active Galactic Nuclei Number Counts and Survey Strategies for Imaging Hard X-Ray Missions
Finding and characterizing the population of active galactic nuclei (AGNs) that produces the X-ray background (XRB) is necessary to connect the history of accretion to observations of galaxy evolution at longer wavelengths. The year 2012 will see the deployment of the first hard X-ray imaging telescope which, through deep extragalactic surveys, will be able to measure the AGN population at the energies where the XRB peaks (~20-30 keV). Here, we present predictions of AGN number counts in three hard X-ray bandpasses: 6-10 keV, 10-30 keV, and 30-60 keV. Separate predictions are presented for the number counts of Compton thick AGNs, the most heavily obscured active galaxies. The number counts are calculated for five different models of the XRB that differ in the assumed hard X-ray luminosity function, the evolution of the Compton thick AGNs, and the underlying AGN spectral model. The majority of the hard X-ray number counts will be Compton thin AGNs, but there is a greater than tenfold increase in the Compton thick number counts from the 6-10 keV to the 10-30 keV band. The Compton thick population shows enough variation that a hard X-ray number counts measurement will constrain the models. The computed number counts are used to consider various survey strategies for the NuSTAR mission, assuming a total exposure time of 6.2 Ms. We find that multiple surveys will allow a measurement of Compton thick evolution. The predictions presented here should be useful for all future imaging hard X-ray missions
An XMM-Newton observation of Ton S180: Constraints on the continuum emission in ultrasoft Seyfert galaxies
We present an XMM-Newton observation of the bright, narrow-line, ultrasoft
Seyfert 1 galaxy Ton S180. The 0.3-10 keV X-ray spectrum is steep and curved,
showing a steep slope above 2.5 keV (Gamma ~ 2.3) and a smooth, featureless
excess of emission at lower energies. The spectrum can be adequately
parameterised using a simple double power-law model. The source is strongly
variable over the course of the observation but shows only weak spectral
variability, with the fractional variability amplitude remaining approximately
constant over more than a decade in energy. The curved continuum shape and weak
spectral variability are discussed in terms of various physical models for the
soft X-ray excess emission, including reflection off the surface of an ionised
accretion disc, inverse-Compton scattering of soft disc photons by thermal
electrons, and Comptonisation by electrons with a hybrid thermal/non-thermal
distribution. We emphasise the possibility that the strong soft excess may be
produced by dissipation of accretion energy in the hot, upper atmosphere of the
putative accretion disc.Comment: 9 pages, accepted for publication in MNRA
Impact of neutron star spin on Poynting-Robertson drag during a Type I X-ray burst
External irradiation of a neutron star (NS) accretion disc induces
Poynting-Robertson (PR) drag, removing angular momentum and increasing the mass
accretion rate. Recent simulations show PR drag significantly enhancing the
mass accretion rate during Type I X-ray bursts, which could explain X-ray
spectral features such as an increase in the persistent emission and a soft
excess. However, prograde spin of the NS is expected to weaken PR drag,
challenging its importance during bursts. Here, we study the effect of spin on
PR drag during X-ray bursts. We run four simulations, with two assuming a
non-spinning NS and two using a spin parameter of , corresponding to a
rotation frequency of 500 Hz. For each scenario, we simulate the disc evolution
subject to an X-ray burst and compare it to the evolution found with no burst.
PR drag drains the inner disc region during a burst, moving the inner disc
radius outward by km in the and by km in the
simulation. The burst enhances the mass accretion rate across the
innermost stable circular orbit times when the NS is not spinning
and times when it is spinning. The explanation for this seemingly
contradictory result is that the disc is closer to the NS when , and
the resulting stronger irradiating flux offsets the weakening effect of spin on
the PR drag. Hence, PR drag remains a viable explanation for the increased
persistent emission and soft excess observed during X-ray bursts in spinning NS
systems.Comment: 9 pages, 8 figures, accepted for publication in MNRA
Liver-specific knockout of arginase-1 leads to a profound phenotype similar to inducible whole body arginase-1 deficiency
Arginase-1 (Arg1) converts arginine to urea and ornithine in the distal step of the urea cycle in liver. We previously generated a tamoxifen-inducible Arg1 deficient mouse model (Arg1-Cre) that disrupts Arg1 expression throughout the whole body and leads to lethality ≈ 2 weeks after gene disruption. Here, we evaluate if liver-selective Arg1 loss is sufficient to recapitulate the phenotype observed in global Arg1 knockout mice, as well as to gauge the effectiveness of gene delivery or hepatocyte transplantation to rescue the phenotype. Liver-selective Arg1 deletion was induced by using an adeno-associated viral (AAV)-thyroxine binding globulin (TBG) promoter-Cre recombinase vector administered to Arg1 "floxed" mice; Arg1(fl/fl) ). An AAV vector expressing an Arg1-enhanced green fluorescent protein (Arg1-eGFP) transgene was used for gene delivery, while intrasplenic injection of wild-type (WT) C57BL/6 hepatocytes after partial hepatectomy was used for cell delivery to "rescue" tamoxifen-treated Arg1-Cre mice. The results indicate that liver-selective loss of Arg1 (> 90% deficient) leads to a phenotype resembling the whole body knockout of Arg1 with lethality ≈ 3 weeks after Cre-induced gene disruption. Delivery of Arg1-eGFP AAV rescues more than half of Arg1 global knockout male mice (survival > 4 months) but a significant proportion still succumb to the enzyme deficiency even though liver expression and enzyme activity of the fusion protein reach levels observed in WT animals. Significant Arg1 enzyme activity from engrafted WT hepatocytes into knockout livers can be achieved but not sufficient for rescuing the lethal phenotype. This raises a conundrum relating to liver-specific expression of Arg1. On the one hand, loss of expression in this organ appears to be both necessary and sufficient to explain the lethal phenotype of the genetic disorder in mice. On the other hand, gene and cell-directed therapies suggest that rescue of extra-hepatic Arg1 expression may also be necessary for disease correction. Further studies are needed in order to illuminate the detailed mechanisms for pathogenesis of Arg1-deficiency
A Correlation Between the Ionization State of the Inner Accretion Disk and the Eddington Ratio of Active Galactic Nuclei
X-ray reflection features observed from the innermost regions of accretion
disks in Active Galactic Nuclei (AGNs) allow important tests of accretion
theory. In recent years it has been possible to use the Fe K line and
reflection continuum to parametrize the ionization state of the irradiated
inner accretion disk. Here, we collect 10 measurements of xi, the disk
ionization parameter, from 8 AGNs with strong evidence for reflection from the
inner accretion disk and good black hole mass estimates. We find strong
statistical evidence (98.56% confidence) for a nearly linear correlation
between xi and the AGN Eddington ratio. Moreover, such a correlation is
predicted by a simple application of alpha-disk accretion theory, albeit with a
stronger dependence on the Eddington ratio. The theory shows that there will be
intrinsic scatter to any correlation as a result of different black hole spins
and radii of reflection. There are several possibilities to soften the
predicted dependence on the Eddington ratio to allow a closer agreement with
the observed correlation, but the current data does not allow for an unique
explanation. The correlation can be used to estimate that MCG-6-30-15 should
have a highly ionized inner accretion disk, which would imply a black hole spin
of ~0.8. Additional measurements of xi from a larger sample of AGNs are needed
to confirm the existence of this correlation, and will allow investigation of
the accretion disk/corona interaction in the inner regions of accretion disks.Comment: 21 pages, 2 figures, accepted by Ap
Obscuring Active Galactic Nuclei with Nuclear Starburst Disks
We assess the potential of nuclear starburst disks to obscure the
Seyfert-like AGN that dominate the hard X-ray background at z~1. Over 1200
starburst disk models, based on the theory developed by Thompson et al., are
calculated for five input parameters: the black hole mass, the radial size of
the starburst disk, the dust-to-gas ratio, the efficiency of angular momentum
transport in the disk, and the gas fraction at the outer disk radius. We find
that a large dust-to-gas ratio, a relatively small starburst disk, a
significant gas mass fraction, and efficient angular momentum transport are all
important to produce a starburst disk that can potentially obscure an AGN. The
typical maximum star-formation rate in the disks is ~10 solar masses per year.
Assuming no mass-loss due to outflows, the starburst disks feed gas onto the
black hole at rates sufficient to produce hard X-ray luminosities of
10^{43}-10^{44} erg s^{-1}. The starburst disks themselves should be detectable
at mid-infrared and radio wavelengths; at z=0.8, the predicted fluxes are ~1
mJy at 24microns and ~10-30 microJy at 1.4GHz. Thus, we predict a large
fraction of radio/X-ray matches in future deep radio surveys. The starburst
disks should be easily distinguished from AGN in future 100microns surveys by
Herschel with expected fluxes of ~5 mJy. Any AGN-obscuring starbursts will be
associated with hot dust, independent of AGN heating, resulting in observable
signatures for separating galactic and nuclear star-formation. Finally, because
of the competition between gas and star-formation, nuclear starbursts will be
associated with lower-luminosity AGN. Thus, this phenomenon is a natural
explanation for the observed decrease in the fraction of obscured AGN with
luminosity.Comment: 13 pages, 12 figures, 3 in color; accepted by Ap
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