2,707 research outputs found
Pomeranchuk effect and spin-gradient cooling of Bose-Bose mixtures in an optical lattice
We theoretically investigate finite-temperature thermodynamics and
demagnetization cooling of two-component Bose-Bose mixtures in a cubic optical
lattice, by using bosonic dynamical mean field theory (BDMFT). We calculate the
finite-temperature phase diagram, and remarkably find that the system can be
heated from the superfluid into the Mott insulator at low temperature,
analogous to the Pomeranchuk effect in 3He. This provides a promising many-body
cooling technique. We examine the entropy distribution in the trapped system
and discuss its dependence on temperature and an applied magnetic field
gradient. Our numerical simulations quantitatively validate the spin-gradient
demagnetization cooling scheme proposed in recent experiments.Comment: 9 pages, 8 figure
A Behavioral Model System for Implicit Mobile Authentication
Smartphones are increasingly essential to users’ everyday lives. Security concerns of data compromises are growing, and explicit authentication methods are proving to be inconvenient and insufficient. Meanwhile, users demand quicker and more secure authentication. To address this, a user can be authenticated continuously and implicitly, through understanding consistency in their behavior. This research project develops a Behavioral Model System (BMS) that records users’ behavioral metrics on an Android device and sends them to a server to develop a behavioral model for the user. Once a strong model is generated with TensorFlow, a user’s most recent behavior is queried against the model to authenticate them. The model is tested across its metrics to evaluate the reliability of BMS
Evidence for a Black Hole and Accretion Disk in the LINER NGC 4203
We present spectroscopic observations from the Hubble Space Telescope that
reveal for the first time the presence of a broad pedestal of Balmer-line
emission in the LINER galaxy NGC 4203. The emission-line profile is suggestive
of a relativistic accretion disk, and is reminiscent of double-peaked transient
Balmer emission observed in a handful of other LINERs. The very broad line
emission thus constitutes clear qualitative evidence for a black hole, and
spatially resolved narrow-line emission in NGC 4203 can be used to constrain
its mass, with M_BH less than 6 x 10^6 solar masses at 99.7% confidence. This
value implies a ratio of black-hole mass to bulge mass of less than
approximately 7 x 10^-4 in NGC 4203, which is less by a factor of ~3 - 9 than
the mean ratio obtained for other galaxies. The availability of an independent
constraint on central black-hole mass makes NGC4203 an important testbed for
probing the physics of weak active galactic nuclei. Assuming M_BH near the
detection limit, the ratio of observed luminosity to the Eddington luminosity
is approximately 10^-4. This value is consistent with advection-dominated
accretion, and hence with scenarios in which an ion torus irradiates an outer
accretion disk that produces the observed double-peaked line emission.
Follow-up observations will make it possible to improve the black-hole mass
estimate and study variability in the nuclear emission.Comment: 10 pages (LaTeX, AASTeX v4.0), 2 postscript figures, accepted for
publication in The Astrophysical Journal Letter
The Broad-Line and Narrow-Line Regions of the LINER NGC 4579
We report the discovery of an extremely broad H-alpha emission line in the
LINER nucleus of NGC 4579. From ground-based observations, the galaxy was
previously known to contain a Type 1 nucleus with a broad H-alpha line of FWHM
= 2300 km/s and FWZI ~ 5000 km/s. New spectra obtained with the Hubble Space
Telescope and a 0.2 arcsec-wide slit reveal an H-alpha component with FWZI ~
18,000 km/s. The line is not obviously double-peaked, but it does possess
shoulders on the red and blue sides which resemble the H-alpha profiles of
double-peaked emitters such as NGC 4203 and NGC 4450. This similarity suggests
that the very broad H-alpha profile in NGC 4579 may represent emission from an
accretion disk. Three such objects have been found recently in two HST programs
which have targeted a total of 30 galaxies, demonstrating that double-peaked or
extremely broad-line emission in LINERs must be much more common than would be
inferred from ground-based surveys. The ratio of the narrow [S II] 6716, 6731
lines shows a pronounced gradient indicating a steep rise in density toward the
nucleus. The direct detection of a density gradient within the inner arcsecond
of the narrow-line region confirms expectations from previous observations of
linewidth-critical density correlations in several LINERs.Comment: 8 pages, includes 3 figures. To appear in The Astrophysical Journa
Double-Peaked Broad Emission Lines in NGC 4450 and Other LINERs
Spectra taken with HST reveal that NGC 4450 emits Balmer emission lines with
displaced double peaks and extremely high-velocity wings. This characteristic
line profile, previously seen in a few nearby LINERs and in a small fraction of
broad-line radio galaxies, can be interpreted as a kinematic signature of a
relativistic accretion disk. We can reproduce the observed profile with a model
for a disk with a radial range of 1000-2000 gravitational radii and inclined by
27 degrees along the line of sight. The small-aperture HST data also allow us
to detect, for the first time, the featureless continuum at optical wavelengths
in NGC 4450; the nonstellar nucleus is intrinsically very faint, with M_B =
-11.2 mag for D = 16.8 Mpc. We have examined the multiwavelength properties of
NGC 4450 collectively with those of other low-luminosity active nuclei which
possess double-peaked broad lines and find a number of common features. These
objects are all classified spectroscopically as "type 1" LINERs or closely
related objects. The nuclear luminosities are low, both in absolute terms and
relative to the Eddington rates. All of them have compact radio cores, whose
strength relative to the optical nuclear emission places them in the league of
radio-loud active nuclei. The broad-band spectral energy distributions of these
sources are most notable for their deficit of ultraviolet emission compared to
those observed in luminous Seyfert 1 nuclei and quasars. The double-peaked
broad-line radio galaxies Arp 102B and Pictor A have very similar attributes.
We discuss how these characteristics can be understood in the context of
advection-dominated accretion onto massive black holes.Comment: To appear in The Astrophysical Journal. Latex, 15 pages, embedded
figures and tabl
The Magnificent Seven: Magnetic fields and surface temperature distributions
Presently seven nearby radio-quiet isolated neutron stars discovered in ROSAT
data and characterized by thermal X-ray spectra are known. They exhibit very
similar properties and despite intensive searches their number remained
constant since 2001 which led to their name ``The Magnificent Seven''. Five of
the stars exhibit pulsations in their X-ray flux with periods in the range of
3.4 s to 11.4 s. XMM-Newton observations revealed broad absorption lines in the
X-ray spectra which are interpreted as cyclotron resonance absorption lines by
protons or heavy ions and / or atomic transitions shifted to X-ray energies by
strong magnetic fields of the order of 10^13 G. New XMM-Newton observations
indicate more complex X-ray spectra with multiple absorption lines. Pulse-phase
spectroscopy of the best studied pulsars RX J0720.4-3125 and RBS 1223 reveals
variations in derived emission temperature and absorption line depth with pulse
phase. Moreover, RX J0720.4-3125 shows long-term spectral changes which are
interpreted as due to free precession of the neutron star. Modeling of the
pulse profiles of RX J0720.4-3125 and RBS 1223 provides information about the
surface temperature distribution of the neutron stars indicating hot polar caps
which have different temperatures, different sizes and are probably not located
in antipodal positions.Comment: 10 pages, 8 figures, to appear in Astrophysics and Space Science, in
the proceedings of "Isolated Neutron Stars: from the Interior to the
Surface", edited by D. Page, R. Turolla and S. Zan
The isolated neutron star X-ray pulsars RX J0420.0–5022 and RX J0806.4–4123 : new X-ray and optical observations
We report on the analysis of new X-ray data obtained with XMM-Newton and Chandra from two ROSAT-discovered X-ray dim isolated neutron stars (XDINs). RX J0806.4−4123 was observed with XMM-Newton in April 2003, 2.5 years after
the first observation. The EPIC-pn data confirm that this object is an X-ray pulsar with 11.371 s neutron star spin period. The X-ray spectrum is consistent with absorbed black-body emission with a temperature kT = 96 eV and N H = 4 × 10 19 cm −2 without significant changes between the two observations. Four XMM-Newton observations of RX
J0420.0−5022 between December 2002 and July 2003 did not confirm the 22.7 s pulsations originally indicated in ROSAT data, but clearly reveal a 3.453 s period. A fit to the X-ray spectrum using an absorbed black-body model yields kT = 45 eV, the lowest value found from the small group of XDINs and N H = 1.0 × 10 20 cm −2. Including a broad absorption line improves the quality of the spectral fits considerably for both objects and may indicate the presence of absorption features similar to those reported from RBS1223, RX J1605.3+3249 and RX J0720.4−3125. For both targets we derive accurate X-ray positions from the Chandra data and present an optical counterpart candidate for RX J0420.0−5022 with B = 26.6 ± 0.3 mag from VLT imaging
Reflection in Seyfert Galaxies and the Unified Model of AGN
We present a deep study of the average hard X-ray spectra of Seyfert
galaxies. We analyzed all public INTEGRAL IBIS/ISGRI data available on all the
165 Seyfert galaxies detected at z<0.2. Our final sample consists of 44 Seyfert
1's, 29 Seyfert 1.5's, 78 Seyfert 2's, and 14 Narrow Line Seyfert 1's. We
derived the average hard X-ray spectrum of each subsample in the 17-250keV
energy range. All classes of Seyfert galaxies show on average the same nuclear
continuum, as foreseen by the zeroth order unified model, with a cut-off energy
of Ec>200keV, and a photon index of Gamma ~1.8. Compton-thin Seyfert 2's show a
reflection component stronger than Seyfert 1's and Seyfert 1.5's. Most of this
reflection is due to mildly obscured (10^23 cm^-2 < NH < 10^24 cm^-2) Seyfert
2's, which have a significantly stronger reflection component
(R=2.2^{+4.5}_{-1.1}) than Seyfert 1's (R<=0.4), Seyfert 1.5's (R<= 0.4) and
lightly obscured (NH < 10^23 cm^-2) Seyfert 2's (R<=0.5). This cannot be
explained easily by the unified model. The absorber/reflector in mildly
obscured Seyfert 2's might cover a large fraction of the X-ray source, and have
clumps of Compton-thick material. The large reflection found in the spectrum of
mildly obscured Seyfert 2's reduces the amount of Compton-thick objects needed
to explain the peak of the cosmic X-ray background. Our results are consistent
with the fraction of Compton-thick sources being ~10%. The spectra of Seyfert
2's with and without polarized broad lines do not show significant differences,
the only difference between the two samples being the higher hard X-ray and
bolometric luminosity of Seyfert 2's with polarized broad lines. The average
hard X-ray spectrum of Narrow line Seyfert 1's is steeper than those of Seyfert
1's and Seyfert 1.5's, probably due to a lower energy of the cutoff.Comment: 19 pages, accepted for publication in Astronomy and Astrophysics,
final versio
Rare central nervous system tumors in adults:a population-based study of ependymomas, pilocytic astrocytomas, medulloblastomas, and intracranial germ cell tumors
BACKGROUND: Ependymomas, pilocytic astrocytomas, medulloblastomas, and intracranial germ cell tumors occur relative frequently in children, but are rare central nervous system (CNS) tumors in adults. In this population-based survey, we established incidence, treatment, and survival patterns for these tumors diagnosed in adult patients (≥18 years) over a 30-year period (1989-2018). METHODS: Data on 1384 ependymomas, 454 pilocytic astrocytomas, 205 medulloblastomas, and 112 intracranial germ cell tumors were obtained from the Netherlands Cancer Registry (NCR) on the basis of a histopathological diagnosis. For each tumor type, age-standardized incidence rates and estimated annual percentage change were calculated. Trends in incidence and main treatment modalities were reported per 5-year periods. Overall survival was calculated using the Kaplan-Meier method, and relative survival rates were estimated using the Pohar-Perme estimator. RESULTS: Incidence and survival rates remained generally stable for pilocytic astrocytomas, medulloblastomas, and germ cell tumors. Increasing incidence was observed for spinal ependymomas, mostly for myxopapillary ependymomas, and survival improved over time for grade II ependymomas (P < .01). Treatment patterns varied over time with shifting roles for surgery in ependymomas and for chemotherapy and radiation in medulloblastomas and germinomas. CONCLUSIONS: The study provides baseline information for highly needed national and international standard treatment protocols, and thus for further improving patient outcomes in these rare CNS tumors
Shedding light on the elusive role of endothelial cells in cytomegalovirus dissemination.
Cytomegalovirus (CMV) is frequently transmitted by solid organ transplantation and is associated with graft failure. By forming the boundary between circulation and organ parenchyma, endothelial cells (EC) are suited for bidirectional virus spread from and to the transplant. We applied Cre/loxP-mediated green-fluorescence-tagging of EC-derived murine CMV (MCMV) to quantify the role of infected EC in transplantation-associated CMV dissemination in the mouse model. Both EC- and non-EC-derived virus originating from infected Tie2-cre(+) heart and kidney transplants were readily transmitted to MCMV-naïve recipients by primary viremia. In contrast, when a Tie2-cre(+) transplant was infected by primary viremia in an infected recipient, the recombined EC-derived virus poorly spread to recipient tissues. Similarly, in reverse direction, EC-derived virus from infected Tie2-cre(+) recipient tissues poorly spread to the transplant. These data contradict any privileged role of EC in CMV dissemination and challenge an indiscriminate applicability of the primary and secondary viremia concept of virus dissemination
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