927 research outputs found
The verifiability approach to deception detection: A preregistered direct replication of the information protocol condition of Nahari, Vrij, and Fisher (2014b)
The verifiability approach to deception detection: A preregistered direct replication of the information protocol condition of Nahari, Vrij, and Fisher (2014b)
Radio Emission and Particle Acceleration in SN 1993J
The radio light curves of SN 1993J are found to be well fit by a synchrotron
spectrum, suppressed by external free-free absorption and synchrotron
self-absorption. A standard r^-2 circumstellar medium is assumed, and found to
be adequate. The magnetic field and number density of relativistic electrons
behind the shock are determined. The strength of the magnetic field argues
strongly for turbulent amplification behind the shock. The ratio of the
magnetic and thermal energy density behind the shock is ~0.14. Synchrotron and
Coulomb cooling dominate the losses of the electrons. The injected electron
spectrum has a power law index -2.1, consistent with diffusive shock
acceleration, and the number density scales with the thermal electron energy
density. The total energy density of the relativistic electrons is, if
extrapolated to gamma ~ 1, ~ 5x10^-4 of the thermal energy density. The
free-free absorption required is consistent with previous calculations of the
circumstellar temperature of SN 1993J, T_e ~ (2-10)x10^5 K. The relative
importance of free-free absorption, Razin suppression, and the synchrotron
self-absorption effect for other supernovae are briefly discussed. Guidelines
for the modeling and interpretation of VLBI observations are given.Comment: accepted for Ap.
The Formation of Cataclysmic Variables with Brown Dwarf Secondaries
The present-day formation of cataclysmic variables (CVs) with brown dwarf
(BD) secondaries (0.013 M_sun < M_sec < 0.075 M_sun) is investigated using a
population synthesis technique. Results from the latest, detailed models for
BDs have been incorporated into the population synthesis code. For our models,
we find that ZACVs with BD secondaries have orbital periods in the range 46 min
to 2.5 hrs. We also find that ZACVs with BD secondaries comprise 18% of the
total, present-day ZACV population. In addition, we find that 80% of ZACVs with
BD secondaries have orbital periods < 78 minutes. This implies that 15% of the
present-day ZACV population should have orbital periods shorter than the
observed orbital period minimum for CVs. We also investigate the dependence of
the present-day formation rate of CVs with BD secondaries on the assumed value
of the common envelope efficiency parameter, alpha_CE, for three different
assumed mass ratio distributions in ZAMS binaries. Surprisingly, we find that
the common envelope process must be extremely inefficient (alpha_CE < 0.1) in
order for CVs with BD secondaries not to be formed. Finally, we find that the
progenitor binaries of ZACVs with BD secondaries have ZAMS orbital separations
< 3 AU and ZAMS primary masses between ~1-10 M_sun, with ~75% of the primary
masses less than ~1.6 M_sun. Interestingly, these ranges in orbital separation
and primary mass place the majority of the progenitor binaries within the
so-called ``brown dwarf desert.''Comment: preprint 27 pages 4 figures; to appear in ApJ April 1, 200
Thermal Instability and Photoionized X-ray Reflection in Accretion Disks
We study the X-ray illumination of an accretion disk. We relax the
simplifying assumption of constant gas density used in most previous studies;
instead we determine the density from hydrostatic balance. It is found that the
thermal ionization instability prevents the illuminated gas from attaining
temperatures at which the gas is unstable. In particular, the uppermost layers
of the X-ray illuminated gas are found to be almost completely ionized and at
the local Compton temperature ( K); at larger depths, the gas
temperature drops abruptly to form a thin layer with K, while at
yet larger depths it decreases sharply to the disk effective temperature. We
find that most of the Fe K line emission and absorption edge are
produced in the coolest, deepest layers, while the Fe atoms in the hottest,
uppermost layers are generally almost fully ionized, hence making a negligible
contribution to reprocessing features in keV energy range. We
provide a summary of how X-ray reprocessing features depend on parameters of
the problem. The results of our self-consistent calculations are both
quantitatively and qualitatively different from those obtained using the
constant density assumption. Therefore, we conclude that X-ray reflection
calculations should always utilize hydrostatic balance in order to provide a
reliable theoretical interpretation of observed X-ray spectra of AGN and GBHCs.Comment: Submitted to ApJ; 16 pages plus 13 figure
Chemical Abundances in the Secondary Star in the Black Hole Binary A0620-00
Using a high resolution spectrum of the secondary star in the black hole
binary A0620-00, we have derived the stellar parameters and veiling caused by
the accretion disk in a consistent way. We have used a chi^2 minimization
procedure to explore a grid of 800 000 LTE synthetic spectra computed for a
plausible range of both stellar and veiling parameters. Adopting the best model
parameters found, we have determined atmospheric abundances of Fe, Ca, Ti, Ni
and Al. The Fe abundance of the star is [Fe/H]=0.14 +- 0.20. Except for Ca, we
found the other elements moderately over-abundant as compared with stars in the
solar neighborhood of similar iron content. Taking into account the small
orbital separation, the mass transfer rate and the mass of the convection zone
of the secondary star, a comparison with element yields in supernova explosion
models suggests a possible explosive event with a mass cut comparable to the
current mass of the compact object. We have also analyzed the Li abundance,
which is unusually high for a star of this spectral type and relatively low
mass.Comment: 32 pages, 5 tables and 11 figures, uses rotate.st
The system parameters of DW Ursae Majoris
We present new constraints on the system parameters of the SW Sextantis star
DW Ursae Majoris, based on ultraviolet (UV) eclipse observations with the
Hubble Space Telescope. Our data were obtained during a low state of the
system, in which the UV light was dominated by the hot white dwarf (WD)
primary. Eclipse analysis, using the full Roche lobe geometry, allows us to set
firm limits on the masses and radii of the system components and the distance
between them: 0.67 \leq M_1/M_sun \leq 1.06, 0.008 \leq R_1/R_sun \leq 0.014,
M_2/M_sun > 0.16, R_2/R_sun > 0.28 and a/R_sun > 1.05. For q = M_2/M_1 < 1.5
the inclination must satisfy i > 71 degrees. Using Smith & Dhillon's
mass-period relation for CV secondaries, our estimates for the system
parameters become M_1/M_sun = 0.77 \pm 0.07, R_1/R_sun = 0.012 \pm 0.001,
M_2/M_sun = 0.30 \pm 0.10, R_2/R_sun = 0.34 \pm 0.04, q =0.39 \pm 0.12, i = 82
\pm 4 degrees and a/R_sun = 1.14 \pm 0.06. We have also estimated the spectral
type of the secondary, M3.5 \pm 1.0, and distance to the system, d =930 \pm 160
pc, from time-resolved I- and K-band photometry. Finally, we have repeated
Knigge et al.'s WD model atmosphere fit to the low-state UV spectrum of DW UMa
in order to account for the higher surface gravity indicated by our eclipse
analysis. In this way we obtained a second estimate for the distance, d = 590
\pm 100 pc, which allows us to obtain a second estimate for the spectral type
of the secondary, M7 \pm 2.0. We conclude that the true value for the distance
and spectral type will probably be in between the values obtained by the two
methods.Comment: 23 pages including 5 figures and 3 tables. Accepted for publication
in Ap
The population of close double white dwarfs in the Galaxy
We present a new model for the Galactic population of close double white
dwarfs. The model accounts for the suggestion of the avoidance of a substantial
spiral-in during mass transfer between a giant and a main-sequence star of
comparable mass and for detailed cooling models. It agrees well with the
observations of the local sample of white dwarfs if the initial binary fraction
is close to 50% and an ad hoc assumption is made that white dwarfs with mass
less than about 0.3 solar mass cool faster than the models suggest. About 1000
white dwarfs brighter than V=15 have to be surveyed for detection of a pair
which has total mass greater than the Chandrasekhar mass and will merge within
10 Gyr.Comment: 15 pages, 7 figures, to appear in Proc. ``The influence of binaries
on stellar population studies'', Brussels, August 2000 (Kluwer, D. Vanbeveren
ed.
SN Ib 1990I: Clumping and Dust in the Ejecta?
Photometry and spectra of the type Ib SN 1990I are presented and analysed,
covering about 400 days of evolution. The presence of optical helium lines is
shown. SN 1990I seems to show higher velocities compared to a sample of type Ib
events. The nebular emission lines display a high degree of asymmetry and the
presence of fine structures. Using the [O I] 6300,64A flux, we estimate a lower
limit on the oxygen mass to fall in the range (0.7-1.35) M_solar. The oxygen
mass requires a filling factor as small as 10^{-2} on day 254, indicating a
highly clumpy distribution of the oxygen material. A blueshift of the order 600
km/s is reported in the [O I] 6300,64A after day 254. The [Ca II] 7307.5A
emission profile appears blueshifted as well at late epochs . We recover the
quasi-bolometric "BVRI" light curve of SN 1990I. The constructed bolometric
light curve shows a change of slope at late phases, with an e-folding time of
60 +/-2 d in the [50:200] d time interval, considerably faster than the one of
{56}Co decay (i.e. 111.3 d), suggesting the gamma-rays escape with lower
deposition, owing to the low mass nature of the ejecta. After day 200, an
-folding time ~47 +/-2.8 d is measured. A simplified gamma-ray deposition
model is applied after adding a contribution of about 35% to the computed
pseudo-bolometric light curves to account for near-IR luminosities to estimate
the ejecta and {56}Ni masses (M({56}Ni)=0.11 M_solar and M_{ej}=3.7 M_solar).
The deficit in luminosity is estimated to be about 50% around day 308. The
observed spectral blueshift combined with the dramatic and sudden drop in the
pseudo-bolometric light curve and (B-V) colour is interpreted to be a
consequence of dust condensation in the ejecta of SN 1990I around day 250.Comment: 17 pages, 11 gigures and 5 tables. To appear in A & A (accepted
Optimizing Cadences with Realistic Light Curve Filtering for Serendipitous Kilonova Discovery with Vera Rubin Observatory
Current and future optical and near-infrared wide-field surveys have the potential of finding kilonovae, the optical and infrared counterparts to neutron star mergers, independently of gravitational-wave or high-energy gamma-ray burst triggers. The ability to discover fast and faint transients such as kilonovae largely depends on the area observed, the depth of those observations, the number of re-visits per field in a given time frame, and the filters adopted by the survey; it also depends on the ability to perform rapid follow-up observations to confirm the nature of the transients. In this work, we assess kilonova detectability in existing simulations of the LSST strategy for the Vera C. Rubin Wide Fast Deep survey, with focus on comparing rolling to baseline cadences. Although currently available cadences can enable the detection of more than 300 kilonovae out to 1400 Mpc over the ten-year survey, we can expect only 3-32 kilonovae similar to GW170817 to be recognizable as fast-evolving transients. We also explore the detectability of kilonovae over the plausible parameter space, focusing on viewing angle and ejecta masses. We find that observations in redder izy bands are crucial for identification of nearby (within 300 Mpc) kilonovae that could be spectroscopically classified more easily than more distant sources. Rubin's potential for serendipitous kilonova discovery could be increased by gain of efficiency with the employment of individual 30s exposures (as opposed to 2x15s snap pairs), with the addition of red-band observations coupled with same-night observations in g- or r-bands, and possibly with further development of a new rolling-cadence strategy
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